# Experimental and calculated detonation cells in gaseous ethanol-oxygen mixtures

Titouan Fessard, Léa Vilasi, Vincent Rodriguez, Pierre Vidal

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Titouan Fessard, Léa Vilasi, Vincent Rodriguez, Pierre Vidal. Experimental and calculated detonation cells in gaseous ethanol-oxygen mixtures. 30th International Colloquium on the Dynamics of Explosions and Reactive Systems (ICDERS 2025), Jul 2025, Ottawa, Canada. hal-05038411

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Distributed under a Creative Commons Attribution 4.0 International License
Experimental and calculated detonation cells
in gaseous ethanol-oxygen mixtures

Titouan Fessard, Léa Vilasi, Vincent Rodriguez, Pierre Vidal

Institut Pprime, UPR 3346 CNRS, ENSMA, 86961 Futuroscope-Chasseneuil, France

# 1 Introduction

Liquid alcohols (e.g., methanol, ethanol) are low-carbon, highly volatile liquid fuels commonly used in industry. Studying the conditions for igniting their detonation is important for preventing accidents during transport or storage. Harnessing their detonation in air or oxygen is also of practical interest as a combustion process in advanced propulsion systems designed to achieve pressure gain for more efficient combustion, such as rotating detonation engines [1–3].

Few studies have been conducted on the detonability of alcohol-oxidizer gas mixtures. Eaton et al. [4] and Diakow [5, 6] studied the width of detonation cells in vaporized methanol/ethanol-oxygen mixtures and compared it to predictions from correlations with calculated lengths of the ZND reaction zone. Diakow et al. [5] and Zevallos et al. [7] investigated the detonation propagation and the deflagration-to-detonation transition (DDT) limits of vaporized ethanol-air mixtures. The propagation of detonation in vapors diffusing from a liquid layer to a gaseous oxidant was addressed by Hieronymus et al. [8, 9] and Dengel et al. [10, 11], who studied the propagation of detonations in methanol-oxygen liquid-gas systems. They showed that the detonation velocity decreases with decreasing pressure and becomes constant after a step, which corresponds to the pressure-flammability limit of the gas phase. These studies mainly describe detonations occurring directly in the gas phase at the surface of the liquid above the flammability limit. Detonation below this limit occurs throughout the gas phase where the liquid has diffused. Hieronymus et al. [9] found no dependence of detonability on liquid layer thickness. Dengel et al. [10, 11] did not observe any influence of the detonation on the liquid surface as it passes over.

In this work, we first experimentally analyze the 3D cellular structure of detonation in gaseous ethanol-
oxygen mixtures using frontal and parietal soot recordings. We then calculate the mean cell width using the
predictive model based on graph theory and geometric probabilities by Monnier et al. [12], as a function
of the equivalence ratio and initial pressure. Section 1 describes the experimental setup and methodology,
Sect. 2 presents and analyzes the results, and Sect. 4 concludes the work.
## 2 Experimental setup and methodology

<figure><img src="image_2.png" /><figcaption>Figure 1: Scheme of the experimental setup.</figcaption></figure>

The setup consists of two sections: the booster and the test section, both with a square cross-section of 50×50 mm² and lengths of 4 m and 0.4 m, respectively. They are separated by an 18 µm thick plastic film (Mylar). The methodology involves initiating a detonation in the booster section with an electric arc and accelerating the transition to a steady detonation with a Shchelkin spiral. Upon impact, the Mylar foil acts as a piston, generating a shock wave that may or may not initiate detonation in the gas of the test section, depending on its initial characteristics - namely, equivalence ratio, initial pressure, and temperature. The advantage of the shock-to-detonation process over the deflagration-to-detonation transition is that it ensures detonation initiation over shorter distances. The booster gas was either a H₂ + ½ O₂ or C₃H₈ + 5 O₂ mixture at ambient temperature (299 K) and the same initial pressures as the test section. The mixtures in the

test section consisted of gaseous ethanol (C₂H₅OH) and oxygen (O₂) at varying initial pressures *p*₀, equivalence ratios (ER), and ambient temperature. The test section was filled with gaseous O₂ and a thin layer of liquid C₂H₅OH (1 mm). After a few minutes, the liquid vaporized and diffused into the gaseous O₂, forming a homogeneous mixture saturated with ethanol. Under these conditions, the equivalence ratio decreases with increasing initial pressure *p*₀. The initial pressures *p*₀ ranged from 15 to 100 kPa.

Optical oxygen probes (Pyroscience PreSens Oxygen Dipping Probe PSt3) were used to measure the oxygen concentration in the test mixture. The detonation velocities and pressures in the booster and test sections were measured using four Kistler 603CAB pressure transducers (Fig. 1, S1-3, 1 µs response time, 300 kHz natural frequency), each coupled to a Kistler 5018A charge amplifier (200 kHz bandwidth). The cellular structure of the detonation was characterized using parietal and frontal soot-coated foils. The parietal recordings were obtained from soot foils covering the entire length of the side wall of the test section, thus providing, in addition to velocity measurements, a criterion for detonation stability based on nearly constant

<figure><img src="image_3.png" /><figcaption>Figure 2: Calculated and measured equivalence ratios (ER) of EtOH-oxygen mixtures relative to the initial pressure *p*₀ in the chamber.</figcaption></figure>
cell geometric properties. The frontal recordings were obtained from soot foils covering the end wall of the test section, perpendicular to the direction of detonation propagation.

We measured the composition of the ethanol-oxygen mixture in the test section using oxygen probes at an initial temperature of 299.15 ± 1 K. No significant variations in the mixture composition were observed with respect to the longitudinal or transverse position of the measurements. Ethanol diffuses into the oxygen as a function of temperature and pressure, and the composition stabilizes after 3-5 minutes under our experimental conditions. This period represents the minimum delay required after injection to ignite the detonation in the booster gas. Figure 2 shows the measured and theoretically estimated equivalence ratios (ER), assuming the gas mixture is saturated with ethanol.

## 3 Results and analysis

<figure><img src="image_4.png" /><figcaption>Figure 3: Pressure signals with $p_0=50$ kPa. Top: shock in $O_2$. Bottom: detonation in EtOH/$O_2$ mixture at ER=0.59.</figcaption></figure>

Pressure signals were used to characterize detonation initiation and propagation. Pressure transducer S1 was located in the booster section, while S2 and S3 were located in the test section. Figure 3 compares the pressure history records without (top) and with (bottom) the EtOH liquid layer in the test section for a shock transmitted to the test section from a stoichiometric $C_3H_8/O_2$ detonation in the booster.

Figures 4 and 5 show parietal and frontal soot recordings for $p_0 = 20$ kPa, with detonation cells indicating detonation propagation. Detonation initiation is observed in the lower left of the recording in Figure 4 as an overdriven detonation propagating transversely to the tube axis. This is indicated by the transverse orientation of very small detonation cells that progressively increase in size. Steady propagation is then achieved, as evidenced by larger cells of nearly uniform size and orientation relative

to the tube axis. The frontal image (Fig. 5) shows the cell patterns during the steady propagation phase at the end of the test section. The edges of these patterns correspond to the positions of the transverse waves that bound the cells at the moment of head-on impact with the soot-coated foil. These recordings show that the detonation propagates steadily through the assumed uniform distribution of EtOH in oxygen.

Figure 6 shows the distributions of the measured detonation cell widths for initial pressures of 20 and 100 kPa. The results show that the mean detonation cell width under these conditions is statistically significant, with values of 4.1 mm and 10 mm for $p_0 = 20$ and 100 kPa, respectively. We recall (Sect. 2) that vaporization leads to equivalence ratios that decrease with increasing initial pressure, which accounts for the larger mean cell width at 100 kPa. The soot recordings, such as those in Figs. 4 and 5, show that the cell widths remain essentially constant across the height of the channel. They also show a large number of cells at the detonation front, i.e., the cell widths are small relative to the transverse dimension of the channel, regardless of the initial pressure.
Non-excessive irregularity, steady propagation, and a sufficiently large number of cells suggest that the cell structure is independent of the channel geometry under these conditions. Cell widths are thus suitable for prediction based on the model of Monnier et al. [12], which combines graph theory, geometric probabilities, and ZND properties derived from detailed chemical kinetic mechanisms. Table 1 compares the measured and calculated mean widths and shows excellent agreement with Konnov's kinetic mechanism.

<figure><img src="image_5.png" /><figcaption>Figure 4: Parietal soot recording of the detonation cellular structure in the EtOH-oxygen mixture at ER=2.12 and $p_0=20$ kPa. The detonation propagates from left to right, lower plates continuing upper ones.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Figure 5: Front-view soot recording of the detonation cellular structure in the EtOH-oxygen mixture at ER=2.12 and $p_0=20$ kPa.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6: Cell width distributions for EtOH-O₂ mixtures with (Left) ER = 2.13 and p₀ = 20 kPa, and (Right) ER = 0.27 and p₀ = 100 kPa.</figcaption></figure>

<table><caption>Table 1: Measured (λₓ) and calculated (λc) [12] cell mean widths for EtOH-oxygen mixtures.</caption><thead><tr><th>p<sub>0</sub><br>(kPa)</th><th>ER</th><th>λ<sub>x</sub><br>(mm)</th><th>λ<sub>c</sub><br>(mm)</th><th>Chemical mechanism</th></tr></thead><tbody><tr><td>20</td><td>2.13</td><td>4.1 ± 1.3</td><td>5.1<br>11.6</td><td>Konnov<br>SanDiego</td></tr><tr><td>100</td><td>0.27</td><td>10 ± 1.8</td><td>10.5<br>17.1</td><td>Konnov<br>SanDiego</td></tr></tbody></table>

## 4 Conclusion

The purpose of this study was to demonstrate the possibility of detonation in ethanol-oxygen mixtures by directly observing the cellular structure of the detonation reaction zone. Homogeneous ethanol-oxygen gas mixtures were generated by diffusion from a layer of liquid ethanol initially placed under an atmosphere of gaseous oxygen. The initiation, propagation, and cellular structure of the detonation were analyzed using pressure transducers and parietal and frontal soot recordings. Steady detonation propagation in ethanol-oxygen gas mixtures was achieved for several equivalence ratios ranging from 0.27 to 2.13. Calculated mean detonation cell widths – based on graph theory, geometric probabilities, and ZND properties from Konnov's chemical mechanism – are in good agreement with the measurements, further confirming that detonation effectively propagated in the saturated ethanol-oxygen gas. Ongoing studies are examining the effect of varying the equivalence ratio at fixed initial pressures.

**Acknowledgements:** This work was supported by the French Research National Agency (ANR) under the project ANR-21-CE05-0002-01.
References

[1] J. Li, Y. Lei, S. Yao, J. Yu, J. Li, and W. Zhang, "Investigation of multi-stage evaporation and wave multiplicity of two-phase rotating detonation waves fueled by ethanol," *Acta Astronautica*, vol. 213, pp. 418-430, 2023. doi: https://doi.org/10.1016/j.actaastro.2023.08.037.

[2] K. Ishihara, K. Yoneyama, T. Sato, H. Watanabe, N. Itouyama, A. Kawasaki, K. Matsuoka, J. Kasahara, A. Matsuo, and I. Funaki, "Visualization and performance evaluation of a liquid-ethanol cylindrical rotating detonation combustor," *Transactions of the Japan Society for Aeronautical and Space Sciences*, vol. 66, no. 2, pp. 46-58, 2023. doi: https://doi.org/10.2322/tjsass.66.46.

[3] S. Yao, X. Tang, and W. Zhang, "Structure of a heterogeneous two-phase rotating detonation wave with ethanol-hydrogen-air mixture," *Physics of Fluids*, vol. 35, no. 3, 2023. doi: https://doi.org/10.1063/5.0144920.

[4] R. Eaton, B. Zhang, J. M. Bergthorson, and H. D. Ng, "Measurement and chemical kinetic model predictions of detonation cell size in methanol-oxygen mixtures," *Shock Waves*, vol. 22, pp. 173-178, 2012. doi: https://doi.org/10.1007/s00193-012-0359-x.

[5] P. A. Diakow, *Detonation characteristics of dimethyl ether, methanol and ethanol air mixtures*. PhD thesis, Queen's University (Canada), 2012.

[6] P. Diakow, M. Cross, and G. Ciccarelli, "Detonation characteristics of dimethyl ether and ethanol-air mixtures," *Shock Waves*, vol. 25, pp. 231-238, 2015. doi: https://doi.org/10.1007/s00193-015-0554-7.

[7] A. A. M. Zevallos, G. Ciccarelli, and J. A. Carvalho Jr, "Ddt limits of ethanol-air in an obstacles-filled tube," *Combustion Science and Technology*, 2018. doi: https://doi.org/10.1080/00102202.2018.1477770.

[8] H. Hieronymus, P. Henschen, M. Hofmann, J. Bender, R. Wendler, J. Steinbach, and B. Plewinsky, "Hazards of surface explosions," pp. 897-908, 2001. doi: https://doi.org/10.1016/B978-044450699-3/50010-0.

[9] H. Hieronymus, M. Hofmann, J. Bender, P. Henschen, R. Wendler, B. Plewinsky, and J. Steinbach, "Characterization of surface explosions," *Archivum combustionis*, vol. 21, no. 2, pp. 117-125, 2001.

[10] J. Dengel, R. Wendler, W. Denzer, J. Buchholz, M. Beckmann-Kluge, J. Steinbach, B. Plewinsky, and H. Hieronymus, "Explosions near the surface of organic liquids," *Process safety and environmental protection*, vol. 83, no. 5, pp. 452-458, 2005. doi: https://doi.org/10.1205/psep.04199.

[11] J. Dengel and H. Hieronymus, "Einfluss der flüssigen phase auf detonationen im zweiphasigen system methanol/sauerstoff," *Chemie Ingenieur Technik*, vol. 78, no. 1-2, pp. 134-138, 2006. doi: https://doi.org/10.1002/cite.200500053.

[12] V. Monnier, P. Vidal, V. Rodriguez, and R. Zitoun, "From graph theory and geometric probabilities to a representative width for three-dimensional detonation cells," *Combustion and Flame*, vol. 256, p. 112996, 2023. doi: https://doi.org/10.1016/j.combustflame.2023.112996.