# Detonation onset mechanisms and fundamental combustion properties in unobstructed channels

Cristian Camilo Mejia-Botero, F. Virot, Luís Fernando Figueira da Silva, J Melguizo-Gavilanes

► To cite this version:

Cristian Camilo Mejia-Botero, F. Virot, Luís Fernando Figueira da Silva, J Melguizo-Gavilanes. Detonation onset mechanisms and fundamental combustion properties in unobstructed channels. 41st International Symposium on Combustion, The Combustion Institute, Jul 2026, Kyoto, Japan. hal-05495928

HAL Id: hal-05495928

https://hal.science/hal-05495928v1

Submitted on 5 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# Detonation onset mechanisms and fundamental combustion properties in unobstructed channels

C.C. Mejía-Botero<sup>a,b,*</sup>, F. Virot<sup>a</sup>, L.F. Figueira da Silva<sup>a</sup>, J. Melguizo-Gavilanes<sup>a,c</sup>

## Abstract

We examine the relationship between fundamental combustion properties (FCP) and detonation onset (DO) mechanisms in a square, unobstructed channel. Hydrogen and hydrogen/methane mixtures in oxygen, with and without nitrogen, helium, or argon dilution are tested. Two DO mechanisms were identified: (i) short, symmetric flames with a single normal precursor wave forming far ahead of the main flame leading to DO after reflected/transmitted shock collisions, and (ii) long, corner-inclined flames with multiple non-normal precursor waves resulting in DO from collisions between supersonic combustion structures propagating close to walls/corners and reflected shocks. Results show that the DO mechanisms identified are successfully grouped using the FCP boundaries used to classify flame morphologies during flame acceleration (FA) highlighting a direct link between flame morphology during FA and DO mechanisms. Distributions of DO on the channel cross-section are also provided.

Keywords: Detonation onset; smooth channel; hydrogen safety; deflagration-to-detonation transition

## 1. Introduction

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

The fundamental nature of the detonation onset (DO) during the deflagration-to-detonation transition remains poorly understood [1–6]. Currently, no tools are available to predict the different DO mechanisms that can be observed in channels with and without obstacles. Here, we investigate whether a relationship exists between fundamental combustion properties (FCP) and DO mechanisms observed in a smooth, square cross-section channel, with the aim of proposing FCP boundaries that separate specific DO characteristics such as simple/multiple flame-shock interactions, wall-induced auto-ignition, among others.

This work builds upon the study presented in [7–9], which examined the influence of FCP on flame morphologies during flame acceleration (FA). Two characteristic behaviors were identified, depending on the mixture: (i) a symmetric flame inversion (tulip flame) in the early stages of FA, followed by a short, symmetric flame in the later stages, accompanied by the formation of a precursor wave far ahead of the flame; and (ii) an asymmetric, wrinkled flame in the early stages, evolving into a large flame with its tip inclined toward the tube wall, along with the formation of multiple precursor waves. The pairs of FCP that most significantly influenced flame morphology were the

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

expansion ratio and the ratio of laminar flame speed to the sound speed in the combustion products, i.e., $(\sigma, \sigma_{s_L}/c_b)$, as well as the latter ratio and the ratio of specific heats, i.e., $(\sigma_{s_L}/c_b, \gamma)$. Boundaries between these FCP were proposed, which accounted for 98% of the observed morphologies.

In this work, these boundaries are tested against DO cases observed for hydrogen ($H_2$) mixtures with and without the dilution of nitrogen ($N_2$), helium (He), or argon (Ar), as well as $H_2$/methane ($CH_4$) blends. This approach enables a direct connection between flame morphology and DO characteristics, and permits the validation of the previously proposed boundaries in the context of DO mechanisms. While the consensus in the community [10–14] has been that the DO process is stochastic in nature here we show that there is order in the apparent chaos.

## 2. Methodology

The experiments were conducted using the simultaneous Schlieren visualization setup (2DS) described in detail in [9, 15]. This configuration consists of a $10 \times 10 \text{ mm}^2$ square channel, 1 m length, where ignition occurs at the closed end and the flame propagates toward the open end. The channel provides full optical access, enabling simultaneous observation from both side views, i.e., the *xy* and *xz* planes, where *x* denotes the direction of flame propagation. This

<footnote><sup>a</sup>Institut Pprime, CNRS, ISAE-ENSMA and Université de Poitiers, Futuroscope Chasseneuil, France</footnote>

<footnote><sup>b</sup>Aix Marseille Université, CNRS UMR7342, Centrale Marseille, IRPHE, Marseille, France</footnote>

<footnote><sup>c</sup>Shell Global Solutions B.V., Major Hazards Management, Energy Transition Campus, 1031 HW Amsterdam, The Netherlands</footnote>

<footnote>*Corresponding author.</footnote>
permits the identification of flame asymmetries dur-
ing acceleration, and to unveil the role of the cor-
ners on DO mechanisms. The setup is equipped
with two cameras: (i) for the *xy* plane view, the
Shimadzu HPV-2, with a maximum frame rate of
1, 000, 000 FPS (250 ns exposure time), and (ii) for
the *xz* plane view, the Shimadzu HPV-X2, with a
maximum frame rate of 5, 000, 000 FPS (110 ns ex-
posure time). See [9, 15] for more information about
the experimental setup.

For the mixtures’ selection, three experimental
campaigns are designed in which some FCP are kept
constant in order to assess their individual effect.
First, mixtures with the same σ but different γ were
used. Specifically, 2H₂ + O₂ + η_I I mixtures where
I = N₂, Ar, and/or He. To keep a fixed value
of σ = 7.9, the diluent moles were chosen as fol-
lows: ηN₂ = 1, ηAr = 2.8, ηHe = 2.8, and
ηAr + ηHe = 2.8. Second, mixtures with the same
σs_L ≈ 80 m/s, but with different Lewis number Le_D,
were selected. H₂/O₂ mixtures with φ = 0.8 and
φ = 1.7 were employed, corresponding to Le_D < 1
and Le_D > 1, respectively. Third, mixtures with sim-
ilar flame thickness δ_f, but different σs_L were used.
These consisted of H₂/CH₄ blends with stoichiomet-
ric O₂, and H₂ mole fractions x_H₂ = 0, 0.5, and 0.8,
where x_H₂ + x_CH₄ = 1.

Figure 1 presents a summary of the FCP for all the
mixtures, using simplified labels for clarity. Approxi-
mately 20 tests were conducted for each mixture, fo-
cusing on the respective DO zone. This permits to
obtain sufficient information to observe potential DO
mechanisms that occur systematically, as well as to
gather statistical data on the run-up distance to deto-
nation (x_D_D_T) [16] and the spatial distribution of DO
on the cross-section of the channel for the different
mixtures.

<figure><img src="image_2.png" /><figcaption>Fig. 1: Fundamental combustion properties summary for the mixtures used in this study.</figcaption></figure>

**3. Detonation onset mechanisms**

Two representative DO mechanism cases were
identified based on the mixture studied. This analysis
excludes H₂/O₂ mixtures diluted with He, since for
this mixture no transition to detonation was observed
in the available channel length.

3.1. Case 1

This case was observed for H₂/O₂ mixtures diluted
with Ar and Ar+He, which exhibited short flames
along with the formation of a main precursor wave
(PW1) during the final stages of flame acceleration
(FA) described in [7–9]. Figure 2 shows a schematic
of the DO process, along with the two-directional
schlieren views. In ①, PW1 approaches another pre-
cursor wave (PW2) generated during the early stages
of FA due to gas expansion at the closed end of the
channel. In ②, since PW1 is faster than PW2, a col-
lision occurs 6.8 µs later, producing a reflected shock
(RS) and a transmitted shock (TS), as depicted in ③.
From the TS frame of reference, as the RS advances
toward the main flame, a slight curvature is observed
in ③, which is more pronounced near the walls. This
curvature may be attributed to interactions between
the RS and the boundary layer, consistent with ob-
servations of reflected shock bifurcations reported in
[17]. In ④, due to shock compression and wall fric-
tion, a supersonic combustion structure (CS1) is auto-
ignited in a corner. It is important to note that CS1
forms separately from the main flame and behind the
shocks, and thus is not an extension of the main flame.
In ⑤, 1 µs later, another combustion structure (CS2)
forms, and simultaneously, DO occurs between RS
and TS, right where the lambda shock forms due to
the boundary layer-RS interaction, where the ther-
modynamic conditions are extreme. This detona-
tion wave then expands and propagates as shown in
⑥. Note that without simultaneous schlieren visual-
ization, it would have been impossible to determine
whether the combustion structures and DO occur on
a wall or in a corner. To our knowledge, this DO case
has only previously been observed by Ballossier et al.
[15] for 2H₂ + O₂ + ηN₂ N₂ with ηN₂ = 2 and 2.5.
However, this is the first time it has been presented
with such detail and linked explicitly to FCP.

3.2. Case 2

This case was observed for H₂/O₂ mixtures diluted
with N₂ (ηN₂ = 1), φ = 0.8, φ = 1.7, and CH₄/H₂
blends with xH₂ = 0, 0.5 and 0.8, which exhibited
long flames with the formation of multiple precur-
sor waves during the late stages of FA [7–9]. Fig-
ure 3 shows the schematic for this configuration. In
①, the long flame, inclined toward one of the chan-
nel corners, induces a supersonic combustion struc-
ture (CS1) at the flame tip. Unlike the CS1 observed
in the previous case (see Figure 2), this CS1 forms di-
rectly at the tip of the main flame. In ②, CS1 deceler-
ates as it approaches the main precursor wave (PW),
while simultaneously generating oblique shocks that
<figure><img src="image_3.png" /><figcaption>Fig. 2: Case 1 - Schematic of DO sequence observed during the collision process between two precursor waves which yields reflected and transmitted shocks with the occurrence of DO between them. The images correspond to those obtained with the Ar-diluted mixture, but the behavior is similar to that observed with the Ar+He-diluted mixture.</figcaption></figure>

reflect off the opposite walls. These reflected oblique shocks ignite a new combustion structure (CS2) in the opposite corner, as presented in (③), which is separate from the main flame. This structure again generates oblique shocks, creating a chain process where combustion structures appear at three corners of the channel, as seen in (④) and (⑤). During these stages, the main PW continuously changes its inclination as it interacts with multiple oblique shocks. All combustion structures (CS1, CS2, and CS3) form abruptly at high speeds but slow down and increase in size as they approach the main PW. Finally, in (⑥), CS4 forms in the remaining corner, accelerates, and finally collides with one of the oblique shocks, triggering the DO. It is worth noting that this final collision can also occur with the main PW, resulting in DO closer to the PW. However, the systematic formation of multiple super-sonic combustion structures in all four corners was consistently observed for the mixtures considered in this DO case.

### 4. DO cases against FCP

This section evaluates the FCP boundaries proposed in [7–9] for flame morphologies during FA

against the DO cases observed here. These boundaries were found to be 98% effective in separating the two flame morphology types observed based on 60 data points collected from the literature.

Figure 4 shows the results, also including three points from [15], corresponding to H₂/O₂/N₂ mixtures, $\phi = 1$, with (i) $\eta_{N2} = 0$, (ii) $\eta_{N2} = 2$, and (iii) $\eta_{N2} = 2.5$. These mixtures exhibited DO mechanisms similar to those observed in this work, i.e., (i) is classified as case 2, whereas (ii) and (iii) are classified as case 1. The boundary defined by $(\sigma_{sL}/c_b, \sigma)$ successfully separates most tests corresponding to case 1 except for two data points corresponding to case 2 (H₂/O₂ mixtures diluted with N₂, and $\phi = 1.7$) which are in a morphology transition state as discussed in [7–9]. The failure of this FCP pair to separate the DO cases is not necessarily surprising since $\sigma$ plays a more prominent role on flame dynamics during the early stages FA. In contrast, the boundary defined by $(\sigma_{sL}/c_b, \gamma)$ correctly separates all the data points. This shows that $\gamma$ strongly influences the DO characteristics, which is to be expected, since it governs the thermodynamic response of gases to compression. Mixtures with a low $\sigma_{sL}/c_b$, and high $\gamma$ result in simple flame and shock structures accompanied by autoignition on the walls (case 1) which make them good candidates for the study of flame/shock interactions during DO, whereas mixtures with a high $\sigma_{sL}/c_b$, and low $\gamma$ yield more complex DO mechanisms including multiple oblique shock reflections.

### 5. DO distribution

Figure 5 presents representative cases of the detonation onset distribution across the channel cross-section for (a) case 1, and (b) case 2. The results are conclusive: most of the detonation onsets occur in the channel corners, as clearly demonstrated by the simultaneous schlieren visualization, and consistent with the findings of Ballossier et al. [15]. These results indicate that extreme conditions develop in the corners, possibly due to viscous effects associated with the boundary layer.

Finally, it is worth mentioning that the nature of the mixture also plays an important role in the number of DO points that may occur (almost) simultaneously. More precisely, for mixtures diluted with Ar, Ar+He, and with $x_{H2} = 0.8$, two consecutive DO points were observed in approximately 25% of the tests. The lean H₂/O₂ mixture ($\phi = 0.8$) exhibited a much more pronounced behavior, with 50% of the tests showing two DO points. For the remaining mixtures, only a single DO point was observed.

### 6. Conclusion

We investigated the relationship between fundamental combustion properties and detonation onset mechanisms in a smooth, square channel. Two detonation onset cases were systematically observed in the mixtures studied:

* Case 1 - occurred for H₂/O₂ mixtures diluted with Ar and Ar+He. These mixtures produced
<figure><img src="image_4.png" /><figcaption>Fig. 3: Case 2 - Schematic of DO sequence observed during one typical combustion propagation after the formation of the LS2 structure. These images were taken for the mixture diluted with N₂, and the behavior is similar to those observed with the mixture φ = 0.8, and for the blends with xH₂ = 0, 0.5 and 0.8.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4: DO cases plotted against FCP pairs (σ, σsL/cb), and (γ, σsL/cb).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 5: DO distribution in the cross-section of the channel for a) case 1 and b) case 2. These maps include the cases where multi DO were observed.</figcaption></figure>

short, symmetric flames and a single precursor wave, with detonation initiated in corners via shock reflections and localized autoignition.

* Case 2 - occurred for H₂/O₂ mixtures diluted with N₂, φ = 0.8 and φ = 1.7, and for CH₄/H₂ blends with xH₂ = 0, 0.5 and 0.8. These mixtures produced long, asymmetric flames and multiple precursor waves. Supersonic combustion structures formed in channel corners sequentially eventually triggering detonation.

Using fundamental combustion properties boundaries originally proposed by the current authors to classify flame morphologies during FA, it was shown that the boundary provided by the pair (σsL/cb, γ) successfully separates all the detonation onset cases, establishing a novel framework connecting combustion

properties, flame behavior, and detonation initiation. Finally, for all mixtures, the onset of detonation occurred primarily at the channel corners, as evidenced by the simultaneous schlieren visualization setup.

## 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.

## Acknowledgements

The authors gratefully acknowledge the financial support from l'Agence Nationale de la Recherche Program JCJC (FASTD ANR-20-CE05-0011-01).
References

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

[1] E. S. Oran, V. N. Gamezo, Origins of the deflagration-to-detonation transition in gas-phase combustion, Combust. Flame 148 (2007) 4–47.

[2] G. Ciccarelli, S. Dorofeev, Flame acceleration and transition to detonation in ducts, Prog. Energy Combust. Sci. 34 (2008) 499–550.

[3] T. Jaravel, O. Dounia, Q. Malé, O. Vermorel, Deflagration to detonation transition in fast flames and tracking with chemical explosive mode analysis, Proc. Combust. Inst. 38 (2021) 3529–3536.

[4] M. Moran, G. Ciccarelli, The transition from CJ-deflagration to detonation in a square channel, Combust. Flame 281 (2025) 114407.

[5] Z. Wang, W. Qin, L. Wei, Z. Zhang, Y. Hui, Advances on deflagration to detonation transition methods in pulse detonation engines, Energies 18 (2025) 2109.

[6] C. C. Mejía-Botero, F. Veiga-López, J. Melguizo-Gavilanes, Minimum tube diameters for detonation propagation in ch4/h2-air mixtures: Implications for natural gas cooktop burners, Journal of Loss Prevention in the Process Industries 80 (2022) 104919.

[7] C. C. Mejía-Botero, F. Virot, L. F. F. da Silva, J. Melguizo-Gavilanes, Flame morphology boundaries and fundamental combustion properties in unobstructed channels, Combust. Flame 286 (2026) 114806.

[8] C. Mejía-Botero, F. Virot, L. F. Figueira da Silva, J. Melguizo-Gavilanes, Flame morphology during acceleration in unobstructed channels: effect of fundamental combustion properties, in: 30th International Colloquium on the Dynamics of Explosions and Reactive Systems (ICDERS 2025), 2025.

[9] C. Mejía-Botero, Flame and shock morphologies and dynamics during acceleration and transition to detonation in unobstructed channels: effect of fundamental combustion properties, Ph.D. thesis, ISAE-ENSMA (2024).

[10] S. Boulal, A. Langenais, L. François, P. Bernigaud, L. Vingert, Large-scale stochastic dynamics of the afterburning in a supersonic fuel-rich plume: Possible evidence of unconfined deflagration-to-detonation transition, Phys. Fluids 37 (2025).

[11] N. Dexter-Brown, J. Jayachandran, Delineating the effects of ignition propensity and markstein number on deflagration-to-detonation transition, Combust. Flame 265 (2024) 113439.

[12] P. Bosnic, M. Henriksen, D. Bjerketvedt, K. Vaagsaether, Mechanistic and statistical characterization of DDT in hydrogen-air explosions through cylindrical obstacles at atmospheric conditions, Combust. Sci. Technol. (2025) 1–23.

[13] A. C. St. George, R. B. Driscoll, V. Anand, E. J. Gutmark, Starting transients and detonation onset behavior in a rotating detonation combustor, in: 54th AIAA aerospace sciences meeting, 2016, p. 0126.

[14] J. Zhao, L. Zhou, L. Zhong, X. Zhang, J. Pan, R. Chen, H. Wei, Experimental investigation of the stochastic nature of end-gas autoignition with detonation development in confined combustion chamber, Combust. Flame 210 (2019) 324–338.

[15] Y. Ballossier, F. Virot, J. Melguizo-Gavilanes, Flame acceleration and detonation onset in narrow channels: simultaneous schlieren visualization, Combust. Flame 254 (2023) 112833.

[16] C. Mejía-Botero, F. Virot, L. F. Figueira da Silva, J. Melguizo-Gavilanes, A detonation run-up distance database: data-driven existing models improvement and new model development, Proc. Combust. Inst. 40 (1-4) (2024) 105444.

68

69

70

71

72

73

74

[17] O. Penyazkov, A. Skilandz, Bifurcation parameters of a reflected shock wave in cylindrical channels of different roughnesses, Shock Waves 28 (2) (2018) 299–309.