# A detailed investigation of swirl spray fuel injectors using conventional and plenoptic shadowgraphy techniques

Mohammed Asad Khan, Adrian Marszalek, Abdallah Alhaffar, Pierre Schleuniger, Jean-Bernard Blaisot, Françoise Baillot

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## To cite this version:

Mohammed Asad Khan, Adrian Marszalek, Abdallah Alhaffar, Pierre Schleuniger, Jean-Bernard Blaisot, et al.. A detailed investigation of swirl spray fuel injectors using conventional and plenoptic shadowgraphy techniques. 33rd Conference on Liquid Atomization & Spray Systems (ILASS Europe 2025), Lund University, Aug 2025, Lund, Sweden. hal-05330519

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# A detailed investigation of swirl spray fuel injectors using conventional and plenoptic shadowgraphy techniques

Mohammed Asad Khan*¹, Adrian Marszalek¹, Abdallah Alhaffar¹, Pierre Schleuniger¹, Jean-Bernard Blaisot¹, Françoise Baillot¹

¹ Université de Rouen Normandie, INSA Rouen Normandie, CNRS, CORIA UMR 6614, 76000 Rouen, France

*Corresponding author: khanm@coria.fr

## Abstract

The swirl spray injector, consisting of a fuel atomizer and air swirler, is a key component of gas turbines and aero-engine combustors. The internal atomizer geometry is one of the important factors that control the spray characteristics of an atomizer. Therefore, slight imperfection in atomizer design due to manufacturing imprecision or damage due to repetitive operation results in poor atomization and in-homogeneous spray, which in turn may negatively affect the steady state and dynamical properties of the combustor. The present study investigates how much of the distorted spray characteristics of the simplex pressure swirl atomizer translate into injector spray characteristics and have the potential to impact flame characteristics. A set of functional and damaged hollow-cone simplex pressure swirl atomizers were studied in combination with a radial swirler with a swirl number of 0.6 or 0.74. The spray droplet size distribution of standalone atomizers and swirl spray injectors is measured in two perpendicular planes by the conventional shadowgraphy technique. Moreover, the spray quality, i.e., symmetry and homogeneity, was evaluated with plenoptic shadowgraphy using a 3D light field camera. The results show that the robust interaction between the swirler and primary spray produced by the atomizer eliminates the atomizer defects. Thus, atomizer spray characteristics have no significant effect on the swirl spray injector characteristics despite the huge difference in spray characteristics of standalone atomizers.

**Keywords:** Spray swirl burner; Light field imaging; 3D measurement; Manufacturing precision; Damaged atomizer

## Introduction

The swirling flows are mainly used to aerodynamically enhance fuel-air mixing and flame stabilization. This swirl flow is generally achieved in gas turbines and aero engines with two important elements: a fuel atomizer and an air swirler. Pressure swirl atomizers are widely used in most practical applications due to geometric simplicity, reliability, ability to generate fine sprays, wide spray angles, and low power requirements. The fuel atomizer is located in the central hub of the air swirler. The air swirler generates radial and axial pressure gradients to raise a central toroidal recirculation zone (CTRZ). The CTRZ helps stabilize the flame under a wide range of operating conditions by promoting the recirculation of hot combustion products. Moreover, the CTRZ increases the rate of ambient air entrainment, which results in spray dispersion and rapid fuel-air mixing [1]. Thus, the swirl spray injector actively controls the steady-state behavior of the combustor in terms of local equivalence ratio, temperature distribution, combustion efficiency, and pollutant emission.

The internal geometry of a pressure swirl atomizer comprises three parts: a swirl chamber, tangential ports, and an exit orifice. Liquid fuel is forced under high pressure from tangential ports to enter the swirl chamber. The liquid rotates inside the swirl chamber and finally discharges through the exit orifice in the form of a conical film and central air core. The thin conical liquid sheet ruptures into ligaments outside the atomizer when internal forces overcome surface tension forces. The liquid ligaments break up into droplets further downstream due to aerodynamic interaction with ambient air. The internal geometric features of a pressure swirl atomizer are important factors that affect the spray characteristics of an atomizer. The ratio of the diameter of the swirl chamber to the diameter of the orifice, the length-to-diameter ratio of the swirl chamber, the length-to-diameter ratio of the orifice, and the length-to-diameter ratio of the atomizer have no significant effect on the spray characteristics of the atomizer.
ratio of the swirl port can significantly affect the discharge coefficient, spray angle, mean droplet diameter, droplet size distribution, and circumferential liquid distribution [2][3]. Moreover, variation in the manufacturing techniques can result in large tolerances to these geometric features. Recently, Jedelský et al. [4] showed that apart from large dimensional tolerances, poor surface quality and shape inaccuracy also result in poor atomization quality in terms of discharge coefficient, Sauter mean diameter, spray cone angle, breakup length, and liquid distribution. Furthermore, the atomizer may be damaged due to thermal stresses during repetitive combustion experiments.

The swirl spray injector has been extensively studied in the literature over the past several decades since it has the utmost importance in liquid fuel combustors. Although the swirl flow fields are complex and three-dimensional, most of these experimental studies are based on planar illumination techniques, e.g., schlieren, shadowgraphy, Planar Laser-Induced Fluorescence (PLIF), Mie scattering, planar particle image velocimetry (PIV), particle tracking velocimetry (PTV), etc. In addition, phase/laser Doppler anemometry (PDA/LDA) measurement provides local droplet size and velocity at different positions in the spray. However, the three-dimensional acquisition of swirl spray characteristics provides more comprehensive information about the atomization process, droplet transport, and overall spray. With rapid development in non-intrusive optical diagnostic techniques for sprays, some methods have successfully achieved the 3D characteristics of non-reacting sprays. Multi-camera tomographic shadowgraphy and PIV techniques were proposed for the three-dimensional characteristics of swirl spray reported to have good agreement with PDA results [5][6]. Wu et al. [7] employed a holography technique on the kerosene swirl spray for the measurement of droplet size, shape, and 3D position. Similarly, Wang et al. [8] studied the liquid sheet morphology and 3D droplet distribution of pressure swirl spray using holography. Recently, Roth et al. [9] proposed a Fringe Projection-Laser Induced Fluorescence (FPLIF) technique for the detailed 3D analysis of spray symmetry, spray angle, liquid sheet deformation, and breakup. In addition, 3D spray measurement can be acquired using a single-shot plenoptic camera as opposed to tomography, where multiple cameras or images are required to reconstruct the 3D image. The plenoptic camera has been successfully employed to analyze the 3D spray characteristic [10].

In this work, the objective is to investigate the potential impact of distorted atomizer spray characteristics due to atomizer failure on the spray characteristics of swirl spray injectors. A combination of conventional 2D shadowgraphy and 3D plenoptic imaging of the swirling spray was carried out to explain the effect of atomizer damage on the overall characteristics of the spray.

# 1 Experimental methods

<figure><img src="image_3.png" /><figcaption>**Figure 1.** (a) Schematic of swirl spray fuel (Figure is not to the scale) (b) Microscopic images of atomizer exit orifice (c) Experimental setup</figcaption></figure>

The swirl spray injector comprises a hollow-cone simplex pressure swirl atomizer, air plenum, air distributor, swirler, and terminal plate, as shown in Figure 1 (a). The atomizers were designed and manufactured by an external supplier to generate a fine spray. No data on the atomizers' internals or dimensions was given to the users. The Mitutoyo Quick Scope microscope was used for exit orifice imaging. The microscopic images in Figure 1 (b) shows the perfectly circular exit orifice with no sign of damage for both atomizers. The compressed air is fed into the air plenum of the swirl spray injector using an EL-
FLOW mass flow meter (Bronkhorst). The six axial hole distributor uniformly channels the air into the six tangential holes of the swirler. The liquid fuel is pumped and controlled using a gear pump and mini CORI-FLOW mass flow meter (Bronkhorst). The n-heptane is used as a liquid fuel. The experiment was conducted at fuel flow rates corresponding to thermal flame power 5.9 kW, 6.6 kW, 7.0 kW and 7.4 kW in combination with airflow rates leading to $\Phi = 0.85$, $\Phi = 0.90$, and $\Phi = 0.95$ equivalence ratio. These operating conditions correspond to the TACC-Spray combustor developed at CORIA Laboratory to study the fundamental mechanisms of thermo-acoustic instabilities [11]. A set of functional and damaged hollow-cone simplex pressure swirl atomizers is tested. Experimental results presented in this paper are obtained under cold conditions, without combustion. First experiments were carried out on standalone functional and damaged atomizers to evaluate the quality of spray produced in terms of droplet distribution, spray angle, breakup length, mean diameter, and liquid distribution in the spray. These atomizers were placed in the swirl spray injector in combination with swirler S707 and S727. The swirler S707 and S727 have swirl numbers 0.6 and 0.74, respectively. The recess length between the injector exit (or terminal plate exit) and atomizer exit is kept constant at $h_r \approx 6.4$ mm. The terminal plate has an exit diameter $D_{out} = 8$ mm leading to $h_r/D_{out} \approx 1$. All the experiments were carried out in an unconfined environment at atmospheric pressure and ambient temperature.

## 1.1 Conventional shadowgraphy technique

The optical setup is shown in Figure 1 (c) for conventional shadowgraphy. It consists of a background light, a diffuser, and a high-resolution CMOS camera (Baumer, LXG-500M) with a Nikon 200 mm f/4 objective. An optical fiber is used to transmit the light from a pulsed laser source (CAVILUX) to a diffuser to generate a uniform background. This optical arrangement provides $\approx 18 \times 14 \text{ mm}^2$ field of view. The background images and dark images are captured without spray and without light, respectively. For each condition, high-resolution (7920 × 6004 pixels) images were captured and processed using an in-house image processing program based on the imaging model presented by Blaisot et al. [12]. The image processing contains normalization, thresholding, and droplet shape and size estimation. The spray images are normalized with captured background images and dark images. Then, image segmentation is performed on normalized images using a combination of a global threshold and a wavelet transform threshold. This allows the detection of the maximum number of objects, including objects with very low contrast. The diameter of each object is computed from the sizing criteria given by relative level as a function of normalized contrast [13]. The depth-of-field calibration was done by using the method of the sharp edge presented by Fdida et al. [14]. The calibration images are then processed to establish the relationship between PSF width and position along the optical axis. The depth of measurement for the present optical system corresponds to $\approx 2$ mm. The average number of droplets counted for each operating condition varies along the operating condition. However, the mean detection rate is about 2500 droplets detected per image. The complete analysis was carried out on an average of $\approx 350000$ droplets. The minimum droplet diameter obtained from the present optical setup is 5 µm.

## 1.2 Plenoptic shadowgraphy technique

The optical arrangement for plenoptic shadowgraphy is similar to the conventional shadowgraphy setup, except that the plenoptic camera was used as a substitute for a high-resolution CMOS camera. The plenoptic camera works on the light-field imaging principle, which involves capturing the position and direction of incoming light rays. It consists of a multiple microscopic lens array (MLA) placed between the main lens and the camera sensor. Each microlens behaves as a micro-camera and captures the virtual image given by the main lens from a different perspective. The raw image of a plenoptic camera contains all micro-images. Neighboring micro-images have redundancy, which is used for depth estimation. The 3D position of an object in the depth-of-field is stored in the form of a "depth map". The "all-in-focus" images can be constructed using information from raw images and depth maps such that all objects appear as focused. In the current experiments, a Raytrix R29 light-field camera was used in combination with a pulsed laser source (CAVILUX). The experimental process involves the following four steps. First, the calibration of MLA was performed for a fixed working distance and magnification. It is an alignment process of MLA to make sure that the lenses are properly specified in the software for the all-in-focus image reconstruction. The second step is depth-of-field calibration, in which a knife edge is placed perpendicular to the optical axis at the farthest plane from the camera. The X and Y axes represent the spray and optical axes of the experimental setup. The origin of a coordinate system is defined as the point of intersection of the atomizer/injector exit plane and the spray axis. The all-in-focus and the depth map images are captured while translating the knife edge across the optical axis (Z) with 0.5 mm step size. These images are processed to establish the relationship between the virtual depth map and the actual position along the optical axis. This gives the depth-of-field of the current optical system in
≈ 4 mm. Then, the spray imaging was carried out by positioning the atomizer/injector at Z = 0 mm, Z = +4 mm, and Z = -4 mm to scan a total depth ≈ 12 mm to cover the spread of the spray. A total of 150 light fields are captured at each position. All depth maps and all-in-focus have 3288 × 2192 pixels resolution and ≈ 18 × 12 mm² field of view. All the depth maps and all-in-focus images are processed using the RxLive software. The all-in-focus images can be treated as images obtained with conventional shadowgraphy and, therefore, processed using the method mentioned in section 1.1. Finally, droplet information from the all-in-focus spray images, depth map of spray, and metric depth maps obtained from depth-of-field calibration are combined to get the 3D position of droplets.

## 2 Comparison of functional and damaged atomizers

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Representative spray images and volumetric reconstruction of droplets from single light field for functional and damaged atomizers at P = 7.4 kW</figcaption></figure>

Figure 2 represents the instantaneous spray images of functional and damaged atomizers obtained from two orthogonal views at a fuel flow rate corresponding to 7.4 kW flame power. The functional atomizer produced a fully developed hollow cone spray for a given flow rate. As expected, the damaged atomizer failed to produce a fully developed spray at any given operating conditions. Surprisingly, a continuous liquid jet is observed for the damaged atomizer, representing the poor atomization quality of this atomizer. The liquid jet is further breakup into ligaments and large droplets at downstream. Earlier, it has been reported that displacement between the orifice exit axis and the swirler axis results in jet formation [4]. The microscopic images of atomizer exit orifice in Figure 1 (b) shows that the exit orifice is both atomizer is not damaged. Thus, the blockage of the swirler port or the displacement of the swirler from the
exit orifice axis could be the reason for the malfunctioning of the damaged atomizer. The 3D information of spray droplets captured from plenoptic shadowgraphy was used for the volumetric reconstruction of the spray. The 3D spray structure also shows a conical spray for a functional atomizer. Most of the droplets in this spray have a size $d < 100 \ \mu m$ with the exception of a few larger drops ($d > 100 \ \mu m$) at the spray periphery. Relatively, spray from a damaged atomizer has most of the droplets with a large droplet diameter $100 \ \mu m < d < 300 \ \mu m$. The liquid jet in the case of a damaged atomizer sheds droplets within a narrow spatial region compared to the fully developed spray of the functional atomizer. The difference between the spatial distribution of damaged and functional atomizers is not shown using an additional plot since it is quite evident from spray images as well as volumetric reconstructed 3D views. The spray angle and liquid breakup length are evaluated to quantitatively compare the spray structure of functional and damaged atomizers. Figure 3 (a) shows that the functional atomizer has a wide spray cone angle that remains almost constant at ≈ 80° for all thermal powers. In addition, it has a short liquid sheet breakup length due to effective swirling of liquid inside the atomizer and functional internal geometry. The liquid sheet breakup length for the functional atomizer was found around ≈ 1.9 mm irrespective of operating condition, as shown in Figure 3 (b). As opposed to a functional atomizer, the damaged atomizer has a large liquid breakup length (≈ 7 mm) and narrow spray cone angle (≈ 1.5°). Figure 3 also highlights that the spray cone angle and liquid breakup length measured from view A (solid lines) and view B (dotted lines) are almost identical. Similarly, no significant difference was found in the spray structure of Figure 2 view A and view B. The number droplet size distribution $F_N$ of damaged and functional atomizers obtained from view A and view B at $P = 7.4 \ kW$ are shown in Figure 3 (c) & (d), respectively. The peak and spread of distribution obtained from view A well match the distribution obtained from view B for both atomizers, which further confirms the orthogonal symmetry of both atomizers. It shows that macroscopic (spray angle and liquid breakup length) and microscopic (droplet size distribution) behaviors of both atomizers are independent of their orthogonal views. Therefore, only view A is used to compare the spray characteristics of functional and damaged atomizers in the subsequent sections.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Comparison of functional (blue) and damaged (red) atomizers in terms of (a) spray angle and (b) mean liquid breakup length and (c-d) $F_N$ number and $C_N$ cumulative droplet size distribution at $P = 7.4 \ kW$</figcaption></figure>

The number and volume drop size distributions for all given thermal powers are shown in Figure 4. The number distribution of functional atomizer shows two peaks at $d \approx 6 \ \mu m$ and $d \approx 20 \ \mu m$ with practically no droplets at $d > 100 \ \mu m$. The number distribution for the damaged atomizer was observed to be slightly shifted to the right with a third mode around $100 \ \mu m$ as a result of poor atomization. This variation in droplet distribution functions results in a huge variation in the mean droplet diameters of both atomizers. At $5.9 \ kW$, the spray obtained from the functional atomizer has an arithmetic mean diameter ($D_{10}$) and a Sauter mean diameter ($D_{32}$) of $22.6 \ \mu m$ and $32.2 \ \mu m$, respectively. Meanwhile, the mean diameters for damaged atomizer was measured $D_{10} = 49.3 \ \mu m$ and $D_{32} = 102.3 \ \mu m$ despite the noticeable peak around $d \approx 6 \ \mu m$ at $5.9 \ kW$. The difference between the functional and damaged atomizers is more pronounced for the volumetric distributions. It was observed that droplets with $d < 100 \ \mu m$ represent 90% of the volume of the spray for the functional atomizer, and only 20% for the damaged atomizer.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Number and volume drop size distributions ($F_N$ and $F_V$) with their cumulative functions ($C_N$ and $C_V$) of damaged and functional atomizers from view A</figcaption></figure>

### 3 Comparison of functional and damaged atomizers with swirlers

In this section, spray characteristics of functional and damaged atomizers with swirling air flow are discussed. The atomizers are characterized using two swirlers S707 and S727 of swirl numbers 0.6 and 0.74. The behavior of both atomizers with swirlers is compared in terms of spray shape, symmetry, droplet size distribution, and mean droplet size. The mean spray images and 3D volumetric reconstruction of droplets for the swirl injector with functional and damaged atomizers are shown in Figure 5 at $P = 5.9 \text{ kW}$ and $\Phi = 0.85$. It is interesting to note that the spray produced by both atomizers has an identical hollow conical shape spray structure with a central recirculation zone. The swirl air stream successfully breaks up the liquid jet emerging from the damaged atomizer and provides a hollow conical spray identical to the functional atomizer. The 3D volumetric reconstruction of droplets in Figure 5 also shows the hollow conical spray structures for both atomizers. Both of the atomizers with S727 swirler have a maximum number of droplets within $d < 100 \ \mu\text{m}$ despite the huge variation when operating as standalone (see Figure 2). A few large droplets $d > 100 \ \mu\text{m}$ were observed for both atomizers close to the injector exit. No liquid core was observed at the exit of the injector for both atomizers and swirlers due to intense air-fuel interaction and high gas-to-liquid mass flow ratio(≈ 15). The spray angle is defined as the angle between the left and right boundaries of spray calculated at a distance of 0.5 times the exit orifice diameter of the terminal plate. The spray angle was calculated from the mean spray images captured from view A. At $P = 5.9 \text{ kW}$ and $\Phi = 0.85$, the damaged and functional atomizers produce spray with almost the same spray angle 44.28°, and 45.67° with S707. While the S727 yields marginally wider spray with spray angles of 58.64°, and 53.31°, for damaged and functional atomizers due to the higher rotation rate of air flow.

The spatial distribution of droplet density and mean droplet diameter is plotted to provide more insight into the spray symmetry and homogeneity in the azimuthal and radial directions. The mean droplet size ($D_{30}$) spatial distributions are plotted in the XZ plane by combining 3D information along the spray axis (Y). As illustrated in Figure 6, the $D_{30}$ spatial distribution appears as a ring shape pattern with a central air core region. With S707, the functional atomizers has uniform $D_{30}$ spatial distribution with $D_{30} \approx 50 \ \mu\text{m}$. Also, some brighter region of larger mean diameter $D_{30} \approx 100 \ \mu\text{m}$ was observed near the edge of the air core, while the central air core region is predominantly empty. In comparison, the damaged atomizer has a less bright region of $D_{30} \approx 30$. Moreover, the air core for the damaged atomizer is not empty. For S727, the difference in $D_{30}$ spatial distribution of both atomizers reduces significantly as a result of relatively higher swirl strength. Both atomizers have $D_{30}$ distribution with most of the region of mean drop size $D_{30} \approx 50 \ \mu\text{m}$. Also, a region of large droplets of $D_{30} > 100 \ \mu\text{m}$ was observed at the periphery of the air core. Both atomizers even have a similar position for the brighter
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Mean spray images and volumetric reconstruction of droplets for swirl injector at $P = 5.9 \text{ kW}$ and $\Phi = 0.85$</figcaption></figure>

region, which clearly highlights the azimuthal and radial similarities in spray shape and structure. The $D_{30}$ spatial distribution has few void regions around $Z \approx +5\text{mm}, +1\text{mm}, -3 \text{ mm}$ due to discontinuity in the spray position during experiments. Figure 7 illustrates the drop number and $D_{30}$ spatial distributions obtained from circumferential integration in the cylindrical coordinates. The $R$ axis represents the radial coordinate with $R = 0$ as the injector axis. At low swirl conditions, a marginal difference was observed between the two atomizers. These differences were mainly observed near the injector exit and within the air core region. The functional atomizer has a relatively uniform drop density distribution compared to the damaged atomizer. The spray with the damaged atomizer shows a high droplet density region near the injector exit. Also, a few droplet within the spray air core was found for the damaged atomizer. The effect of higher swirl strength in homogenizing and distributing liquid droplets can be seen clearly with S727. The strong swirl flow effects eliminate the defects in an atomizer and provide identical drop numbers as well as $D_{30}$ circumferential spatial distributions. The droplet density increases and then decreases with radial direction, resulting in a maximum value at the center of the spray lip. The concentration of droplets is found to be maximum near the injector exit and decreases in the downstream direction. The droplet detection at further downstream locations is not possible because of the presence of fine droplets due to secondary atomization. Similarly, no notable difference was observed for the $D_{30}$ circumferential spatial distribution of both atomizers. The both atomizers have large droplet size region ($D_{30} > 100\mu\text{m}$) near the injector exit ($Y < 2 \text{ mm}$). This is induced by a poor atomization of liquid ligaments attached to the nozzle plate. Those big elements were not taken into account to determine the drop size distributions,
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Mean droplet size ($D_{30}$) spatial distribution of injector at $P = 5.9 \text{ kW}$ and $\Phi = 0.85$</figcaption></figure>

thanks to a morphological filtering retaining only spheroid objects. As secondary atomization of droplets progresses, the region of large mean droplet size gradually decreases in a downstream direction. Finally, the region of $D_{30} < 30 \ \mu\text{m}$ was observed at $Y > 6 \ \text{mm}$ for both atomizers.

The typical number and volumetric drop size distributions of functional and damaged spray are shown in Figure 8. The functional and damaged atomizers have an identical number distribution function with a peak at $d \approx 6.5 \ \mu\text{m}$. Both atomizers in the presence of an air swirler show unimodal number drop size distribution compared to the bimodal spray distribution of the standalone atomizer. Also, distribution with swirlers is relatively shifted to the left with no droplets at $d > 100 \ \mu\text{m}$ due to enhanced atomization in the presence of air. The number and volumetric distribution perfectly overlap with each other for both atomizers, except for some inconsistency near the peak values. The difference between the droplet size distribution of functional and damaged atomizers remains negligible, irrespective of variation in thermal power and equivalence ratio. With S727, the functional and damaged atomizers have mean droplet size $D_{32} = 18.2 \ \mu\text{m}$ and $D_{32} = 20.7 \ \mu\text{m}$ at $P = 5.9 \text{ kW}$ and $\Phi = 0.85$. At $P = 5.9 \text{ kW}$ and $\Phi = 0.95$, the mean diameter values increase up to $D_{32} = 19.2 \ \mu\text{m}$ and $D_{32} = 21.3 \ \mu\text{m}$ for functional and damaged atomizers due to reduction in gas-to-liquid mass flow ratio. The maximum difference between both atomizers in terms of arithmetic mean and Sauter mean diameters is reported as < 3 µm and < 1 µm, respectively.

## Conclusions

The present work investigates the spray characteristics of a damaged atomizer due to repetitive combustion experiments, comparing its performance to that of a functional atomizer, both with and without an air swirler. Two radial swirlers S707 and S727 of swirl numbers 0.6 and 0.74 were used in the injector assembly. Conventional shadowgraphy and 3D plenoptic imaging techniques were successfully employed to compare both atomizers. The conventional shadowgraphy results were used to measure the liquid breakup length, spray angle, and spray granulometry. The spatial drop distribution and 3D structure of the spray were constructed from volumetric information captured using plenoptic imaging. Without swirlers, the internal atomizer geometry of the pressure swirl atomizer plays a prominent role in determining the overall spray characteristics. As a result, a huge difference was observed in the spray produced from both atomizers in terms of liquid breakup length, spray angle, droplet size distribution, mean droplet size, and spatial distribution of droplets. The functional atomizer forms a fully developed hollow cone spray with practically no drop of $d > 100 \ \mu\text{m}$ while the damaged atomizer produces a continuous liquid jet with fewer large droplets, representing poor atomization. With air swirlers, the distorted spray characteristics of a malfunctioned pressure-swirl atomizer do not necessarily translate into steady-state spray characteristics of a swirl spray injector. Both atomizers behave similarly, producing a hollow conical spray structure with a central recirculation zone. A notable difference was observed in the spatial drop number and volumetric distributions between the two atomizers with S707. The difference in spray symmetry and spatial distribution of droplets was less pronounced for S727. A negligible difference was found in the drop size distribution function and mean droplet size of both atomizers, irrespective of swirler type, at given operating conditions. The large gas-to-liquid mass ratio (> 15), strong swirl air flow (≥ 0.6), and considerable recess length allow robust air-fuel interaction resulting in identical spray characteristics of swirl spray injector for both functional and damaged atomizers.
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Circumferential integrated drop number and mean droplet size ($D_{30}$) spatial distribution of injector at $P = 5.9 \text{ kW}$ and $\Phi = 0.85$</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 8.** Number and volume drop size distribution functions ($F_N$ and $F_V$) with their cumulative functions ($C_N$ and $C_V$) of injector at $P = 5.9 \text{ kW}$ from view A</figcaption></figure>
## Acknowledgements

The SAFER project has received funding from the European Union's Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement N°101034329. This project is a beneficiary of the WINNING Normandy program supported by the Normandy Region, France. The authors acknowledge support from the French National Research Agency (ANR) through the project FlySAFe (ANR-22-CE05-0022), from the Normandy Region through the project MEDIC, and from the University of Rouen Normandy through the project ARELIA3D. The authors also acknowledge technical support from the RENADIAG platforms of CORIA.

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