Distorted Tulip Flame Research Articles

Flame evolution and pressure dynamics of methane-hydrogen-air explosion in a horizontal rectangular duct

This work is aimed at exploring flame evolution and pressure dynamics of methane-hydrogen-air explosion experimentally and numerically by changing equivalence ratio and hydrogen content. For the flame evolution, the smooth distortion vanishes at equivalence ratio of Ф = 1.4, the prominent cellular structure and distorted tulip flame start to form with increasing hydrogen content. For the pressure dynamics, as the equivalence ratio increases, both maximum explosion overpressure and maximum pressure rise rate increase and then decrease, and both of them continue to increase with increasing hydrogen content. H + O2 = O + OH is the main elementary reaction with positive effect on the laminar burning velocity. The main elementary reactions with negative effects are H + O2(+M) = HO2(+M) and CH3 + H(+M) = CH4(+M). H radical always dominates on the rich side or the lean, stoichiometric side with high hydrogen content. As the hydrogen content increases, the main oxidation pathways of CH4 undergoes the transfer from CH4 → CH2(S) → CH2 → CO to CH4 → CH2(S) → CH2 → CH → CO → CO2. The research results are helpful to improve combustion efficiency of internal combustion engine and ensure process safety of hydrogen doped natural gas pipeline transportation.

Study on premixed hydrogen-ammonia-air flame evolution in a horizontal rectangular duct

The hydrogen-ammonia mixture exhibits significant potential as a carbon-free fuel for internal combustion engines. To gain a deeper understanding of the combustion characteristics of hydrogen-ammonia-air flames in internal combustion, the flame evolution and pressure dynamics are systematically investigated in a closed duct by varying ammonia mixing ratio and equivalence ratio. Concerning flame dynamics, the distorted tulip flame disappears and the inclined flame is formed as the ammonia mixing ratio increases. Additionally, the depression and protrusion structure distributed on flame front is diminished. For pressure dynamics, as the equivalence ratio increases, both the maximum explosion overpressure and maximum pressure rise rate initially increase and then decrease. As the ammonia mixing ratio increases, both of them continue to decrease, and the equivalence ratio corresponding to the peak value transfers from Ф = 1.2 to Ф = 1.0. In terms of detailed chemistry analysis, the H and OH radical dominate when the ammonia mixing ratio is lower than Ω = 40%. NH2 and OH radical dominate at ammonia mixing ratio higher than Ω = 80% on the lean and stoichiometric side, but NH2 and H radical dominate at ammonia mixing ratio higher than Ω = 40% on the rich side.

Numerical investigation on dynamic behavior of premixed hydrogen/air flame propagation in a closed tube

In this paper, the large eddy simulation (LES) model of OpenFOAM was used to study the flame propagation characteristics of hydrogen-air premixed flame in a closed tube with an aspect ratio of 12. The numerical simulation reproduces the characteristic flame shape evolution observed in the experiment, such as classical tulip flame, T-shaped flame, and distorted tulip deformation. The predicted flame front velocity, overpressure, and its fluctuation are all in good agreement with the measurements. The simulation demonstrates that after the classic tulip flame is formed, a series of waves cause an obvious adverse pressure gradient at the front of the flame. This adverse pressure gradient drives reverse flow which pulls the backward tip further into the combustion zone, forming an elongated tulip flame. A small amount of unburnt gas trapped in the cusp rapidly consumes to form a featured T-shaped flame. Then the T-shaped flame transforms into the second finger-shaped flame, and finally the tulip distortion occurs. The vortex and the reverse flow near the flame front generated by the interaction between the pressure field and the flow field accounts primarily for the formation of distorted tulip flame.

Effect of Vent Area on Dynamic Characteristics of Premixed Methane/Air Explosions in End-vented Channels with Different Length/Diameter Ratios

ABSTRACT Further elucidate the interaction mechanism of pressure-flame-wall in channels under the effects of length/diameter ratio (L/D) and vent area, which helps to propose more effective decompression means. Experiments of premixed methane-air explosions in end-vented channels with L/D of 10, 15 and 20 were conducted under initial conditions (301 ± 2 K and 1 atm). The results demonstrate that L/D and vent area are critical external factors causing tulip flames and distorted tulip flames. When the vent area is 60 mm × 60 mm~40 mm × 40 mm, the distorted tulip flame can be generated at L/D of 20. Notably, the critical intrinsic factor causing distorted tulip flame is the interaction of reflected pressure waves-flame. It is deduced that flames stagnate or decelerate during the initial stages of irregular flames because rarefaction waves attenuate the driving force of flames. The new overpressure peaks (Ptulip, Pdis-tulip, and Pirr-tulip) induced by tulip flames, distorted tulip flames and irregular tulip flames are identified. Moreover, the drop size and drop ratio of the maximum overpressure decrease with increasing L/D.

Numerical study of the stability of premixed flames propagating in half-open tubes

This paper studies premixed flame dynamics in half-open tubes by solving the two-dimensional, fully compressible, reactive Navier-Stokes equations on a dynamically adapting mesh using a high-order algorithm. A simplified chemical-diffusive model was used to describe the chemical reaction and diffusive transports in a stoichiometric hydrogen-air mixture. The influence of the length scale was examined by considering four tube heights at a fixed aspect ratio α = 7. The numerical simulations show that the flame evolves into a tulip flame (TF) in all the tubes shortly after being ignited at the open end. Variation in tube size leads to differences in the evolution of TF and generation of expansion waves. In a sufficiently large tube (d > 0.5 cm), the TF further develops into a series of more unstable distorted tulip flames (DTFs). By contrast, in a small tube (d < 0.5 cm), the TF shape remains until the end of the combustion. In addition, both the flame and pressure oscillate significantly almost in the entire process of flame propagation in the large tubes, while the oscillating behaviour in flame or pressure is negligible in the small tube after TF forms. It was found that the TF formation mechanism is length-scale dependent even for the same type of geometry and condition. A detailed examination of the interactions between flame, boundary layer, and pressure waves showed two mechanisms of TF formation: (1) boundary layer effect for the larger tubes (d ≥ 0.5 cm), and (2) Rayleigh–Taylor instability driven by compression waves for the smallest tube (d = 0.25 cm). The DTF formation in the half-open tubes is closely related to the expansion waves generated by the collapse of the TF cusp. The expansion waves cause a reverse flow in the boundary layer ahead of the flame front and consequently initiate the DTF.

Comparison of the premixed flame dynamics of CH4/O2/CO2 mixtures in closed and half-open ducts

The premixed flame dynamics of CH4/O2/CO2 mixtures ignited in two duct configurations, i.e., both ends closed and half-open (open at one end and closed at the opposite end), were investigated. The influence of the combustion-generated gas compression on the chemical kinetics was analyzed. The results show that CH4/O2/CO2 mixtures formed thrice distorted tulip flames (DTF) in the closed duct. The shortened time scale β impairs the interaction between the flame front and the Rayleigh-Taylor instability, resulting in the gradual reduction of the flame distortion degree. Moreover, the deeper primary cusps generated by the stronger Darrieus-Landau instability is favorable for forming a distorted tulip flame with more times of distortions. The duct configuration exerts a significant influence on the relationship between the maximum flame velocity (Vmax) and pressure (Pmax) versus the peak mole fraction [H + O + OH]max. The higher flow velocity of unburned gas induced by the duct-venting and the expansion waves generated by the collapses of secondary cusps as an additional contribution promote the flame velocity for the half-open duct and the pressure rise for the closed duct, respectively. In addition, R1: H + O2 = O + OH exhibits the most positive effect on the laminar burning velocity Sl. R97: CH3 + H (+M) <=> CH4 (+M) and R39: HCO + OH <=> H2O + CO show the most negative influence on Sl for the oxygen fraction γ ≤ 0.45 and γ > 0.45, respectively. The gas compression owing to the pressure waves especially discernible in the closed duct inhibits [H + O + OH]max, and is further enhanced by the oxygen enrichment. This entails to a certain extent a much lower flame velocity in the closed duct as compared to in the half-open duct.

Experimental study of explosion dynamics of syngas flames in the narrow channel

This article introduced the experimental study of the propagation of a syngas premixed flame in a narrow channel. The structural evolution, flame front position and velocity characteristics of lean and rich premixed flames were investigated at different hydrogen volume fractions as the flame was ignited at the open end of the pipe and propagated to the closed end. The comparative study of the syngas fuel characteristics, flame oscillation frequency and overpressure oscillation frequency prove that the syngas explosion flame oscillation in the narrow passage has a strong coupling relationship with overpressure and fuel heat release rate. The results was shown that the flame structure was strongly influenced by the hydrogen volume fraction of the syngas and the fuel concentration. The distorted tulip flame only appears in lean mixture. At 30% of hydrogen volume fraction, the flame exhibits intense and unstable propagation, manifested as the reciprocating and alternating movement of the flame front. As the volume fraction of hydrogen increased, the velocity of flame propagation and the frequency of oscillation increased. When the hydrogen volume fraction γ ≥ 0.4 at the equivalence ratio of Φ = 0.8, the pressure oscillation amplitude gradually increases and reaching the peak after 200–320 ms. Significantly, when γ = 0.3, the pressure peak increases abnormally. This work can provide support for the safe use of syngas in industry by experimental study of various explosion parameters in the narrow channel.

Comparative study on flame instability and combustion characteristics of CH4/O2/CO2 and CH4/O2/N2 mixtures

To investigate flame instabilities and combustion characteristics of oxygen-enriched methane in a closed chamber, a series of combustion experiments on the CH4/O2/CO2 and CH4/O2/N2 mixtures were conducted at the initial temperature and pressure of 301 ± 2 K and 1.0 atm. The results show that the distorted tulip flames and corrugated flames, which can be differentiated by a 2D plane of firstly wall-touching pressure (Pwall/P0) versus flame position (Ztipwall/L), are for the first time observed in the oxygen-enriched methane mixtures. Particularly, the corrugated flames can emerge by increasing the reactivity of CH4/O2/N2 mixtures. There are distinct flame instabilities between the CO2/N2 diluents. Specifically, the CH4/O2/CO2 mixtures always show stronger Thermal-Diffusive instability whereas the CH4/O2/N2 mixtures always exert stronger Darrieus-Landau instability. There is a linearly positive correlation between the Darrieus-Landau instability and oxygen enrichment ratio for both mixtures. In contrast, the oxygen enrichment ratio plays positive (N2 dilution) and negative (CO2 dilution) role for the Thermal-Diffusive instability. A linear prediction between the maximum combustion pressures and Sl·E/lfg is proposed by considering the Darrieus-Landau instability. However, the Rayleigh-Taylor instability, as an additional mechanism for flame distortion, makes the linearity less accurate. As the oxygen enrichment ratio is further increased, the Richtmyer-Meshkov instability instead of the Rayleigh-Taylor instability plays a role in converting distorted tulip flames to corrugated flames, and could revalidate this linear prediction.

Propagation of premixed hydrogen-air flame initiated by a planar ignition in a closed tube

Numerical simulations were used to study the dynamics of premixed flames propagating after planar ignition in a closed tube filled with stoichiometric hydrogen-air mixture. The two-dimensional fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusive model were solved using a high-order numerical method and adaptive mesh refinement. The results show that the flame evolves from an initially planar flame to a double-cusped tulip flame, subsequently to a multi-cusped tulip flame, and finally to a series of distorted tulip flames (DTFs). The DTF forms one after another until the end of combustion. The initial flame lips of the double-cusped tulip flame are produced due to the stretching effect of nonuniform flow caused by the wall friction. The multi-cusped tulip flame forms as secondary cusps are created on the leading flame tips near the sidewalls. The formation of DTFs here is thought to be closely connected to pressure waves generated in the flame propagation process. The first DTF is caused by the combined effects of the vortex motions and the Rayleigh–Taylor (RT) instability driven by pressure waves, while the subsequent DTFs form due to reverse flows and RT instability. Nevertheless, both the vortex motions and reverse flows are essentially induced by the interactions between pressure waves and flow fields. Furthermore, the numerical results were compared to that in the case with a semicircular ignition. It was found that although there are significant differences in the early flame acceleration and tulip formation stages between the two differently shaped ignitions, the dynamics of DTFs are substantially consistent.

Numerical study of premixed flame dynamics in a closed tube: Effect of wall boundary condition

Numerical simulations were conducted to study the dynamics of premixed flames propagating in a closed tube by solving the fully compressible reactive Navier–Stokes equations using a high-order numerical method on a dynamically adapting grid. A simplified chemical-diffusive model was used to describe the reactions and energy release in a stoichiometric hydrogen-air mixture. The influence of wall boundary condition on the flame dynamics was explored by considering three different types of condition on the walls: adiabatic no-slip, adiabatic free-slip, and isothermal. The calculations show that the wall boundary condition has a significant effect on the generation and amplification of pressure waves and consequently on the flame dynamics. In the early stages of flame propagation, the flame behaves in a similar manner for different boundary conditions, that is, the flame develops a tulip shape that further evolves into a distorted tulip flame (DTF) through Rayleigh-Taylor instability arising from acoustic-flame interaction. Significant differences, however, arise after DTF formation in the late stages, especially when the primary acoustic wave is amplified to form a shock wave in the adiabatic free-slip and isothermal cases. The shock-flame interactions facilitate the formation of a series of increasingly corrugated flames by triggering the Richtmyer–Meshkov instabilities. The way how the lateral flame fronts touch the tube sidewalls to generate the primary acoustics and the heat conduction through the tube sidewalls play an important role in the generation and amplification of the pressure waves.

Experimental study on explosion characteristics of premixed syngas/air mixture with different ignition positions and opening ratios

This paper explains the basic knowledge to understand the relationship between overpressure wave behavior and the structure of flame propagation, especially the effects of the ignition position and opening ratio on the combustion dynamics. The explanations are based essentially on the experimental investigations of premixed syngas/air explosions carried out at a constant equivalence ratio of 1 and the hydrogen volume fraction of 0.3 in a rectangular half-open combustion chamber. The initial temperature T0 and pressure p0 were 298 K and 0.1 MPa, respectively. The four ignition positions were set at 0, 150, 450 and 750 mm off the left of the combustion chamber. Four different opening ratios of the duct were employed, including 0.25, 0.5, 0.75, and 1. The obtained results show that ignited at IP0, the increased opening ratio would cause the distorted tulip flame to disappear, accompanied by the propagation direction change of flame front. At the same ignition position, when the opening ratio increased, the oscillation amplitude of overpressure, and the velocities of the right-hand and left-hand flame all rose, but the overpressure growth rate and maximum overpressure decreased. As the ignition position approached the opening end, the overpressure and the flame front oscillations became more intense. It is indicated that different ignition positions and opening ratios exerted obvious effect on the flame front propagation and the overpressure.

On the propagation dynamics of lean H2/CO/air premixed flame

Syngas is a promising alternative fuel. The combustion of lean H2/CO meets the demand of high efficiency with low emission. In this paper, the propagation dynamics of lean H2/CO/air premixed flame in a closed duct were experimentally investigated at an atmospheric condition. Propagation of the premixed flame is recorded by using high-speed cinematograph, and results show that the tulip flame is attained in all cases while the occurrence of tulip distortion is determined by both equivalence ratios and hydrogen volume fractions. The flame tip location and speed are attained through using image processing technique. Results show that the analytical theory, which is firstly proposed to model the flame acceleration in tulip flame propagation phase, can be used to predict the early phase flame tip motion in some limited cases. In fuel-lean combustion, with the effect of the diffusive-thermal instability, the analytical theory underestimates the flame tip motion, but overestimates the time instant of the flame initial touching the lateral walls. The overpressure build-up is specially scrutinized in conjunction with the scaled flame tip speed. The disorderly scaled flame tip speed of distorted tulip flame indicates that the distorted tulip flame is a phenomenon which relates to pressure waves. The pressure wave is of importance for tulip distortion, but it cannot change the overpressure build-up directly. Finally, both the flame-wall interaction and the combustion instability have a crucial effect on flame propagation dynamics, and these factors should be accounted for the build of flame propagation models.

Experimental study on explosion characteristics of syngas with different ignition positions and hydrogen fraction

The influence of different ignition positions and hydrogen volume fractions on the explosion characteristics of syngas is studied in a rectangular half-open tube. Three ignition positions were set at the axis of the tube, which are 0 mm, 600 mm and 1100 mm away from the closed end, respectively. A range of hydrogen volume fraction (φ) from 10% to 90% were concerned. Experimental results show that different ignition positions and hydrogen volume fraction have important influence on flame propagation structure. When ignited at 600 mm from the closed end on the tube axis, distorted tulip flame forms when flame propagates to the closed end. The formations of the tulip flame and the distorted tulip flame are accompanied by a change in the direction of the flame front propagation. The flame propagation structure and pressure are largely affected by the ignition position and the hydrogen volume fraction. At the same ignition position, flame propagation speed increases with the growing of hydrogen volume fraction. And the pressure oscillates more severe as the ignition location is closer to the open end. And pressure oscillations bring two different forms. The first form is that the pressure has a periodic oscillation. The amplitude of the pressure oscillation gradually increases. It takes several cycles from the start of the oscillation to the peak. For the second form, the pressure reaches the peak of the oscillation in the first cycle of the start to the oscillation.

A comparative investigation of premixed flame propagation behavior of syngas-air mixtures in closed and half-open ducts

A comparative investigation has been developed in conjunction with high-speed cameras and piezoelectric gauge to gain deep insight into the propagation behavior of premixed syngas-air flames. Experiments were carried out in two constant rectangular ducts, one closed duct (C-D) and one half-open duct (HO-D). One important finding is that the flame behavior in the C-D is different than that in the HO-D. The repeatable distorted tulip flame forms in the C-D while an unrepeatable distorted tulip shape forms in the HO-D. The repeated pressure-flame interactions result in a repetitive process of the tulip distortion. The downstream opening end of the duct has a significant influence on the flame propagation characteristics after the flame surface reaches the lateral walls. The opening end results in a higher maximum flame tip velocity, a lower overpressure and a longer plane formation time in the HO-D. Meanwhile, the position of the plane flame formation decreases as hydrogen fraction increases in the C-D while the opposite is true in the HO-D. Furthermore, the pressure build-up in both ducts are examined, and the maximum flame tip velocity is of importance for distorted tulip flame formation.

An experimental investigation into the behavior of premixed flames of hydrogen/carbon monoxide/air mixtures in a half-open duct

In this paper, the premixed flame behavior of hydrogen/carbon monoxide/air mixtures is investigated experimentally in a half-open duct with a large aspect ratio (L/D = 20), employing a high-speed camera and a pressure transducer. The flame front structure, flame tip position, and flame tip velocity of the mixtures are obtained by analysing the high-speed images. Meanwhile, the overpressure history is obtained and analysed to examine their effects on the flame propagation behaviour. Additionally, the effects of the PVC membrane on the flame propagation behaviour is specifically scrutinized. The experimental results indicate that the effects of the PVC membrane on the flame front shape are negligible, but they can affect the flame tip and overpressure dynamics more or less in some specific conditions. When the flame propagates along the half-open duct, four stages of the tulip flame formation can be obtained in all cases. Additionally, the tulip distortion phenomenon occurs in most of the cases. This indicates that the distorted tulip flame may be an intrinsic and universal phenomenon. The most interesting phenomenon is that there is a flame front stretch process in the stage where the flame’s skirt touches the sidewalls. This stretched process cannot affect the flame front inversion and tulip distortion. In this work, an observable oscillation in the overpressure history results from the changes in the flame’s surface area. In addition, the flame stretch process indicates that the pressure waves cannot change the flame front structure. This is evidence that the pressure waves are not the incitation factor for the flame inversion and tulip distortion in the half-open duct.

Combined effect of ignition position and equivalence ratio on the characteristics of premixed hydrogen/air deflagrations

Premixed hydrogen/air deflagrations were performed in a 100 mm × 100 mm × 1000 mm square duct closed at one end and opened at the opposite end under ambient conditions, concerning with the combined effect of ignition position IP and equivalence ratio ∅. A wide range of ∅ ranging from 0.4 to 5.0, as well as multiple IPs varying from 0 mm to 900 mm off the closed end of the duct were employed. It is indicated that IP and ∅ exerted a great impact on the flame structure, and the corresponding pressure built-up. Except for IP0, the flame can propagate in two directions, i.e., leftward and rightward. A regime diagram for tulip flames formation on the left flame front (LFF) was given in a plane of ∅ vs. IP. In certain cases (e.g. the combinations of ∅ = 0.6 and IP500 or IP700), distorted tulip flames were also observed on the right flame front (RFF). Furthermore, the combinations of IP and ∅ gave rise to various patterns of pressure profiles. The pressure profiles for ignition initiated at the right half part of the duct showed a weak dependence on equivalence ratio, and showed no dependence on ignition position. However, the pressure profiles for ignition initiated at the left half part of the duct were heavily dependent on the combination of IP and ∅. More specifically, in the leanest (∅ = 0.4) and the richest (∅ = 4.0–5.0) cases, intensive periodical oscillations were the prime feature of the pressure profiles. With the moderate equivalence ratios (∅ = 0.8–3.0), periodical pressure oscillations were only observed for IP900. The maximum pressure peaks Pmax were reached at ∅ = 1.25 rather than at the highest reactivity ∅ = 1.75 irrespective of ignition position. The ignition positions that produced the worst conditions were different, implying a complex influence of the combination of IP and ∅.

Experimental study of premixed syngas/air flame deflagration in a closed duct

The propagation behaviour of a deflagration premixed syngas/air flame over a wide range of equivalence ratios is investigated experimentally in a closed rectangular duct using a high-speed camera and pressure transducer. The syngas hydrogen volume fraction, φ, ranges from 0.1 to 0.9. The flame propagation parameters such as flame structure, propagation time, velocity and overpressure are obtained from the experiment. The effects of the equivalence ratio and hydrogen fraction on flame propagation behaviour are examined. The results indicate that the hydrogen fraction in a syngas mixture greatly influences the flame propagation behaviour. When φ, the hydrogen fraction, is ≥0.5, the prominently distorted tulip flame can be formed in all equivalence ratios, and the minimum propagation time can be obtained at an equivalence ratio of 2.0. When φ < 0.5, the tulip flame distortion only occurs in a hydrogen fraction of φ = 0.3 with an equivalence ratio of 1.5 and above. The minimum flame propagation time can be acquired at an equivalence ratio of 1.5. The distortion occurs when the maximum flame propagation velocity is larger than 31.27 m s−1. The observable oscillation and stepped rise in the overpressure trajectory indicate that the pressure wave plays an important role in the syngas/air deflagration. The initial tulip distortion time and the plane flame formation time share the same tendency in all equivalence ratios, and the time interval between them is nearly constant, 4.03 ms. This parameter is important for exploring the quantitative theory or models of distorted tulip flames.

Experimental study of premixed syngas/air flame propagation in a half-open duct

The flame propagation characteristics of premixed syngas/air mixtures are investigated in a half-open duct over a wide range of hydrogen fractions at various equivalence ratios. A high-speed camera and a pressure transducer are used to acquire the flame images and overpressure dynamics. The results indicate that the flame shape changes as the hydrogen fraction and equivalence ratio vary. There are two different styles of tulip flame inversion. When the equivalence ratios, Ф, are 2.5 and 3.0 and the hydrogen fraction, φ, is higher than 0.3, the flame inversion is more complicated than the classical tulip flame inversion. Moreover, the distorted tulip flame, which is always obtained in closed duct, is observed in a half-open duct in three cases, i.e., Ф = 1.0 and φ = 0.1, Ф = 1.0 and φ = 0.3, and Ф = 3.0 and φ = 0.1. Both the equivalence ratio and the hydrogen fraction can affect the flame propagation velocity and the pressure dynamics. Dramatic flame reacceleration and pressure oscillation occur after the flame inversion. The hydrogen fraction has a significant influence on the characteristics of premixed syngas/air flame. The influence is significant when hydrogen fraction was small (φ < 0.5), and the influence is moderate when hydrogen fraction was large (φ ≥ 0.5).

Effects of pressure waves on the stability of flames propagating in tubes

Numerical simulations were performed to study the effects of pressure waves on the evolution and stability of flames propagating in tubes. The model is the fully compressible reactive Navier–Stokes equations coupled to a calibrated one-step chemical-diffusive model for combustion in a stoichiometric hydrogen–air mixture. The numerical solution method is high order in space and time, and adaptive mesh refinement provides adequate resolution of flames, boundary layers, acoustic waves, and all of their interactions. The influence of tube length scale and aspect ratio was examined for a range of tube sizes that produce tulip flames (TFs) and series of distorted tulip flames (DTFs). The simulations show that pressure waves and acoustic properties of the tube play an important role in the flame evolution, specifically in the formation and evolution of the series of increasingly wrinkled DTFs. A time scale and pressure wave analysis of the results, combined with a linear analysis to give the growth rate of the Rayleigh–Taylor instability (RTI), shows that the RTI is a primary cause for the initiation of DTFs.

Formation and evolution of distorted tulip flames

The development and evolution of tulip and distorted tulip flames in closed channels were simulated by solving the fully compressible reactive Navier–Stokes equations using a high-order numerical method and a single-step Arrhenius model for the reactions and energy release in a stoichiometric mixture of hydrogen and air. Important features of the simulations include (1) the development and propagation of acoustic waves and their effects on flame evolution, (2) the formation and collapse of flame cusps, both at the flame front and near the sidewalls, and the effects of cusp collapse on flame propagation, and (3) the appearance of adverse pressure gradients at the onset of the tulip or a distorted tulip flame, which result in reverse flow in the unburned gas. The simulations highlight the coupling between pressure waves, adverse pressure gradients, boundary layers, and the propagating flame front. Whereas the formation of the tulip flame can be attributed to several effects (such as pressure waves, vortex motion and Landau–Darrieus instabilities), the onset of the distorted tulip flame is strongly influenced by the Rayleigh–Taylor instability.