# Polarization Ratio with Structured Laser Illumination Planar Imaging for Droplet Sizing in Flash Boiling Ammonia Spray

Utkarsha Sonawane, Sebastien Garcia, Vassily Kornienko, Camille Hespel, Christine Mounaïm-Rousselle

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Utkarsha Sonawane, Sebastien Garcia, Vassily Kornienko, Camille Hespel, Christine Mounaïm-Rousselle. Polarization Ratio with Structured Laser Illumination Planar Imaging for Droplet Sizing in Flash Boiling Ammonia Spray. 33rd Conference on Liquid Atomization & Spray Systems (ILASS Europe 2025), Aug 2025, Lund (Sweden), Sweden. hal-05385788

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# Polarization Ratio with Structured Laser Illumination Planar Imaging for Droplet Sizing in Flash Boiling Ammonia Spray

Utkarsha Sonawane*<fnref n="1" />, Sebastien Garcia<fnref n="2" />, Vassily Kornienko<fnref n="2" />, Camille Hespel<fnref n="1" />,<fnref n="3" />,
Christine Rousselle<fnref n="1" />,<fnref n="3" />, Edouard Berrocal<fnref n="2" />

## Abstract

Liquid-phase ammonia, a carbon-free alternative fuel, is increasingly being considered for improving overall efficiency, reducing system complexity, and lowering costs across various thermal applications such as engines, gas turbines, and burners. However, the physical properties of ammonia differ significantly from those of carbon-based fuels, as it is prone to flash boiling upon injection. Therefore, experimental characterization of liquid ammonia sprays, specifically the spatial distribution of droplet sizes, is crucial for enhancing the understanding of the evaporation process and its mixing with ambient air. However, flash boiling sprays severely suffer from multiple scattering, limiting the use of investigatory optical techniques. Structured Laser Illumination Planar Imaging (SLIPI) is an effective and widely accepted technique developed to suppress such parasitic optical phenomena. Upon combination with a novel Polarization Ratio technique, droplet sizing can be performed on these highly scattering flash boiling events. This manuscript presents a first attempt to measure droplet sizes, *D*₂₁, across a flash-boiling ammonia spray injected into an optical vessel using a Spray G (Engine Combustion Network) multi-hole injector. While the conventional planar polarization Mie scattering imaging is highly affected by the detection of multiple light scattering, we show that the use of 2-phase SLIPI allows for an efficient suppression of these unwanted photons to obtain, to the best of the authors' knowledge, a first reliable measurement of droplet sizes in flash boiling ammonia spray under such challenging conditions.

**Keywords:** Flash boiling spray, Ammonia, Droplet sizing, SLIPI- Polarization ratio, Multiple light scattering.

## Introduction

<footnote n="1">¹Université d'Orléans, INSA CVL, PRISME, Orléans, France</footnote>

<footnote n="2">²Division of Combustion Physics, Department of Physics, Lund University, Sweden</footnote>

<footnote n="3">³IUF, Institut Universitaire de France, Paris, France, Orléans, France</footnote>

<footnote>*Corresponding author: utkarsha-sugriv.sonawane@univ-orleans.fr</footnote>
Ammonia is increasingly being considered not only as a hydrogen carrier but also as a carbon-free fuel in its own right. For practical reasons — including storage volume optimisation, reduced transport costs, and simpler injection — it is typically used in liquid form. This can be achieved at around 9 bar pressure at 20 °C, or at atmospheric pressure at -33 °C. Due to its high saturation pressure, liquid ammonia is highly susceptible to flash boiling upon injection, resulting in rapid evaporation. Under such conditions, the accelerated atomisation and wider radial spread of droplets are expected to improve spatial distribution and air-fuel mixing [1]. However, the high heat of vaporisation can cause local temperature drops that may hinder combustion [2]. These competing effects make it essential to characterise both the spray morphology and the droplet size distribution [3].

Flash boiling of ammonia and its effect on sprays has attracted growing research interest in recent years. Several experimental studies have shown that the extent of flash boiling increases with reduced ambient pressure (P<sub>amb</sub>) and/or higher fuel temperature (T<sub>fuel</sub>), often resulting in shorter axial penetration due to greater radial spray expansion under these conditions [1,4,5]. Fang et al. [6] observed intense cavitation inside the nozzle during flash boiling, influenced by both fuel properties and injector geometry. For multi-hole injectors, plume interaction can lead to spray collapse, with small changes producing shorter penetration and wider spray angles. Conversely, strong spray collapse has also been reported to increase penetration length and narrow spray angles, while local cooling effects cause liquid fuel to accumulate in the plume interaction zone [7–8,11]. Wang et al. [12] demonstrated that the strong phase change under flash boiling can produce more pronounced spray expansion, higher velocities, and rapid atomization driven by bubble breakup, accompanied by a local temperature drop in the spray region. Despite these findings, there remains a significant body of work to be performed in order to investigate the combined influence of fuel properties, injector design, and operating conditions for widespread use of such sprays. This underlines the need for advanced diagnostic techniques capable of resolving droplet-scale dynamics in such complex sprays.

Laser-based diagnostics are widely employed for quantitative spray droplet measurements, with Phase Doppler Anemometry (PDA) being one of the most established methods for accurate, calibration-free determination of droplet sizes and velocities [13]. However, PDA has important limitations. It measures at a single spatial point, making it time-consuming for full spray characterization; thus, it is unsuitable for transient sprays. The method is also biased towards detecting larger droplets, often overlooking finer ones, and it struggles with multiple light scattering in dense sprays. To address these issues, planar laser sheet imaging techniques provide a reliable alternative, offering two-dimensional, snapshot data acquisition. Depending on the specific approach, the measured image intensities can be related to mean droplet diameters through methods such as the Polarization ratio [14], the Laser-Induced Fluorescence (LIF)/Mie ratio [15], or the Raman/Mie ratio [16]. LIF/Mie approaches require a fluorescent tracer in the fuel, but under high-temperature evaporation conditions, the LIF signal loses its direct dependence on droplet volume, leading to sizing biases [17]. Raman/Mie methods, while tracer-free, typically suffer from low signal levels, long acquisition times, and poor signal-to-noise ratios, particularly for small droplets. In contrast, the additive-free, Mie-scattering-based Polarization ratio technique offers strong potential for
two-dimensional droplet sizing in flash-boiling sprays. In this approach, the non-calibrated surface mean diameter ($D_{21}$) is estimated from the ratio of perpendicularly polarized (S-pol, proportional to $D^2$) to parallel polarized (P-pol, proportional to $D$) scattered light images. Since the ratio depends on factors such as laser wavelength, refractive index, scattering angle, and the droplet size distribution [19,20], a calibration is then performed, either experimentally using PDA on a comparable system [18] or theoretically using Lorenz-Mie scattering calculations. This calibration is then applied to convert the measured ratio into $D_{21}$ values.

Planar laser imaging techniques, including the polarization ratio method, provide reliable droplet sizing when single light scattering dominates. However, this assumption breaks down in optically dense sprays, where droplets undergo multiple scattering events. In flash-boiling sprays, light transmission can drop below 10%, highlighting the significant role of multiple scattering and the resulting challenges for optical droplet measurements. To address this, Structured Laser Illumination Planar Imaging (SLIPI) can be applied to suppress contributions from multiple scattering [21]. In short, the SLIPI technique applies a spatial intensity modulation to the incident laser sheet such that any detected light that does not maintain this structure is removed in the post-processing step, effectively removing the multiple light scattering contribution from the measurement. Recently, the SLIPI approach has been combined with the polarization ratio method for droplet sizing [22], demonstrating that this combined technique is effective for the measurement of droplet sizes in optically dense and evaporative sprays without requiring additives such as dyes.

In this manuscript, we apply the combined SLIPI-polarization ratio technique to obtain two-dimensional droplet size maps for flash-boiling ammonia sprays generated by a multi-hole ECN injector. The experiments were conducted at fuel temperatures of 20 °C and -10 °C, with an injection pressure of 75 bar, under ambient conditions (~1 bar, ~20 °C). By capturing the spatial and temporal evolution of droplet sizes, these measurements provide a detailed view of spray morphology under flash-boiling conditions. The results highlight how liquid ammonia temperature influences droplet size distribution and spray dynamics, offering insights that are essential for both fundamental understanding and practical optimization of ammonia fuel injection.

# 1. Experiment

## 1.1 General description of SLIPI

Upon illumination of an optically dense spray with a spatially modulated laser sheet, photons that undergo multiple scattering scatter off their intended path and lose the modulated information when captured by a camera. In contrast, photons that scatter only once retain the modulation, appearing in the captured image with the intended structure. This makes it possible to isolate these contributions and extract the true single-scattering signal. There are two main types of SLIPI technique:
- **2-phase SLIPI:** where two sub-images are used with a 180° phase shift of the spatial modulation (modulation is shifted half of the spatial period)

- **3-phase SLIPI:** where three sub-images are used with a 120° shift of the spatial modulation (the modulation is shifted one third of the spatial period)

For all these approaches, the sub-images are post-processed to extract the amplitude of the spatial modulation and construct the final image where the contribution of the non-modulated component is suppressed [22]. For the general case, the modulation is shifted n-1 times and n images are recorded with a phase shift of $\phi = 2\pi/n$ between images, the final SLIPI image is constructed using equation (1):

$$SLIPI = \frac{\sqrt{2}}{n} \cdot \sqrt{\left(\sum_{i=1}^{n-1} \sum_{j=i+1}^{n} [I_i - I_j]^2\right)} \quad (1)$$

Here, $I_j$ and $I_k$ are the intensity maps of the two different images and the subtraction $[I_j - I_k]$ removes their common features arising from the diffuse, multiply scattered light. Unique features from singly scattered light are, on the other hand, preserved. Note that the conventional image, corresponding to a non-modulated light sheet, can also be reconstructed by summing up and averaging the images, as shown in equation (2):

$$Conv. = \frac{1}{n} \cdot (\sum_{i=1}^{n} I_i) \quad (2)$$

In general, 3-phase SLIPI results in a full solution to the reconstructed image, preserving its spatial resolution. In contrast, 2-phase SLIPI is not however function as an adequate option for preserving the majority of the initial spatial resolution (when a spatial modulation in the range of 10-15 pixels per period) while simultaneously reducing the temporal footprint of the experiment, suitable for challenging transient sprays.

## 1.2 Description of the experimental setup

The Engine Combustion Network (ECN) spray G 8-hole injector is detailed in the following webpage:

https://ecn.sandia.gov/gasoline-spray-combustion/target-condition/spray-g-plume-orientation/
is mounted on a stainless-steel spherical vessel of 4.2 L.
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Description of the experimental set-up, showing in detail how the spatially modulated laser sheet is optically formed to illuminate an ammonia spray that is injected from a G8 multi-hole injector from ECN. A picture of the setup is also given on the top-left of the figure.</figcaption></figure>

Ammonia is pressurized to pressures of up to 120 bar with helium and injected in its liquid state at 75 bar fuel injection pressure. The injection duration of 1000 µs was controlled with an injector power driver (EFS 8427). The ammonia spray is injected into the spray vessel as schematized in Figure 1, triggered by a National Instrument CRIO 9024. Flash boiling occurs when the injected fuel experiences a vessel pressure, $P_{amb}$, below that of the fuel saturation pressure, $P_{sat}$. The sudden reduction of pressure during the injection results in the superheating of the fuel spray. This leads to bubble nucleation, bubble growth, and the two-phase flow outlined in the introduction. The degree of superheating, $R_p$, can then be defined as the ratio between $P_{sat}$ at $T_{fuel}$ to $P_{amb}$, as shown in equation (3):

$$R_p = \frac{P_{sat}(T_{fuel})}{P_{amb}} \qquad (3)$$

$R_p$ is commonly used to quantify the extent of flash boiling. Variations in this parameter, along with fuel properties, influence vapor pressure and surface tension, which are the main factors controlling bubble formation. Beyond $R_p$, fuel characteristics and nozzle geometry are also key parameters, affecting both bubble growth inside the nozzle and the subsequent breakup of the external spray. Table 1 exhibits the experimental variations of these parameters within the presented experimental investigation. As for the illumination, a CW laser (Model Verdi G from COHERENT) running at 532 nm and reaching up to 20 W output power was utilized together with a SLIPI device (from LaVision).
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Final image generated from the 2-phase SLIPI shown on the bottom right, by means of two spatially modulated images shown on the top left of the figure, and by applying a correction of remaining “white lines”.</figcaption></figure>

By combining the two instruments, a structured laser sheet of desired thickness and size was created. In short, within the SLIPI device, a Diffractive Optical Element (DOE) divides the incident laser beam into two diffracted orders with an efficiency of >90%. Upon recombination with a vertically oriented cylindrical, the interference of these orders results in the spatial modulation. A horizontally oriented cylindrical lens is then added to finalise the creation of the thin spatially modulated sheet that is incident on the ammonia spray. This light polarization is rotated to the desired angle with a $\lambda/2$ waveplate specified at 532 nm. The detection of the scattered light is performed with a high-speed Phantom V1611 camera, recording images at 16 kHz. The objective lens is a Nikon AF Micro of 105 mm focal length, providing a final pixel resolution of 50 µm/pixel over a matrix of 800×1280 pixels, leading to a 40x64 mm² field of view. A linear polarization filter was placed on a rotating holder in front of the camera objective to switch the detection between the perpendicular and parallel polarization of the scattered light.

## 1.3 Methodology and image post-processing
For the use of SLIPI, two images with spatial modulations shifted by 180° are recorded. For this, the DOE is mounted on a piezoelectric motor (PiMotor) is which displaces the modulation vertically. After the acquisition of two sets of averaged high-speed video sequences (see below), one in s-pol and one in p-pol, the post-processing extraction of the singly scattered light from each set is performed via the following specific form of eq. (1):

$$I_{2p} = \frac{\sqrt{2}}{2} \cdot \sqrt{[I_0 - I_{180}]^2} \quad (4)$$

Here, $I_0$ and $I_{180}$ are the light intensity maps for the spatial modulation phase of 0° and 180°, respectively. This process is schematically shown in Figure 2 for a p-pol case, where it is clear that there is a significant DC component of light under the modulation patterns in the zoomed inset. Upon performance of eq. (4), the singly scattered light is extracted from the two images; however, a clear striped pattern is still visible. An in-house developed wavelet-based, Fourier filtering algorithm is then applied in order to “destripe” the image with a slight loss of spatial resolution. A further confirmation that multiply scattered light has been effectively removed can be noticed in the color bar of the final image, as compared to the raw images – only about 15% of the initial image intensity is maintained.

## Convergence of intensity maps

As this droplet sizing technique is an intensity-based ratio, convergence of the signal intensities as a function of camera parameters is crucial. Due to the transient and challenging nature of flash-boiling ammonia sprays, a 4 µs exposure was deemed suitable for eliminating motion blur. At such low exposure times, single-shot droplet size images could not be acquired. Instead, high-speed video sequences were averaged.

<figure><img src="image_4.png" /><figcaption>Figure 3. Convergence of the final averaged images over a number of recorded image data</figcaption></figure>
This involved recording multiple repetitions of the same injection condition and averaging the images at each time stamp. To ensure that this averaging did not affect the measured intensities, a convergence study was performed as shown in Figure 3.

The average Lorenz-Mie intensity changes drastically from 50 to 65 images. Further increments in the number of averages do not significantly alter the intensity profile across the spray; however, for 65 images, a large amount of background noise is observed with intense scattering from the spray core. By setting a limiting intensity fluctuation of a single pixel at <10%, an averaging over 80 images is deemed to satisfy statistical convergence. Furthermore, as shown in Figure 3, similar results are obtained for an average of 100 images. Hence, the measurements were based on the recording of 80 high-speed video sequences at 16 kHz with 4 µs exposure times, followed by an averaging at each time stamp, for both S-pol and P-pol acquisitions.

## Flat field correction

The camera objective utilized in the setup is not telecentric. Thus, the collection angle varies from one pixel location to another, throughout the captured image [26]. Therefore, a correction of the detected scattering intensity with respect to the Field of View (FoV) must be performed. To proceed, we have taken into consideration the camera lens specifications and calculated the total angle variation in the image field over 5°. Figure 4(a) shows the correction coefficient for S-pol and P-pol with scattering angle at the object plane. Results show that at $θ = 90°$, the correction coefficient is equal to 1. For other values of $θ$, the intensity coefficient varies for both polarizations. Figure 4(b) represents local correction factors for the variation of intensity across the FoV for both polarizations, ranging from a lower value of ~0.8 on the lower part of the sensor to an upper value of ~1.5 on the upper part. This flat-field image correction is applied to the averaged images before taking the S-pol and P-pol polarization ratio in order to extract droplet sizes. The calculation of intensity variation for scattering angle from 85° to 95° is performed with the MiePlot software: http://www.philiplaven.com/mieplot.htm

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>
**Figure 4.** Flat field image correction for S-pol and P-pol. In (a), angular correction coefficient curves are given, and in (b), images of a flat field illumination show the angular dependence of light intensity detection.

**Intensity ratio to droplet size calibration**

As previously described, a pixel-to-pixel ratio is performed between the p-pol and s-pol images in order to extract an uncalibrated 2-dimensional $D_{21}$ map. In order to convert this into real droplet sizes, a theoretical calibration is performed in Figure 5, once again using the MiePlot software [27]. First, the experimental parameters are entered into the simulation: incident laser wavelength ($\lambda$ = 532 nm), refractive index of ammonia under relevant conditions (n = 1.33 + 0i, [28] and the light collection parameters (scattering angle of the collecting lens: $\theta$ = 90 $\pm$ 5°). Second, from previous experimental analysis by PDPA, a droplet size distribution shape is defined using a distribution function (i.e., Gaussian shape) and the corresponding scattered light is calculated with Lorenz-Mie theory. Finally, a proportionality coefficient is defined and applied to convert polarization ratio into surface mean diameter $D_{21}$.

<figure><img src="image_7.png" /></figure>

**Figure 5.** Calibration curve theoretically deduced using the MiePlot software, where the polarization ratio is assumed to be linear to the droplet size $D_{21}$. In such calculations, a Gaussian droplet size distribution is assumed for each size.

**2. Results and discussion**

Figure 6 shows three P-pol and S-pol images at different time steps during the injection of a transient ammonia spray at 28 °C and ambient pressure at a degree of superheating of $R_p$ = 8.5. It is important to note that only one of the eight injectors is illuminated in its center from the right. The upper panel presents conventional images (from Equation 2) of the scattered light, which are severely blurred with
poorly defined edges due to multiple scattering. In contrast, the lower panel shows the destriped SLIPI images of the same injection (from Equation 4), revealing much sharper structures, though at lower overall intensity (see color bar). The higher intensity on the right side of the spray, compared to the conventional image, also reveals the entrance direction of the laser sheet, while the asymmetry in the SLIPI images is attributed to laser extinction as the beam passes through the highly scattering medium. Altogether, this establishes that SLIPI effectively isolates the targeted information along the laser sheet from deep within the spray: the singly scattered light that reflects droplet sizes.

Near the nozzle outlet of the spray, the s-pol images (proportional to D²) exhibit relatively high intensity while the injection is on (≤ 1000 µs) in contrast to when it is off (> 1000 µs). On the other hand, the p-pol images (proportional to D) exhibit the opposite behavior, indicating a large concentration of droplets in the nozzle outlet during the injection. These are then dispersed into the vessel in a hollow type structure – as the spray penetrates, its core becomes less dense. The continued presence of droplets inside this area, albeit at a lower liquid volume fraction, is attributed to the recirculation of droplets back into the spray centre as a result of vortical air flow [22]. The bottom right lobe of the spray exhibits high p-pol intensity as compared to the s-pol images. This difference indicates a large amount of small droplets, showing that the spray plume is well-formed and spread.

Performing the ratio between the s- and p-pol images results in an intensity ratio, $Y$, shown in Figure 7(a). These images represent an uncalibrated droplet size map of the ammonia spray at the given conditions. Applying the conversion between the Lorenz-Mie intensity of the ratio and the droplet sizes of Figure 5, an absolute $D_{21}$ map is extracted and shown in Figure 7(b). Due to the non-sphericity of the droplets near the nozzle, droplet sizing according to said calibration is not possible. Therefore, this area of the spray has been omitted. At the start of the injection, large droplets with surface mean diameters exceeding 30 µm propagate as elongated liquid ligaments. Plume-to-plume interactions and collapse in this region lead to coarser droplets at the spray center. Additionally, the leading edge of the spray exhibits larger droplets, an effect attributed to drag forces that induce droplet collision and coalescence [29]. After the injection event ends, the spray plume exhibits more homogeneous, smaller droplets, reflecting a high degree of breakup and mixing with the ambient gas.
<figure><img src="image_8.png" /></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Comparison of conventional and SLIPI for S-pol and P-pol for 8.5 Rp. It is seen that a maximum signal of 1800 counts obtained for conventional imaging is reduced to 150 with SLIPI, after suppression of unwanted multiple scattering. One jet from eight is illuminated from the right side.</figcaption></figure>
**Figure 7.** Final results of the polarization ratio images in (a) at three different injection times. Using the calibration curve shown in Fig. 6, the final droplet size $D_{21}$ of the spray shown in (b), where the size of the droplets is observed to reduce over time.

**Conclusion**

For the first time, SLIPI and polarization ratio techniques are merged to provide droplet size distributions for flash boiling ammonia sprays injected with a highly relevant Engine Combustion Network (ECN) spray G 8-hole injector. There are multiple advantages to using this technique as compared to conventional laser-based droplet sizing diagnostics. Firstly, as a result of the direct detection of light scattering off droplets, performing a polarization ratio eliminates the need for additives that are unreliable in flash boiling situations and maintains a high signal-to-noise ratio. Secondly, being an intrinsically 2-dimensional technique, allows for fast acquisition of entire spray structures, revealing the intricate spatiotemporal dependencies of the flash boiling phenomenon. Furthermore, as the experimental setup is placed outside of the vessel, these measurements can be performed in real-life settings. Finally, the application of SLIPI allows for the reliable detection of the relevant physics from deep within the spray, devoid of the blurring effect of the multiple scattering signal, a severe problem when measuring on these types of systems.

As for the droplet sizing itself, it has been shown that the region near the nozzle consists of large liquid ligaments with droplets of large diameters. For these flash-boiling conditions, significant plume-to-plume interactions and spray collapse lead to coarser droplet formation near the spray center. Similarly, larger droplet diameters are seen in the leading edge of the spray due to drag effects.

Ultimately, unlocking the potential of ammonia as a carbon-free fuel demands advanced diagnostics. Techniques like SLIPI polarization ratio are essential to reveal the fundamentally different spray physics of ammonia injections compared to conventional hydrocarbon fuels, and we view these measurements as a crucial step toward fully understanding and harnessing ammonia sprays.

**Funding**

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was partially funded by Adonis (project from the Call on Sustainable Hydrogen Technology as Affordable and Clean Energy of the EIG Concert Japan 8° Call)—ANR-22-HTCE-0001b and Labex CAPRYSSES ANR-11-. LABX-0006-01. This project is partly funded by E-COOL (grant agreement 101130315), a European Union HORIZON-EIC Pathfinder action.

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