# The effect of low-temperature chemistry in nitrogen-diluted flames under microgravity

Roman Glaznev, Christian Schwenzer, Fabien Halter, Raik Hesse, Christian

Chauveau, Chaimae Bariki, Heinz Pitsch, Joachim Beeckmann

► To cite this version:

Roman Glaznev, Christian Schwenzer, Fabien Halter, Raik Hesse, Christian Chauveau, et al. The effect of low-temperature chemistry in nitrogen-diluted flames under microgravity. Proceedings of the Combustion Institute, 2025, 41, pp.105986. 10.1016/j.proci.2025.105986. hal-05410523

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# The effect of low-temperature chemistry in nitrogen-diluted flames under microgravity

Roman Glaznev<sup>a, ID</sup>,*, Christian Schwenzer<sup>a, ID</sup>, Fabien Halter<sup>b,c ID</sup>, Raik Hesse<sup>a ID</sup>, Christian Chauveau<sup>b ID</sup>, Chaimae Bariki<sup>a</sup>, Heinz Pitsch<sup>a</sup>, Joachim Beeckmann<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

- Low-temperature chemistry

- Microgravity

- Laminar flame speed

- Flame propagation

- Flame structure

## ABSTRACT

In this work, a flame propagation behavior analysis was conducted for outwardly propagating flames of diethoxymethane (DEM) and ethanol as exemplary fuels with and without low-temperature chemistry (LTC). Flame temperatures were decreased by N₂-dilution of the oxidizer to enhance the possible low-temperature chemistry effect, leading to decreased flame propagation speed. Slowly expanding spherical flames are known to be affected by radiation and particularly by buoyancy, deforming the flame's shape and violating underlying existing post-processing assumptions. Experiments were conducted under microgravity conditions during parabolic flight campaigns aboard the Airbus A310 Zero-G to isolate the effects of radiation from buoyancy-induced flame deformation. Extracted unstretched laminar flame speeds at initial pressures of 3 to 10 bar were compared with planar flame simulations. The approach by Yu et al. (2014) was used to correct the experimental laminar flame speeds for radiation. For lean mixtures, significant flame acceleration was observed with a strong nonlinear propagation speed dependence on stretch for both fuels. In contrast, rich DEM/oxidizer mixtures consistently maintain self-sustained flame propagation, while ethanol/oxidizer mixtures tend to extinguish. This is linked to the effectiveness of low-temperature branching reactions in DEM oxidation. Flame structure analyses for DEM/oxidizer mixtures revealed that the reaction zone broadening is correlated to the distribution of oxidized DEM radicals predominantly present at a lower temperature of T ~ 800 K.

## 1. Introduction

The global transition to minimum CO₂ emissions causes growing interest in non- fossil alternative fuels. Various alternative energy carriers with low-temperature chemistry (LTC) are utilized in many applications. They differ from fuels without LTC by having a wider flammability range and lower soot formation [1], which complicates their fire detection. Stagni et al. [2] found that removing low-temperature reaction pathways reduces the ignition region in the ignition-no-ignition diagrams of n-heptane as an exemplary LTC fuel, which highlights the importance of LTC on flame propagation limits. Diethoxymethane (DEM, C₂H₅-OCH₂O-C₂H₅) is a bio-hybrid fuel with complex LTC [1]. The laminar flame speed (S₀>L,u), a fundamental reactivity and exothermicity metric of a combustible mixture, is of key importance for kinetic model validation. Understanding the effect of LTC on DEM's laminar flame speed requires high-fidelity experiments under various conditions.

One of the widely-used experimental methods to measure S₀>L,u is based on the measurement of a centrally-ignited outwardly propagating flame. The stretch rate of the spherical flame is defined as

$K = (2/R_f)\dot{R}_f$, where $R_f$ and $\dot{R}_f$ denote the flame radius and its rate of change in the laboratory frame, respectively. Since the burned gas can be assumed to be motionless for fast flames, the flame propagation speed with respect to the burned gas is given by $S_{L,b} = \dot{R}_f$. This assumption is challenged for the flames in focus of the current study because of the kinematic radiation effect of the burned gas [3]. The corresponding correction of this effect is included in the radiation correction approach described below. The flame speed can be extrapolated to zero stretch rate using nonlinear methods [4,5] resulting in $S_0>L,b$. The laminar flame speed is then obtained using the mass continuity equation as $S_0>L,u = \sigma S_0>L,b$, where $\sigma = \rho_b/\rho_u$ is the density ratio between burned and unburned gases. The last can be extracted from the equilibrium calculations.

The effect of LTC on DEM ignition was studied in literature [1,2,6]. Unlike ignition, the LTC effect on laminar flame speed is generally considered negligible. However, under critical conditions, such as flame propagation near the extinction limits and oxidizer dilution, the impact

<footnote><sup>a</sup> Institute for Combustion Technology, RWTH Aachen University, 52056 Aachen, Germany</footnote>

<footnote><sup>b</sup> CNRS ICARE, Avenue de la Recherche Scientifique, 45071 Orléans, France</footnote>

<footnote><sup>c</sup> University of Orléans, 45100 Orléans, France</footnote>

<footnote>* Corresponding author.
E-mail address: r.glaznev@itv.rwth-aachen.de (R. Glaznev).</footnote>
<table><caption>**Table 1**
Experimental conditions investigated under microgravity.</caption><thead><tr><th>Fuel</th><th>Oxidizer [% in vol.]</th><th>T<sub>u</sub> [K]</th><th>p<sub>u</sub> [bar]</th><th>Equivalence ratio φ [–]</th></tr></thead><tbody><tr><td>Ethanol</td><td>12 O<sub>2</sub>/88 N<sub>2</sub></td><td>373</td><td>2.8</td><td>0.87</td></tr><tr><td>Ethanol</td><td>12 O<sub>2</sub>/88 N<sub>2</sub></td><td>373</td><td>3</td><td>1.66, 1.8, 1.96</td></tr><tr><td>DEM</td><td>12 O<sub>2</sub>/88 N<sub>2</sub></td><td>373</td><td>2</td><td>1.3</td></tr><tr><td>DEM</td><td>12 O<sub>2</sub>/88 N<sub>2</sub></td><td>373</td><td>3</td><td>0.86, 1.0, 1.3, 2.05, 2.66, 2.81</td></tr><tr><td>DEM</td><td>12 O<sub>2</sub>/88 N<sub>2</sub></td><td>373</td><td>3.9</td><td>0.99</td></tr><tr><td>DEM</td><td>12 O<sub>2</sub>/88 N<sub>2</sub></td><td>373</td><td>4</td><td>1.52, 2.77</td></tr><tr><td>DEM</td><td>12 O<sub>2</sub>/88 N<sub>2</sub></td><td>373</td><td>5</td><td>0.86, 1.03, 1.75, 2.94</td></tr><tr><td>DEM</td><td>12 O<sub>2</sub>/88 N<sub>2</sub></td><td>373</td><td>10</td><td>0.86, 1.03, 2.02</td></tr></tbody></table>

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Laminar flame speed of the DEM flames obtained using adiabatic unstretched simulations at $X_{O_2}^{air} = 12\%$, $T_u = 373$ K, $p_u = 3$ bar. Full model - the detailed kinetic model by Jacobs et al. [1]; No LTC - the detailed model with deactivated low-temperature chemistry reaction pathways.</figcaption></figure>

of low-temperature chemistry on $S_{L,u}^0$ can be enhanced. Fig. 1 presents an increasing deviation in $S_{L,u}^0$ between the full kinetic model and the one with extracted LTC reaction pathways up to 8% at $\phi = 2.0$ and 20% at $\phi = 3.0$. The heat release rate shows a 20% decrease of the peak value once the LTC reaction pathways are excluded (Fig. S1 in Supplementary materials, SM). A similar peak decrease is observed for the peak OH mole fraction, explaining the change in $S_{L,u}^0$. The LTC also enhances heat release in the pre-heat zone, where the elevated temperatures can change the composition of the flame, significantly affecting chain branching reactions and flame propagation limits. A high-fidelity experimental investigation of flame propagation under the specified critical conditions is therefore necessary, followed by an analysis of the processes occurring in the preheating zone.

Ruan et al. [7] recently highlighted that more experimental studies are warranted for the enhancement of the existing DEM kinetic models. Experimental investigation of the LTC effect on flame propagation dynamics requires careful selection of the experimental conditions. In this work, an experimental study of $N_2$-diluted outwardly propagating DEM flames was conducted. Nitrogen dilution considerably decreases flame propagation speed, causing significantly longer experimental timescales, which was shown for ethanol flames by Bariki et al. [8]. To highlight the role of LTC, a comparison of ethanol (non-LTC fuel) and DEM (LTC fuel) flame propagation characteristics is presented. The analysis of slowly propagating flames is a complex process, since their evolution is influenced by radiation and buoyancy. The last can cause the flame to deviate from its spherical shape, invalidating the underlying assumptions [8]. To avoid buoyancy-induced deformation of the flames, experiments were conducted under microgravity conditions during parabolic flight campaigns. The collected data provides a solid basis to answer the following research questions:

* How similar are the behaviors of ethanol (non-LTC fuel) and DEM (LTC fuel) near flame propagation limits?

* What chemical processes occur in LTC flames that support flame propagation?

## 2. Experimental framework

Experiments were performed under microgravity to avoid the influence of buoyancy on slowly propagating flames within a dual chamber designed for pressure release, ensuring isobaric conditions and compliance with aircraft safety standards [8–10]. The details of the setup are discussed by Bariki et al. [8], and only a brief description of the major points and an included schlieren setup is given here. Microgravity conditions were realized through parabolic flights aboard the Airbus A310 Zero-G, achieving an effective gravity level of $10^{-2}$ g. A photo of the experimental apparatus is shown in Fig. 2, which features a 1 L spherical combustion chamber encased within an 11 L high-pressure chamber.

Both chambers feature aligned windows that provide optical access for observation of the flame. A schlieren imaging system is employed to visualize the outer contour of the flame. An Energetiq laser-driven light source (LDLS) operates in conjunction with light parallelization lenses, a pinhole, and a high-speed camera (Phantom v1611), which continuously captures flame propagation at a rate of 3000 frames per second, covering a field of view of 768 × 768 pixels². This setup achieves a magnification ratio of 18.9 px/mm. The temporal evolution of the flame radius is derived from the schlieren images obtained during the experiments.

For a limited set of experiments, the setup was modified from schlieren imaging to natural luminosity imaging by not using the LDLS, pinhole, and parallelization lenses. Here, the flame's radius is obtained from the light emitted by the combustion process. The $\dot{R}_f$ values extracted using both techniques agree with each other once the ignition effects have decayed. A comparison of the images obtained using both techniques is shown in Fig. S2 in SM.

To extract the unstretched propagation speed, the expansion of the nonlinear asymptotic expression proposed by Kelley et al. [5] is used, with the detailed formula given in the SM. The data range used to fit the model is carefully selected based on the flame propagation regimes described below, ensuring self-sustained flame propagation.

Decreasing oxygen content is one way to obtain a lower adiabatic flame temperature ($T_{ad}$). A preliminary parametric study was conducted to determine the appropriate oxygen content. It was found that a synthetic mixture of 12% $O_2$ and 88% $N_2$ in volume is sufficient to obtain flames with $1260 K \le T_{ad} \le 1750 K$, which causes pronounced LTC reaction pathways, as shown for the sensitivity coefficients in Section 4.1 and reaction fluxes in Fig. S3 in SM.

For the low flame speeds, especially those below 10 cm/s, the radiation effect must be considered. The approach by Yu et al. [11] was used to account for the radiation heat losses in experimental laminar flame speed data, with the exact formula and its applicability for DEM discussed in the SM, where the results of transient simulations have shown an uncertainty of ≈ 15%. This model was developed by employing CFD and a statistical narrow-band (SNB) model. It is based on several types of fuels, from $C_1$ to $C_8$ hydrocarbon fuels, syngas $H_2$/CO at various ratios, and fuels containing oxygen functional groups such as dimethyl ether. A wide range of equivalence ratios and different initial pressures and temperatures were chosen to quantify the uncertainty associated with radiation. This formula was previously applied for $N_2$-diluted
<figure><img src="image_5.png" /><figcaption>Fig. 2. Photo of the experimental setup for natural luminosity imaging.</figcaption></figure>

ethanol flames, resulting in agreement between the experimental and simulation results within the experimental uncertainty [8]. Thus, the model is applied in the present study to DEM flames. The model was applied to the experimental data extracted with a similar extrapolation range ($R_f < 2$ cm) as the one used by Yu et al. [11].

An overview of the investigated mixtures is presented in Table 1. The initial temperature was fixed at 373 K for all conditions. The repeatability of the results was ensured by conducting two to three experiments for each condition, except for super-rich ($\phi > 2.5$) mixtures, where ignition challenges lead to artifacts caused by the ignition spark momentum and thus hindered repetitions. A limited set of experiments was conducted with ethanol as an exemplary fuel without the low-temperature chemistry to reference the effect on the flame propagation limits.

### 3. Numerical framework

One-dimensional simulations of stationary planar flames were performed using the freely available program FlameMaster [12]. Isobaric conditions are preserved by zero gradient outflow conditions. Gradients are properly resolved using a dynamic grid refinement algorithm. At least 50 grid points resolve the flame thickness.

A recently developed model by Jacobs et al. [1], containing 654 species and 6497 elementary reactions with forward and backward reactions counted separately, is used to simulate DEM's chemical kinetics. It is based on the NUIG ver. 0.9 model [13]. Ruan et al. [6] modified the model by Jacobs et al. showing a better agreement in ignition delay time values at 30 bar. These changes are expected to have a minor effect on the flame propagation dynamics at $p_u \le 10$ bar investigated here.

A reduced version of the C3.3 kinetic model [14], containing 178 species and 2725 elementary reactions, was used for ethanol simulations. The reduction was conducted to exclude larger fuel molecules and maintain $S_{L,u}^0$ and ignition delay times (IDTs) of ethanol.

### 4. Results and discussion

This section is structured as follows. First, a sensitivity analysis is conducted for DEM flames with varying oxygen mole fraction in the oxidizer ($X_{O_2}^{air}$), highlighting the effect of $N_2$ dilution on LTC-fuel combustion. Then, the dependence of flame propagation speed on stretch is discussed, resulting in $S_{L,u}^0$ being extracted for various $\phi$ at several pressures. Finally, the observed effects in lean and rich flame propagation limits are discussed.

#### 4.1. Sensitivity analysis of rate coefficients

The sensitivity analysis was conducted to investigate the effect of LTC on laminar flame speeds at $X_{O_2}^{air} = 21$ and 12%. A similar investigation was pursued by Bariki et al. [8] for ethanol, revealing the sensitivity coefficients remaining qualitatively similar with decreasing $X_{O_2}^{air}$ for a non-LTC fuel.

The normalized sensitivity coefficients for DEM flames are presented in Fig. 3. Among the most sensitive reactions for both dilutions are

<figure><img src="image_6.png" /><figcaption>Fig. 3. Normalized sensitivity coefficients for $S_{L,u}^0$ with respect to frequency factors of rate coefficients in planar DEM flames at $T_u = 373$ K, $p_u = 3$ bar, and $\phi = 2.0$.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. DEM flame propagation speed evolution as a function of flame stretch at $T_u = 373$ K, $p_u = 3$ bar, and $X_{O_2}^{air} = 0.12$ for different equivalence ratios shown by different colors. The dotted lines represent iso-lines of the corresponding flame radius.</figcaption></figure>

$(5f) O_2 + H = O + OH$ and $(43f) CH_3 + H + M = CH_4 + M$. The same reactions were found to be sensitive for ethanol by Bariki et al. [8].

However, in contrast to the non-LTC fuel, the sensitivity coefficients of other reactions in DEM flames have a more complex dependence on $X_{O_2}^{air}$ variation. The absolute values of the coefficients of three H-abstraction reactions (2710f), (2686f), and (2711f) increase significantly for lower $X_{O_2}^{air}$, while being almost negligible for normal air. Detailed reaction fluxes describing the radical branching are illustrated in Fig. S3 in SM. The pronounced sensitivities of these reactions, along with their opposing influence on flame speeds, enable the validation of their branching ratios using our experimental data. This, in turn, can support further development of the DEM kinetic model and enhance the kinetic understanding of similar reactions in structurally analogous fuels within the framework of rate rules.

The sensitivity coefficient increase with lower $X_{O_2}^{air}$ is particularly pronounced for the reaction (2834f) involved in LTC, highlighting the increased role of the LTC in the diluted mixtures. However, since the kinetic model was not yet validated against experimental flame speeds at the selected conditions, careful comparison with the high-fidelity experiments is required before further analysis of the LTC effects, described in the following sections.

#### 4.2. Flame propagation speed

Fig. 4 depicts the characteristic evolution of the measured experimental propagation speed over stretch rate. The curves for expanding flames are read from high to low stretch rate. Three flame propagation regimes are usually identified for outwardly propagating flames: (1) the ignition-energy-driven flame kernel propagation regime, (2) the
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Laminar flame speed of DEM (a) and ethanol (b) flames at $T_u = 373$ K, $p_u = 3$ bar, and $X_{O_2}^{air} = 0.12$. Lines represent simulation results. Experimental and numerical ethanol data is adapted from Bariki et al. [8].</figcaption></figure>

transition regime, and (3) the self-sustained (normal) flame propagation regime. These regimes are detailed below and illustrated by arrows in Fig. 4 for the initial pressure of $p_u = 3$ bar and mixtures of $\phi = 1.0$. In the first regime, the propagation of the flame kernel is primarily driven by the ignition energy deposition. As the ignition kernel expands outwardly, both flame propagation and flame stretch gradually decrease until the kernel reaches a critical radius, which is $R_f = 1.0$ cm for $\phi = 0.86$ and $R_f = 0.8$ cm and $\phi = 1.0$. It is important to note that successful ignition occurs only when the flame kernel surpasses this critical radius, leading to a self-sustaining flame. Here, $\dot{R}_f$ is only affected by the stretch rate, and no effect of ignition energy deposition can be observed. The third regime must be used to deduce the unstretched laminar flame speed and the burned-gas Markstein length $L_b$, representing the sensitivity of the flame speed to the stretch rate [5]. Note that the derived $L_b$ of a slowly propagating flame is radiation-affected.

One can see that $\dot{R}_f$ changes almost linearly with $K$ for all the measured mixtures, except for $\phi = 0.86$ and $\phi = 1.0$ at $p_u = 3$ bar. For the stoichiometric fuel-air mixtures, the dependence becomes almost linear for $R_f > 1.2$ cm, while for $\phi = 0.86$, a strong nonlinear dependence causes significant extrapolation uncertainties for all the measured flame radii. Thus, the unstretched flame propagation speed $S_{L,b}^0$ is not extracted for the equivalence ratio of 0.86. Similar nonlinearities are observed at $p_u = 5$ bar in Fig. S6 in SM.

For the richer mixtures shown in Fig. 4, the propagation speed $\dot{R}_f$ decreases with the decreasing stretch rate $K$ as the flame front radius increases, highlighting negative radiation-affected burned-gas Markstein length. A similar behavior was observed in a study of lean flammability limits of methane flames under microgravity by Wang et al. [15]. It is expected for methane to have negative $L_b$ on the lean side because of the methane's lower molar mass compared to the oxidizer. Note that the relationship between stretch and flame speed depends on the equivalence ratio and on the fuel.

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Microgravity schlieren images of ethanol and DEM flames close to the rich flame propagation limit at $T_u = 373$ K, $p_u = 3$ bar, and $X_{O_2}^{air} = 0.12$.</figcaption></figure>

### 4.3. Laminar flame speed

The extracted laminar flame speeds corrected for radiation using the Yu-formula are shown in Fig. 5(a) for DEM and in Fig. 5(b) for ethanol. The lines represent laminar flame speeds obtained from the planar unstretched adiabatic premixed simulations. Experimental and numerical ethanol data is adapted from Bariki et al. [8]. The uncorrected values for DEM are given in Fig. S8. In stoichiometric DEM mixtures, an overprediction of the experimentally measured values is observed at $p_u = 10$ bar, while an underprediction occurs at $p_u = 3-5$ bar. It suggests that further refinement of the kinetic model may be necessary at these conditions, which requires careful assessment of additional DEM data (IDTs, extinction strain rate) in the literature. However, it is beyond the scope of the present work.

The deviation decreases for richer mixture. The experimental results underpredict the simulation results by 10% at $\phi = 1.3$ for DEM after the radiation correction. The offset becomes negligible for $1.5 < \phi < 2.1$. A slightly larger discrepancy with simulations is observed for the richer mixtures ($\phi > 2.5$), which can be explained by a single successful experiment for each mixture and thus not sufficient repeatability, caused by the ignition artifacts affecting the flames at these conditions. However, the obtained experimental data, which were extremely difficult to measure and post-process, can be used as a reference data set to validate chemical kinetic models of DEM as long as more sophisticated radiation correction approaches are not developed.

Laminar flame speeds of both ethanol and DEM tend to have a minor equivalence ratio and pressure dependence for $\phi > 1.5$ (Fig. 5). Ethanol flames were studied in a lower equivalence ratio range because of their ignition challenges for $\phi > 1.6$ discussed below. A wider flame propagation limit of DEM can be expected because of the LTC effect. Thus, the flame propagation behavior analysis close to the propagation limits might gain new insights into the understanding of DEM combustion dynamics.

### 4.4. Flame propagation limits

Fig. 4 illustrates a strong stretch dependence for lean DEM mixtures at $p_u = 3$ under microgravity. A similar dependence was captured for lean ethanol flames. This effect was observed previously for rich methane and lean iso-octane mixtures [4], but here it is highlighted because of the unusually low minimum flame propagation speed, and, consequently, radiation effect. That strong stretch dependence makes accurate extrapolation to zero stretch with existing extrapolation models impossible. Microgravity experiments in a larger vessel would allow for capturing larger flames, decreasing the stretch-related uncertainty and permitting quantitative characterization of the nonlinear dependence. On the other, hand radiative heat loss would be more ponounce.
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Structure of planar DEM flames for varying equivalence ratio. Left: major flame components; right: LTC radicals and OH. The origin of the flame coordinate x is the OH peak position.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Structure of diethoxymethane (DEM) and its major fuel radicals with low-temperature pathways indicated by Jacobs et al. [1].</figcaption></figure>

The transient adiabatic spherical flame simulations were conducted for $\phi = 0.85$, in order to estimate the sufficient extrapolation range. The results have shown low extrapolation uncertainty at a radius of 3.0-3.5 cm, meaning the minimum vessel radius of $\approx 7$ cm. However, the stretch dependence must be corrected for radiation-caused inward gas motion, which is expected to be significant and non-trivial for the flames with $R_f > 2$ cm. Then, the data extracted can be compared with the one captured here, and a new extrapolation model can be developed for smaller spherical vessels. The complexity of this approach, particularly with respect to conducting high-fidelity microgravity experiments and their correction for radiation, must be taken into account here.

The flame propagation behavior of ethanol and DEM flames changes close to the rich flammability limit. At $p_u = 3$ bar, ethanol mixtures with diluted oxidizer experienced transition to self-sustained flame propagation up to $\phi = 1.66$. All the richer mixtures were observed to extinguish once the ignition energy was disposed into the system, as can be seen in Fig. 6(b). Conversely, DEM flames propagate in the self-sustained regime after ignition energy deposit for mixtures up to $\phi = 2.81$. Conducting experiments with mixtures of $\phi > 2.81$ led to technical challenges and were thus avoided.

Thus, the flame propagation of ethanol flames is limited to an equivalence ratio of $\phi = 1.66$, while that of DEM extends to equivalence ratios near 3.0, potentially due to LTC. In order to gain more insights into DEM combustion, an analysis of the flame structure was performed

for $\phi = 1.0$ and 2.0, where the LTC effect should start to kick in. The mole fractions of the major flame components such as DEM, $O_2$, CO, and $CO_2$ are presented in Fig. 7 in the left column. Note that only a section of the wider computational domain is presented in the figure. The reaction zone, indicated by the consumption of reactants (DEM and $O_2$) and production of $CO_2$, is broadened with increasing equivalence ratio: from $\sim 0.1$ cm at $\phi = 1.0$ to $\sim 0.45$ cm at $\phi = 2.0$.

Jacobs et al. [1] analyzed DEM oxidation, showing that DEM consumption proceeds through three major fuel radicals with low-temperature pathways: $R1\dot{O}_2$, $R2\dot{O}_2$, and $R4\dot{O}_2$. These radicals differ from each other by the location of the O-O radical, which is either bonded to the primary C–C site ($R1\dot{O}_2$), secondary C–C site ($R2\dot{O}_2$), or the central O-C-O site of DEM ($R4\dot{O}_2$), as shown in Fig. 8. The concentration profiles of these radicals are presented in the right column of Fig. 7. For both equivalence ratios, the peak concentration of the radicals is observed at a temperature of $\sim 600$ K. Note that the peak concentration of fuel radicals increases with richer flames, as the greater amount of fuel serves as the source molecule. The width of the concentration profile widens with a higher equivalence ratio, mainly due to the more pronounced low-temperature region of the profile. This results in an expanded reaction zone and, consequently, thicker flames at elevated equivalence ratios. Therefore, the broader distribution of LTC radicals contributes to a wider flame propagation regime of DEM compared to ethanol.

To gain further insights into the flame structure of ethanol as a representative non-LTC fuel compared to DEM, the fuel radical concentration profiles are presented for $\phi = 1.0$ and $\phi = 2.0$ in Fig. 9. The unstretched simulation for ethanol converged at $\phi = 2.0$ because of the zero radiation heat loss assumption. The leading fuel radicals formed by H atom abstraction from the first carbon atom of the fuel molecule are selected here for consistency: $CH_2-CH_2-OCH_2-O-C_2H_5$ for DEM ($R_{1,D}$) and $CH_2-CH_2-OH$ for ethanol ($R_{1,E}$). A bell-like shape is evident for both fuel radicals at $T \approx 1300$ K for $\phi = 1.0$. The shape persists with a lower peak concentration at the same position for the ethanol radical at $T \approx 1050$ K for $\phi = 2.0$, when $R_{1,D}$ peak concentration shifts to the unburned side and is observed at $T \approx 800$ K. The $R_{1,D}$ concentration profile exhibits a double-peak shape at $\phi = 2.0$, indicating more complex chemical interactions for DEM under rich conditions compared
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** The fuel radical's concentration profiles of DEM (a) and ethanol (b) flames. The origin of the flame coordinate *x* is the OH peak position.</figcaption></figure>

to a non-LTC fuel. Thus, LTC enhances fuel decomposition at lower temperatures.

Jacobs et al. highlighted DEM decomposition through $R2\ddot{O}_2$ radical. Here, 81% of this radical is consumed via internal isomerization forming $Q2OOHc$ ($C_2H_5O\ddot{C}HOCH(OOH)CH_3$), with its rate of production presented in Fig. S7. The thermal dissociation of $Q2OOHc$ (reaction 2826) competes with the second molecular oxygen addition (reaction 2834), with the latest being initiated at lower temperatures. Similar behavior was observed for DEM by Jacobs et al. in a constant volume reactor [1], leading to the chain-branching and decreasing ignition delay time. Here, the peak production rate of $Q2OOHc$ is located at 0.1 cm before the flame front, that is a typical location for the higher heat release rates caused by LTC in rich DEM flames (Fig. S1). In conjunction with the importance of the $Q2OOHc$ radical for chain-branching, these reactions are concluded to be essential for DEM flame sustenance. Since ethanol combustion lacks a similar pathway, its flames are unable to sustain at similar conditions.

## 5. Conclusion

An experimental study was carried out to investigate the LTC effect on flame propagation. Diethoxymethane (DEM, LTC-fuel) and ethanol (non-LTC fuel) mixtures with 12% $N_2$-dilution of the oxidizer were analyzed. The flames of these mixtures exhibit very low propagation speeds that are difficult to measure with good accuracy. The buoyancy effect was suppressed under microgravity conditions in a series of parabolic flights. Radiation-corrected laminar flame speeds were obtained by applying the empirical correlation presented by Yu et al. [11] to the experimental data. Unstretched planar 1-D freely propagating flame simulations were performed. The experimental and simulated results agree within the uncertainty for $1.3 < \phi < 2.1$. The newly provided data can be used as a starting point for the subsequent investigation of DEM's flame propagation. A detailed experimental and numerical

analysis of the temperature distribution on LTC intensity would benefit a deeper understanding of DEM combustion.

Flame propagation on the rich side was found to be different for ethanol and DEM mixtures. Ethanol flames did not experience self-sustained propagation at $\phi \ge 1.8$ at $p_u = 3$ bar, while DEM flames propagated even at $\phi = 2.81$. Unstretched flame structure analysis was conducted for DEM mixtures up to $\phi = 2.0$ to gain insights into the wider flame propagation regime. It was found that the reaction zone increase is associated with fuel radicals' peaks widening toward the low-temperature preheating flame zone. Since these radicals are the key components of the low-temperature reaction pathways, they are the reason for the DEM sustained flame propagation in a wide range of conditions.

Finally, a comparison of the main DEM and ethanol fuel radicals was performed. It was shown, that in case of non-LTC fuel, the radical peak concentration is decreasing and its position is steady with increasing $\phi$. For LTC fuel, the radical peak is shifted to the unburned side, and its shape is changing, leading to the fuel consumption at lower temperatures and supporting further radical formation and chain branching.

## Novelty and significance statement

This study investigates the effect of low-temperature chemistry (LTC) on flame propagation. For this, combustion dynamics of two fuels are compared: ethanol and diethoxymethane (DEM), as an exemplary biofuel with LTC. Nitrogen dilution is shown to enhance the LTC reaction pathway in DEM combustion and the most sensitive reactions are discussed. The novelty of this study lies in the unique microgravity data captured during parabolic flight campaigns, which allows for the validation of the kinetic model against experimental flames that are otherwise buoyant under these conditions. Our results reveal a significant difference in the flame propagation limit of ethanol and DEM at 3 bar, where the LTC effects in DEM mixtures are shown to be particularly pronounced. Fuel radical formation closer to the unburned gas was observed for the LTC-fuel, enabling self-sustained flame propagation.

## CRediT authorship contribution statement

**Roman Glaznev:** Conceptualization, Methodology, Investigation, Data analysis, Visualization, Writing – original draft. **Christian Schwenz-zer:** Investigation, Data analysis, Visualization, Writing – review & editing. **Fabien Halter:** Investigation, Data analysis, Scientific discussion, Writing – review & editing. **Raik Hesse:** Investigation, Data analysis, Visualization, Writing – review & editing. **Christian Chauveau:** investigation, Scientific discussion, Writing – review & editing. **Chaimae Bariki:** Conceptualization, analysis, Writing – review & editing. **Heinz Pitsch:** Supervision, Scientific discussion, Writing – review & editing. **Joachim Beeckmann:** Supervision, Funding acquisition, Scientific discussion, Writing – review & editing.

## 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 gratefully acknowledge the financial support provided by the Deutsches Zentrum für Luft- und Raumfahrt (DLR, German Aerospace Center), Grant no. 50WM2258., the Centre National d'Études Spatiales (CNES, French Space Agency) and the Centre National de la Recherche Scientifique (CNRS), GDR MFA no. 2799. We are thankful to DLR and CNES for funding and Novespace for providing parabolic flight campaigns.
# Appendix A. Supplementary data

Supplementary material related to this article can be found online at https://doi.org/10.1016/j.proci.2025.105986.

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