# Combustion of NH3/CH4 mixtures in a swirl burner: Study of non-premixed flames with radial fuel injection

Antoine Morel, Toufik Boushaki

► To cite this version:

Antoine Morel, Toufik Boushaki. Combustion of NH3/CH4 mixtures in a swirl burner: Study of non-premixed flames with radial fuel injection. Experimental Thermal and Fluid Science, 2026, 171, pp.111630. 10.1016/j.expthermflusci.2025.111630. hal-05344703

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# Combustion of NH<sub>3</sub>/CH<sub>4</sub> mixtures in a swirl burner: Study of non-premixed flames with radial fuel injection

Antoine Morel, Toufik Boushaki*

<figure><img src="image_2.png" /></figure>

ICARE-CNRS, University of Orléans, Orléans, France

## ARTICLE INFO

### Keywords:

Ammonia combustion
Swirling flame
Flame stability
NOx

## ABSTRACT

The transition to renewable energy is essential in addressing climate change. While natural gas plays a significant role in this transition, it still produces CO₂ emissions. Ammonia (NH₃) is being investigated as a promising alternative fuel. However, ammonia combustion presents several technical challenges, such as low flame velocity, limited calorific value, difficulties with flame stabilization, and high NOx emissions. This study examines the impact of ammonia addition to methane, equivalence ratio, and swirl number on pollutant emissions (NO, CO, CH₄, and CO₂), exhaust gas temperature, and flame stability. Experiments are carried out using a swirl burner with a radial fuel injection in a 1-meter high combustion chamber. The burner consists of two concentric tubes, with the inner tube supplying fuel and the outer tube supplying air. The fuel is injected radially through eight holes at the burner exit. The ammonia fraction ranges from 0 to 100 %, the equivalence ratio from 0.8 to 1.0, and the swirl number from 0.8 to 1.4, with a constant flame power of 10 kW. Emissions of NO, CO, CH₄, and CO₂ are measured in the dry exhaust gases using a multi-gas analyzer, inside chamber temperatures are measured and the flame structure is analyzed via OH* and NH₂* chemiluminescence and velocity measurements by LDA technique. The results show that both the swirl number and equivalence ratio significantly alter flame geometry, affecting combustion zones and flame height. Axial velocity measurements indicate that the recirculation zone shrinks with ammonia addition, while a high swirl number increases axial velocity, promoting ammonia combustion and upstream flame propagation. Up to 30 % ammonia, the flame remains stable, but higher ammonia levels lead to fluctuations in stabilization. High ammonia content in the fuel mixture results in reductions of NO, CO, and CO₂ emissions, though there is a potential increase in unburned gases. As expected, the inside chamber temperatures decrease as the ammonia fraction increases.

## 1. Introduction

With climate change, the constant increase in greenhouse gas emissions, and a growing energy demand, the energy transition has become a global priority. Current projections show that, to meet the Paris Agreement targets and limit warming to 1.5 °C, it is crucial to drastically reduce CO₂ emissions across all sectors, including industry and transportation.

In this context, exploring new combustion technologies that are both cleaner and more efficient is essential. Ammonia (NH₃), as a carbon-free fuel, has great potential to address these challenges [1,2]. However, despite its advantages, using ammonia in combustion processes presents several technical challenges, particularly in terms of nitrogen oxides (NOx) emissions and flame stabilization due to its low combustion characteristics (such as laminar flame velocity, flammability limits, and

calorific value) compared to those of methane (CH₄) or hydrogen (H₂). Many recent studies have highlighted the differences between the combustion properties of NH₃, CH₄ and H₂ [3–5] as well as the use of NH₃/H₂ blends [6–9]. However, significant challenges remain, particularly regarding hydrogen storage, transportation, and distribution. Given its abundance and widespread use, methane presents a strong alternative for blending with ammonia.

Industrial applications, such as furnaces and gas turbines, require a deep understanding of non-premixed turbulent combustion, given the wide variety of burner and combustion chamber configurations employed. This process involves complex interactions between chemistry, thermodynamics, and fluid mechanics. In high-power flame systems, turbulence plays a crucial role and can be controlled through the use of swirling jets, enhancing flame anchoring through recirculation zones, as demonstrated in several studies [10–12], as well as better

<footnote>* Corresponding author at: ICARE, CNRS, 1C, Avenue de la Recherche Scientifique, 45071 Orléans, France.</footnote>

<footnote>E-mail address: toufik.boushaki@cnrs-orleans.fr (T. Boushaki).</footnote>
mixing between fuel, oxidant and burnt gases. Under swirling jet conditions, many studies have been conducted with premixed flames of ammonia blends to determine stability and NOx emissions. Introducing swirl creates a central toroidal recirculation zone (CTRZ) that stabilizes the flame by recirculating hot gases into the unburnt mixture. This promotes ignition, improves mixing, and enhances thermal homogeneity, potentially increasing combustion efficiency and reducing some pollutants [13–16]. However, the effect of swirl on pollutant emissions, particularly NOx, is complex and non-monotonic. Some studies have reported that excessive swirl can induce hydrodynamic instabilities or even flame blow-off and may increase NOx formation (Jojka and Sleafarski [17]), whereas other works have observed the opposite trend. For example, Wang et al. [18,19] demonstrated through direct NO measurements and NO-PLIF imaging that increasing swirl intensity can significantly reduce NOx emissions by improving mixing and lowering local flame temperatures. Liu et al. [20] further highlighted that both very low and very high swirl intensities can result in elevated NOx levels, and that a moderate swirl intensity providing an optimal balance between mixing efficiency and temperature control can minimize NOx formation. These discrepancies highlight that the impact of swirl strongly depends on operating conditions such as ammonia fraction, global equivalence ratio, pressure, and heat losses. Therefore, swirl remains a critical but highly sensitive parameter for flame stabilization and emission control in ammonia-methane combustion systems.

To further elucidate the mechanisms behind these complex trends, it is essential to examine the detailed radical chemistry governing NO formation and reduction in CH₄/NH₃ combustion. From studies on NO formation kinetics, 4 radicals are the main responsible for NO formation mechanism, more particularly from fuel-NO and thermal-NO pathways: OH, NH, NH₂ and HNO. Their formation is very complex under NH₃/CH₄ non-premixed flames and not studied under the specific configuration of this study. But it is still possible to understand and give suggestions on some underlying process happening in the flames.

First of all, OH radical production is highly boosted by methane due to its faster oxidation chemistry. As explained by Ariemma et al. [21], this high production of OH radical added with NH₃ allows the fast production of NH₂ radical (NH₃ + OH = NH₂ + H₂O) and the production of NO through the pathway: NH₃ → NH₂ → NH → HNO → NO. At low NH₃ proportion, this fast production/consumption of NH₂ limits the concentration of the DeNOx agent forming N₂ from the equation NH₂ + NO = N₂ and leading to an increase in NOx emissions. When the proportion of ammonia become significant in the fuel blend, Elbaz et al. [22] explain that NO concentration decreases due to decrease of O, H and OH radicals. They explain the decrease of NO by an abundance of NHi termination reactions.

These mechanistic trends have been corroborated by several numerical and experimental studies. For instance, Romano et al. [23] used a CFD-CRN approach for non-premixed flames with 30 % and 50 % NH₃. They identified three main NO-forming reactions (H + HNO = H₂ + NO; N + OH = H + NO; NH + O = H + NO) and two dominant NO-consuming reactions (N + NO = N₂ + O; NH + NO = H + N₂O). Their results show that increasing the NH₃ proportion from 30 % to 50 % doubles the NO consumption rate and increases NO formation by 50 %, highlighting a dynamic balance between formation and destruction pathways.

Building on this, Wang et al. [24] provide a complementary and more mechanistically detailed view by performing a sensitivity analysis using Chemkin for both premixed and dual flame modes at $\phi$ = 0.7. In premixed flames, the reaction NH₂ + O = HNO + H exhibits the highest positive sensitivity coefficient, indicating that NH₂ consumption through HNO formation is the dominant step in NO production. This coefficient increases sharply with NH₃ content. Additionally, the reaction N₂O + O = ₂NO gains prominence, while key NO reduction reactions such as NH₂ + NO = NNH + OH and NH + NO = N₂O + H become less effective at high NH₃ levels. This suggests that the increased NH₃ inhibits NO-consuming channels, contributing further to NO build-

up. In dual flame configurations, the dominant reactions shift. The OH-forming reaction H + O₂ = O + OH becomes the most sensitive, highlighting the role of flame structure and local stoichiometry. Here too, NH₂ + NO = N₂ + H₂O remains a crucial NO-reducing step but shows diminished sensitivity at higher NH₃ fractions. Interestingly, the reaction NH₂ + NO = NNH + OH switches from an inhibiting role at low NH₃ (10–30 %) to a promoting one at 50 %, indicating a regime-dependent behavior of NO pathways tied to radical availability and temperature fields. These results from Wang et al. thus reinforce the critical role of NH₂ and OH radicals in governing both the fuel-NO and thermal-NO mechanisms, with their interaction tightly modulated by flame type, equivalence ratio, and NH₃/CH₄ ratio.

These mechanistic perspectives are further complemented by the effect of flame temperature and radical availability. Somarathne et al. [25], through numerical analysis of CH₄/NH₃/air flames in a two-stage combustor, observed that NO emissions peak at NH₃ energy fractions (ENH₃) between 20 % and 30 %, coinciding with maximum local OH concentrations and flame temperatures around a local equivalence ratio ($\phi$ local) of 0.9. Notably, the NO concentration increases exponentially above approximately 2100 K, driven by a corresponding rise in OH radical concentration. As the NH₃ addition increases beyond this range, both temperature and OH radical concentrations decrease, contributing to the observed drop in NO emissions. This demonstrates that the interplay between flame temperature and OH radical availability strongly governs NO production in CH₄/NH₃ flames, complementing the chemical pathway explanations by Ariemma et al. [21] and Elbaz et al. [22]. Together, these findings highlight the critical role of flame dynamics, temperature, radical chemistry, and flow recirculation in controlling NO emissions across varying NH₃ proportions.

In summary, these studies collectively demonstrate that NO emissions in CH₄/NH₃ flames result from a delicate interplay of radical chemistry, flame temperature, equivalence ratio, swirl-induced recirculation, and fuel injection configuration. Understanding these coupled effects is essential for optimizing combustion strategies and NOx mitigation.

The equivalence ratio ($\phi$) also plays a crucial role in pollutant emissions and flame stability in premixed flames. As reported by Zhang et al. [26], under fuel-lean conditions, adding small amounts of methane or hydrogen to ammonia flames enhances flame stabilization but significantly increases NOx emissions, particularly NO and NO₂. Moreover, N₂O formation is prominent in lean mixtures due to lower flame temperatures and reduced radical concentrations, which limit its decomposition. Similarly, experimental results from flat premixed NH₃–CH₄-air flames [27] show that increasing ammonia content leads to a substantial rise in NO and NO₂ emissions across the entire flamability range, including lean conditions. This indicates that ammonia does not chemically reduce NOx, even at lower flame temperatures.

On the other hand, under fuel-rich conditions, the behavior of NOx emissions is more complex and depends strongly on the fuel blend and operating conditions. Zhang et al. [26] report that NO emissions tend to decrease with increasing $\phi$ for a given ammonia concentration, due to suppression of radical-driven oxidation pathways, favoring conversion of NHi species directly to N₂. However, NO levels remain higher for ammonia-containing mixtures than for pure methane flames because of the additional nitrogen from ammonia. Contrasting, experimental results by Ariemma et al. [21] show that for pure methane, NOx emissions increase from about 8 ppm at $\phi$ = 0.5 to 20 ppm at $\phi$ = 0.7, then sharply decrease to below 10 ppm at $\phi$ ≥ 1.4. Pure ammonia flames exhibit a monotonic decrease in NOx emissions from ~340 ppm at $\phi$ = 0.7 down to ~10 ppm at $\phi$ = 1.3·NH₃/CH₄ blends show the highest NOx levels, peaking near 1000 ppm at $\phi$ ≈ 0.7–0.8 before abruptly dropping toward rich conditions. This non-monotonic trend highlights the complex interplay between fuel composition and flame stoichiometry in pollutant formation.

Despite extensive research on premixed ammonia-based flames, their practical implementation in industrial combustors remains limited. As
noted by Valera-Medina et al. [28], fully premixed injection strategies are ill-suited for optimized ammonia combustion, particularly at moderate swirl numbers where strong flame instabilities are observed. This underscores the need for alternative injection strategies—such as non-premixed configurations—and the use of lower swirl intensities to ensure stable and efficient ammonia-based power generation.

In this context, non-premixed combustion emerges as a more viable approach. It offers spatial control over mixing, reduces pre-ignition risks, and enhances flame stability through localized fuel-oxidizer interactions. Findings from the two studies by Elbaz et al. [22,29], which allow a direct comparison between premixed and non-premixed NH₃/CH₄ flames, show that non-premixed swirling combustion provides clear advantages in terms of flame stabilization and emission control—especially at high ammonia fractions. While swirl effectiveness decreases at very high NH₃ content, non-premixed systems still offer a broader operational window and greater flexibility for optimizing flame behavior and minimizing pollutants. In contrast, premixed ammonia-rich flames are more prone to instability and tend to produce higher NO emissions under both lean and rich conditions, complicating their practical use without risking blow-off or emission spikes. In terms of swirl effect on non-premixed flames, studies reported same effect as for premixed flames, the only but important difference comes from fuel-oxidant mixing as reported by Terasaki and Hayashi [30] which allows control on NOx emission as well as flame temperature.

Lean combustion strategies are commonly employed for NOx mitigation in hydrocarbon-fueled systems. However, studies reveal a different trend when ammonia is used. According to Somarathne et al. [31], while lean combustion reduces NOx emissions in both NH₃ and CH₄ flames, it is significantly less effective for ammonia due to persistent NO formation pathways mediated by OH radicals. In ammonia combustion, lean operation alone is insufficient; it must be combined with additional measures such as air preheating or optimized swirl to improve combustion efficiency and limit unburnt NH₃ emissions. Yamashita et al. [32] confirmed the central role of OH radicals in this context, noting that their concentration tends to be lower in lean NH₃ flames. This reduction directly impacts NO formation and explains the high sensitivity of lean ammonia combustion to fuel-air mixing. They emphasize that achieving low emissions under lean conditions requires tight control of the equivalence ratio due to a narrow operating window. Biebl et al. [33] further demonstrated that although lean ammonia flames can be stabilized, they may exhibit increased NOx emissions. Interestingly, increasing the equivalence ratio toward stoichiometric conditions can reduce NOx levels, primarily due to lower oxygen availability for thermal NO formation.

In contrast, rich combustion conditions offer more favorable emission profiles for ammonia-based flames. Tu et al. [34] observed a significant decrease in NO emissions in rich non-premixed flames compared to premixed configurations. Similarly, Yamashita et al. [32] reported a notable reduction in NOx emissions under rich conditions compared to lean combustion, identifying an optimal equivalence ratio—located in the rich regime—that minimizes both NOx and unburned NH₃. However, they also caution that increasing the NH₃ fraction in CH₄/NH₃ blends narrows this low-emission window, making the system more sensitive to equivalence ratio variations. Furthermore, Somarathne et al. [31] explain that operating under rich conditions limits NO formation not only due to oxygen deficiency but also because NO is produced through less efficient, non-thermal pathways. However, despite these promising reductions in pollutant emissions, operating under rich combustion conditions presents notable practical and industrial limitations. From a safety standpoint, rich flames are generally associated with an increased risk of incomplete combustion, flame instability, and potential accumulation of unburned fuel, which can pose explosion hazards. In addition, rich combustion is inherently less efficient, as excess fuel remains unoxidized, leading to increased fuel consumption without corresponding energy output. This trade-off between emission control and combustion efficiency raises concerns regarding

the viability of sustained rich operation in real-world applications, particularly in large-scale industrial systems where fuel economy and operational safety are critical considerations.

Many recent studies have investigated the effects of swirl intensity and mixture composition on ammonia-methane combustion. However, these investigations primarily focus on axial fuel injection configurations. While it is well established that swirl significantly enhances flame stabilization and helps reduce pollutant emissions [35–37], the fuel injection strategy itself also plays a crucial role in governing mixture formation and flame dynamics in the primary combustion zone. Specifically, axial injection often leads to a diffusion-like flame structure, where the cold fuel jet intermittently penetrates the central recirculation zone. This can result in localized over-rich regions and, in some cases, the formation of luminous soot plumes. In contrast, radial injection facilitates a more uniform and rapid mixing between the fuel and the swirling air, promoting a near-premixed flame regime with improved stability and significantly reduced soot formation. These observations suggest that injection geometry, often overlooked in ammonia-methane combustion studies, may have a non-negligible impact on flame stabilization mechanisms, pollutant formation, and overall combustion performance.

Despite the increasing interest in NH₃ combustion for low-carbon energy systems, key knowledge gaps persist concerning flame stability, pollutant emissions, and the interaction between chemical kinetics and turbulence in non-premixed NH₃/CH₄ flames—especially in radial injection swirl burners.

This study aims to address these gaps through a comprehensive experimental analysis of NH₃/CH₄ swirl flames with radial injection. Unlike coaxial or simply swirl-stabilized configurations, radial injection plays a crucial role in flame stabilization, significantly altering the interactions between fresh and burnt gases. Additionally, this configuration affects pollutant emissions behavior, which remains poorly understood in such burners. Therefore, it is essential to investigate the impact of key parameters such as ammonia content, swirl number, and equivalence ratio on emissions (NOx, CO, etc.). To achieve this, the study relies on an experimental approach integrating optical diagnostics (OH*, NH₂*, LDA), temperature measurements, and a detailed evaluation of emissions. The objectives are to characterize chemiluminescence, flame topology, emissions, and temperature distributions in the chamber across various ammonia-methane mixtures, from pure methane to pure ammonia, under different equivalence ratios and swirl numbers. Additionally, this work provides axial velocity data useful for validating numerical models, thereby enhancing the accuracy of simulations and supporting the practical implementation of NH₃/CH₄ combustion systems.

## 2. Experimental setup and techniques

### 2.1. Burner and measurement setup

The components of the burner are illustrated in Fig. 1. The burner consists of two concentric tubes through which the fuel and air are supplied. The fuel (methane and ammonia) is delivered through the central tube, while the oxidant (air) is supplied through the annular section. The fuels are mixed prior to reaching the burner. The outer diameter of the fuel tube is 15 mm, and the inner diameter of the air supply tube is 38 mm. The radial injector features 8 symmetrically distributed holes around the tube, each with a diameter of 3 mm. A swirler is placed inside the outer tube, surrounding the fuel tube injection point, and is positioned at a height of $h = 50$ mm from the burner outlet.

The swirl number, as described by Beér and Chigier [38], is analyzed in this study. Specifically, the geometrical swirl number (Sn) defined by Sheen et al. [39] is considered:
<figure><img src="image_3.png" /><figcaption>Fig. 1. Schematic of the swirl burner with radial fuel injection [36]. a) 3D view of burner's outlet, b) cross section of the burner, c) real view of the swirler.</figcaption></figure>

$$S_n = \frac{1}{1 - \psi} \left(\frac{1}{2}\right) \frac{1 - (R_h/R)^4}{1 - (R_h/R)^2} \tan\alpha_0 \quad (1)$$

where $\alpha_0$ is the vane angle, $\psi$ is the blockage factor, R and $R_h$ are nozzle and vane pack hub radii, respectively. The swirl number was varied by adjusting the vane angle ($\alpha_0$) of the swirler. Two distinct values, 0.8 and 1.4, were selected to represent low and high swirl conditions, respectively. Further details on the determination of the swirl number for this burner are provided in [40].

The scheme of the experimental bench is shown in Fig. 2. It consists of a stainless-steel square based combustion chamber of 48 x 8 cm² and 1 m high operating at atmospheric pressure. The walls and the base of the combustion chamber are water cooled on the outside and lined with refractory (10 mm thick panels) on the inside. The combustion chamber is equipped of 24 windows (6 per side) to allow optical access to the flame. The exhaust section is closed by a convergent with an opening of 10 cm in diameter. The flue gases are exited by ventilation duct allowing non stagnation of the flue gases inside the combustion chamber.

## 2.2. Measurement techniques

In this study, three different variables were measured under multiple conditions: combustion products, chemiluminescence from two different radicals, and temperatures inside the combustion chamber. The experiments were conducted at atmospheric conditions (ambient temperature and pressure) with a constant theoretical thermal power of 10 kW. Three parameters were varied: the ammonia proportion (ranging from 0 to 100 % in 10 % increments), the swirl number (with two values:

0.8 and 1.4), and the equivalence ratio at the injection (with two values: 0.8 and 1.0).

Table 1 summarizes the flow rates of fuels and oxidant for different NH₃ proportions and equivalence ratios. As noted, the fuel flow rates remain constant across different equivalence ratios to maintain the theoretical thermal power. Flow rates are controlled with flow meters, which have an instrument-related error of approximately 0.6 %.

The measurement setup can be divided into four distinct components. The first setup (Fig. 2, a) is the gas analyzer (HORIBA PG250 and ABB AO2020) used to measure the flue gases exiting the combustion chamber. This study focuses on the emissions of CO, NO, CO₂, and CH₄. For the measurements, CO and CH₄ were detected using non-dispersive

<table><caption>Table 1 Operating conditions for different ammonia proportions and equivalence ratios.</caption><thead><tr><th rowspan="2">%NH<sub>3</sub></th><th rowspan="2">Q<sub>fuel</sub> (l/mn)</th><th rowspan="2">Q<sub>CH<sub>4</sub></sub> (l/mn)</th><th rowspan="2">Q<sub>NH<sub>3</sub></sub> (l/mn)</th><th colspan="2">Q<sub>air</sub> (l/mn)</th></tr><tr><th>φ = 0.8</th><th>φ = 1</th></tr></thead><tbody><tr><td>0</td><td>17.64</td><td>17.64</td><td>0</td><td>209.7</td><td>167.76</td></tr><tr><td>10</td><td>18.77</td><td>16.89</td><td>1.88</td><td>209.1</td><td>167.30</td></tr><tr><td>20</td><td>20.05</td><td>16.04</td><td>4.01</td><td>208.5</td><td>166.79</td></tr><tr><td>30</td><td>21.52</td><td>15.06</td><td>6.46</td><td>207.7</td><td>166.19</td></tr><tr><td>40</td><td>23.22</td><td>13.93</td><td>9.29</td><td>206.9</td><td>165.51</td></tr><tr><td>50</td><td>25.22</td><td>12.61</td><td>12.61</td><td>205.9</td><td>164.72</td></tr><tr><td>60</td><td>27.58</td><td>11.03</td><td>16.55</td><td>204.7</td><td>163.78</td></tr><tr><td>70</td><td>30.43</td><td>9.13</td><td>21.30</td><td>203.3</td><td>162.65</td></tr><tr><td>80</td><td>33.95</td><td>6.79</td><td>27.16</td><td>201.6</td><td>161.26</td></tr><tr><td>90</td><td>38.38</td><td>3.84</td><td>34.54</td><td>199.4</td><td>159.51</td></tr><tr><td>100</td><td>44.15</td><td>0</td><td>44.15</td><td>196.5</td><td>157.24</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>Fig. 2. Schematic of the experimental setup. a) Flue gases analysis setup, b) chemiluminescence technique setup, c) Temperature measurement setup, d) LDA measurement setup.</figcaption></figure>
infrared (NDIR) sensors, NO was measured using chemiluminescence, and CO₂ was detected using NDIR with pyroelectric sensors. The gases are transported through a duct heated to 150 °C to prevent water condensation, which could compromise the gas analysis (absorption of CO₂ and possibly other pollutants). A cooling unit (PSS5 pre-sampler) is placed before the analyzer to remove water vapor from the combustion gases before they enter the analyzers, ensuring that the measurements are taken on dry flue gases. The response time for both analyzers is approximately ten seconds. The analyzed gas flow rate is 0.4 NL/min for the PG250 and between 0.8 and 1 NL/min for the AO2020. The gas sample probes used were PSP4000-HCT for the HORIBA/ABB analyzers positioned 120 cm downstream of the burner exit (Fig. 2, a). The estimated uncertainty for emissions is approximately 3 %, which includes 1 % due to instrument-related errors (as indicated by the manufacturer), 1 % for measurement fluctuations, and 1 % for reading errors. Emissions were acquired after the system reached steady-state conditions, with a waiting period of approximately 5 min after the start of the measurement.

The second setup involves chemiluminescence imaging to qualitatively investigate flame dynamics, including global flame structure, fluctuations, and lift-off height. This system includes an Intensified Charge-Coupled Device (ICCD) PI-MAX 4 camera equipped with a UV lens (Nikon PF10545MF-UV) and wavelength-specific filters (Fig. 2, b). The camera is mounted on a 3D translation stage to position the bottom of the image at the burner outlet, thereby maximizing the observation area. Light emitted by the flame passes through the filter to the camera, with the resulting signal transmitted to a controller and then to an acquisition computer. For each condition, the average images were obtained from 200 instantaneous images with an exposure time of 5 ms. Chemiluminescence measurements were performed for two radicals: OH\* and NH₂\*. These radicals were chosen for specific reasons: OH\* is a reliable indicator of CH₄-air combustion as well as NH₃-air combustion [41–43], and NH₂\* is closely associated with ammonia oxidation and exhibits distinct features compared to other radicals like NH\*, as highlighted in several studies [44,45]. The wavelengths selected for these radicals correspond to strong emission bands. While it is known that NH₂\* emission can overlap with broadband signals from species such as H₂O\*, H₂O₂, and CO₂, the attribution of this wavelength to NH₂\* remains a widely accepted convention in the literature to ensure consistency with previous studies. This approach, used in multiple works [44–47], facilitates comparison across different investigations. In addition, the filter used in this setup is very narrow, with a FWHM of 5 nm, which allows only the NH₂\* peak intensity to pass, minimizing the influence of other broadband emissions. However, potential contributions from other species are acknowledged, and caution is exercised in the quantitative interpretation of the measured signals. For OH\*, a bandpass filter centered at 310 nm with a full width at half maximum (FWHM) of 5 nm was used, and for the NH₂\* radical, a bandpass filter centered at 632 nm with a FWHM of 5 nm was employed. Although OH\* and NH₂\* radicals were not measured simultaneously, the operating conditions were carefully controlled and monitored using solenoid valves to maintain stable fuel and air flow rates. In addition to chemiluminescence, direct photographs were taken to compare the visual appearance of the flame with the chemiluminescence results, using a CANON EOS 77D camera with an EFS 18–55 mm lens and an exposure time of 1/8 s. The positioning of the ICCD camera for chemiluminescence imaging was done by aligning the sight at the center of the burner. The measurement field covers 109x109 mm² with a projection pixel size of 9.4 pixels/mm. The uncertainty associated with the conversion factor is estimated at 0.8 %, 3.5 % for the threshold value, and 1 % for the burner position. Therefore, the total uncertainty is approximately 3.7 %.

The third setup involves temperature measurements inside the combustion chamber (Fig. 2, c). Four measurement points were defined: one near the wall T<sub>wall</sub> with the thermocouple tip protruding 5 mm from the refractory and 53 mm above the burner exit; one near the flame at

the bottom of the combustion chamber T<sub>bottom</sub>, located 65 mm from the burner's axis of symmetry and 78 mm above the burner exit; one at the chamber center relative to the burner axis T<sub>middle chamber</sub>; and a final point at the chamber exit, aligned with the gas sample probe Texit. T<sub>wall</sub>, T<sub>bottom</sub> and T<sub>middle chamber</sub> were measured using type K thermocouples, while Texit was measured using FeCu-Ni thermocouple for φ = 0.8 and NiCr-Ni thermocouple for φ = 1.0 measurements. A similar approach was used for temperature measurements to determine uncertainties, which were acquired under the same conditions as emissions. The uncertainty for temperature measurements includes 0.75 % from instrument-related errors (as indicated by the manufacturer), 1 % from measurement fluctuations, and 1 % from reading errors, giving a total uncertainty of about 2.75 %.

Finally, the fourth experimental setup is designed for Laser Doppler Anemometry (LDA) to precisely measure the axial velocity component in reactive flows (Fig. 2, d). The TR-SS-2D Laser Doppler Velocimetry (LDV) system, which operates with solid-state lasers, consists of a PowerSight probe, a PDM1000 signal filtering module, and an FSA3500 signal processor for data acquisition and analysis. In this study, only laser beams with a wavelength of 561 nm are used. These beams intersect at a focal point, forming an ellipsoidal measurement volume where particle velocities are accurately determined based on the Doppler effect. A 350 mm focal length lens is employed to focus the laser beams, ensuring optimal convergence at the measurement volume. To prevent thermal expansion due to the proximity of the flame, the lens is maintained at a temperature of 298 K with a fan. Zirconium oxide (ZrO₂) tracer particles, selected for their high melting point and resistance to extreme temperatures, are used to ensure reliable measurements in the combustion environment. These particles are preheated to 383 K to minimize agglomeration. The Doppler signals obtained are processed to calculate velocity values with an accuracy better than 0.5 %. A high-precision 3D positioning system, identical to the one used in the chemiluminescence setup, is utilized to align the laser relative to the burner. This system offers a positioning resolution of 0.1 mm, allowing for detailed mapping of the flow field. Measurements were conducted along four horizontal lines intersecting the burner's central axis at heights of 10 mm, 20 mm, 38 mm, and 60 mm. These specific heights were chosen to correspond to the flame's position, ensuring a comprehensive representation of the flow field.

## 2.3. Post-processing techniques

The lift-off height H<sub>lo</sub> was calculated based on the flame luminous

<figure><img src="image_5.png" /><figcaption>Fig. 3. Definition graph of lift-off height parameter.</figcaption></figure>
intensity distribution derived from instantaneous chemiluminescence images of NH₂* and OH*, as illustrated in Fig. 3. First, the image was binarized to extract the flame contour. A threshold of 25 % of the maximum intensity was applied uniformly across all cases to eliminate background noise and weak chemiluminescence regions. Using the same threshold value facilitates the comparison between different conditions. This threshold was chosen as a balance between excluding low-intensity background noise and preserving the primary flame structure, ensuring an accurate determination of H₁₀. The lift-off height is determined separately for each radical to compare their respective distributions and assess potential differences. Using this contour and the known pixel-to-millimeter conversion scale, the lift-off height H₁₀ was determined. It is important to note that the definition of the lift-off height in this context is dependent on the measurement method used, including the binarization technique and threshold criteria. As such, this definition may not have general applicability and can vary depending on experimental conditions and methodological choices. Other studies using different techniques (e.g., direct imaging [48], LIF [49], or alternative threshold criteria) may yield slightly different results.

Since the intensity distribution was not perfectly axisymmetric, likely due to short exposure times and fluctuations, the deconvolution was performed on half-images split along the burner axis, assuming local axisymmetric on each half. This approach mitigates the effects of asymmetry and allows meaningful reconstruction of radical distributions. To compare these distributions with direct photographs, the chemiluminescence results for OH* and NH₂* radicals were superimposed. Fig. 4 illustrates the method used for superimposing the images. First, the time-averaged chemiluminescence images for NH₂* and OH* were subjected to an inverse Abel transform. The intensity scales of each image were then normalized by dividing by their respective maximum intensities. The normalized images were subsequently multiplied to produce the final superposition. Before generating the final image, radical production zone contours were extracted from the inverse Abel transform using a threshold of 35 %, chosen to accurately delineate radical production boundaries. This threshold differs from the one used for lift-off height determination due to the specific processing requirements of the inverse Abel transform. The resulting superimposed image provides a clearer visualization of the combustion zones, highlighting overlapping regions (depicted in green) and areas where the radicals remain distinct (red for NH₂* and blue for OH*). The observed fluctuations in the images are primarily attributed to the asymmetry of the flame.

## 3. Results and Discussions

The results are organized into three sections. The first section presents the chemiluminescence observations of OH* and NH₂* radicals

along with axial velocity measurements obtained using Laser Doppler Anemometry (LDA), providing an overview of the global flame structure and flow field characteristics. The second section investigates the emissions of pollutants, including NO, CO, CO₂, and CH₄, and compares these emissions with the observed flame features and temperature data. The final section analyzes the temperature measurements within the combustion chamber, evaluating their relationship with the swirl number and equivalence ratio.

### 3.1. Flame topology and reactive zone features

#### 3.1.1. Direct chemiluminescence analysis

Chemiluminescence has been investigated for two wavelengths corresponding to the strong emission bands of OH* and NH₂* radicals. Figs. 5, 6, 7, and 8 display time-averaged chemiluminescence images for ammonia proportions of 0 %, 30 %, 60 %, and 80 %, with swirl numbers of 0.8 and 1.4, and equivalence ratios of 0.8 and 1.0. In addition to chemiluminescence images, direct photographs are provided for each case to identify the flame zones associated with radical production. To facilitate comparison across different ammonia proportions, each intensity bar is normalized to the maximum intensity measured from the averaged images of the corresponding filter.

As NH₃ proportion increases, a consistent color transition is observed in all configurations: the characteristic blue of soot-free methane combustion gradually shifts to a mix of dark orange and white, eventually becoming a light orange flame at high ammonia concentrations, typical of NH₃ combustion. These color changes reflect the underlying complexity of combustion chemistry, which varies with ammonia proportion.

Regarding flame behavior, significant differences arise. Flame attachment to the burner, for instance, is highly dependent on the swirl number and equivalence ratio. Notably, only the case with Sn = 0.8 and φ = 1.0 shows lifted flames at 60 % and 80 % ammonia. Additionally, distinct changes in flame shape are visible. For ammonia proportions above 0 %, the flame tends to adopt a conical shape before experiencing base strangulation as the ammonia proportion increases. This strangulation occurs at lower ammonia levels for an equivalence ratio of 1.0 and is less pronounced at a swirl number of 1.4.

The chemiluminescence intensities for both filters follow a consistent trend across all cases: OH* radical intensity decreases with increasing ammonia, while NH₂* radical intensity rises with higher NH₃ content.

Initially, it is observed that in each case, NH₂* radical appears earlier in the flame than OH* radical. According to Davidson et al. [50], the primary reactions in ammonia decomposition are:

$$NH_3 + M \rightarrow NH_2 + H + M \text{ and } NH_3 + H \rightarrow NH_2 + H_2.$$

Moreover, flame temperature plays a critical role, as the rate coefficient of a reaction is temperature-dependent. According to the

<figure><img src="image_6.png" /><figcaption>Fig. 4. Construction schema of the superposition image of inverse Abel transform OH* and NH₂*chemiluminescence.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. Chemiluminescence of OH\* and NH₂\* radicals with direct photograph depending on NH₃ proportion, φ = 0.8, Sn = 0.8.</figcaption></figure>

generalized reaction scheme for ammonia oxidation presented by Zhu et al. [9] and the observations of Smyth et al. [51], the reaction temperature must be around 2080 K to produce significant OH\* radicals, whereas NH₂\* radicals can form at lower temperatures. As highlighted by Vigueras-Zuniga et al. [44], a high NH₂\* concentration is noticeable in regions in contact with recirculation zones and hot products, meaning that part of the ammonia reacts with recirculating hot burned gases, leading to NH₂\* production. This explains why the production zones of OH\* and NH₂\* become more distinct as the ammonia proportion increases.

As ammonia content increases, the OH\* production zone elongates along its length, while the NH₂\* production zone contracts. This phenomenon is attributed to changes in flow dynamics as well as flame chemistry. The central recirculation zone (CRZ), a crucial flame stabilizer, anchors the flame around it, and its position is highly dependent on axial (Fig. 14 and Fig. 15) and tangential flow velocities. These velocities change with ammonia addition to maintain constant theoretical flame power. With radial fuel injectors, a cross-flow is induced, modifying the velocity field and affecting the CRZ's location and shape. Notably, for high NH₃ proportions, the strength of the recirculation zone just above the burner exit is significantly greater than for lower NH₃ proportions. Consequently, burned gases are drawn further downward, allowing more ammonia to interact with them in this region, which explains the contraction of the NH₂\* production zone. Conversely, the

central recirculation zone, located further downstream of the burner, appears to change shape, as evidenced by axial velocity measurements at 60 mm. This could potentially explain the elongation of the OH\* production zone, in addition to the probable modifications in flame chemistry at high NH₃ concentrations. As the equivalence ratio increases while maintaining the same theoretical power, the air injection flow rate decreases, enhancing the cross-flow effect, which significantly impacts the dynamics [52]. This explains the observable flame strangulation at the flame base at lower NH₃ proportions. Additionally, a higher swirl number appears to reduce the intensity of this strangulation.

Fig. 9 presents the normalized chemiluminescence intensity for each filter as a function of ammonia proportion, swirl number, and equivalence ratio. It shows that OH\* intensities decrease with increasing ammonia content, indicating that CH₄ is a more effective OH\* radical producer compared to NH₃. Additionally, a non-zero NH₂\* signal is observed even in pure methane flames. This can be attributed to broadband emissions from other excited species such as H₂O\*, H₂O₂, and CO₂, which contribute weakly at 632 nm, the central wavelength of the NH₂\* filter. This background emission explains the apparent NH₂\* signal and does not necessarily indicate the presence of actual NH₂\* radicals in CH₄-air flames. However, when the ammonia proportion is increased even slightly (e.g., by 10%), a noticeable jump in intensity at this wavelength is observed, indicating the onset of true NH₂\* chemiluminescence due to the presence of ammonia in the mixture.
<figure><img src="image_8.png" /><figcaption>Fig. 6. Chemiluminescence of OH\* and NH₂\* radicals with direct photograph depending on NH₃ proportion, φ = 0.8, Sn = 1.4.</figcaption></figure>

Furthermore, ammonia addition increases NH₂\* production, which is consistent with its role in the ammonia oxidation pathway. For Sn = 0.8 and φ = 0.8, NH₂\* production increases up to 60% NH₃. Beyond this point, a decrease in NH₂\* intensity is observed, coinciding with a change in flame shape, as shown in Fig. 5 from chemiluminescence images and direct photographs at 80% NH₃. Subsequently, the intensity increases again until flame extinction. For Sn = 1.4 and φ = 0.8, the intensity curve follows a similar trend to Sn = 0.8, but the change in flame shape does not significantly affect NH₂\* production. For φ = 1.0, the curves for both swirl numbers are very similar, with a slight peak at 40% NH₃ for Sn = 1.4, corresponding to a change in flame shape as previously noted.

Finally, OH\* radical production for the two swirl numbers is very similar for ammonia proportions ranging from 30% to extinction for φ = 0.8 (with the swirl effect being noticeable at low NH₃ proportions) and across all spectra for φ = 1.0. These observations suggest that the swirl number not only influences the overall flame structure, but also affects the chemical processes by modifying the air-fuel mixing characteristics.

Fig. 10 illustrates instantaneous chemiluminescence images of NH₂\* and OH\* for φ = 0.8. Each image is normalized by the maximum intensity of the corresponding filter. As shown by Vigueras-Zuniga et al. [44], high ammonia blends are associated with lower OH\* concentrations and higher NH₂\* concentrations. In the OH\* chemiluminescence images, regions of high OH\* emission appear more detached with increasing ammonia content and exhibit random motion patterns, which

qualitatively reflect variations in the reactive zone structure. This contrasts with pure methane flames, where the high-production zone appears stable and remains in a fixed position. For the NH₂\* radical, the high production zones in the 0% NH₃ case correlate with those of OH, but the overall intensity remains significantly lower than in ammonia-containing flames. At 60% NH₃, the swirl effect is particularly noticeable, as the production zones are located closer to the burner exit and become more concentrated when the swirl number is high. These images suggest that for Sn = 0.8, the flame is more perturbed, with the production zone contracting and relaxing more than for Sn = 1.4. This behavior may be related to instabilities in the recirculation zone. When burned gases enter the recirculation region, they interact with fresh fuel, leading to either the production or consumption of NH₂\*.

radicals. For instance, NH₂ is known to contribute to NO consumption, as reported by Mashruk et al. [53], while it can also be produced through reactions involving ammonia and OH\* radicals, according to Mendiara and Glarborg [54]. These instabilities can create a feedback loop, where changes in flame chemistry alter the temperature, which in turn affects the flow field, further driving instabilities.

### 3.1.2. Flame morphology

Fig. 11 presents the inverse Abel transform of direct photographs and compares them with the superimposed OH\* and NH₂\* chemiluminescence images. The analysis focuses on the case where φ = 0.8, as
<figure><img src="image_9.png" /><figcaption>Fig. 7. Chemiluminescence of OH\* and NH₂\* radicals with direct photograph depending on NH₃ proportion, φ = 1.0, Sn = 0.8.</figcaption></figure>

the observations from this case are also applicable to φ = 1.0. From the cutaway views of the flame shown in Fig. 11, three distinct flame shapes can be identified as the ammonia proportion varies: truncated torus, V-shape, and Y-shape.

The first observation from Fig. 11 is that the Abel transform of the direct photograph aligns closely with the superimposed OH and NH₂* chemiluminescence, indicating that in this flame configuration, the visible light emitted by the flame corresponds to the potential combustion zones highlighted by radical chemiluminescence. The overlapping OH\* and NH₂\* production zone (shown in red) appears to shift upstream as the ammonia proportion increases, while also elongating at higher ammonia levels. This behavior reflects the influence of the recirculation zone and the increased dominance of ammonia in the fuel mixture.

Fig. 12 consolidates all the information regarding the flame shapes across different cases. The flame shapes presented in Fig. 12 are derived from the superimposed OH\* and NH₂\* chemiluminescence profiles shown in Fig. 11, with the extracted profiles corresponding to an intensity contour of 0.5.

The following discussion about observations and flame shape transition are supported by the data shown in Figs. 11 and 12.

The transition from a truncated torus shape to a V-shape (0 % NH₃ to 10 % NH₃) is primarily influenced by chemical dynamics, specifically the production of NH₂\* radicals from ammonia combustion. In contrast,

the shift from a V-shape to a Y-shape (10 % NH₃ to 80 % NH₃) is mainly driven by fluid mechanics effects. The fuel injector acts as a bluff body from the airflow perspective, leading to the formation of a central recirculation zone (CRZ) under the influence of swirling flow. The employed swirlers are sufficiently high to cause the CRZ to lift off the flame, resulting in a V-shaped flame configuration. However, with increasing ammonia addition, a reduction in the flame open angle is observed; the more ammonia is added, the smaller the flame open angle becomes. A sudden decrease in the flame open angle occurs as the flame transitions to a Y-shape. In the Y-shape configuration, the CRZ is attached to the burner and is smaller than in V-shaped flames (approximately the size of the bluff body or even smaller), and probably a Vortex Breakdown Bubble (VBB) appears above the strangled flame zone. According to Kallifronas et al. [55], V-shaped flames are more likely to occur at high swirl numbers, whereas Y-shaped flames are associated with lower swirl numbers.

Additionally, Tong et al. [56] demonstrated that the flame open angle decreases due to reduced centrifugal effects of the swirler. These observations suggest that the cross-flow induced by fuel velocity injection-necessary to maintain constant power for varying NH₃ proportions reduces the tangential velocity, thereby decreasing the apparent swirl effect and leading to a transition from V-shaped to Y-shaped flames.

Additionally, a transition between the V-shape and Y-shape was
<figure><img src="image_10.png" /><figcaption>Fig. 8. Chemiluminescence of OH\* and NH₂\* radicals with direct photograph depending on NH₃ proportion, Φ = 1.0, Sn = 1.4.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 9. Intensity normalized by the maximum of the corresponding filter depending on ammonia proportion for the different filters.</figcaption></figure>

observed for Sn = 0.8 and 60 % NH₃, where the flame started to oscillate between these two shapes. Candel et al. [15] explain this phenomenon as a result of the interaction between acoustic waves and vorticity waves, which can induce oscillations in the flame angle, as seen in our observations. Further research is needed to fully understand this specific behavior.

Finally, the flame's behavior, from a Y-shape to extinction, varies depending on the swirl number. For Sn = 0.8, the flame oscillates at its base, alternating between an attached and a lifted position, while

maintaining a Y-shape (at 90 % NH₃) before eventually blowing off. In contrast, for Sn = 1.4, the flame blows off directly without any intermediate phases.

Another observation was made for a swirl number of 1.4 and the two studied equivalence ratios. A ring-shaped flame front was observed inside the burner, as shown in Fig. 13, for ammonia proportions ranging from 0 % to 20 %. A similar phenomenon was reported by Mansouri and Boushaki [57] for a pure methane flame in a non-premixed swirl burner with radial injection under the same swirl conditions (Sn = 1.4). They
<figure><img src="image_12.png" /><figcaption>Fig. 10. Instantaneous chemiluminescence images of NH₂* and OH* for φ = 0.8.</figcaption></figure>

observed the formation of a recirculation bubble generated downstream of the swirler, whose size is controlled by the swirler and cross-flow dynamics. This bubble occupies approximately half of the annular section, promoting rapid mixing of air and methane. The shear layer created between the recirculation bubble and the fuel jet stabilizes the combustion, leading to the formation of the ring-shaped flame front. Consequently, the “pre-combustion” stage involves the presence of flue gases prior to the complete combustion of the fuel, which may influence the flame chemistry.

### 3.1.3. Flow velocity

Fig. 14 presents the axial velocity results with the inverse Abel transform of OH\* and NH₂\* chemiluminescence superimposed for φ = 1. The mean zero axial velocity line is represented in blue, the maximum velocity line in red and the outer jet flow boundary in green. Each velocity profile has its own origin, represented by a dotted red line. The velocity scale is 1.2 m/s per mm.

Firstly, from the direction change boundary, the velocity profiles suggest that the recirculation zone transitions from a truncated torus shape (at 0 % NH₃) to a Y-shaped flame (at 80 % NH₃). Indeed, as ammonia is added, the recirculation zone retracts, altering the flame geometry, as the flame continuously seeks stability. This can be explained by the fact that the maximum laminar burning velocity of ammonia (~0.07 m/s) is approximately five times lower than that of methane (~0.37 m/s), according to Kobayashi et al. [3]. As a result, methane flames remain stable in higher velocity flow fields compared to ammonia flames.

This effect is further supported by chemiluminescence superposition, where the production zones of OH\* and NH₂\* correspond to low-velocity regions for high-ammonia flames and high-velocity regions for methane flames. Moreover, temperature plays a crucial role in this behavior. Early ammonia combustion with burned gases leads to an increase in temperature, which, in turn, accelerates the flow field. This is particularly evident when comparing downward velocity profiles, where the maximum downward velocity is up to twice as high for 80 % NH₃ compared to 0 % NH₃.

At first glance, comparing different swirl numbers appears to contradict conventional expectations. A higher swirl number is typically expected to decrease axial velocity while increasing radial and

tangential velocity due to momentum conservation. However, in this case, the maximum downward velocity is around – 4 m/s for Sn = 0.8, whereas it reaches – 6 m/s for Sn = 1.4, suggesting an unexpected increase in axial velocity. This can be explained by the fact that a higher swirl number reduces axial velocity, which facilitates ammonia combustion and allows the flame to propagate upstream. As this propagation occurs, it elevates the local temperature, accelerating the flow caused by gas expansion and increasing the overall flow velocity. An equilibrium is then established, where the velocity reaches its maximum intensity while still allowing the flame to remain attached to the burner. This behavior appears to be specific to ammonia flames, as it was never observed for pure methane flames with this burner. Despite these velocity changes, no major alteration in the recirculation zone shape is observed. When comparing chemiluminescence profiles, we can see that the flame shape and position correspond to the same axial flow velocity range. The attached Y-shaped flame at the burner appears to be a direct continuation of the elevated Y-shaped flame.

Fig. 15 presents axial velocity measurements for ammonia proportions ranging from 0 to 90 % under conditions where φ = 1.0 and Sn = 0.8, as well as the effect of swirl at 80 % NH₃. All measurements were conducted along the horizontal line located 20 mm above the burner outlet. For clarity, uncertainty bars are shown only for the swirl effect.

In Fig. 15a, two distinct axial velocity profiles can be observed: one exhibiting a plateau in

the interval [-10;10] mm, which becomes progressively less pronounced as the ammonia proportion increases (with a velocity difference of approximately 1 m/s between 0 % and 30 % NH₃), and another characterized by an inverted peak within the same interval. A comparison with Fig. 12 for φ = 1.0 and Sn = 0.8 indicates that the truncated toroidal and V-shaped flames exhibit nearly identical velocity profiles. This suggests that the primary difference between these flame shapes arises from the flame chemistry, particularly the interaction between ammonia combustion and burned gases occurring just at the burner exit, leading to a slight increase in downward velocity. Regarding Y-shaped flames, no significant differences in velocity profiles are observed except at the highest ammonia proportion before extinction. At 90 % NH₃, the velocity peaks contract by approximately 0.7 m/s. This reduction in velocity appears to be a precursor to flame extinction or weakening. Several factors may explain this decrease, including reduced combustion
<figure><img src="image_13.png" /><figcaption>Fig. 11. Inverse Abel transform of direct photograph and OH\* and NH₂\* chemiluminescence superposition for Sn = 0.8 and 1.4, φ = 0.8.</figcaption></figure>

efficiency leading to lower local temperatures, which constrain the expansion of fresh gases, or the increasing influence of cross-flow, which begins.

to alter the velocity field. The difference between the two observed velocity profile types (plateau and inverted peak) likely results from cross-flow dynamics. When the cross-flow is significant, it increases the pressure drop at the burner center, leading to higher downward velocity.

Fig. 15b illustrates the impact of swirl on the axial velocity profile. A significant difference in velocity peak amplitude, approximately 2 m/s, is observed between the two swirl numbers, with a slight shift to the right, likely due to the swirl positioning within the burner (causing asymmetry in the flame). This difference results in steeper velocity gradients between the peaks, further limiting the region where the flame can stabilize. This effect may explain why flame extinction occurs at 90 % NH₃ for Sn = 1.4, while it occurs at a later stage for Sn = 0.8. RMS uncertainties for both swirl numbers indicate higher values at the burner center, where the descending flows of the recirculation zone are located, highlighting the turbulent nature of this region. Moreover, the mean velocity profile for Sn = 0.8 is within the RMS uncertainty range for Sn = 1.4, suggesting the possibility of transient flame detachment for Sn =

1.4.

For V-shaped and truncated toroidal flame configurations, previous numerical simulations in the literature align with the present results regarding axial velocity trends, consistent with bluff-body or radial injection burner configurations [57–59]. When both cross-flow effects and ammonia proportion are high, velocity differences can be observed in Fig. 14. To date, no studies have reported similar findings. Slightly similar yet distinct results have been reported in the work of Zaidaoui [60], where CO₂ and H₂O were diluted in the oxidizer for a methane flame, leading to a temperature decrease that altered gas density and reduced internal flame pressure, thereby enhancing the jet depression responsible for flame stabilizing in the central recirculation zone.

Comparing with other swirl configurations, axial burners with thick lips exhibit significantly weaker downward axial velocity, as demonstrated by Zhang et al. [61]. Similarly, in the case of purely axial swirling flames (without radial injection), Anand et al. [62] reported an absence of downward axial velocity. This suggests that flame chemistry and aerodynamic interactions differ significantly compared to radial swirl burners, where strong recirculation zones enhance fuel–air mixing and promote the stabilization. Additionally, radial swirl burners tend to
<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Evolution of flame shapes with varying NH₃ proportions (%NH₃) for different swirl numbers (Sn) and equivalence ratios (φ). Blue shapes correspond to the truncated torus shape, brown shapes correspond to the V-shape, yellow shapes correspond to the Y-shape, truncated yellow shapes correspond to lifted Y-shape flames, and red “X” marks represent flame extinction points.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 13.** Crown shape flame front observed inside the burner for Sn = 1.4.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 14.** Axial velocity measurement compared with OH\* and NH₂\* chemiluminescence superposition for φ = 1.0.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>Fig. 15. Axial velocity measurement compared with OH\* and NH₂\* chemiluminescence superposition for φ = 1.0.</figcaption></figure>

produce higher turbulence intensity and more distributed heat release, which can influence combustion efficiency and pollutant formation. These differences highlight the importance of burner design in optimizing performance for specific applications.

### 3.1.4. Flame stability

**Blow-off limits:** For both equivalence ratios, the blow-off limit is observed just before reaching 100% ammonia for a swirl number of Sn = 0.8, whereas for Sn = 1.4, it occurs prior to reaching 90% ammonia. Consequently, flame extinction occurs at lower ammonia proportions with the higher swirl number. This phenomenon can be attributed to the fact that a higher swirl number induces greater dilution between the fuel and air, which in turn creates very lean regions within the flame, leading to local extinction that eventually culminate in global flame blow-off.

While prior research generally suggests that higher swirl intensity enhances flame stabilization, several studies in the literature indicate that beyond a certain threshold, excessive swirling turbulent intensity caused by high swirling flow can destabilize the flame [38,63]. Consistent with these studies, the present results suggest that while moderate swirl intensity can enhance flame stabilization, exceeding a critical swirl threshold may lead to excessive turbulent intensity, which disrupts the flame structure and induces instability. Yu et al. [64], who studied LES/PDF modeling of non-premixed methane/air flames, present conclusions for explaining extinction. According to the study, flame extinction is primarily driven by high scalar dissipation rates. The dissipation rate creates quenching of the flame when it is high. Furthermore, excessive air diffusion into the reaction zone enhances cooling and leads to a reduction in flame temperature and heat release. Local flame regions can extinguish even at scalar dissipation rates lower than the theoretical extinction limit for steady laminar flames, highlighting the complex interplay between turbulence, mixing, and chemistry.

Zhang et al. [65], through experiments and LES simulations, demonstrated that ammonia flames are particularly prone to blow-off

due to their low laminar flame speed, low adiabatic flame temperature, and weak heat release rate. Their results showed that pure ammonia flames cannot stabilize in the CRZ, even at relatively high equivalence ratios. Increasing the swirl number does not significantly improve the blow-off limits for pure ammonia, indicating that swirl alone cannot overcome the fundamental limitations of ammonia combustion. However, in ammonia/methane blends (e.g., 50% NH₃ / 50% CH₄), methane contributes to enhanced stability and extended blow-off limits. In such cases, swirl more effectively promotes stabilization. They also observed that, near blow-off, large-scale instabilities emerge especially under low swirl conditions serving as early indicators of flame destabilization.

**Lift-off height:** Figs. 16 and 17 illustrate the flame lift-off height as a function of NH₃ proportion for various swirl numbers and equivalence ratios, based on measurements of OH\* and NH₂\* radicals respectively. The error bars denote the standard deviation. The lift-off height refers to the distance between the burner exit and the base of the flame. In this study, OH\* and NH₂\* are used as key indicators of combustion, as they are particularly relevant for methane and ammonia flames, respectively. The discrepancies in lift-off height measurements between these radicals can be attributed to differences in their formation regions and reaction kinetics. Indeed, OH\* is predominantly found in high-temperature regions [41], whereas NH₂\* can be produced in cooler zones [66]. Evaluating the accuracy of these metrics, OH\* is a well-established indicator, closely linked to the heat release zone, but it may underestimate the lift-off height if combustion starts earlier. NH₂\*, on the other hand, is particularly relevant for ammonia flames but is less commonly used as a universal marker, as it may appear in cooler regions, potentially underestimating the lift-off height. These observations suggest that the choice of diagnostic species significantly impacts the measured lift-off height. A combination of multiple species, or complementary diagnostic techniques, could improve accuracy and provide a more comprehensive understanding of flame stabilization mechanisms.

From 0% to 20% NH₃, the trend lines for all cases are quite similar,

<figure><img src="image_18.png" /><figcaption>Fig. 16. Flame lift-off height depending on NH₃ proportion from OH\* chemiluminescence.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>Fig. 17. Flame lift-off height depending on NH₃ proportion from NH₂* chemiluminescence.</figcaption></figure>

although distinctions based on equivalence ratio are apparent: higher equivalence ratios correspond to greater lift-off heights. Beyond 20 % NH₃, noticeable differences emerge between the cases. The discontinuities in the curves can primarily be attributed to changes in flame geometry, which alter the recirculation zones and, consequently, the mixing and production zones of the radicals involved. From OH* chemiluminescence, it is observed that between 40 % and 70 % NH₃, the flame experiences a “plateau” where the production zone of OH* remains stable. Beyond 70 % NH₃, the lift-off height decreases. This can be explained by changes in flame geometry: at high cross-flow conditions, the apparent swirl number decreases, leading to vortex breakdown elongation and explaining the observed reduction in lift-off height at high NH₃ proportions. The equivalence ratio generally results in an increase in lift-off height.

However, the effect of swirl number becomes more pronounced for NH₃ proportions above 30 %. For NH₃ concentrations greater than 30 %, a high swirl number significantly lowers the lift-off height. This can be explained by the fact that a higher swirl number strengthens the central recirculation zone (CRZ), bringing it closer to the burner compared to lower swirl conditions. The particular case where $\phi = 0.8$ and $Sn = 1.4$ stands out due to a global decrease in lift-off height, which can be attributed to the combined effects of the cross-flow induced by fuel injection and the high swirl number. Analysis of NH₂* chemiluminescence suggests that the reduction in lift-off height is due to the influence of radial cross-flow. The cross-flow increases to maintain theoretical power, which facilitates flame attachment to the burner. However, when the ammonia proportion becomes too high, the combustion velocity of ammonia becomes the limiting factor. The increased air flow velocity prevents the flame from rising through the flow. The case with $Sn = 0.8$ and $\phi = 1.0$ is particularly affected by the air flow conditions; the swirled air flow is insufficient for proper mixing of air and fuel just above the burner, resulting in a lower lift-off height for smaller NH₃ proportions compared to other cases.

A general observation is that for ammonia proportions greater than 30 %, the swirl number significantly impacts the lift-off height. An increase in swirl number corresponds to a decrease in lift-off height. Additionally, larger error bars indicate greater oscillations and stabilization issues in the flame, as discussed in the previous subsection. But the fluctuations remain relatively stable, except for the last case before extinction. Yu et al. [64] give explanations for understanding this behavior. They show that flame lift-off is closely linked to the fuel jet velocity and the swirl-induced flow structure. As the fuel jet velocity increases, the effective swirl number decreases (in their study they used axial fuel injection but it is still valuable for radial injection since cross-flow induces effective swirl number to change), weakening the central recirculation zone (CRZ). This causes the flame to detach from the burner exit and stabilize further downstream, where the scalar dissipation rate is lower and conditions are more favorable for combustion. In regions near the burner, high scalar dissipation and excessive mixing prevent stable flame anchoring. However, farther downstream, where mixing is less intense, lifted flames can re-establish through a

combination of auto-ignition and partially premixed flame propagation.

## 3.2. Pollutant emissions

To elucidate the impact of NH₃/CH₄ fuel composition, equivalence ratio, and swirl number on pollutant formation, Figs. 18 and 20 present the emission profiles of key species (CO, NO, CH₄, and CO₂). These trends are interpreted in terms of the underlying chemical kinetics and flow dynamics that govern radical production, thermal conditions, and flame stability.

Fig. 18 presents the emission profiles for CO and NO. For $\phi = 0.8$, CO emissions initially decrease (up to 10 %NH₃) and then increased with the NH₃ proportion, peaking at 50 % NH₃, with slightly higher levels observed at a swirl number of 0.8. NO emissions, on the other hand, peaked at approximately 10 % NH₃ before significantly decreasing as the NH₃ proportion increased, showing minimal sensitivity to the swirl number. For $\phi = 1.0$, CO emissions peaked between 60 and 70 % NH₃ and were more influenced by the swirl number, with $Sn = 1.4$ consistently producing higher CO levels. NO emissions at this equivalence ratio followed a similar trend, peaking around 20 % NH₃ before rapidly declining at higher NH₃ concentrations, with $Sn = 1.4$ resulting in lower NO emissions. Overall, increasing the equivalence ratio shifts the CO and NO emissions peaks to higher NH₃ percentages and generally increases NO levels, while CO emissions consistently stay at the same levels regardless of $\phi$.

This complex behavior results from the intertwined influence of swirl-induced flow dynamics and radical-driven chemical pathways. As previously discussed in the blow-off analysis, higher swirl numbers intensify recirculation zones, promoting better mixing and higher flame temperatures. The axial velocity data confirms that $Sn = 1.4$ significantly enhances the downward velocity in the central recirculation region, thereby increasing the entrainment of burned gases. This, in turn, modulates local concentrations of reactive radicals such as OH, O, HNO and NHᵢ (i.e., NH, NH₂), which play key roles in both NO formation and destruction.

Now, the influence of the swirl number on pollutant emissions, particularly NO and CO, stems from its dual role in shaping both the flow field and the thermochemical environment of the flame. As discussed in the blow-off limit analysis, higher swirl numbers enhance turbulence and strengthen the central recirculation zone, promoting heterogeneous mixing with alternating lean and rich pockets. This contributes to elevated CO levels at $Sn = 1.4$ for $\phi = 1.0$ due to incomplete oxidation in fuel rich regions. Simultaneously, the intensified recirculation increases the entrainment of hot combustion products into the reaction zone, modifying local radical concentrations and temperature gradients.

Axial velocity profiles confirm that $Sn = 1.4$ induces a significantly stronger downward flow than $Sn = 0.8$, reinforcing the internal recirculation loop. This enhanced backflow brings burned gases—rich in reactive species such as OH, O, and NHᵢ—into contact with fresh reactants, thereby amplifying both thermal and chemical feedback loops. ReaxFF-MD simulations by Guo et al. [67] indicate that under fuel-lean
<figure><img src="image_20.png" /><figcaption>Fig. 18. Emission graphs of NO and CO with %NH₃ for Sn = 0.8 and Sn = 1.4.</figcaption></figure>

<figure><img src="image_21.png" /><figcaption>Fig. 19. NO emission in [mg/kWh] as a function of NH₃ proportion.</figcaption></figure>

NH₃/CH₄ conditions, these radicals favor the oxidation of NH₃ intermediates into NO, enhancing NOx formation. However, the same simulations also reveal that higher temperatures can facilitate NO reduction via thermally activated pathways, implying that swirl-induced thermal stratification may favor NO destruction in localized zones.

Experimental observations by Wang et al. [24] using a premixed swirl burner further support this interpretation: they observed a decrease in NO emissions with increasing swirl number and a peak in NO at low NH₃ concentrations. Their findings reinforce the notion that strong recirculation zones—whether in premixed or non-premixed configurations—support complex NO dynamics governed by radical redistribution and thermal history. In contrast, Biebl et al. [68] reported the opposite trend, with higher swirl leading to increased NO emissions. They attributed this to burner design: their axial injection system lacked a bluff body, resulting in weak recirculation and poor mixing between burned and fresh gases. This underlines the crucial role of burner geometry in controlling the effectiveness of swirl-induced mechanisms.

Moreover, it is essential to consider that apparent reductions in NO at high NH₃ blending ratios could also stem from incomplete ammonia

oxidation rather than purely aerodynamic or thermal effects. If unburned NH₃ escapes the flame, measured NO levels may not reflect true formation-reduction dynamics. Therefore, complementary diagnostics—such as NH₃ and NO₂ measurements—are necessary to disentangle combustion completeness from genuine NO suppression mechanisms.

Another important phenomenon to consider is flame morphology. Changes in flame geometry are strongly influenced by thermodynamic effects. Variations in the fuel mixture alter the local temperature, which in turn affects local flow dynamics and consequently modifies the recirculation zone's morphology. As the ammonia fraction increases, the recirculation zone tends to shrink, leading to a reduction in the surface area where burnt and fresh gases interact. This reduction can promote termination reactions before NO formation occurs and may also result in unburnt reactants escaping the reaction zone. Additionally, the crossflow effect, induced by variations in the fuel inlet velocity, plays a role in this process. A higher fuel velocity can enhance air-fuel mixing, leading to the formation of ultra-lean regions that do not sustain combustion, thereby contributing to lower NO emissions. When comparing these results to axial fuel injection in non-premixed swirl burners from other studies, significant differences are observed. In the work of Tu et al. [34], the NO emission curve shifts markedly: the peak NO concentration moves from 10–20 % NH₃ in the radial configuration to 40–50 % NH₃ in the axial configuration, and no inflection point is observed. They also report lower flame stability, with extinction occurring at 60 % NH₃. Similarly, Biebl et al. [69] observed this trend in axial swirl burners with swirl numbers greater than 0.5. However, for a swirl number of 0.3, the NO emission profile closely resembles that of radial injection, even though the peak remains shifted toward higher NH₃ concentrations in the axial case. This discrepancy is largely attributed to differences in the flow field. At low swirl numbers (Sn = 0.3), CH₄-NH₃ blended flames tend to be lifted, as opposed to being attached in higher swirl cases. Fokaides et al. [70] attribute this behavior to differences in global flame residence time: attached flames are associated with strong recirculation zones and thus longer residence times, while lifted flames correspond to weaker recirculation and shorter residence times. Shorter residence times typically result in lower NO formation.

<figure><img src="image_22.png" /><figcaption>Fig. 20. Emission graphs of CO₂ and CH₄ with %NH₃ for Sn = 0.8 and Sn = 1.4.</figcaption></figure>
Regarding CO emissions, Chai et al. [5] compiled data from the literature on premixed flames and concluded that CO emissions tend to decrease with high ammonia proportions, a trend that is also observed in the present study for NH₃ proportions above 80 %. Zhang et al. [61] investigated premixed methane-ammonia flames with a swirl number of approximately 0.7 and found lower CO emissions for $\phi = 0.8$, suggesting that equivalence ratio and flame configuration play a crucial role in CO formation mechanisms. The CO emission peak is a sign of incomplete combustion and can be attributed to several factors. First, the mixing of fuel and air might be suboptimal under certain conditions. The variation in NH₃ proportion influences the cross-flow to maintain the same theoretical flame power. This can lead to a more heterogeneous mixture when the cross-flow is high, creating richer and leaner combustion zones, which in turn affects the completeness of combustion. Second, in regions with high NH₃ concentration, local flame quenching may occur due to NH₃'s higher heat capacity and lower reactivity compared to CH₄. This quenching can prevent the complete oxidation of CO to CO₂. Lastly, chemical kinetics play a crucial role, as NH₃ can interact with combustion radicals, such as OH, which are essential for the conversion of CO to CO₂. Since the OH radical is highly involved in numerous formation reactions particularly in NO formation it is possible that the NO and CO peaks are shifted due to the limited availability of OH radicals. These numerous interactions can cause a temporary accumulation of CO, leading to the observed emission peak.

Fig. 19 shows NO emissions expressed in mg/kWh as a function of NH₃ percentage in the fuel blend. This representation better reflects the environmental efficiency of the combustion system by eliminating the influence of flue gas dilution. The conversion from ppm to mg/kWh is based on the EN 267 standard [71], and follows the equation:

$$NO_{[mg/kWh]} = NO_{meas} \cdot \rho_{NO} \cdot \left( \frac{\frac{21}{21-0_2_{meas}}}{H_i} \right) \cdot \left( \frac{V_{A, th, tr, min}}{H_i} \right) \quad (2) \text{With } NO_{[mg/kWh]}$$

NO emission in mg/kWh, $NO_{meas}$ the measured NO concentration, $\rho_{NO}$ the density of NO, $0_2_{meas}$ the measured $0_2$ concentration, $V_{A, th, tr, min}$ the theoretical reference volume of the dry flue gas and $H_i$ the low heat value (LHV). The trends in mg/kWh closely mirror those observed in the NO concentration (ppm), but allow for more meaningful comparison across operating conditions. For ammonia proportions below 50 %, the $\phi = 0.8$ case shows lower NO emissions. However, at higher NH₃ levels, the stoichiometric case ($\phi = 1.0$) achieves significantly lower emissions –up to two times lower- compared to the leaner condition.

Fig. 20 shows the emission profiles of CH₄ and CO₂. At an equivalence ratio of $\phi = 0.8$, CH₄ emissions are initially low but begin to increase sharply around 60 % NH₃, reaching a peak at 80 % NH₃. This increase is more pronounced for a swirl number $Sn = 1.4$. Typically, a higher swirl number enhances mixing between fuel and oxidizer, promoting more efficient combustion. However, under these conditions, a higher swirl appears to generate very lean zones, leading to incomplete combustion of both methane and ammonia. This phenomenon could account for the reduction in NO emissions—due to the lower flame temperature and shorter residence time—as well as the increase in unburnt methane. In contrast, CO₂ emissions decrease steadily with increasing NH₃ concentration. This trend indicates that as the NH₃ content increases, the methane combustion becomes less complete, leading to higher levels of unburnt CH₄ and lower CO₂ production. It is worth noting that a similar trend likely applies to unburnt NH₃. Although ammonia concentration is not measured in this study, Ahmed et al. [72] indicate that a reduction in NO emissions may lead to an increase in unburnt ammonia. Furthermore, experimental data from methane-ammonia non-premixed flames [33] confirm this tendency: N₂O emissions peak around 50 % NH₃, particularly under lean conditions ($\phi = 0.83$), and coincide with elevated levels of unburnt NH₃. This behavior suggests a shift in reaction pathways that favors the formation of intermediate nitrogen species (e.g., NH₃, N₂O) at the expense of NO production, highlighting the trade-off between NOx reduction and ammonia slip in such operating regimes.

For $\phi = 1.0$, a similar trend is observed: CH₄ emissions remain negligible up to approximately 70 % NH₃, after which they increase sharply, reaching a peak at 90 % NH₃ for $Sn = 0.8$. In contrast, no CH₄ emissions are detected for $Sn = 1.4$, indicating that methane combustion is more efficient under these conditions. However, the presence of CO emissions suggests that combustion is still not complete. As in the leaner case, CO₂ emissions decrease with increasing NH₃ concentration, although the decline is more gradual compared to $\phi = 0.8$. The higher equivalence ratio appears to shift the onset of efficient methane combustion to higher NH₃ contents. This suggests that richer mixtures delay the degradation of methane combustion efficiency but eventually lead to a higher CH₄ emission peak as NH₃ becomes dominant in the fuel blend. The combined influence of equivalence ratio and swirl number reveals a nonlinear interaction: for $\phi = 0.8$, methane combustion is more effective at $Sn = 0.8$ than at $Sn = 1.4$, whereas for $\phi = 1.0$, the opposite trend is observed, with better combustion at $Sn = 1.4$ than at $Sn = 0.8$. This highlights the complex interplay between chemical kinetics and flow dynamics, emphasizing the importance of their combined effects in non-premixed ammonia-methane combustion systems.

While presenting CO₂ and CH₄ concentrations provides useful information about the combustion process, it is acknowledged that directly presenting combustion efficiency would offer a more comprehensive assessment. In the case of combustion involving both methane and ammonia, a more accurate determination of overall combustion efficiency would require measuring additional species, such as NH₃, N₂, and potentially NOx. However, given that the current dataset includes only CO₂, CH₄, and CO concentrations, the combustion efficiency of methane is estimated using the formula:

$$\eta = \frac{[CO2]}{[CO2] + [CO] + [CH4]} \quad [3]$$

The corresponding combustion efficiency curve is presented in Fig. 21. In this figure, it can be observed that for $\phi = 0.8$, a small amount of NH₃ allows for good methane combustion, but its efficiency decreases exponentially as the NH₃ proportion increases. A similar trend is observed for both swirl configurations at this equivalence ratio. For $\phi = 1.0$, a significantly higher ammonia proportion still maintains good methane combustion efficiency, although the performance differs between swirl numbers, from 70 % NH₃ to extinction. These results highlight the importance of the initial equivalence ratio, with stoichiometric combustion proving to be more efficient than lean combustion for methane consumption.

In summary, ammonia proportion significantly impacts flame efficiency, as evidenced by peaks in CO emissions and an increase in

<figure><img src="image_23.png" /><figcaption>Fig. 21. Combustion efficiency of methane.</figcaption></figure>
unburned methane. This loss in efficiency partially explains the observed decrease in overall flame temperature. Notably, at ammonia proportions greater than 60 %, the concentrations of NO and CO are either comparable to or lower than those observed in pure methane flames, with CO emissions being particularly reduced.

### 3.3. Inside chamber temperatures

Fig. 22 presents the temperature profiles for all study cases. These measurements were obtained using type K thermocouples placed near the chamber walls, providing temporally and spatially averaged values. Due to their positioning and response time, these readings do not capture the peak flame temperatures or the highly localized and transient variations that occur in the reaction zone. As such, while they offer useful trends on global chamber behavior, caution must be exercised when interpreting these values in relation to combustion completeness, local equivalence ratio effects, or pollutant formation mechanisms. The overall measurement uncertainty of the thermocouples is estimated to be 2.75 % of the measured temperature.

Overall, temperature profiles are largely similar across different swirl numbers. However, a slight trend is observed: wall temperatures decrease by approximately 30 °C and bottom temperatures increase by around 40 °C as the swirl number increases. From 0 % to 50 % NH₃, the curves exhibit a plateau, followed by a decrease due to the lower adiabatic flame temperature of ammonia compared to methane.

For an equivalence ratio of $\phi = 1.0$, temperature differences between swirl numbers become more pronounced at higher NH₃ fractions. Higher swirl appears to reduce the global average chamber temperature, which could be influenced by a combination of factors such as changes in flame shape, enhanced mixing (and thus dilution), and increased crossflow. These interpretations, however, remain qualitative due to the limitations of wall-based thermocouple data.

A general increase in flue gas temperature is observed with increasing equivalence ratio. For example, the mid-chamber temperature increases from ~ 600 °C to 650 °C, and the exit temperature rises from 500 °C to 590 °C. This may be linked to reduced excess air and higher adiabatic flame temperatures. Additionally, OH* chemiluminescence images show a slight increase in flame height at $\phi = 1.0$, possibly resulting in less heat exchange with the surroundings. Nonetheless, the estimated increase in flame length (~30 mm) seems insufficient to explain the full temperature difference (~100 °C at the outlet), suggesting that other effects such as incomplete mixing or local heat losses may be involved.

When comparing temperature data with emissions trends, some correlations are observed. At $\phi = 0.8$, high levels of unburned methane in the flue gas suggest that chemical energy is not fully released, possibly contributing to lower measured chamber temperatures. At $\phi = 1.0$ and NH₃ > 80 % with Sn = 0.8, methane slip remains significant, and a corresponding decrease in temperature is observed. Interestingly, the decrease in temperature with increasing NH₃ is more abrupt for $\phi = 0.8$ ($\Delta T \approx 200$ °C at the bottom) than for $\phi = 1.0$ ($\Delta T \approx 50$ °C), which may

reflect differences in combustion efficiency and dilution effects. The apparent onset of temperature drop also coincides with a CO emission peak, potentially indicating a transition in flame chemistry. However, direct causality cannot be confirmed with the current measurement setup.

Regarding NO emissions, a qualitative correspondence is observed between maximum NO levels and high chamber temperatures, which may suggest a contribution of the thermal NOx formation pathway (Zeldovich mechanism). Still, given the nature of the temperature measurements, this interpretation should be considered indicative rather than conclusive.

Flame shape, as observed in imaging, does not show a direct correlation with temperature levels. For $\phi = 0.8$ and 60 % NH₃, both swirl cases exhibit a V-shaped flame and decreasing temperature. For $\phi = 1.0$, V-shaped flames maintain similar temperatures across NH₃ contents. In contrast, Y-shaped flames are associated with decreasing temperatures at $\phi = 0.8$, and only above 50 % NH₃ for $\phi = 1.0$. These observations suggest that while flame geometry may influence the flow and mixing, it is not a reliable predictor of thermal behavior based on the current diagnostic method.

## 4. Conclusion

The study of non-premixed flames of ammonia-methane mixtures in a swirl burner with radial fuel injection are investigated. This study provides a detailed analysis of the flame structure, temperature distribution, and pollutant emissions in combustion systems using varying ammonia-methane fuel mixtures. The experiments, conducted with different swirl numbers and equivalence ratios, revealed key insights into the influence of ammonia proportion on flame behavior, stability, and emissions.

**Flame Topology:** Chemiluminescence analysis of OH* and NH₂* radicals shows that ammonia strongly influences flame morphology and radical production. Increasing ammonia content alters the flame shape (from truncated torus to V- and Y-shaped), mainly due to fluid mechanics effects, swirl number, and fuel injection on the central recirculation zone. However, overall flame stability decreases with higher ammonia fractions. Axial velocity measurements confirm that ammonia addition retracts the recirculation zone, changing flame geometry, while a higher swirl number enhances ammonia combustion and upstream flame propagation, leading to local temperature rises and flow acceleration.

**Temperature Profiles:** Temperature measurements showed that while overall trends were consistent across different swirl numbers, notable differences appeared at higher ammonia levels. As ammonia concentration increased, temperature variations between swirl numbers became more significant, indicating stronger dilution and crossflow effects. Moreover, changes in temperature slopes coincided with CO emission peaks, marking a transition in flame chemistry, while NO peaks aligned with maximum chamber temperatures, consistent with the Zeldovich thermal NOx formation mechanism.

<figure><img src="image_24.png" /><figcaption>Fig. 22. Temperatures taken inside the combustion chamber.</figcaption></figure>
**Pollutant Emissions:** The emissions data highlighted a complex relationship between ammonia proportion, swirl number, and equivalence ratio. NO emissions are high at low NH₃ levels due to faster methane oxidation but decrease as ammonia content rises because of reactions that limit NO formation. CO emissions increase with higher swirl numbers, indicating local rich zones despite enhanced turbulence. Unburned methane rises with high ammonia content, reflecting less complete combustion. The addition of ammonia to more than 80% in the fuel blend yields promising results in reducing NO, CO and CO₂ emissions, despite potentially leading to an increase in unburned fuel gases. The addition of ammonia to more than 80% in the fuel blend yields promising results in reducing NO, CO and CO₂ emissions, despite potentially leading to an increase in unburned fuel gases.

In conclusion, this study highlights the trade-offs between flame stability, temperature control, and pollutant emissions in ammonia-methane combustion. While increasing ammonia content offers a pathway to reducing NO emissions, it introduces challenges related to flame stability and incomplete combustion, as reflected in the CO emission peaks. The swirl number plays a crucial role in modulating these effects, affecting both the overall flame structure and pollutant formation. These findings provide valuable insights to set up combustion systems using ammonia as a fuel, balancing performance with environmental considerations.

## CRediT authorship contribution statement

**Antoine Morel:** Writing – review & editing, Writing – original draft, Methodology, Investigation. **Toufik Boushaki:** Writing – review & editing, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors are grateful for the financial support of this work provided by Valormac JRP CNRS-Africa and LEAP-RE Biotherap.

## Data availability

Data will be made available on request.

## References

[1] D.R. MacFarlane, P.V. Cherepanov, J. Choi, B.H.R. Suryanto, R.Y. Hodgetts, J. M. Bakker, F.M. Ferrero Vallana, A.N. Simonov, A Roadmap to the Ammonia Economy, Joule 4 (2020) 1186–1205.

[2] A. Valera-Medina, F. Amer-Hatem, A.K. Azad, I.C. Dedoussi, M. de Joannon, R. X. Fernandes, P. Glarborg, H. Hashemi, X. He, S. Mashruk, J. McGowan, C. Mounaim-Rouselle, A. Ortiz-Prado, A. Ortiz-Valera, I. Rossetti, B. Shu, M. Yehia, H. Xiao, M. Costa, Review on Ammonia as a potential fuel: from Synthesis to Economics, Energy Fuels 35 (2021) 6964–7029.

[3] H. Kobayashi, A. Hayakawa, K.D. Kunkuma, A. Somarathe, C.E. Okafor, Science and technology of ammonia combustion, Proc. Combust. Inst. 37 (2020) 109–133.

[4] M. Aziz, F.B. Juangsa, A.R. Irhamna, A.R. Irsyad, H. Hariana, A. Darmawan, Ammonia utilization technology for thermal power generation: a review, J. Energy Inst. 111 (2023) 101365.

[5] W.S. Chai, Y. Bao, P. Jin, G. Tang, L. Zhou, A review on ammonia, ammonia-hydrogen and ammonia-methane fuels, Renew. Sustain. Energy Rev. 147 (2021) 111254.

[6] O. Kekul, M. Ilbas, S. Karyeyen, Hydrogen concentration effects on a swirl-stabilized nonpremixed burner using ammonia, Int. J. Hydrogen Energy 52 (2024) 1288–1305.

[7] J. Li, H. Huang, N. Kobayashi, Z. He, Y. Nagai, Study on using hydrogen and ammonia as fuels: Combustion characteristics and NOx formation, Int. J. Energy Res. 38 (2014) 1214–1223.

[8] J. Otomo, M. Koshi, T. Mitsumori, H. Iwasaki, K. Yamada, Chemical kinetic modeling of ammonia oxidation with improved reaction mechanism for ammonia/air and ammonia/hydrogen/air combustion, Int. J. Hydrogen Energy 43 (2018) 3004–3014.

[9] Y. Zhu, H.J. Curran, S. Girhe, Y. Murakami, H. Pitsch, K. Senecal, L. Yang, C. W. Zhou, The combustion chemistry of ammonia and ammonia/hydrogen mixtures: a comprehensive chemical kinetic modeling study, Combust. Flame 260 (2024) 113239.

[10] S. Mishra, K. Mishra, Effect of swirl number on structural and emission characteristics of moderate size burner flames, Process Saf. Environ. Prot. 189 (2024) 1151–1160.

[11] A. Rowhani, S. Tabejamaat, Experimental study of the effects of swirl and air dilution on biogas non-premixed flame stability, Therm. Sci. 19 (2015) 2161–2169.

[12] R. Santhosh, S. Basu, Transitions and blowoff of unconfined non-premixed swirling flame, Combust. Flame 164 (2016) 35–52.

[13] A. Gupta, M. Lewis, S. Qi, Effect of Swirl on Combustion Characteristics in Premixed Flames, J. Eng. Gas Turbines Power 120 (1998) 488–494.

[14] Y. Huang, V. Yang, Effect of swirl on combustion dynamics in a lean-premixed swirl-stabilized combustor, Proc. Combust. Inst. 30 (2005) 1775–1782.

[15] S. Candel, D. Durox, T. Schuller, J.F. Bourgouin, J.P. Moeck, Dynamics of swirling flames, Annu. Rev. Fluid Mech. 46 (2014) 147–173.

[16] C. Külshheimer, H. Büchner, Combustion dynamics of turbulent swirling flames, Combust. Flame 131 (2002) 70–84.

[17] J. Jójká, R. Šlefarski, Emission characteristics for swirl methane-air premixed flames with ammonia addition, Energies 14 (2021) 662.

[18] G. Wang, T. Guiberti, W. Roberts, Exploring the Effects of Swirl Intensity on NO Emission in Ammonia-Methane-Air Premixed Swirling Flames, J. Ammonia Energy 1 (2023) 074–082.

[19] G. Wang, T. Guiberti, S. Cardona, C. Jimenez, W. Roberts, Effects of residence time on the NOx emissions of premixed ammonia-methane-air swirling flames at elevated pressure, Proc. Combust. Inst. 39 (2023) 4277–4288.

[20] C. Liu, H. Yang, C. Ruan, L. Yu, X. Lu, Influence of swirl intensity on combustion dynamics and emissions in an ammonia-enriched methane/air combustor, Phys. Fluids 36 (2024) 034123.

[21] G.B. Ariemma, G. Sorrentino, R. Ragucci, M. de Joannon, P. Sabia, Ammonia/Methane combustion: Stability and NOx emissions, Combust. Flame 241 (2022) 112071.

[22] A.M. Elbaz, Z.O. Hassan, W.L. Roberts, Exploring the influence of swirl intensity on stability, emissions, and flame structure in non-premixed NH3/CH4 swirling flames, Proc. Combust. Inst. 40 (2024) 105644.

[23] C. Romano, M. Cerutti, G. Babazzi, L. Miris, R. Lamioni, C. Galletti, L. Mazzotta, D. Borello, Ammonia blends for gas-turbines: Preliminary test and CFD-CRN modelling, Proc. Combust. Inst. 40 (2024) 105494.

[24] S. Wang, C. Chong, S. Sheykhbaglou, J. Ng, B. Tian, A. Valera-Medina, Revealing the NO Formation Kinetics for NH3/CH4 Blends under Dual-Flame and Premixed Swirl Flame Configurations, Energies 17 (2024) 6090.

[25] K. Somarathe, E. Okafor, D. Sugawara, A. Hayakawa, H. Kobayashi, Effects of OH concentration and temperature on NO emission characteristics of turbulent non-premixed CH4/NH3/air flames in a two-stage gas turbine like combustor at high pressure, Proc. Combust. Inst. 38 (2021) 5163–5170.

[26] M. Zhang, Z. An, L. Wang, X. Wei, B. Jianayihan, J. Wang, Z. Huang, H. Tan, The regulation effect of methane and hydrogen on the emission characteristics of ammonia/air combustion in a model combustor, Int. J. Hydrogen Energy 46 (2021) 21013–21025.

[27] P. Henshaw, T. D'Andrea, K. Mann, D. Ting, Premixed ammonia-methane-air combustion, Combust. Sci. Technol. 177 (2005) 2151–2170.

[28] A. Valera-Medina, R. Marsh, J. Runyon, D. Pugh, P. Beasley, T. Hughes, P. Bowen, Ammonia-methane combustion in tangential swirl burners for gas turbine power generation, Appl. Energy 185 (2017) 1362–1371.

[29] A.M. Elbaz, Z.O. Hassan, A.M. Albalawi, M.M.A. Ahmed, M. Abdullah, E. Cenker, W.L. Roberts, Investigating NO emissions, stability, and flame structure in co-fired premixed NH3/CH4/air swirling flames, Combust. Flame 272 (2025) 113892.

[30] T. Terasaki, S. Hayashi, The effects of fuel-air mixing on NOx formation in non-premixed swirl burners, Symp. (Int.) Combust. 26 (1996) 2733–2739.

[31] K. Somarathe, E. Okafor, A. Hayakawa, T. Kudo, O. Kurata, N. Iki, H. Kobayashi, Emission characteristics of turbulent non-premixed ammonia/air and methane/air swirl flames through a rich-lean combustor under various wall thermal boundary conditions at high pressure, Combust. Flame 210 (2019) 247–261.

[32] H. Yamashita, A. Hayakawa, E. Okafor, S. Colson, K. Somarathe, T. Tsujimura, S. Ito, M. Uchida, T. Kudo, H. Kobayashi, Optimum primary equivalence ratio for rich-lean twostage combustion of non-premixed ammonia/methane/air and ammonia/hydrogen/air flames in a swirling flow, Fuel 368 (2024) 131598.

[33] M. Biebl, J. Leicher, A. Giese, G. Wieland, A comprehensive study of non-premixed combustion of ammonia and its blends: Flame stability and emission reduction, Fuel 386 (2025) 134501.

[34] Y. Tu, S. Xu, H. Liu, Combustion and emission characteristics of NH3/CH4/air in a model swirl combustor: Comparison between premixed and non-premixed modes, International J. Hydrogen, Energy 48 (2023) 17311–17323.

[35] A. Olivani, G. Solero, F. Cozzi, A. Coghe, Near field flow structure of isothermal swirling flows and reacting non-premixed swirling flames, Exp. Therm Fluid Sci. 31 (2007) 427–436.

[36] T. Boushaki, N. Merlo, C. Chauveau, I. Gokalp, Study of pollutant emissions and dynamics of non-premixed turbulent oxygen enriched flames from a swirl burner, Proc. Combust. Inst. 36 (2017) 3959–3968.
[37] T.S. Cheng, Y.-C. Chao, D.-C. Wu, T. Yuan, C.-C. Lu, C.-K. Cheng, J.-M. Chang, Effects of fuel-air mixing on flame structures and NOx emissions in swirling methane jet flames, Symp. (Int.) Combust. 27 (1998) 1229-1237.

[38] J.M. Beér, N.A. Chigier, Combustion Aerodynamics, Applied Science Publishers Ltd, London, 1972.

[39] H.J. Sheen, W.J. Chen, S.Y. Jeng, T.L. Huang, Correlation of swirl number for a radial-type swirl generator, Exp. Therm Fluid Sci. 12 (1996) 444-451.

[40] T. Boushaki, N. Merlo, S. de Persis, C. Chauveau, I. Gökalp, Experimental investigation of CH4-air-O2 turbulent swirling flames by Stereo-PIV, Exp. Therm Fluid Sci. 106 (2019) 87-99.

[41] M. DeLeo, A. Saveliev, L. Kennedy, S. Zelepouga, OH and CH luminescence in opposed flow methane oxy-flames, Combust. Flame 149 (2007) 435-447.

[42] Y. Liu, J. Tan, M. Wan, X. Yao, OH* and C* chemiluminescence characteristics in low swirl methane-air flames, AIP Adv. 10 (2020) 055318.

[43] D.S. Dandy, S.R. Vosen, Numerical and Experimental Studies of Hydroxyl Radical Chemiluminescence in Methane-Air Flames, Combust. Sci. Technol. 82 (1992) 131-150.

[44] M.O. Vigueras-Zúniga, M.E. Tejeda-del-Cueto, S. Mashruk, M. Kovaleva, C. L. Ordónez-Romero, A. Valera-Medina, Methane/Ammonia Radical Formation during High Temperature Reactions in Swirl Burners, Energies 14 (2021) 6624.

[45] S. Mashruk, X. Zhu, W.L. Roberts, T.F. Guiberti, A. Valera-Medina, Chemiluminescent footprint of premixed ammonia-methane-air swirling flames, Proc. Combust. Inst. 39 (2023) 1415-1423.

[46] A. Hayakawa, T. Goto, R. Mimoto, T. Kudo, H. Kobayashi, NO formation/reduction mechanisms of ammonia/air premixed flames at various equivalence ratios and pressures, Mechanical, Engineering Journal 2 (2015) 14-00402.

[47] D. Pugh, J. Runyon, P. Bowen, A. Giles, A. Valera-Medina, R. Marsh, B. Goktepe, S. Hewlett, An investigation of ammonia primary flame combustor concepts for emissions reduction with $oH^*, NH2^*$ and $nH^*$ chemiluminescence at elevated conditions, Proc. Combust. Inst. 38 (2021) 6451-6459.

[48] J. Zheng, F. Tang, S. Chung, L. Hu, Characteristics of liftoff, blowout and instability in nonpremixed jet flames with NH3/CH4 mixture fuels, Proc. Combust. Inst. 40 (2024) 105591.

[49] D.S. Jeon, G.J. Hwang, N.I. Kim, Lift-off characteristics of non-premixed jet flames of CH4 and cracked NH3 mixtures in laminar/turbulent transition, Combust. Flame 265 (2024) 113462.

[50] D.F. Davidson, K. Kohse-Hoinghaus, A.Y. Chang, R.K. Hanson, A pyrolysis mechanism for ammonia, Int. J. Chem. Kinet. 22 (1990) 513-535.

[51] K.C. Smyth, P.J.H. Tjossem, A. Hamins, J.H. Miller, Concentration measurements of OH and equilibrium analysis in a laminar methane-air diffusion flame, Combust. Flame 79 (1990) 366-380.

[52] F. Cozzi, A. Coghe, Effect of air staging on a coaxial swirled natural gas flame, Exp. Therm Fluid Sci. 43 (2012) 32-39.

[53] S. Mashruk, H. Xiao, A. Valera-Medina, Rich-Quench-Lean model comparison for the clean use of humidified ammonia/hydrogen combustion systems, Int. J. Hydrogen Energy 46 (2021) 4472-4484.

[54] T. Mendiara, P. Glarborg, Ammonia chemistry in oxy-fuel combustion of methane, Combust. Flame 156 (2009) 1937-1949.

[55] D.P. Kallifronas, P. Ahmed, J.C. Massey, M. Talibi, A. Ducci, R. Balachandran, N. Swaminathan, K.N.C. Bray, Influences of heat release, blockage ratio and swirl on the recirculation zone behind a bluff body, Combust. Sci. Technol. 195 (2023) 3785-3809.

[56] Y. Tong, X. Liu, Z. Wang, M. Richter, J. Klingmann, Experimental and numerical study on bluff-body and swirl stabilized diffusion flames, Fuel 217 (2018) 352-364.

[57] Z. Mansouri, T. Boushaki, Experimental and numerical investigation of turbulent isothermal and reacting flows in a non-premixed swirl burner, Int. J. Heat Fluid Flow 72 (2018) 200-213.

[58] D. Fuzesi, S. Wang, V. Jozsa, C. Chong, Ammonia-methane combustion in a swirl burner: Experimental analysis and numerical modeling with Flamelet Generated Manifold model, Fuel 341 (2023) 127403.

[59] S. Chakchak, A. Hidouri, H. Zaidaoui, M. Chrigui, T. Boushaki, Experimental and Numerical Study of Swirling Diffusion Flame provided by a Coaxial Burner: effect of Inlet Velocity Ratio, Fluids 6 (2021) 159.

[60] H. Zaidaoui, Etude des flammes turbulentes non-prémélangées stabilisées par un swirler : effets de la recirculation des gaz de combustion et de l'enrichissement à l'oxygène, Ph.D. thesis, University of Orléans, France, 2022, https://theses.hal.science/tel-03682759v2.

[61] J. Zhang, C. Sui, B. Zhang, J. Li, Effects of swirl intensity on flame stability and NO emission in swirl-stabilized ammonia/methane combustion, Appl. Energy Combust. Sci. 14 (2023) 100138.

[62] M. Anand, F. Takahashi, M. Vangsness, M. Durbin, W. Schmoll, An Experimental and Computational Study of Swirling Hydrogen Jet Diffusion Flames, J. Eng. Gas Turbines Power 119 (1997) 305-314.

[63] T. Boushaki, J. Sautet, B. Labegorre, Control of flames by tangential jet actuators in oxy-fuel burners, Combust. Flame 156 (2009) 2043-2055.

[64] S. Yu, X. Liu, X. Bai, A. Elbaz, W. Roberts, LES/PDF modeling of swirl-stabilized nonpremixed methane/air flames with local extinction and re-ignition, Combust. Flame 219 (2020) 102-119.

[65] M. Zhang, X. Wei, J. Wang, Z. Huang, H. Tan, The blow-off and transient characteristics of cofiring ammonia/methane fuels in a swirl combustor, Proc. Combust. Inst. 38 (2021) 5181-5190.

[66] H. Tang, D. Ezendeeva, G. Magnotti, Simultaneous measurements of NH2 and major species and temperature with a novel excitation scheme in ammonia combustion at atmospheric pressure, Combust. Flame 250 (2023) 112639.

[67] Y. Guo, H. Shi, H. Liu, Y. Xie, Y. Guan, Reactive molecular dynamics simulation and chemical kinetic modeling of ammonia/methane co-combustion, Fuel 354 (2023) 129341.

[68] M. Biebl, J. Leicher, A. Giese, C. Wieland, A Comprehensive Study of Non-Premixed Combustion of Ammonia and its Blends. Challenges and Solutions for Flame Stability and Emission Reduction, Available at SSRN, 2024.

[69] M. Biebl, J. Leicher, A. Giese, K. Görner, Experimental and simulation investigations of the combustion of blends of ammonia with hydrogen and natural gas in industrial non-premixed burners, Proceedings of the European Combustion Meeting 2023, 11th European Combustion Meeting, Rouen, France, 2023, p.26-28.

[70] P. Fokaides, P. Kasabov, N. Zarzalis, Experimental investigation of the stability mechanism and emissions of a lifted swirl nonpremixed flame, J. Eng. Gas Turbines Power 130 (2008) 011508.

[71] DIN e.V., EN 267:2020, "Forced draught burners for liquid fuels"; German version, Beuth-Verlag, Berlin, Germany, 2021.

[72] M.M.A. Ahmed, L. Xu, X.-S. Bai, Z.O. Hassan, M. Abdullah, J. Sim, E. Cenker, W. L. Roberts, A.M. Elbaz, Flame stabilization and pollutant emissions of turbulent ammonia and blended ammonia flames: a review of the recent experimental and numerical advances, Fuel Commun. 20 (2024) 100127.