Deflagration-to-detonation Transition Process Research Articles

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.

Theoretical analysis on deflagration-to-detonation transition**Project supported by the National Natural Science Foundation of China (Grant Nos. 11672312 and 11532014).

The study on deflagration-to-detonation transition (DDT) is very important because this mechanism has relevance to safety issues in industries, where combustible premixed gases are in general use. However, the quantitative prediction of DDT is one of the major unsolved problems in combustion and detonation theory to date. In this paper, the DDT process is studied theoretically and the critical condition is given by a concise theoretical expression. The results show that a deflagration wave propagating with about 60% Chapman–Jouguet (CJ) detonation velocity is a critical condition. This velocity is the maximum propagating velocity of a deflagration wave and almost equal to the sound speed of combustion products. When this critical condition is reached, a CJ detonation is triggered immediately. This is the quantitative criteria of the DDT process.

Effect of actuating voltage and discharge gap on plasma assisted detonation initiation process

The influence of actuating voltage and discharge gap on plasma assisted detonation initiation by alternating current dielectric barrier discharge was studied in detail. A loose coupling method was used to simulate the detonation initiation process of a hydrogen–oxygen mixture in a detonation tube under different actuating voltage amplitudes and discharge gap sizes. Both the discharge products and the detonation forming process assisted by the plasma were analyzed. It was found that the patterns of the temporal and spatial distributions of discharge products in one cycle keep unchanged as changing the two discharge operating parameters. However, the adoption of a higher actuating voltage leads to a higher active species concentration within the discharge zone, and atom H is the most sensitive to the variations of the actuating voltage amplitude among the given species. Adopting a larger discharge gap results in a lower concentration of the active species, and all species have the same sensitivity to the variations of the gap. With respect to the reaction flow of the detonation tube, the corresponding deflagration to detonation transition (DDT) time and distance become slightly longer when a higher actuating voltage is chosen. The acceleration effect of plasma is more prominent with a smaller discharge gap, and the benefit builds gradually throughout the DDT process. Generally, these two control parameters have little effect on the amplitude of the flow field parameters, and they do not alter the combustion degree within the reaction zone.

On the deflagration-to-detonation transition (DDT) process with added energetic solid particles for pulse detonation engines (PDE)

In this paper, numerical simulations are performed to study the dynamics of the deflagration-to-detonation transition (DDT) in pulse detonation engines (PDE) using energetic aluminum particles. The DDT process and detonation wave propagation toward the unburnt hydrogen/air mixture containing solid aluminum particles is numerically studied using the Eulerian–Lagrangian approach. A hybrid numerical methodology combined with appropriate sub-models is used to capture the gas dynamic characteristics, particle behavior, combustion characteristics, and two-way solid-particle–gas flow interactions. In our approach, the gas mixture is expressed in the Eulerian frame of reference, while the solid aluminum particles are tracked in the Lagrangian frame of reference. The implemented computer code is validated using published benchmark problems. The obtained results show that the aluminum particles not only shorten the DDT length but also reduce the DDT time. The improvement of DDT is primarily attributed to the heat released from surface chemical reactions on the aluminum particles. The temperatures associated with the DDT process are greater than the case of non-reacting particles added, with an accompanying rise in the pressure. For an appropriate range of particle volume fraction, particularly in this study, the higher volume fraction of the micro-aluminum particles added in the detonation chamber can lead to more heat energy released and more local instabilities in the combustion process (caused by the local high temperature), thereby resulting in a faster DDT process. In essence, the aluminum particles contribute to the DDT process of successfully transitioning to detonation waves for (failure) cases in which the fuel gas mixture can be either too lean or too rich. With a better understanding of the influence of added aluminum particles on the dynamics of the DDT and detonation process, we can apply it to modify the geometry of the detonation chamber (e.g., the length of the detonation tube) accordingly to improve the operational performance of the PDE.

Evaluation of critical blockage ratio and pulse length in a pulse detonation engine using CFD and MATLAB

A Pulse Detonation Engine (PDE) is a new invented propulsion device that takes advantage of the pressure rise inherent to the efficient burning of fuel-air mixtures via detonations. Detonation initiation is a critical process that occurs in the cycle of a PDE. A practical method of detonation initiation is Deflagration-to-Detonation Transition (DDT), which describes the acceleration of a subsonic deflagration created using low initiation energies to a supersonic detonation. The DDT process is not well understood due to a wide range of time and length scales involving complex chemistry, turbulence and unsteady pressure waves. This paper discuss about the effects of blocking ratio in the augmentation of detonation pressure and velocity inside a cylindrical tube of diameter 0.0254m and a length of 1 m. The blockages are rectangular in shape placed at 2/3rd distances of the length of the tube and the heights of the blockages are varied in terms of the diameter of the tube as 1/4th, 1/3rd, ½, 2/3rd and 3/4th the diameter of the tube. The setup is then analyzed in MATLAB using the physics of Friedlander’s equation, which formulate the decay time duration of pressure across the tube length, with and without the blockage. Further, a 2D CFD analysis through ANSYS Workbench is conducted which gave the effective blocking ratio in a rectangular type of blockage placed at the 2/3rd position of the length of the tube and the results are compared. For variable pressures ranging from 1 MPa to 100 MPa input, the effective pulse length is around 0.25 seconds after which the decay of pressure and temperature attain the critical limit. Also it is found that the maximum feasible velocity occurs for an inlet pressure of 10 MPa and 2/3rd height of the blockage where the corresponding outlet velocity is 4692m/s and outlet total pressure being 10.542 MPa.

Brief review on passive and active methods for explosion and detonation suppression in tubes and galleries

Gaseous and dust (including hybrid) explosions often occur in mines, grain elevators, and industrial plants, and always lead to severe damage. Strategies for suppressing or mitigating explosions are developed based on an understanding of their physical mechanism. This study briefly summarized previous studies on explosions, detonations, and the deflagration-to-detonation transition (DDT), and then discussed potential passive/active or hybrid methods for suppressing explosions and detonation in tubes and galleries. Suppression can be achieved using methods that are focused on eliminating or mitigating factors that can either promote the reactive process or break down the DDT process. These methods involve lowering temperature and pressure, diluting the mixture concentration, and venting the closed system. The method of tuning wall materials (using aerogel) is also validated for the effectiveness of detonation suppression. By using various sensors to detect propagating flames or pressure increases in tubes or galleries, active systems can disperse a suppressing agent to extinguish combustion in a timely manner. It is commonly regarded that the active systems are superior to passive methods that operate without additional control units in some ways, but there are always limitations on the application due to reasons such as the cost of equipment, regular maintenance as well as reliability of the complicated integrated-systems. Practice applications show that there is no method of explosion prevention is absolutely safe. In reality, people must consider various factors in the development of an explosion/detonation suppression system to reduce the risk to an acceptable level.

Experimental study on incident wave speed and the mechanisms of deflagration-to-detonation transition in a bent geometry

The study of deflagration-to-detonation transition (DDT) in bent tubes is important with many potential applications including fuel pipeline and mine tunnel designs for explosion prevention and detonation engines for propulsion. The aim of this study is to exploit low-speed incident shock waves for DDT using an S-shaped geometry and investigate its effectiveness as a DDT enhancement device. Experiments were conducted in a valveless detonation chamber using ethylene–air mixture at room temperature and pressure (303 K, 1 bar). High-speed Schlieren photography was employed to keep track of the wave dynamic evolution. Results showed that waves with velocity as low as 500 m/s can experience a successful DDT process through this S-shaped geometry. To better understand the mechanism, clear images of local explosion processes were captured in either the first curved section or the second curved section depending on the inlet wave velocity, thus proving that this S-shaped tube can act as a two-stage device for DDT. Owing to the curved wall structure, the passing wave was observed to undergo a continuous compression phase which could ignite the local unburnt mixture and finally lead to a local explosion and a detonation transition. Additionally, the phenomenon of shock–vortex interaction near the wave diffraction region was also found to play an important role in the whole process. It was recorded that this interaction could not only result in local head-on reflection of the reflected wave on the wall that could ignite the local mixture, and it could also contribute to the recoupling of the shock–flame complex when a detonation wave is successfully formed in the first curved section.

Effects of a jet turbulator upon flame acceleration in a detonation tube

Allowing a fuel/air mixture to flow through bluff-body structures enhances the deflagration-to-detonation transition (DDT) process in pulse-detonation engines by increasing the turbulence intensity within the detonation chamber. However, these structures give rise to resistance and thrust loss. The use of jets as turbulators to reduce the obstacle-blockage ratio may reduce this loss. A jet turbulator composed of multiple evenly spaced jets on the tube wall was designed, and its effects upon flame acceleration were analysed. The results show that fuel/air-mixture jets combined with a low obstacle-blockage ratio can accelerate flame propagation whereas pure-air jets cannot. The jet turbulator offers an improvement over bluff-body structures in terms of acceleration of the DDT process.

Numerical study of nonequilibrium plasma assisted detonation initiation in detonation tube

Nonequilibrium plasma has shown great merits in ignition and combustion nowadays, which should be especially useful for hypersonic propulsion. A coaxial electrodes configuration was established to investigate the effect of alternating current (AC) dielectric barrier discharge nonequilibrium plasma on the detonation initiation process in a hydrogen-oxygen mixture. A discharge simulation-combustion simulation loosely coupled method was used to simulate plasma assisted detonation initiation. First, the dielectric barrier discharge in the hydrogen-oxygen mixture driven by an AC voltage was simulated, which takes 17 kinds of particles (including positively charged particles, negatively charged particles, and neutral particles) and 47 reactions into account. The temporal and spatial characteristics of the discharge products were obtained. Then, the discharge products were incorporated into the combustion model of a detonation combustor as the initial conditions for the later detonation initiation simulation. Results showed that the number density distributions of plasma species are different in space and time, and develop highly nonuniformly from high voltage electrode to grounded electrode at certain times. All the active species reach their highest concentration at approximately 0.6T (T denotes a discharge cycle). Compared with the no plasma case, the differences of flowfield shape mainly appear in the early stage of the deflagration to detonation transition process. None of the sub-processes (including the very slow combustion, deflagration, over-driven detonation, detonation decay, and propagation of a self-sustained stable detonation wave) have been removed by the plasma. After the formation of a C-J detonation wave, the whole flowfield remains unchanged. With the help of plasma, the deflagration to detonation transition (DDT) time and distance are reduced by about 11.6% and 12.9%, respectively, which should be attributed to the active particles effect of nonequilibrium plasma and the local turbulent enhancing effect by the spatial characteristics of discharge. In addition, as the duration of forming a shock wave in the combustor is shortened by approximately 8.1%, it can be inferred that the plasma accelerates the DDT process more significantly before the flow becomes supersonic.

Deflagration to detonation transition in mechanoactivated mixtures of ammonium perchlorate with aluminum

Deflagration-to-detonation transition (DDT) in aluminum-ammonium perchlorate (Al/AP) loose-packed charges (80% porosity) has been studied. The charges were manufactured from preliminary mechanoactivated mixtures. The mixtures placed in steel tubes 10 mm in diameter were ignited by Nichrome wire. It was found that it is possible to distinguish three parts corresponding to different stages of DDT process development. Steady-state detonation velocity reached the level of 2500 m/s at the distance of 90 mm from the ignition point.

Mechanism of detonation transition from accelerating flames in a channel

In order to clarify the mechanisms associated with a local explosion leading to deflagration to detonation transition (DDT) in a rectangular channel, detonation initiation behind the incident shock wave with a weak spark discharge was caused. In this approach the incident shock wave plays a role of the precursor shock wave, while the propagating flame is replaced with a flame kernel generated by the spark discharge. This method was successful in controlling the time and location of detonation initiation with good repeatability. The DDT process was visualized in two perpendicular directions to understand the three-dimensional behavior of the flame and the shock wave generated from flame acceleration. The following conclusions on the DDT process are deduced. (1) A shock wave coupled with a propagating flame eventually reflects off the wall as Mach reflection, behind which an accelerating flame and a shock wave ahead of it are caused. (2) The shock wave interacts with the other shock wave (shock–shock interaction) with more than a Mach number of 3. (3) This shock–shock interaction increases the temperature and pressure of the mixture locally, leading to the generation of a sufficiently strong shock wave with a larger MCJ. (4) The induced chemical energy release sustains the shock wave that decays to MCJ, resulting in propagation as detonation. Because the emergence of the accelerating flame and the shock–shock interaction process are completely three-dimensional phenomena, an understanding of three-dimensional flame and shock behavior is essential to reveal the DDT mechanism.

Effects of boundary layer on flame propagation generated by forced ignition behind an incident shock wave

To study the effects of the boundary layer on the deflagration to detonation transition (DDT) process, the mixture behind an incident shock wave was ignited using laser breakdown. Ignition timing was controlled so that the interaction of the resulting flame with a laminar or turbulent boundary layer could be examined. In the case of the interaction with a laminar boundary layer, wrinkling of the flame was observed after the flame reached the corner of the channel. On the other hand, interaction with the turbulent boundary layer distorted the flame front and increased the spreading rate of the flame followed by prompt DDT. The inner structure of the turbulent boundary layer plays an important role in the DDT process. The region that distorted the flame within the turbulent boundary layer was found to be the intermediate region $$0.01< y/\delta < 0.4$$ , where y is the distance from the wall and $$\delta $$ is the boundary layer thickness. The flame disturbance by the turbulent motions is followed by the flame interaction with the inner layer near the wall, which in turn generates a secondary-ignition kernel that produced a spherical accelerating flame, which ultimately led to the onset of detonation. After the flame reached the intermediate region, the time required for DDT was independent of the ignition position. The effect of the boundary layer on the propagating flame, thus, became relatively small after the accelerating flame was generated.

Numerical Simulation of Deflagration to Detonation Transition in a Straight Duct: Effects of Energy Release and Detonation Stability

AbstractNumerical simulation based on the Euler equation and one-step reaction model is carried out to investigate the process of deflagration to detonation transition (DDT) occurring in a straight duct. The numerical method used includes a high resolution fifth-order weighted essentially non-oscillatory (WENO) scheme for spatial discretization, coupled with a third order total variation diminishing Runge-Kutta time stepping method. In particular, effect of energy release on the DDT process is studied. The model parameters used are the heat release atq= 50,30,25,20,15,10 and 5, the specific heat ratio at 1.2, and the activation temperature atTi= 15, respectively. For all the cases, the initial energy in the spark is about the same compared to the detonation energy at the Chapman-Jouguet (CJ) state. It is found from the simulation that the DDT occurrence strongly depends on the magnitude of the energy release. The run-up distance of DDT occurrence decreases with the increase of the energy release forq= 50 ~ 20, and increases with the increase of the energy release forq= 20 ~ 5. This phenomenon is found to be in agreement with the analysis of mathematical stability theory. It is suggested that the factors to strengthen the DDT would make the detonation more stable, and vice versa. Finally, it is concluded from the simulations that the interaction of the shock wave and the flame front is the main reason for leading to DDT.

Understanding explosions – From catastrophic accidents to creation of the universe

This paper focuses on two extremes of explosions: the general class of Type Ia supernova (SNIa), which are surprisingly uniform thermonuclear explosions of white dwarf stars, and a specific gasoline vapor-cloud explosion that occurred at the Buncefield fuel depot in 2005. In both cases, recurring questions are whether an initial spark or small, local ignition could result in a detonation, and if so, how could this happen? The broader question is: What is the origin of the deflagration-to-detonation transition (DDT) in confined, partially confined, and unconfined systems? The importance of DDT to SNIa is based on the use of these objects as cosmological “standard candles” that are used for measuring distances and curvature in the universe. The importance of DDT to Buncefield is related to design and operational safety of industrial plants and fuel storage facilities. Combinations of observations, specific laboratory experiments, and selected numerical simulations have given us information and some understanding of the DDT process and its likelihood. Numerical simulation both of large- and small-scale phenomena in these reactive flows were important ingredients in the studies. The invention and discovery of numerical algorithms, including (but not limited to) monotone methods, implicit large-eddy simulation, and adaptive mesh refinement, enabled these simulations certainly as much as the increase in computer speed and memory. Unresolved issues that arose in these studies include the nonequilibrium, non-Kolmogorov properties of the turbulence and turbulent fluctuations in these flows, how these prepare the system for transitions, and how to represent the chemical reactions and energy release in the high temperatures and pressures that are near and might signal a transition.

Deflagration-to-Detonation Transition Induced by Hot Jets in a Supersonic Premixed Airstream

Detonation is initiated through a hot jet in a supersonic premixed mixture of H2 and air, which is produced by using a air heater. The results show that initiation fails in the low-equivalence-ratio premixed gas. With the increase of equivalence ratio, the hot jet can induce deflagration to detonation transition (DDT) in the premixed mixture, which an indirect initiation of detonation. Further studies show that the DDT process is due to the combined effect of a local hemispherical explosion shock wave, the bow shock, and the flame produced by the hot jet.

Effects of high activation energies on acoustic timescale detonation initiation

Acoustic timescale Deflagration-to-Detonation Transition (DDT) has been shown to occur through the generation of compression waves emitted by a hot spot or reaction centre where the pressure and temperature increase with little diminution of density. In order to compensate for the multi-scale nature of the physico-chemical processes, previous numerical simulations in this area have been limited to relatively small activation energies. In this work, a computational study investigates the effect of increased activation energy on the time required to form a detonation wave and the change in behaviour of each hot spot as the activation energy is increased. The simulations use a localised spatially distributed thermal power deposition of limited duration into a finite volume of reactive gas to facilitate DDT. The Adaptive Wavelet-Collocation Method is used to solve efficiently the 1-D reactive Euler equations with one-step Arrhenius kinetics. The DDT process as described in previous work is characterised by the formation of hot spots during an initial transient period, explosion of the hot spots and creation of an accelerating reaction front that reaches the lead shock and forms an overdriven detonation wave. Current results indicate that as the activation energy is raised the chemical heat release becomes more temporally distributed. Hot spots that produce an accelerating reaction front with low activation energies change behaviour with increased activation energy so that no accelerating reaction front is created. An acoustic timescale ratio is defined that characterises the change in behaviour of each hot spot.

COMPUTATIONAL STUDY OF DEFLAGRATION TO DETONATION TRANSITION IN A STRAIGHT DUCT: EFFECT OF ENERGY RELEASE

Numerical simulation based on the Euler equation and one-step reaction model is carried out to investigate the process of deflagration to detonation transition (DDT) occurring in a straight duct. The numerical method used includes a high resolution fifth-order weighted essentially nonoscillatory scheme for spatial discretization, coupled with a third order total variation diminishing Runge-Kutta time stepping method. In particular, effect of energy release on the DDT process is studied. The model parameters used are the heat release at q =50, 30, 25, 20, 15, 10 and 5, the specific heat ratio at 1.2, and the activation temperature at Ti =15, respectively. For all the cases, the initial energy in the spark is about the same compared to the detonation energy at the Chapman-Jouguet (CJ) state. It is found from the simulation that the DDT occurrence strongly depends on the magnitude of the energy release. The run-up distance of DDT occurrence decreases with the increase of the energy release for q =50~20, and increases with the increase of the energy release for q =50~20. It is concluded from the simulations that the interaction of the shock wave and the flame front is the main reason for leading to DDT.

Deflagration-to-detonation transition in isopropyl nitrate mist/air mixtures

The characteristics and stages of the deflagration-to-detonation transition (DDT) in isopropyl nitrate (IPN) mist/air mixtures are studied and analyzed. A self-sustained detonation wave forms, as is observed from the existence of a transverse wave and a spinning wave structure. The run-up distance of the DDT process and the pitch size of the self-sustained spinning detonation wave in IPN/air mixtures are analyzed. Moreover, a retonation wave forms during the DDT process. Two propagation modes, the high-speed deflagration mode and the self-sustained detonation mode, of the shock-reaction complex (SRC) in IPN mist/air mixtures are found and analyzed. The influence of the mist concentration on the SRC propagation mechanism is studied. The minimum and the optimum IPN mist concentrations for DDT occurrence in IPN mist/air mixtures are determined. The propagation velocity and overpressure of the self-sustained detonation wave in IPN mist/air mixtures are measured and calculated.

Explosions and deflagration-to-detonation transitions in epoxy propane/air mixtures

The explosion and deflagration-to-detonation transition (DDT) in epoxy propane (E.P.) vapor/air mixture clouds under weak ignition conditions has been studied in an experimental tube of diameter 199 mm and length 29.6 m. E.P. vapor clouds were formed by injecting liquid E.P. into the experimental tube and evaporating of the fine E.P. droplets. The dimension and the evaporating process of the E.P. droplet were measured and analyzed. The E.P. vapor/air mixture clouds were ignited by an electric spark with an ignition energy of 40 J. The characteristics and the stages of the DDT process in the E.P. vapor/air mixtures have been studied and analyzed. A self-sustained detonation wave formed, as was evident from the existence of a transverse wave and a cellular structure. Moreover, a retonation wave formed during the DDT process in the E.P. vapor/air mixture. The influence of the E.P. vapor concentration on the DDT process has been studied. The minimum E.P. vapor concentration for the occurrence of the DDT in the E.P. vapor/air mixture has been evaluated and the variation of DDT distance with E.P. vapor concentration has been analyzed.

Experimental study on transmission of an overdriven detonation wave from propane/oxygen to propane/air

Two sets of experiments were performed to achieve a strong overdriven state in a weaker mixture by propagating an overdriven detonation wave via a deflagration-to-detonation transition (DDT) process. First, preliminary experiments with a propane/oxygen mixture were used to evaluate the attenuation of the overdriven detonation wave in the DDT process. Next, experiments were performed wherein a propane/oxygen mixture was separated from a propane/air mixture by a thin diaphragm to observe the transmission of an overdriven detonation wave. Based on the characteristic relations, a simple wave intersection model was used to calculate the state of the transmitted detonation wave. The results showed that a rarefaction effect must be included to ensure that there is no overestimate of the post-transmission wave properties when the incident detonation wave is overdriven. The strength of the incident overdriven detonation wave plays an important role in the wave transmission process. The experimental results showed that a transmitted overdriven detonation wave occurs instantaneously with a strong incident overdriven detonation wave. The near-CJ state of the incident wave leads to a transmitted shock wave, and then the transition to the overdriven detonation wave occurs downstream. The attenuation process for the overdriven detonation wave decaying to a near-CJ state occurs in all tests. After the attenuation process, an unstable detonation wave was observed in most tests. This may be attributed to the increase in the cell width in the attenuation process that exceeds the detonability cell width limit.