Flame acceleration and transition from deflagration to detonation in hydrogen explosions

Computational Fluid Dynamics solvers are developed for explosion modelling and hazards analysis in Hydrogen air mixtures. The work is presented in two parts. These include firstly a numerical approach to simulate flame acceleration and deflagration to detonation transition (DDT) in hydrogen–air mixture and the second part presents comparisons between two approaches to detonation modelling. The detonation models are coded and the predictions in identical scenarios are compared. The DDT model which is presented here solves fully compressible, multidimensional, transient, reactive Navier–Stokes equations with a chemical reaction mechanism for different stages of flame propagation and acceleration from a laminar flame to a highly turbulent flame and subsequent transition from deflagration to detonation. The model has been used to simulate flame acceleration (FA) and DDT in a 2-D symmetric rectangular channel with 0.04 m height and 1 m length which is filled with obstacles. Comparison has been made between the predictions using a 21-step detailed chemistry as well as a single step reaction mechanism. The effect of initial temperature on the run-up distances to DDT has also been investigated.     

Effects of obstacle layout and blockage ratio on flame acceleration and DDT in hydrogen-air mixture in a channel with an array of obstacles

Numerical simulations were performed to study flame acceleration and deflagration-to-detonation transition (DDT) in hydrogen-air mixture in a channel with a two-dimensional array of cylindrical obstacles. A high-order numerical algorithm with adaptive mesh refinement was applied to solve reactive Navier-Stokes equations. The effect of obstacle layout was examined by considering three layouts at a fixed blockage ratio of 0.5: staggered, inline-concentrated, and inline-scattered. Three blockage ratios, 0.33, 0.5, and 0.67, were used for the case of staggered obstacles to explore the influence of blockage ratio. The results show that both obstacle layout and blockage ratio have significant effects on flame acceleration and DDT occurrence, although the basic mechanism of detonation initiation is consistent for all the cases involving shock focusing. In the staggered case, the head-on collisions of flame and pressure waves with obstacles greatly promote the growth of flame surface area and thus lead to the fastest flame acceleration and shortest detonation onset time. In the inline-concentrated case, flame propagates slower than that in the staggered case due to smaller flame surface area. However, compared to the zigzag path in the staggered case, the straight passages parallel to flame propagation direction in the inline-concentrated case are more conducive to producing strong shock focusing and thus result in the shortest DDT distance. In the inline-scattered case, the straight passage along the centerline of channel facilitates the early acceleration of flame, but it has the slowest flame propagation in lateral directions and thereby the longest DDT time and distance. For the staggered obstacles at different blockage ratios, flame acceleration rate increases with increasing blockage ratio. The occurrence of DDT is hindered by the most congested obstacles, because the shock focusing is insufficiently strong to initiate detonation after passing through the excessively narrow gaps.     

Effect of solid obstacle distribution on flame acceleration and DDT in obstructed channels filled with hydrogen-air mixture

Ensuring hydrogen safety has become of great significance nowadays, whose leakage can possible result in the deflagration to detonation (DDT). The numerical study aims to explore the effect of solid obstacle distribution on the DDT in a homogeneous hydrogen-air mixture. Results show that the detonation initiation process can be classified into two types: i) local spherical detonation caused by the coupling of flame surface and high-pressure region; ii) detonation triggered by the interaction between the upper wall and multiple compression waves in front of the flame. Also, this study finds that the flame acceleration (FA) experiences two periodic “acceleration-deceleration” processes before the detonation initiation, and the initiation distance and time are the shortest when the obstacles are symmetrically distributed. Further, the higher the unilateral blockage ratio, the more unfavorable DDT occurs. The present results highlight the effect of different solid obstacle distribution patterns on the FA and DDT process.     

Flame acceleration and DDT of hydrogen-oxygen gaseous mixtures in channels with no-slip walls

Hydrogen-oxygen flame acceleration and transition from deflagration to detonation (DDT) in channels with no-slip walls were studied theoretically and using high resolution simulations of 2D reactive Navier-Stokes equations, including the effects of viscosity, thermal conduction, molecular diffusion, real equation of state and a detailed chemical reaction mechanism. It is shown that in “wide” channels (D > 1 mm) there are three distinctive stages of the combustion wave propagation: the initial short stage of exponential acceleration; the second stage of slower flame acceleration; the third stage of the actual transition to detonation. In a thin channel (D < 1 mm) the flame exponential acceleration is not bounded till the transition to detonation. While velocity of the steady detonation waves formed in wider channels (10, 5, 3, 2 mm) is close to the Chapman-Jouguet velocity, the oscillating detonation waves with velocities slightly below the CJ velocity are formed in thinner channels (D < 1.0 mm). We analyse applicability of the gradient mechanism of detonation ignition for a detailed chemical reaction model to be a mechanism of the deflagration-to-detonation transition. The results of high resolution simulations are fully consistent with experimental observations of flame acceleration and DDT in hydrogen-oxygen gaseous mixtures.     

Effect of obstacles on the detonation diffraction and subsequent re-initiation

The DCRFoam solver (density-based compressible solver) built on the OpenFOAM platform is used to simulate the reflection and diffraction processes that occur when detonation waves collide with various objects. Static stoichiometric hydrogen–oxygen mixtures diluted with 70% Ar are used to form stable detonation waves with large cells, with initial conditions of 6.67 kPa pressure and 298 K temperature. The diameters of the cylindrical obstacle range from 6 mm to 22 mm, with x = 230 mm, x = 244 mm, and x = 257 mm being the chosen position. Cylindrical, square, triangular, and inverted triangular obstacles are used, and the quenched detonation re-initiation processes behind them are investigated. In the detonation diffraction process, four triple points exist at the same time due to the effect of cylindrical obstacles of smaller diameters. The re-initiation distance of the detonation wave increases with the increase of cylindrical obstacle diameter. Both the Mach reflection angle and the decoupled angle decrease as the diameter increases. When the location of the cylindrical obstacles is changed, the detonation wave dashes into the obstacles with its different front structures, it is easier to realize the detonation re-initiation when the weak incident shock at the front of a detonation wave strikes the obstacles, and the re-initiation distance decreases by 17.1% when compared with the longest re-initiation distance. The detonation re-initiation distance is shortest under the action of cylindrical obstacles, however the quenched detonation cannot be re-initiated when the inverted triangle and square obstacles are used. The suppression effects of inverted triangle and square obstacles on detonation waves are more evident.     

Effect of fluidic obstacles on flame acceleration and DDT process in a hydrogen-air mixture

Using solid obstacles to accelerate the deflagration to detonation transition (DDT) process induces additional thrust loss, and fluidic obstacles can alleviate this problem to a certain extent. A detailed simulation is conducted to investigate the effects of multiple groups of fluidic obstacles on the flame acceleration and DDT process under different initial velocities and gas types. The results show that, initially, the propagation of reflected shock wave formed by jet impingement is opposite to the flame acceleration direction, thus increasing the initial jet velocity will hinder the flame acceleration. Later, the vortex structure and enhanced turbulence can promote flame acceleration. As the flame accelerates, the virtual blockage ratio of the fluidic obstacles decreases, and increasing initial jet velocity or using reactive jet gases both affect the virtual blockage ratio. Further, increasing initial jet velocity or using reactive jet gases can shorten the detonation initiation time and distance. Compared with solid obstacles, it is concluded that fluidic obstacles can achieve faster detonation initiation with a smaller blockage ratio. Overall, the detonation phenomena in this study are all triggered by hot spots formed by the interaction between reflected waves and distorted flame, but the formation of reflected waves varies.     

Deflagration-to-detonation transition via the distributed photo ignition of carbon nanotubes suspended in fuel/oxidizer mixtures

Here the promotion of flame acceleration and deflagration-to-detonation transition (DDT) using the distributed photo ignition of photo-sensitive nanomaterials suspended in fuel/oxidizer mixtures is demonstrated for the first time. Distributed photo ignition was carried out by suspending single-walled carbon nanotubes (SWCNTs) with Fe impurity in quiescent C₂H₄/O₂/N₂ mixtures and flashing them with an ordinary Xe camera flash. Following the flash, the distributed SWCNTs photo ignite and subsequently provide a quasi-distributed ignition of the C₂H₄/O₂/N₂ mixture. In a closed detonation tube the quasi-distributed photo ignition at one end of the tube leads to the promotion of flame acceleration and DDT and, for sensitive C₂H₄/O₂ mixtures, appears to lead to direct detonation initiation or multiple combustion fronts. The DDT run-up distance, the distance required for the transition to detonation, was measured using ionization sensors and was found to be approximately a factor of 1.5× to 2× shorter for the distributed photo ignition process than for traditional single-point spark ignition. It is hypothesized that the increased volumetric energy release rate resulting from distributed photo-ignition enhances DDT due to the decreased ignition delay and greater early-time flame area and turbulence levels, which in turn result in accelerated formation and amplification of the leading shock and accelerated DDT.     

Numerical simulation of flame acceleration and deflagration to detonation transition in hydrogen-air mixture

A numerical approach has been developed to simulate flame acceleration and deflagration to detonation transition in hydrogen-air mixture. Fully compressible, multidimensional, transient, reactive Navier–Stokes equations are solved with a chemical reaction mechanism which is tuned to simulate different stages of flame propagation and acceleration from a laminar flame to a turbulent flame and subsequent transition from deflagration to detonation. Since the numerical approach must simulate both deflagrations and detonations correctly, it is initially tested to verify the accuracy of the predicted flame temperature and velocity as well as detonation pressure, velocity and cell size. The model is then used to simulate flame acceleration (FA) and transition from deflagration to detonation (DDT) in a 2-D rectangular channel with 0.08 m height and 2 m length which is filled with obstacles to reproduce the experimental results of Teodorczyk et al.The simulations are carried out using two different initial ignition strengths to investigate the effects and the results are evaluated against the observations and measurements of Teodorczyk et al.     

Sensitization of pentane-oxygen mixtures to DDT via cool flame oxidation

The effect of cool flame partial oxidation on the detonation sensitivity of a hydrocarbon fuel was investigated experimentally. The detonation sensitivity was quantified by measuring the run-up distance required for a deflagration to transit to a detonation wave (DDT) in a rough tube. Fuel rich pentane-oxygen mixtures at sub-atmospheric initial pressures were studied. Subsequent to the injection of the mixture into a heated detonation tube, the mixture underwent cool flame oxidation after a well-controlled delay time, dependent on the temperature of the tube. Typical delays ranged from 0.5 to 2 s (depending on temperature) and were reproducible to within one hundred milliseconds. This delay permitted the mixture in the detonation tube to be spark-ignited at various stages of the cool flame process using an igniter driven by a delay generator. The results show that increasing mixture temperature from room temperature to values below the cool flame region (below 250°C) resulted in an increase in run-up distance. However, as the mixture began to undergo cool flame oxidization, a significant reduction in the run-up distance was obtained (as large as 50%). The sensitization effect was found to occur only at the initial stage of the cool flame oxidation reaction. If the mixture was ignited at times long after the onset of cool flame, the mixture was found to be desensitized and the run-up distance increased. The sensitizing effect of the cool flame partial oxidation may be attributed to the presence of peroxides and free radicals associated with the initial cool flame process. However, these radical species are consumed as the cool flame reaction proceeds and the mixture becomes insensitive again.     

Experiments on combustion regimes for hydrogen/air mixtures in a thin layer geometry

A series of experiments in a thin layer geometry performed at the HYKA test site of the KIT. Experiments on different combustion regimes for lean and stoichiometric H₂/air mixtures were performed in a rectangular chamber with dimensions of 200 × 900 x h mm³, where h is the thickness of the layer (h = 1, 2, 4, 6, 8, 10 mm). To model a gap between a fuel cell assembly and a metal housing, three different layer geometries were investigated: (1) a smooth channel without obstructions; (2) a channel with a metal grid filled 25% of chamber length and (3) a metal grid filled 100% of chamber length. The blockage ratio of metal grid has changed from 10 to 60% of cross-section. Detail measurements of H₂/air combustion behavior including flame acceleration (FA) and DDT in closed rectangular channel have been done. Five categories of flame propagation regimes were classified. Special attention was paid to analysis of critical condition for different regimes of flame propagation as function of layer thickness and roughness of the channel. It was found that thinner layer suppresses the detonation onset and even with a roughness, the flame may quench or, in thicker layer, is available to accelerate to speed of sound. The detonation may occur only in a channel thicker than 4 mm.     

The role of shock-flame interactions on flame acceleration in an obstacle laden channel

Flame acceleration was investigated in an obstructed, square-cross-section channel. Flame acceleration was promoted by an array of top and bottom surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. This work is based on a previous investigation of the effects of blockage ratio on the early stage of flame acceleration. This study is focused on the later stage of flame acceleration when compression waves, and eventually a shock wave, form ahead of the flame. The objective of the study is to investigate the effect of obstacle blockage on the rate of flame acceleration and on the final quasi-steady flame-tip velocity. Schlieren photography was used to track the development of the shock-flame complex. It was determined that the interaction between the flame front and the reflected shock waves produced from contact of the lead shock wave with the channel top, channel bottom, and obstacle surfaces govern the late stage of flame acceleration process. The shock-flame interactions produce oscillations in the flame-tip velocity similar to that observed in the early stage of flame acceleration, but only much larger in magnitude. Eventually the flame achieves a globally quasi-steady velocity. For the lowest blockage obstacles, the velocity approaches the speed of sound of the combustion products. The final quasi-steady flame velocity was lower in tests with the higher obstacle blockage. In the quasi-steady propagation regime with the lowest blockage obstacles, burning pockets of gas extended only a few obstacles back from the flame-tip, whereas burning pockets were observed further back in tests with the higher obstacle blockage.     

Transmission of near-limit detonation wave through a planar sudden expansion in a narrow channel

Transmission of single-cell and spinning detonation waves in C₂H₄ + 3(O₂ + βN₂) mixtures through a 2-D sudden expansion experimentally studied using high-speed cinematography and soot film visualization. Nitrogen dilution ratio, β, is utilized to control cell size and detonation mode. Detonation wave of ethylene/oxygen/nitrogen mixture was initiated via DDT in the 1 mm × 1 mm cross-section and 250 mm long initiator channel before propagating into the 3 mm × 1 mm receptor channel. Visualizations show that detonation waves were extinct and accompanied with abrupt decrease in visible reaction front propagating velocities right after passing through the sudden expansion. However, re-acceleration of the reaction front and re-initiation of the detonation wave were observed downstream in the expanded receptor section. Two re-initiation modes with large disparity in the re-initiation distance were experimentally characterized. For mixtures with nitrogen dilution ratio equals 0.3 or less, the cellular detonation front propagated with single cell in the initiator section before entering the sudden expansion. The re-initiation distance was less than 50 mm and was likely achieved via shock reflection. Velocity characterization shows that steady propagating speed of the detonation wave is ～100 m/s higher in receptor section than in the initiator section. Since the cell size became larger than 1 mm for mixtures with β ? 0.3, the detonation wave propagated in spinning detonation mode before transmitting into the expanded section. The reaction front would have to go through another DDT process to reach detonation state in the receptor section, and the re-initiation distance was increased to more than 150 mm. Moreover, step height of the sudden expansion was proposed as the characteristic length scale to obtain a unified non-dimensional correlation between re-initiation distance and detonation cell size.     

Optical and thrust measurement of a pulse detonation combustor with a coaxial rotary valve

We developed a rotary valve for a pulse detonation engine (PDE), and confirmed its basic characteristics and performance. In a square cross-section combustor, we visualized a multi-shot of a pulse detonation rocket engine (PDRE) cycle at an operation frequency of 160 Hz by using a high-speed camera (time resolution: 3.33 μs, space resolution: 0.4 mm) and a Schlieren method. The propellant filling process and the purge process were confirmed, and each process was modeled. Moreover, we confirmed the processes of detonation wave generation and burned gas blowdown. In addition, we investigated the impact of shortening the passage width of a combustor and negative-time ignition (ignition time is earlier than the end-time of the propellant filling process) on the deflagration-to-detonation transition (DDT) distance and time. The DDT distance did not depend on the passage width of a combustor and decreased under the negative-time ignition condition. With a passage width of 20 mm, the DDT distance decreased by 22% under the negative-time ignition condition to a minimum value (76 ± 8 mm). The DDT time from spark time reached a minimum value (69 ± 14 μs) under the condition of a passage width of 10 mm and negative-time ignition. The detonation initiation time and the DDT distance were represented by the time until the flame expanded toward the tube-axis one-dimensionally from ignition (characteristic time). We also carried out thrust measurement using a PDRE system composed of a circular cross-section combustor and the newly developed valve. We obtained a stable time-averaged thrust in a wide range of operation frequency (40–160 Hz) and confirmed the increase of specific impulse due to a partial-fill effect. At a maximum operation frequency of 159 Hz, we achieved a maximum propellant-based specific impulse of 232 s and a maximum time-averaged thrust of 71 N.     

Fast turbulent deflagration and DDT of hydrogen–air mixtures in small obstructed channel

An experimental study of flame propagation, acceleration and transition to detonation in hydrogen–air mixture in 2-m long rectangular cross-section channel filled with obstacles located at the bottom wall was performed. The initial conditions of the hydrogen–air mixture were 0.1 MPa and 293 K. Three different cases of obstacle height (blockage ratio 0.25, 0.5 and 0.75) and four cases of obstacle density were studied with the channel height equal to 0.08 m. The channel width was 0.11 m in all experiments. The propagation of flame and pressure waves was monitored by four pressure transducers and four in-house ion probes. The pairs of transducers and probes were placed at various locations along the channel in order to get information about the progress of the phenomena along the channel. To examine the influence of mixture composition on flame propagation and DDT, the experiments were performed for the compositions of 20%, 25% and 29.6% of H₂ in air by volume. As a result of the experiments the deflagration and detonation regimes and velocities of flame propagation in the obstructed channel were determined.     

Effect of obstacles behind the pre-detonator tube on the re-initiation of diffracted detonation wave

The re-initiation of diffracted detonation wave is simulated by the DCRFoam. The detonation waves propagated from the pre-detonator tube to the main chamber are formed by 2H₂-O₂-7Ar. Due to the sudden expansion of cross-section, the detonation wave will be attenuated by the rarefaction effect, resulting in detonation failure. Introducing obstacles behind the pre-detonator is an effective method to realize the diffracted detonation re-initiation. This study aims to optimize the detonation transmission by considering the height (h) of obstacles and the distance (w) between obstacles and the exit of the pre-detonator. Results show that, when 15.31311w²-1342.52507w+29435.20137≤h ≤ 0.00239w²+0.38038w+10.95694 (10 mm ≤ h ≤ 30 mm, 10 mm ≤ w ≤ 50 mm), the diffracted detonation wave can realize re-initiation. When w is constant, the formation distance of stable detonation first decreases and then increases with the increase of h. The same law of w can be found when h is unchanged.     

Hydrogen–oxygen flame acceleration and deflagration-to-detonation transition in three-dimensional rectangular channels with no-slip walls

Hydrogen–oxygen flame acceleration and the transition from deflagration to detonation (DDT) in channels with no-slip walls are studied using high resolution simulations of 3D reactive Navier–Stokes equations, including the effects of viscosity, thermal conduction, molecular diffusion, real equation of state and detailed (reduced) chemical reaction mechanism. The acceleration of the flame propagating from the closed end of a channel, which is a key factor for understanding of the mechanism of DDT, is thoroughly studied. The three dimensional modeling of the flame acceleration and DDT in a semi-closed rectangular channel with cross section 10 × 10 mm and length 250 mm confirms validity of the mechanism of deflagration-to-detonation transition, which was proposed earlier theoretically and verified using 2D simulations. We show that 3D model contrary to 2D models allows to understand clearly the meaning of schlieren photos obtained in experimental studies. The “numerical schlieren” and “numerical shadowgraph” obtained using 3D calculations clarify the meaning of the experimental schlieren and shadow photos and some earlier misinterpretations of experimental data.     

The flame propagation characteristics and detonation parameters of ammonia/oxygen in a large-scale horizontal tube: As a carbon-free fuel and hydrogen-energy carrier

As a carbon-free fuel and a hydrogen-energy carrier, ammonia is a potential candidate for future energy utilization. Therefore, in order to promote the application of ammonia in detonation engines and to evaluate the safety of ammonia related industrial process, DDT experiments for ammonia/oxygen mixtures with different ERs were carried out in a large-scale horizontal tube. Moreover, pressure transducers and self-developed temperature sensors were applied to record the overpressure and the instantaneous flame temperature during DDT process. The results show that the DDT process in ammonia/oxygen mixtures contains four stages: Slow propagation stage, Flame and pressure wave acceleration stage, Fast propagation and detonation wave formation stage, Detonation wave self-sustained propagation stage. For stoichiometric ammonia/oxygen mixtures, flame front and the leading shock wave propagate one after another with different velocity, until they closely coupled and propagated together with one steady velocity. At the same time, it is found that an interesting retonation wave propagates backward. The peak overpressure, detonation velocity, and flame temperature of the self-sustained detonation are 2 MPa, 2000 m/s and 3500 K, respectively. With the ER increased from 0.6 to 1.6, the detonation velocities and peak overpressures ranged from 2310 m/s to 2480 m/s and 25.6 bar–28.7 bar, respectively. In addition, the detonation parameters of ammonia were compared with those of methane and hydrogen to evaluate the detonation performance and destructiveness of ammonia.     

Characteristics of gasoline–air mixture explosions with different obstacle configurations

The effects of obstacle distance from ignition point, the blockage ratio of obstacle (BR) and the separation distance of obstacles on the characteristics of gasoline–air mixture explosions have been examined by a series of contrast experiments in a semi-confined organic glass pipe (with a square cross section size of 100 mm*100 mm and 1000 mm long, L/D = 10, V = 0.01 m³). It was shown that before the flame fronts propagated to the obstacle, the flame fronts remained regular shape and spread in a low speed, while passed across the obstacle, the flame fronts could be sharply accelerated and became distorted. And it was obvious that the shorter the distance between obstacle and ignition point, the earlier the flame was accelerated, and eventually led to a higher maximum flame speed. Meanwhile, the maximum overpressures and maximum rates of overpressure rise were obtained at L_i = 400 mm, and the shorter the distance between the obstacle and ignition point, the shorter the time taken to reach the maximum overpressure. Three kinds of blockage ratios (BR = 36.4%, 49.8%, 71.7%) were tested, and it was found that the maximum flame speeds, maximum overpressures, average rates of overpressure rise and maximum rates of overpressure rise increased with the growth of blockage ratio. It was also discovered that the maximum effect of the combined obstacles on flame acceleration behavior could be obtained at an obstacle separation distance of 1 time to 4 times the length of pipe diameter. And the time taken to obtain the maximum overpressures became shorter with the growth of the obstacle separation distance, while the maximum overpressures and maximum rates of overpressure rise were obtained at an obstacle separation range from D_i/D = 3 to 5 (or 300 mm–500 mm).     

Numerical simulation and validation of flame acceleration and DDT in hydrogen air mixtures

Combustion of hydrogen can take place in different modes such as laminar flames, slow and fast deflagrations and detonations. As these modes have widely varying propagation mechanisms, modeling the transition from one to the other presents a challenging task. This involves implementation of different sub-models and methods for turbulence-chemistry interaction, flame acceleration and shock propagation. In the present work, a unified numerical framework based on OpenFOAM has been evolved to simulate such phenomena with a specific emphasis on the Deflagration to Detonation Transition (DDT) in hydrogen-air mixtures. The approach is primarily based on the transport equation for the reaction progress variable. Different sub-models have been implemented to capture turbulence chemistry interaction and heat release due to autoignition. The choice of sub-models has been decided based on its applicability to lean hydrogen mixtures at high pressures and is relevant in the context of the present study. Simulations have been carried out in a two dimensional rectangular channel based on the GraVent experimental facility. Numerical results obtained from the simulations have been validated with the experimental data. Specific focus has been placed on identifying the flame propagation mechanisms in smooth and obstructed channels with stratified initial distribution. In a smooth channel with stratified distribution, it is observed that the flame surface area increases along the propagation direction, thereby enhancing the energy release rate and is identified to be the key parameter leading to strong flame acceleration. When obstacles are introduced, the increase in burning rate due to turbulence induced by the obstacles is partly negated by the hindrance to the unburned gases feeding the flame. The net effect of these competing factors leads to higher flame acceleration and propagation mechanism is identified to be in the fast deflagration regime. Further analysis shows that several pressure pulses and shock complexes are formed in the obstacle section. The ensuing decoupled shock-flame interaction augments the flame speed until the flame coalesces with a strong shock ahead of it and propagates as a single unit. At this point, a sharp increase in propagation speed is observed thus completing the DDT process. Subsequent propagation takes place at a uniform speed into the unburned mixture.