Fast Deflagration Research Articles

A grain-scale modeling on the surface burning and agglomeration of the metalized solid propellants

This study delves into the dynamic behavior of reactive grains within metalized propellants during surface deflagration. Our grain-scale simulations account for both fast and slow deflagration processes involving two distinct metal fuels, namely zirconium and magnesium, with potassium perchlorate (KClO4) as the common oxidizer. Our primary goal is to develop a model and numerical technique capable of replicating the surface reactions observed in experiments. To achieve this, we utilize the compressible Navier–Stokes equations, incorporating the Arrhenius rate law and Johnson-Cook stress model. We pay particular attention to the melt layer, where surface burning occurs in an open atmosphere, and we develop a mechanism to describe the interaction between reactive grains and exhaust gases. We differentiate between two limiting scenarios: convective burning, where unreacted particles experience high-speed compression waves leading to reduced thermal diffusion within the cold reactant, and diffusive burning, characterized by insufficient pressure from particles undergoing both reaction and deformation to erode the burning surface. This results in pronounced agglomeration and the formation of gaseous metal oxides above the melt layer. The results shed light on critical factors influencing surface combustion, including the thermal softening of reactive particles, the time required for particle agglomerations, and tangential forces at multi-element interfaces within the grain-scale simulation domain.

Eulerian–Lagrangian modeling of deflagration to detonation transition in n-decane/oxygen/nitrogen mixtures

In this work, to promote deflagration to detonation transition (DDT), a designed hot jet in a pre-detonator is produced to initiate detonations in the main detonation tube. We perform two-dimensional simulations of the DDT process for low-volatile fuel (n-decane) mixed with nitrogen and oxygen based on the Eulerian–Lagrangian approach. The effects of fuel atomization, vaporization, and shock focusing on the flame acceleration and DDT are discussed under different nitrogen dilution ratio and droplet size conditions. The results show that the flame acceleration process can be divided into slow and fast deflagration stages. Additionally, initiation times are mainly determined by the fuel atomization and evaporation in the slow deflagration stage, which dominates the entire DDT time. Furthermore, there are different intensities of hot jets rather than stable detonation waves formed at the pre-detonator exit. Moreover, local decoupling and re-initiation events are detected near the internal wall of the U-bend, inducing the overdriven detonation decaying into stable detonation waves in the smooth tube. The results also demonstrate that the detonation pressure and velocity decrease by 13.56% and 12.55%, respectively, as the nitrogen dilution ratio increases from 0.5 to 2. In particular, as the nitrogen dilution ratio continued to increase to 2.25, the development in DDT is similar, but the jet intensity is significantly weakened. While as the droplet size increases from 10 to 40 μm, the detonation pressure and velocity decrease only by 2.69% and 1.49%, respectively.

Numerical Simulation of the Influence of Hydrogen Concentration on Detonation Diffraction Mechanism

In this study, the impact of hydrogen concentration on deflagration to detonation transition (DDT) and detonation diffraction mechanisms was investigated. The combustion chamber was an ENACCEF facility, with nine obstacles at a blockage ratio of 0.63 and three mixtures with hydrogen concentrations of 13%, 20%, and 30%. Detonation diffraction mechanisms were numerically investigated by a density-based solver of OpenFOAM CFD toolbox named ddtFoam. In this simulation, for the low Mach numbers, the pddtFoam solver was applied, and for high speeds, the pddtFoam solver switched to the ddtFoam solver to simulate flame propagation without resolving all microscopic details in the flow in the CFD grid, and to provide a basis for simulating flame acceleration (FA) and the onset of detonation in large three-dimensional geometries. The results showed that, for the lean H2–air mixture with 13% hydrogen concentration, intense interaction between propagating flame and turbulent flow led to a rapid transition from slow to fast deflagration. However, the onset of detonation did not occur inside the tube. For the H2–air mixture with 20% hydrogen concentration, the detonation initiation appeared in the acceleration tube. It was also found that following the diffraction of detonation, the collision of transverse waves with the wall of the tube and the reflection of transverse waves were the most essential and effective parameters in the re-initiation of the detonation. For the H2–air mixture with 30% hydrogen concentration, the detonation initiation occurred while passing through the obstacles. Subsequently, at detonation diffraction, the direct initiation mechanism was observed.

Flame acceleration and transition to detonation in a pre-/main-chamber combustion system

Numerical simulations are performed to study the mechanism of deflagration to detonation transition (DDT) in a pre-/main-chamber combustion system with a stoichiometric ethylene–oxygen mixture. A Godunov algorithm, fifth-order in space, and third-order in time, is used to solve the fully compressible Navier–Stokes equations on a dynamically adapting mesh. A single-step, calibrated chemical diffusive model described by Arrhenius kinetics is used for energy release and conservation between the fuel and the product. The two-dimensional simulation shows that a laminar flame grows in the pre-chamber and then develops into a jet flame as it passes through the orifice. A strong shock forms immediately ahead of the flame, reflecting off the walls and interacting with the flame front. The shock–flame interactions are crucial for the development of flame instabilities, which trigger the subsequent flame development. The DDT arises due to a shock-focusing mechanism, where multiple shocks collide at the flame front. A chemical explosive mode analysis (CEMA) criterion is developed to study the DDT ignition mode. Preliminary one-dimensional computations for a laminar propagating flame, a fast flame deflagration, and a Chapman–Jouguet detonation are conducted to demonstrate the validity of CEMA on the chemical-diffusive model, as well as to determine the proper conditioning value for CEMA diagnostic. The two-dimensional analysis with CEMA indicates that the DDT initiated by the shock-focusing mechanism can form a strong thermal expansion region at the flame front that features large positive eigenvalues for the chemical explosive mode and dominance of the local autoignition mode. Thus, the CEMA criterion proposed in this study provides a robust diagnostic for identifying autoignition-supported DDT, of which the emergence of excessive local autoignition mode is found to be a precursor. The effect of grid size, initial temperature, and orifice size are then evaluated, and results show that although the close-chamber DDT is highly stochastic, the detonation initiation mechanism remains robust.

2D Combustion Modeling of Cell Venting Gas in a Lithium-Ion Battery Pack

With the rapid development of lithium-ion battery technology, powertrain electrification has been widely applied in vehicles. However, if thermal runaway occurs in a lithium-ion battery pack, the venting gas in the cells will spread and burn rapidly, which poses a great threat to safety. In this study, a 2D CFD simulation of the combustion characteristics of cell venting gas in a lithium-ion battery pack is performed, and the possibility of detonation of the battery pack is explored. First, a numerical model for the premixed combustion of venting gas is established using a two-step combustion mechanism. The combustion characteristics are then simulated in a 2D channel for the stoichiometric combustible mixture, and the variations in the flame velocity and pressure increment in the flow channel are analyzed. Next, the effects of the initial conditions inside the battery pack, including the pressure, temperature, and excess air coefficient, on the flame propagation process and pressure variation are evaluated. The results indicate that the flame velocity increases with the increase in the initial pressure or temperature and that the influence of the initial temperature is more acute. The maximum flame speed is achieved with a slightly rich mixture, about 450 mm·s−1. When the excess air coefficient is around 0.9, the flame propagation changes from a slow deflagration to a fast deflagration, which causes a high risk of explosion for the battery pack.

Code validation and application of hydrogen mitigation by passive autocatalytic recombiner in small modular reactor

Hazard of hydrogen fast deflagration or explosion could highly lead to an over-pressurization of nuclear reactor containment in case of severe accidents. In order to mitigate the hydrogen hazard, Passive Autocatalytic Recombiners (PARs) are wildly used in water-cooled reactor containment via the hydrogen recombination with the oxygen. The Computational Fluid Dynamics (CFD) code GASFLOW-MPI, which provides the hydrogen concentration details for calculating the recombination rate, is appropriate to analyze PARs performance aiming at the prevention or mitigation of hydrogen risk. The validation of the PARs model implemented in GASFLOW-MPI against THAI-HR experiment is comprehensively carried out. The comparisons indicate good agreements between the predictions and the experimental data, which increase the confidence of using GASFLOW-MPI in a full-scale rector containment. Thereafter, the validated code is used to analyze the severe accident in a water-cooled Small Modular Reactor (SMR), specifically with regard to the hydrogen distribution, the risk posed by hydrogen, the mitigation by PARs and the H2 combustion under accidental ignitions. The analysis methodology is improved according to the GASFLOW-MPI features, and the simulation results indicate that: 1. Hydrogen distribution is fairly uniform that reduces the limit for the PARs installation since the hydrogen recombination efficiencies of the installed PARs are similar, 2. PARs (8 units) mitigate the hydrogen risk well in the long-term phase, but a supplementary combustion analysis is needed, since the potential of flame acceleration exists in the intermediate short time period.

Investigation on C2H4-Air combustion mode in a non-premixed rotating detonation combustor

Based on the working characteristics of the rotating detonation combustor, the combustion mode of C2H4-Air under non-premixed injection conditions is experimentally studied in this paper. By changing the equivalence ratio, we observed the acoustic deflagration mode, fast deflagration mode, stable detonation mode, and weak detonation mode in the combustor. The velocity and pressure of the shock wave increase gradually as the equivalence ratio increases from 0.6 to 1.8. The stable detonation region appears near the stoichiometric ratio and the velocity of the detonation wave is relatively stable. When the equivalence ratio of the mixture is larger than 1.32, the stable detonation wave will suddenly extinguish, forming a weak detonation mode until the end of the combustor operation. The combustion mode of weak detonation is greatly affected by the fuel injection pressure ratio, and the release rate of energy is the main reason for the formation of deflagration mode or detonation mode.

Screening Effects on Electron-Capture Rates and Type Ia Supernova Nucleosynthesis

Nucleosynthesis of iron-group elements in Type Ia supernovae is studied for single-degenerate models with the use of electron-capture rates updated with the new shell-model Hamiltonian in pf -shell. An over-production problem of neutron-rich iron-group isotopes compared with the solar abundances is now found to be suppressed within a factor of about twice for the updated weak rates. Effects of screening on nucleosynthesis are investigated for explosion models of fast deflagration and slow deflagration with delayed detonation. The e-capture rates are reduced by the screening, especially by the screening effects on the ions. The production yields of most neutron-rich isotopes such as 50Ti, 54Cr and 58Fe are found to be suppressed most by the screening. The inclusion of the screening is desirable for precise evaluation of abundances of neutron-rich nuclides.

Submicron Potassium Perchlorate: a Key Component to Reach Detonation in Binary Mixtures with Titanium Hydride

AbstractThe pyrotechnic compositions made up of potassium perchlorate (KClO4) and titanium hydride (TiH2), known as THKP, have a fast deflagration velocity (∼500 m/s), along with low sensitivity and high stability. In this research, a new kind of THKP was formulated from a submicron powder of KClO4 (50–400 nm) prepared by the Spray Flash‐Evaporation (SFE) process. The use of fine KClO4 not only ensures better oxidation of TiH2, but also leads to a transition to detonation in the THKP. This transition is observed in loose powders placed in small diameter tubes (3 mm). The distance of transition is relatively short (17–22 mm) and increases with the KClO4 content of the THKP mixture. The detonation front propagates steadily, at a velocity of ∼1250 m/s in THKP powders with 86 % of porosity. The shockwave velocity varies little with the perchlorate content in the domain of composition studied (55–74 wt.% of KClO4). Conversely, in the classical THKP mixtures prepared from micron‐sized KClO4 and tested in the same conditions, no transition to detonation is observed; the combustion slows down and eventually stops. Finally, owing to their high sensitivity thresholds to impact (SImp.≥44.7 J), friction (SFr.≥192 N), and electrostatic discharge (SESD≥34.7 mJ), THKP mixtures prepared from submicron KClO4 can be classified as low‐sensitivity primary explosives.

Numerical study on ignition and failure mechanisms of hydrogen explosion accident in Fukushima Daiichi Unit 1

This paper studied ignition and failure mechanisms of hydrogen explosion accident in the container building of Fukushima Daiichi Nuclear Power Plant Unit 1, which occurred a day after the Great East Japan Earthquake in 2011. To understand the mechanism of hydrogen explosion, an unsteady three-dimensional numerical analysis of the flow field in the container building was conducted in real time by assuming that the high-temperature hydrogen gas (500 °C) issued from the head flange of the reactor vessel. The numerical results show that the hydrogen gas spread not only on the 5th floor but also on the 4th floor of the container building covering the stairs. It was found that the hydrogen concentration on the 5th and 4th floors was highly uniform owing to the long real-time accumulation (15 h) of hydrogen, and it spread gradually from the 5th to the 4th floor through the stair. As a result, the hydrogen concentration reached the ignition level on the 4th floor (5%), which could result in the ignition of hydrogen by a tank heater on the 4th floor and the flame propagated to the 5th floor, where fast deflagration and detonation might occur because of the high concentration of hydrogen gas (19%). This scenario is consistent with the observations of failure mechanism after the explosion accident.

Experimental Study of the Effect of Confining on the Development of Fire in a Closed Compartment

Backdraft is a complex phenomenon which occurs during cases of confined fires. It appears by a fast deflagration which occurs after the introduction of oxygen into a compartment filled with hot gases rich in unburned combustible vapor. Practically, this situation could occur at the time of intervention of firemen who break the door or when a window breaks under the action of thermal stresses. Based on a strong experimental campaign, the present paper aimed to make a quantitative investigation of the effect of confining on a totally closed fire. With this focus, fire tests were carried out in a completely closed room of dimensions 1.20 m × 1.20 m × 1.02 m, with five sources of fire of different heat release rates. The same fire sources were also tested in a free atmosphere in order to get reference data. After a statistical study of data, a comparative analysis between both results has been done. Its outcome is that confining has a major impact on the quality of combustion and on the fire duration. More precisely, it has been noticed comparatively to fire tests in free atmosphere that confining increases the fire duration by 14.85 percent while it decreases the heat release rate by 21.72 percent.

Effects of inert gas jet on the transition from deflagration to detonation in a stoichiometric methane-oxygen mixture

Detonation is an energetic combustion mode augmenting high flow momentum and thermodynamic efficiency, it has been applied in detonation engines, such as pulse detonation engines (PDEs) and rotating detonation engines (RDEs), they have become potential aerospace propulsion equipment. Recently, fluidic jet-in-cross flow (JICF) has been demonstrated experimentally and numerically that can accelerate the deflagration-to-detonation transition (DDT) process. Nonetheless, most of previous studies focused on the jets using combustible mixture or oxygen, which may bring additional risk for turbulence-generated system in detonation engines. In this study, a more safe and controllable inert gas (i.e., Ar) is applied for JICF, experiments are carried out to investigate effects of argon jet as an enhancement method on promoting the DDT in a stoichiometric methane-oxygen mixture. The effects of local argon concentration, turbulence intensity and injection position on the DDT process are systematically examined. Two-dimensional numerical simulations are also performed to elucidate the details of the injection evolution. The experimental results show that turbulence generated by the argon injection can promote flame acceleration and the onset of detonation only in the fast deflagration regime. The enhancing effect is more prominent at higher turbulence intensity by increasing jet injection pressure and shorter injection time. Too long injection duration increases argon local concentration that leads to an adverse effect prohibiting the DDT occurrence. During the initial laminar flame acceleration, referred to as the slow deflagration regime, no enhancement by the argon jet on DDT can be observed. By looking numerically at the flow structure of the argon jet, the vortical features enhance the transport and mixing between reactants and products. The interaction between the reactive travelling wave and the jet structure further induces turbulence and thus accelerates the chemical reaction rate. With the time elapsed, the injected argon entrains largely and dilutes the ambient combustible mixture, and restrains the DDT. Furthermore, a novel dimensionless criterion and a characteristic parameter Turc are proposed, quantitatively analyzing the dominate mechanism in flame propagation and the initial stage of DDT as inert jet is introduced.

The precursor shock wave and flame propagation enhancement by CO2 injection in a methane-oxygen mixture

Successful initiation of a detonation in a confined combustor with limited length is a formidable task for designing the detonation-based engines, shortening the length for the run-up distance of a detonation greatly reduces the weight of the detonation engine, rendering the detonation propulsion become possible in the advanced aerospace propulsion systems. In this work, we proposed a new method that using a non-reactive gas jet (CO2) to facilitate the formation of the precursor shock wave and the flame propagation speed in a deflagration wave in a stoichiometric methane-oxygen mixture, by changing a series of the injection parameters, i.e., the initial pressure of mixture, the jet delay time, the jet pressure and its position, the effects of those parameters on the deflagration propagation are systematically investigated. The experimental results show that, the development processes of pressure rise are divided into two stages prior to the DDT: 1) slow deflagration stage, which characterized by two discrete shock waves named as the 1st shock wave and 2nd shock wave; 2) Fast deflagration stage, the two discrete waves merge into a strong precursor shock wave, and the velocities of precursor shock and flame achieve to the “quasi-detonation” state. The experimental results also illustrate that, introducing the CO2 jet in the first stage prior to the DDT promotes the formation of precursor shock wave and accelerates the flame speed, which speeds up the transformation of a detonation in a shorter distance. By comparing different jet parameter combinations, it is found that high jet pressure and long jet time increase the local concentration of CO2, which adversely affect the formation of a detonation. Thereby there exists an optimal condition of injection, whose injection pressure ratio is 15, jet delay time is 20 ms and jet position is found to be 0.6 m, those parameters are verified to have prominent detonation-enhancement effects on DDT, the underlying mechanisms for those optimal conditions are explored and explained in this study. This study provided new experimental evidences supporting the theory of turbulence-enhancement effect on the initiation of a detonation.

Investigation of rotating detonation fueled by a methane–hydrogen–carbon dioxide mixture under lean fuel conditions

Rotating detonation combustor (RDC) operation is investigated under the main composition of the coal and supercritical water gasification process (a methane–hydrogen–carbon dioxide mixture), and air is selected as an oxidizer. Experiments are performed under four flow rates (79.1, 101.2, 125.8, and 158.2 g/s), and the equivalence ratio (ER) ranges from 0.23 to 0.87. The detonation delay time and propagation characteristics of the rotating detonation wave (RDW) are investigated, and the nonlinear time series analysis method is applied to the pressure signal using the wavelet entropy algorithm and phase space reconstruction. It is demonstrated that a decrease in the flow rate can increase the detonation delay time near the lean fuel boundary; however, the detonation delay time is affected only slightly when the ER is greater than 0.4. The ER significantly affects the wave velocity but is affected only slightly by the flow rate. Contrary to the wave velocity performance, the pressure of the RDW increases with the flow rate. The lean fuel boundary becomes wider when the flow rate increases. Five combustion modes are investigated: failure, fast deflagration, sawtooth wave, single-wave, and two-counter rotating wave modes. The fast deflagration and sawtooth wave modes may occur near the detonation operating boundary, and a higher flow rate tends to increase the occurrence probability of a multiwave mode. The phase space pattern and distribution of wavelet entropy can be used to distinguish the combustion modes intuitively.

Turbulent deflagrations of mildly flammable refrigerant-air mixtures

With current concerns around global climate change, new hydrofluorocarbons with low Global Warming Potential (GWP) are being evaluated as alternative refrigerants. These alternative refrigerants, however, may be mildly flammable (as defined by the A2L safety group classification) and pose safety concerns for the heating, ventilation, air conditioning, and refrigeration (HVAC/R) industry. Consequently, careful assessments of different flammability characteristics and risks for these refrigerants are essential for their safe use in actual applications. In this study, deflagration propagation measurements for different mildly flammable refrigerants, including difluoromethane (R-32) and 2,3,3,3-tetrafluoropropene (R-1234yf), were undertaken in different geometries including a 9.1-m long conduit test rig and a closed cubical 12.5 m3 volume. Different tests were conducted for full volume deflagrations as well as with and without obstructions. Turbulent deflagration speeds for well-mixed, refrigerant-air mixtures have been shown to be orders of magnitude larger than their corresponding laminar flame speed values that are used in classifying flammable refrigerants in safety standards. Testing has also quantified the resulting severity as measured by the event overpressure which was shown to worsen with increased congestion or confinement as a consequence of increased induced turbulence. This work illustrates the importance for severity evaluations for actual large-scale or congested geometries of concern in practical applications. Even for mildly flammable refrigerants characterized by laminar flame speeds <2 cm/s, which is lower than the 10 cm/s limit for A2L refrigerants, relatively fast deflagrations can be generated for very congested geometries where downstream turbulence is generated as the flame front passes over obstacles in these situations.

Fluid structure interactions modelling in vented lean deflagrations

The standard 20 ft ISO containers are studied both experimentally and numerically with model obstacles to ascertain the peak overpressures generated in case of an accidental fast deflagrations. Apart from overpressure its often important to know the maximum deflections of the enclosures to examine the structural integrity. The container walls are not rigid; they not only contribute through acoustic response to the pressure waves but also through structural resonance response to the generated overpressures, to capture these effects its necessary to do the Fluid Structure Interaction (FSI) or in simple terms the coupled Computational Fluid Dynamics (CFD) and Finite element analysis (FE) simulations. Alternatively, the structural response can be coupled to CFD simulation in either one-way or two-way interactions to reduce the associated computation efforts. Numerical simulations have been conducted with a pseudo two-way coupled CFD & FSI, to aid our understanding of the combination of various factor contributing for the generation of overpressures using an in-house solver developed based on open source CFD code OpenFOAM named as HyFOAM. The CFD solver solves the compressible Navier-stokes equations along with the spring-mass-damper system's single degree of freedom motion equation for coupled fluid structure interactions. The turbulent flame deflagrations are modelled using the flame area combustion model, the necessary modification are incorporated to governing equations to include the dominant flame instabilities present during the vented deflagration process. For lean hydrogen mixtures the Lewis number effects are important hence suitable modelling improvements are added to the combustion model. The numerical results are validated against the recent experiments conducted at Gexcon, Norway as part of the HySEA project supported by the Fuel Cells and Hydrogen 2 Joint Undertaking (FCHJU) under the Horizon 2020 Framework Programme for Research and Innovation. Numerical predictions of CFD and FSI coupled, are assessed against the experimental results to study the contributing factors affecting the generated overpressure with in the ISO containers in view to overall improve the numerical predictions.

Thermoacoustic Instabilities in an Annular Rotating Detonation Combustor Under Off-Design Condition

Experiments are performed to investigate the combustion features in a laboratory-scale hydrogen ()/air annular rotating detonation combustor under the off-design condition of low air mass flow rates. The air mass flow rate is ranged from 10 to 115 g/s, and the overall equivalence ratio is ranged from 0.6 to 1.0. The unstable longitudinal pulsed deflagration (ULPD), rotating fast deflagration (RFD), and stable longitudinal pulsed deflagration (SLPD) are found to exist in the combustor according to the analysis of pressure traces obtained at different circumferential and axial positions. The RFD is such a combustion instability coupled with the first tangential acoustic mode, but it occurs very rarely. The propagation speed of the RFD waves is approximately 500 m/s, and the pressure wave amplitudes are as low as 0.2 MPa. The SLPD appears with stronger and periodic pressure oscillations, and it is coupled with the first longitudinal acoustic mode of the combustor. The ULPD found in the present experiment can be regarded as the longitudinal mode even though the main frequency band is wider. The pressure oscillations are very irregular in the ULPD situation, but they are traveling forward and backward along the axial direction of the combustor. The variation of the air mass flow rate has more influence on the oscillation intensity of the deflagration wave than the variation of the equivalence ratio. It is also found that the pressure oscillations in the air and hydrogen plenums are forced by the unstable combustion process in the annular combustor. The unchoked condition of the injectors contributes the system coupling of the combustor and plenum.

Do not believe the hype: Using case studies and experimental evidence to show why the HSE is wrong about excluding deflagration‐to‐detonation transitions

A recent report by the UK's Health and Safety Executive postulated that severe explosions can propagate at subsonic speeds but generate overpressures of several bar in open areas and this “new” type of explosion is episodic in nature. The UK Health and Safety Executive (HSE) paper based their findings on a specific interpretation of historical data and “empirical evidence” from previous incidents and selected experimental data. They indicate that their results should guide plant design and risk assessment. The report fails to take account of key evidence from recent large‐scale experimental studies and incident investigations in proposing a hypothetical explosion mechanism over those that are known to occur and are well understood. This has the potential to misdirect efforts to manage such events.The first part of the article will review a number of incidents and demonstrate that each of these incidents can indeed be explained by fast deflagrations or transition to detonations, in complete contradiction to the HSE findings. The results found at the incident sites will be directly compared against results from recent DDT work and damage. The second part of the paper will present a novel technique for determining the DDT transition velocity from high‐speed video footage of experiments performed at large scales. © 2018 American Institute of Chemical Engineers Process Saf Prog 38: e11998, 2019

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.

Hydrogen combustion experiments in a vertical semi-confined channel

Experiments in an obstructed semi-confined vertical combustion channel with a height of 6 m (cross-section 0.4 × 0.4 m) inside a safety vessel of the hydrogen test center HYKA at the Karlsruhe Institute of Technology (KIT) are reported. In the work, homogeneous hydrogen-air-mixtures as well as mixtures with different well-defined H2-concentration gradients were ignited either at the top or at the bottom end of the channel. The combustion characteristics were recorded using pressure sensors and sensors for the detection of the flame front that were distributed along the complete channel length. In the tests slow subsonic and fast sonic deflagrations as well as detonations were observed and the conditions for the flame acceleration (FA) to speed of sound and deflagration-to-detonation transition (DDT) are compared with the results of similar experiments performed earlier in a larger semi-confined horizontal channel.