Stoichiometric Hydrogen-air Mixture Research Articles

Hot surface ignition of stoichiometric hydrogen-air mixtures

Hot surface ignition is relevant in the context of industrial safety. In the present work, two-dimensional simulations with detailed chemistry, and study of the reaction pathways of the buoyancy-driven flow and ignition of a stoichiometric hydrogen-air mixture by a rapidly heated surface (glowplug) are reported. Experimentally, ignition is observed to occur regularly at the top of the glowplug; numerical results for hydrogen-air reproduce this trend, and shed light on this behavior. The simulations show the importance of flow separation in creating zones where convective losses are minimized and heat diffusion is maximized, resulting in the critical conditions for ignition to take place.

Numerical study of oblique detonation wave initiation in a stoichiometric hydrogen-air mixture

Two-dimensional, oblique detonations induced by a wedge are simulated using the reactive Euler equations with a detailed chemical reaction model. The focus of this study is on the oblique shock-to-detonation transition in a stoichiometric hydrogen-air mixture. A combustible, gas mixture at low pressure and high temperature, corresponding to the realistic, inflow conditions applied in oblique detonation wave engines, is presented in this study. At practical flight conditions, the present numerical results illustrate that oblique detonation initiation is achieved through a smooth transition from a curved shock, which differs from the abrupt transition depicted in the previous studies. The formation mechanism of this smooth transition is discussed and a quantitative analysis is carried out by defining a characteristic length for the initiation process. The dependence of the initiation length on different parameters including the wedge angle, flight Mach number, and inflow Mach number is discussed. Despite the hypothetical nature of the simulation configuration, the present numerical study uses parameters we deem relevant to practical conditions and provides important observations for which future investigations can benefit from in reaching toward a rigorous theory of the formation and self-sustenance of oblique detonation waves.

Numerical Investigation of Spontaneous Formation of Multiple Detonation Wave Fronts in Rotating Detonation Engine

This article presents a three-dimensional (3D) numerical study on rotating detonation engine (RDE), using the premixed stoichiometric hydrogen-air mixture. Different from a conventional numerical setting, intervals are set in the gas intake configuration to bring in effects of nonuniform injection in numerical simulation, which is more semblable to the actual condition in experiments. The simulation successfully shows the spontaneous formation of multiple detonation wave fronts. Discussion is carried out about the factors that affect the phenomenon of multiple detonation wave fronts, which may be helpful to enhance the stability and repeatability of the detonation initiation in RDE experiments. Then subsequently, the simulation result of a typical case is thoroughly described. Three stages of the spontaneous formation of multiple detonation wave fronts after initiation are analyzed. This typical case is also compared to both the conventional simulation and the experimental results to verify its effectiveness.

LES of flame acceleration and DDT in hydrogen–air mixture using artificially thickened flame approach and detailed chemical kinetics

A large eddy simulation is performed to study the deflagration to detonation transition phenomenon in an obstructed channel containing premixed stoichiometric hydrogen–air mixture. Two-dimensional filtered reactive Navier–Stokes equations are solved utilizing the artificially thickened flame approach (ATF) for modeling sub-grid scale combustion. To include the effect of induction time, a 27-step detailed mechanism is utilized along with an in situ adaptive tabulation (ISAT) method to reduce the computational cost due to the detailed chemistry. The results show that in the slow flame propagation regime, the flame–vortex interaction and the resulting flame folding and wrinkling are the main mechanisms for the increase of the flame surface and consequently acceleration of the flame. Furthermore, at high speed, the major mechanisms responsible for flame propagation are repeated reflected shock–flame interactions and the resulting baroclinic vorticity. These interactions intensify the rate of heat release and maintain the turbulence and flame speed at high level. During the flame acceleration, it is seen that the turbulent flame enters the ‘thickened reaction zones’ regime. Therefore, it is necessary to utilize the chemistry based combustion model with detailed chemical kinetics to properly capture the salient features of the fast deflagration propagation.

Stabilization of detonation combustion of a high-velocity combustible gas mixture flow in a plane channel

Several kinetic models of hydrogen oxidization are compared for the purpose of selecting the reaction mechanism to describe the chemical interaction in numerically modeling the detonation combustion of a hydrogen-airmixture. Within the framework of the chosen kinetic model the possibility of stabilizing a detonation wave in a stoichiometric hydrogen-airmixture arriving in a plane channel at a supersonic velocity is discussed. For certain inflow Mach numbers a method for determining the shape of the channel, in which a stabilized detonation wave can be formed without energy supply, is proposed. In the case of the M0 = 5.5 combustible mixture flow past a semi-infinite symmetric plane body aligned with the flow the structure of the detached detonation wave stabilized ahead of the obstacle is studied.

Measurement of Ignition Limits and Induction Times of Hydrogen–Air Mixtures Behind the Incident Shock Wave Front at Low Temperatures

We have made an experimental study of the ignition of a stoichiometric hydrogen–air mixture behind the incident shock wave front in a shock tube. The ignition limits and the induction times of the mixture behind the wave front in the range of temperatures T =550–960 K and pressures P =0.01–1.1 MPa have been determined. Much lower values of the ignition temperature compared to the experimental results obtained in reflected shock waves, as well as to the classical Lewis and Elbe, have been registered. The measured induction times in the above temperature and pressure ranges are in good agreement with theoretical estimates.

Detonation combustion of hydrogen in a convergent-divergent nozzle with a central coaxial cylinder

The feasibility of steady detonation combustion of a hydrogen-air mixture entering at a supersonic velocity in an axisymmetric convergent-divergent nozzle with a central coaxial cylinder is considered. The problem of the nozzle starting and the initiation of detonation combustion is numerically solved with account for the interaction of the outflowing gas with the external supersonic flow. The modeling is based on the gasdynamic Euler equations for an axisymmetric flow. The calculations are carried out using the Godunov scheme on a fine fixed grid which allows one to study in detail the interaction of an oblique shock wave formed in the diffuser with the nozzle axis. It is shown that a central coaxial cylinder ensures the starting with the formation of supersonic flow throughout the entire nozzle and stable detonation combustion of a stoichiometric hydrogen-air mixture in the divergent section of the nozzle.

Optimizing the design of a hydrogen engine with pilot diesel fuel ignition

A diesel engine was converted to dual–fuel hydrogen operation, ignition being started by a 'pilot' quantity of diesel fuel but with 65 to 90% of the energy being supplied as hydrogen. With later injection timing, use of delayed port admission of the gas, and a modified combustion chamber, thermal efficiencies were achieved nearly 15% greater than those for diesel as the sole fuel. A 'solid' water injection technique was used to curb knock under full load conditions when the power output equalled or exceeded that of a similar diesel engine. The indicator diagrams under these conditions closely approach those of the Otto cycle. The development was assisted by computer simulation using a novel self–ignition and flame propagation model. The very fast burning rates obtained with stoichiometric hydrogen–air mixtures show combustion to occur within 5 degrees of crank rotation yet Otto cycle thermal efficiency was not achieved. However, greenhouse gases are shown to be reduced by more than 80%, nitrogen oxides by up to 70%, and exhaust smoke by nearly 80%.

CFD modeling of hydrogen deflagration in a tunnel

In this paper CFD modeling techniques are used to simulate deflagration in homogenous, near stoichiometric hydrogen–air mixture in a model of a tunnel. The tunnel is 78.5 m long. Hydrogen–air mixture is located in a 10 m long region in the middle of the tunnel. Two cases are studied: one with a complete empty tunnel and one with the presence of four vehicles near the center of the tunnel. The combustion model is based on the turbulent flame speed concept. The turbulent flame speed is a modification of Yakhot's equation, in order to account for additional physical mechanisms. A sensitivity analysis for the ψ parameter of the combustion model and for the mesh resolution was made. The agreement between experimental and computational results concerning the value of the maximum pressure, and the time it appears, was satisfactory in both empty and non-empty tunnel case. The sensitivity analysis for the parameter of the combustion model showed that even small changes in it can have impact on the simulating results, whereas the sensitivity analysis of the mesh resolution did not reveal any significant differences. Finally, the effect of the turbulence model is examined (LES and RANS type of model). The only significant difference in the results between LES and RANS model was the arrival time of the pressure peak. A delay in the arrival time in the case of the RANS model was observed.

Critical radius for hot-jet ignition of hydrogen–air mixtures

This study addresses deflagration initiation of lean and stoichiometric hydrogen–air mixtures by the sudden discharge of a hot jet of their adiabatic combustion products. The objective is to compute the minimum jet radius required for ignition, a relevant quantity of interest for safety and technological applications. For sufficiently small discharge velocities, the numerical solution of the problem requires integration of the axisymmetric Navier–Stokes equations for chemically reacting ideal-gas mixtures, supplemented by standard descriptions of the molecular transport terms and a suitably reduced chemical-kinetic mechanism for the chemistry description. The computations provide the variation of the critical radius for hot-jet ignition with both the jet velocity and the equivalence ratio of the mixture, giving values that vary between a few tens microns to a few hundred microns in the range of conditions explored. For a given equivalence ratio, the critical radius is found to increase with increasing injection velocities, although the increase is only moderately large. On the other hand, for a given injection velocity, the smallest critical radius is found at stoichiometric conditions.

Investigation of certain techniques for stabilizing detonationwaves in a supersonic flow

The salient features of detonation wave propagation in a supersonic flow of a stoichiometric hydrogen-air mixture in plane channels of both constant and variable cross-section are numerically investigated for the purpose of determining the conditions ensuring stabilized detonation. The propagation of a detonation wave formed in a variable-cross-section channel is studied. For different inflow Mach numbers the geometric parameters of a channel providing the detonation combustion stabilization are determined. An investigation of detonation wave stabilization in a supersonic flow of a combustible gas mixture in a plane channel with parallel walls using additional weak discharges is continued. The effects of the flow Mach number, the additional discharge energy, and the discharge location on detonation wave stabilization are studied.

Regimes of chemical reaction waves initiated by nonuniform initial conditions for detailed chemical reaction models

Regimes of chemical reaction wave propagation initiated by initial temperature nonuniformity in gaseous mixtures, whose chemistry is governed by chain-branching kinetics, are studied using a multispecies transport model and a detailed chemical model. Possible regimes of reaction wave propagation are identified for stoichiometric hydrogen-oxygen and hydrogen-air mixtures in a wide range of initial pressures and temperature levels, depending on the initial non-uniformity steepness. The limits of the regimes of reaction wave propagation depend upon the values of the spontaneous wave speed and the characteristic velocities of the problem. It is shown that one-step kinetics cannot reproduce either quantitative neither qualitative features of the ignition process in real gaseous mixtures because the difference between the induction time and the time when the exothermic reaction begins significantly affects the ignition, evolution, and coupling of the spontaneous reaction wave and the pressure wave, especially at lower temperatures. We show that all the regimes initiated by the temperature gradient occur for much shallower temperature gradients than predicted by a one-step model. The difference is very large for lower initial pressures and for slowly reacting mixtures. In this way the paper provides an answer to questions, important in practice, about the ignition energy, its distribution, and the scale of the initial nonuniformity required for ignition in one or another regime of combustion wave propagation.

Effect of inert and reactive admixtures on the initiation and propagation of laminar spherical flames

The initial stage of the propagation of discharge-initiated laminar spherical flame in stoichiometric natural gas- and isobutylene-oxygen mixtures containing krypton or carbon dioxide and in the hydrogen-air mixture at atmospheric pressure in a constant-volume bomb has been investigated by high-speed color cinematography. Dilution of the combustible mixtures with the admixtures increases the steady-state flame front formation time by a factor larger than 10. The phlegmatizing effect of carbon dioxide on hydrocarbon combustion is stronger than that of krypton. For given combustible mixtures, the ratio of steady-state flame front formation times is inversely proportional to the ratio of the temperature rise values for the flames. The introduction of a minor amount of a reactive admixture (1.2% isobutylene) into the stoichiometric hydrogen-air mixture greatly increases the steady-state flame front formation time.

Hydroxyl Radical Kinetics in Repetitively Pulsed Hydrogen–Air Nanosecond Plasmas

Absolute hydroxyl radical (OH) concentration is determined in stoichiometric hydrogen-air mixtures at P = 54-94 torr and initial temperature of T = 100°C-200°C, which are both functions of time, after the application of a single approximately 25-ns-duration approximately 20-kV discharge pulse and 60 μs after the final pulse of a variable-length burst of pulses, using single-photon laser-induced fluorescence (LIF). Relative LIF signal levels are put on an absolute number density scale by means of calibration with a standard atmospheric-pressure near-adiabatic Hencken flat-flame burner. By obtaining OH LIF data in both the plasma and the flame and correcting for differences in the collisional quenching and vibrational energy transfer rates, absolute OH number density has been determined. For a single discharge pulse, the absolute OH temporal profile is found to rise rapidly during the initial ~0.1 ms after discharge initiation and decay relatively slowly, with a characteristic time scale of ~1 ms. In repetitive burst mode, the absolute OH number density is observed to rise rapidly during the first approximately ten pulses (0.25 ms) and then level off to a near steady-state plateau. In all cases, a large secondary rise in OH number density is also observed, which is clearly indicative of ignition, with ignition time ranging from 5 to 10 ms, for initial temperatures of 100 °C and 200°C and pressures in the range of 54-94 torr. Plasma kinetic modeling predictions capture this trend quantitatively, using both a full 22-hydrogen-air-chemical-reaction set and a reduced 9-reaction set.

Effect of flow-geometry on turbulence-scalar interaction in premixed flames

Turbulent combustion of stoichiometric hydrogen-air mixture is simulated using direct numerical simulation methodology, employing complex chemical kinetics. Two flame configurations, freely propagating and V-flames stabilized behind a hot rod, are simulated. The results are analyzed to study the influence of flame configuration on the turbulence-scalar interaction, which is critical for the scalar gradient generation processes. The result suggests that this interaction process is not influenced by the flame configuration and the flame normal is found to align with the most extensive strain in the region of intense heat release. The combustion in the rod stabilized flame is found to be flamelet like in an average sense and the growth of flame-brush thickness with the downstream distance is represented well by Taylor theory of turbulent diffusion, when the flame-brushes are non-interacting. The thickness is observed to saturate when the flame-brushes interact, which is found to occur in the simulated rod stabilized flame with Taylor micro-scale Reynolds number of 97.

Effects of wall thermal conductivity on entropy generation and exergy losses in a H 2-air premixed flame microcombustor

Microcombustion is a promising method for fulfilling the energy requirements of small-scale systems currently powered by portable batteries. However, its applications rely upon mitigation of heat losses, which adversely affect flame stability and performance. Heat losses in turn depend upon wall properties, especially thermal conductivity. It is thus necessary to systematically investigate the relationship between wall thermal conductivity and microcombustor performance using the exergy analysis. In this work, entropy generation rates of different irreversible processes in an annular microcombustor were computed for stoichiometric hydrogen–air mixture using CFD simulations of reactive flow for wall thermal conductivities in the range 0.1–325 W/m K. Chemical reaction, heat conduction, and mass diffusion were the dominant contributors to entropy generation, in the decreasing order. Irreversibilities due to combustion decreased as thermal conductivities increased. Diffusion contributions were most sensitive to the changes in thermal conductivity but chemical reaction and heat conduction contributions changed marginally. Results showed that walls did not contribute significantly to entropy generation, but increased wall heat losses at higher thermal conductivities adversely affected the exergetic performance of microcombustor through availability losses and by influencing the flow gradients. Based on the results of this study, wall thermal conductivity in the range 0.1–1.75 W/m K was found suitable in order to obtain uniform wall temperature profiles and high exergetic efficiencies.

Flame stability studies in a hydrogen–air premixed flame annular microcombustor

Flame stability in an annular heat recirculating microcombustor burning stoichiometric hydrogen–air mixture was explored by means of a rigorous thermal analysis. The analysis is based on computational fluid dynamics model of reacting fluid flow accounting for interactions in flow, species, and conjugate thermal field in fluid and solid. Consideration of thermal diffusion effects in the model was necessary for realistic predictions in all the cases. Flame stability under different inlet velocity and wall thermal conductivities was studied. Results showed that a stable flame could stabilize in this combustor in the velocity range of 3–35 m/s. However, the upper stability limit widened for lower wall thermal conductivity. Low velocity flashback and high velocity blowout bounded the stability region with respect to inlet velocity for lower thermal conductivity wall material. Lower flame stability limit was influenced by thermal design of the microcombustor that prevented flame extinction and ability of flame to stabilize at the heated wall even at higher inlet velocity controlled the upper flame stability limit. Flame established well within the combustor for the lowest wall thermal conductivity without blowout and approached flashback for the highest conductivity when wall thermal conductivity was varied at constant inlet velocity. Relative importance of axial and radial wall heat conduction in flame stabilization was explored at the extremes of operating conditions. Both the components played equally important roles in flame stabilization by influencing heat recirculation and losses within the microcombustor. A suitable combination of structural materials could provide a stable flame with high surface temperatures in a lightweight system.

Two‐dimensional Numerical Simulation on the MHD Flow Behavior in a Pulse‐Detonation‐Driven MHD Electrical Power Generator

AbstractThe magnetohydrodynamic (MHD) flow behavior in a pulse‐detonation‐driven Faraday‐type linear MHD electrical power generator with a converging–diverging nozzle connected to a linear pulse‐detonation tube has been examined by time‐dependent two‐dimensional numerical simulations. Cesium‐seeded stoichiometric hydrogen‐air mixture as the working gas is detonated under atmospheric pressure in the generator. The two‐dimensional temporal evolutions of the propagating detonation wave and the subsequent supersonic flow in the generator are successfully revealed, which result in two peaks of the power output against time. The first peak, provided by the propagation of the detonation wave, hardly deteriorates even in the continuous electrode configuration because of a low Hall parameter due to the high pressure behind the detonation wave front. Such a robust detonation wave, however, can be moderated under a strong MHD interaction. In the supersonic flow following the propagation of the detonation wave, the Hall parameter can be high enough to affect the performance. Thus, if continuous electrodes are used, the second peak shrinks owing to Hall effect, as is well known in the conventional MHD power generator. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

An intercomparison exercise on the capabilities of CFD models to reproduce a large-scale hydrogen deflagration in open atmosphere

Within the HySafe Network of Excellence, several organizations with experience in numerical combustion modeling participated to Standard Benchmark Exercise Problem 2 (SBEP-V2), trying to reproduce numerically the explosion of a stoichiometric hydrogen-air mixture in a 10 m radius balloon. Different codes and models have been applied in the validation exercise. The simulation results and experimental data for the flame speed, the maximum pressures, the rate of pressure rise and the maximum impulse are discussed and compared by means of statistical analysis. An overall satisfactory agreement for the flame speed and maximum pressure is found.

Applicability of Zel’dovich’s theory of chain propagation of flames to combustion of hydrogen-oxygen mixtures

The applicability of Zel’dovich’s theory to the propagation velocity of flames with chain-branching reactions was tested by calculations based on a detailed mechanism and using Zel’dovich’s approximate formulas with a refined mechanism of hydrogen atom recombination for stoichiometric hydrogen-oxygen and hydrogen-air mixtures at pressures of 47.5–760 torr. Good agreement of the results was obtained. Zel’dovich’s approximate theory for a flame with chain-branching reactions dominated by recombination processes gives a reasonable physical picture and provides approximate formulas for flame propagation velocity.