Stoichiometric Hydrogen-air Mixture Research Articles

Flame acceleration and deflagration-to-detonation transition in narrow channels filled with stoichiometric hydrogen-air mixture

Understanding flame acceleration (FA) and deflagration-to-detonation transition (DDT) is important for combustion applications. However, DDT is a high-speed, nonlinear and complex process and the exact underlying mechanisms remain unclear, especially for narrow channels with a width smaller than detonation cell size. In this paper, numerical simulations were carried out to study the FA and DDT in a stoichiometric hydrogen-air mixture in narrow channels with width on the order of magnitude comparable to laminar flame thickness. The fully-compressible reactive Navier-Stokes equations were solved by using a high-order numerical method. Hydrogen-air combustion was taken into account using a calibrated simplified chemical-diffusion model. Four different channel widths, i.e., d = 2.0 mm (∼6 δl), 1.0 mm (∼3 δl), 0.5 mm (∼1.5 δl), 0.25 mm (∼0.7 δl), were considered in the simulations to explore the size effect, where δl is the laminar flame thickness. The results show that the FA is mainly determined by boundary layer effects for all the channels considered. In the early stages, the flame keeps accelerating due to the stretching effect of boundary layer that leads to a significant increase in flame surface area. In the later stages of acceleration, an ultra-fast flame develops in the boundary layer ahead of the flame front due to the viscous heating effect associated with wall friction. It was found that the DDT mechanism is essentially consistent in all the channels. Detonation arises from the contact of two neighboring flame fronts in the shocked region that is vulnerable to a mild temperature gradient or fluctuation. The contact of the two high-temperature reaction fronts instantly causes a very high concentration of heat flux and consequently a detonation wave emerges from the contact spot. The detonation propagation after DDT, however, depends on the channel width. When d ≥ 1.5 δl, a quasi-steady or a single-head detonation forms and sustains until the end of the combustion. For the case of d < 1.5 δl, the detonation decays into a high-speed flame. When the channel width is sufficiently small, e.g., d ∼0.7 δl, detonation initiation can occur occasionally, but soon decouples into a shock and a flame.

Promoting of foam copper with chaotic distributed pores on the deflagration of premixed hydrogen-air flame in the tube

The deflagration flame of stoichiometric hydrogen-air mixtures is studied in this paper. Combined with the foam copper structure, the deflagration characteristic is analyzed in this paper. The experimental results show that the foam copper can improve the flow instability at the flame front and the degree of turbulence in the propagation process, the flame propagates more rapidly in turbulence state, and the number of stages of flame morphology during the propagation process in-creases. The classic ?tulip? flame can be transformed into a distorted ?tulip? flame and a fractal ?tulip? flame before it collapses. When the flame passes through the foam copper, the flame front velocity increases as the number of structural layers increases. The flame front velocity propagates at supersonic speed through the accumulation of three layers of foam copper. The instability of overpressure in the propagation process will cause oscillation. More layers of the structure, the oscillation frequency, and the amplitude of the overpressure are increased significantly. Foam copper structure has a reverse action on over-pressure. When the overpressure value is low in the early stage, the structure promotes the propagation, but the overpressure value is large in the later stage, the structure has a blocking effect.

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

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

Modeling of spontaneous Raman scattering for detonation wave imaging

To determine if spontaneous Raman scattering can be employed to perform temperature measurements in detonation waves propagating in a stoichiometric hydrogen–air mixture, a detailed rotational–vibrational Raman scattering model for nitrogen has been implemented. The Raman scattering cross-sections were calculated over the wavelength range 337–532 nm and show reasonable agreement with experimental data. The Raman model was coupled to steady one-dimensional and unsteady two-dimensional simulations of detonation to determine under which conditions temperature measurement can be performed with an appropriate signal-to-noise ratio. Based on the calculated Raman signal and signal-to-noise ratio fields, it was concluded that, through an integrated intensity ratio of Stokes Raman scattering and anti-Stokes Raman scattering, the Raman scattering temperature measurement might be possible for large incident laser light intensity and minimized background emission noise.

Energy efficiency of detonation combustion in supersonic ramjet engines

The energy efficiency of a supersonic ramjet engine with detonation combustion of a hydrogen-air mixture in an axisymmetric Laval nozzle is considered. First of all, within the framework of the one-dimensional (1D) theory, it is shown that extreme combustion corresponding to the Chapman-Jouguet (C.-J.) deflagration, like constant pressure combustion, surpasses detonation in energy efficiency. However, this advantage becomes insignificant with an increasing the free-flow Mach number. Based on the results of the 1D theory, an axisymmetric Laval nozzle is constructed, which provides steady-state detonation combustion of a stoichiometric hydrogen-air mixture at an altitude of about 40 km and with freestream Mach numbers 9 and 12. The efficiency of detonation combustion in this case is about 35% instead of the maximum 65% according to the 1D theory.

Predictions of flame acceleration, transition to detonation, and detonation propagation using the Chemical-Diffusive Model

This paper presents numerical simulations of the evolution of premixed flames in channels with obstacles using the Chemical-Diffusive Model (CDM) coupled to a compressible Navier-Stokes solver. The CDM is a parametric model for chemical reaction and diffusive transport, and it is calibrated to reproduce a set of properties of one-dimensional laminar flame and the Zel’dovich-Neumann-Döring (ZND) detonation. In this work, we use two different CDMs, one calibrated using the theoretical half-reaction distance of the ZND detonation and the other calibrated with the experimental detonation cell size, to simulate a stoichiometric hydrogen-air mixture and investigate flame acceleration, deflagration-to-detonation transition (DDT), and detonation propagation in obstacle-laden channels. As the channel geometry varies, both CDMs predict propagation of fast flames in the choking, quasi-detonation, and CJ-detonation regimes. When the channel width is much greater or smaller than the minimal scale requirement for the onset of DDT, both CDMs predict similar choked fast flames and CJ detonations. In cases where the channel widths are close to the critical conditions, deflagration waves are more likely to undergo DDT if the CDM is calibrated for the half-reaction distance, which produces smaller computed detonation cells than experiments. In numerical simulations of channels with the same geometries as prior experiments, using the cell size-constrained CDM improves predictions of DDT.

Modelling of a spherical deflagration at constant speed

We build in this paper a numerical solution procedure to compute the flow induced by a spherical flame expanding from a point source at a constant expansion velocity, with an instantaneous chemical reaction. The solution is supposed to be self-similar and the flow is split in three zones: an inner zone composed of burnt gases at rest, an intermediate zone where the solution is regular and the initial atmosphere composed of fresh gases at rest. The intermediate zone is bounded by the reactive shock (inner side) and the so-called precursor shock (outer side), for which Rankine-Hugoniot conditions are written; the solution in this zone is governed by two ordinary differential equations which are solved numerically. We show that, for any admissible precursor shock speed (and, in some cases, for a reaction heat large enough), the construction combining this numerical resolution with the exploitation of jump conditions is unique, and yields decreasing pressure, density and velocity profiles in the intermediate zone. In addition, the reactive shock speed is larger than the velocity on the outer side of the shock, which is consistent with the fact that the difference between these two quantities is the so-called flame velocity, i.e. the (relative) velocity at which the chemical reaction progresses in the fresh gases. Finally, we also observe numerically that the function giving the flame velocity as a function of the precursor shock speed is increasing; this allows to embed the resolution in a Newton-like procedure to compute the flow for a given flame speed (instead of for a given precursor shock speed). The resulting numerical algorithm is applied to stoichiometric hydrogen-air mixtures.

Effect of channel geometry and porous coverage on flame acceleration in hydrogen–air mixture

Flame propagation in a hydrogen–air mixture in the presence of porous materials was investigated experimentally in channels with different dimensions and cross–sections. In this study, experiments were performed in a rectangular channel with one or two walls covered with porous material to study flame propagation in stoichiometric hydrogen–air mixtures at room temperature and atmospheric pressure. Depending on the channel configuration, the porous coating of the internal walls ranged from 1/4 to 1/2 of the channel area. Four types of polyurethane foam with a number of pores per inch (PPI) ranging from 10 to 80 were used to cover the channel walls. Flame propagation was visualized using a Schlieren device and high-speed camera. The largest flame acceleration in the porous channel relative to the solid channel was observed in the 20×20mm channel. The ratio of the velocities in the porous channel to the velocity in the solid channels was 6–7 for the porous material with the largest (2.5mm) pores. In the case of a 10×10mm channel, the flame velocity in the porous channel was higher than the flame velocity in the solid channel after 350mm only when using porous coatings with 2.5mm and 1.3mm pores. When using a porous coating with smaller pores the flame velocity was lower than in the solid channel. Schlieren images show different stages of flame propagation from a turbulent flame to a supersonic flame with shock waves.

Schlieren visualization of the interaction of jet in crossflow and deflagrated flame in hydrogen-air mixture

The objective of this study is to investigate the interaction of a jet-in-crossflow (JICF) and a deflagrated flame in a stoichiometric hydrogen-air mixture. The JICF is introduced prior the arrival of a deflagrated flame, therefore the JICF has a certain time to disturb the flow. By adjusting the time delay before the flame contacting the jet (td) and the pressure ratio (pJ/p0), the interaction between JICF and deflagrated flame and the subsequent flame evolution are recorded by a high-speed Schlieren visualization apparatus. The results show that the deflagrated flame without the perturbation of JICF has a quasi-parabolic shape, which is due to the flame suffering strong shear stress from wall boundaries. At higher initial pressure, the propagation velocity is faster. By introducing JICF, the flame speed is promoted, this is because JICF establishes pressure differential regions in the two sides of the jet, forming pressure gradients and flow instabilities, and increasing the turbulent transport between reactant and product, which accelerates the flame propagation. At relative low and medium pressure ratio (i.e., pJ/p0 = 10 and 20), shorter td has a better performance on enhancing the flame speed, nevertheless, at higher pressure ratio (pJ/p0 = 30), longer td conversely better promotes the deflagrated flame speed. This is because, at the condition of weak pressure ratio and long td, the abruptly burst of N2 JICF results in a prominent effect because of its dilution attribute, which prohibits the chemical reaction and the further development of a deflagrated flame. However, as the jet pressure tends to stronger, the corresponding turbulence is greater; the recirculation effect induced by robust JICF overcomes its dilution effect and gradually plays a more dominant role in the flame acceleration.

Morphology of oblique detonation waves in a stoichiometric hydrogen–air mixture

Abstract

Suppression of hydrogen–air detonation using porous materials in the channels of different cross section

Propagation of a detonation wave in a porous channel with different cross–section was experimentally studied. Experiments were performed in three rectangular channels with cross–sectional dimensions of 20 × 40 mm, 10 × 40 mm and 10 × 30 mm with two opposite walls covered with porous material to study the detonation suppression in stoichiometric hydrogen–air mixtures at atmospheric pressure. Detonation was initiated in 3000 mm long circular channel 20 mm in diameter. Porous material was covering 1/2 or 1/3 of the channel internal surface. Polyurethane foam with a number of pores per inch ranging from 10 to 80 was used for detonation attenuation. Piezoelectric pressure sensors were used to obtain the shock wave pressure. Detonation decay into the shock wave and the flame front was visualized using schlieren photography. Shock wave velocity was also calculated using high–speed schlieren image sequences. The strongest pressure attenuation was recorded in a 10 mm wide channel with a porous coating with largest pores (2.5 mm) covering 1/3 of the internal walls. The results indicate that even covering 1/3 of the internal surface of the channel leads to detonation decay and significant shock wave attenuation.

A numerical investigation of oblique detonation waves in hydrogen-air mixtures at low mach numbers

Oblique detonation waves (ODWs) have potential applications in hypersonic propulsion, but the boundary of steady ODWs has not yet been examined comprehensively. In this study, Euler equations coupled with detailed chemical reaction models are used to simulate ODWs in hydrogen-air mixtures with relatively low flight Mach numbers (from 7 to 8) and different flight altitudes (30 km and 25 km). Decreases in the flight Mach number and altitude are shown to result in unsteady ODWs in a stoichiometric hydrogen-air mixture. These ODWs can be re-stabilized by decreasing the fuel‒air equivalence ratio. Regardless of different parameters of Mach number and altitude, unsteady ODWs appear only when the velocity in the induction zone exceeds that of the corresponding Chapman-Jouguet detonation. A low equivalence ratio also induces a long initiation length, limiting the availability of decreases in the equivalence ratio to maintain a steady ODW in practical applications. A flow‒combustion criterion is proposed for the application of ODWs, based on the steadiness of ODWs and fast initiation.

Двумерное моделирование ослабления детонационной волны при ее прохождении через область с круговыми препятствиями

The results of numerical simulation of the detonation wave interaction with a circular obstacles staggered are presented. The calculations were carried out for a prepared stoichiometric hydrogen-air mixture. The influence of the rows number, the step between them and the scale effects on the detonation suppression was investigated. The modes of detonation suppression and re-initiation were obtained.

Numerical study of detonation flows in a supersonic annular chamber

The presented paper is devoted to the study of detonation structures in a flow-type supersonic chamber of a new design, previously proposed by us. The detonation chamber is an annular cylindrical channel located between the inner and outer cylindrical walls. The oblique detonation wave is formed by a compression body inside the annular channel, shaped as a continuous monofilar helix with a constant pitch angle. Numerical simulations are performed for a supersonic flow of a stoichiometric hydrogen-air mixture with inflow Mach number M0=3 and the pitch angle a<aCJ. A mathematical model of the reacting flow in the combustor is developed in a two-dimensional unsteady formulation. A two-stage model of the detonation kinetics is used to describe the chemical process in reacting gas. The flow evolution for the different conditions for starting the detonation chamber operation is numerically studied. The final structures of the steady flow in the combustor are obtained. The bifurcation effect of a stationary solution according to the initial conditions of the problem is discovered for some combinations of the geometric parameters of the detonation combustor.

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

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

Experimental investigation on the propagation of flow and flame in a confined combustion chamber equipped with a single-hole perforated plate

The effects of the hole size and perforated plate position on the propagation of flame and flow and pressure oscillation were explored in a constant volume combustion chamber (CVCB) with a single-hole perforated plate. Stoichiometric hydrogen-air mixture was used in the experiments, and the propagation of the flame and jet flow were recorded by high-speed schlieren photography. The results demonstrated that the flame velocity firstly increases and then decreases with the increasing hole size. With the hole size of 13 mm, the flame velocity, peak pressure, and pressure oscillation reached the maximum under the current experimental conditions. Moreover, the influence of the position of the perforated plate was investigated. It was found that the jet flow before the flame front is prominently distinct when the perforated plate is in positions A and B, resulting in an apparent difference in the flame shape behind the perforated plate, which means that the movement of the jet flow plays a leading role in the development of the flame. In Position B, the flame front overlapped with the jet flow and has the same shape. Besides, an interesting phenomenon was captured: with perforated plate in Position B, a secondary flame front was generated with higher flame tip velocity before the primary flame front under the effect of the flame-vortex interaction.

Suppression of hydrogen-air explosions by hydrofluorocarbons

This study examines the suppression effectiveness of fluorinated species CHF3 and C2HF5 on the premixed hydrogen-air explosions experimentally and numerically. The results demonstrate that as for stoichiometric and rich hydrogen-air mixtures, both CHF3 and C2HF5 can effectively reduce thermal expansion ratio and increase flame thickness, and then reduce the influence of hydrodynamic instability on the flame acceleration. Laminar burning velocity, the maximum of explosion pressure, the maximum rate of pressure rise and the absolute value of pressure impulse all decrease with increasing suppressant concentration at various equivalent ratios. The maximum of explosion pressures for stoichiometric flames is decreased by 11.54 % with 2 % CHF3 added, and decreased rapidly by 40.39 % with 2 % C2HF5 added. The hydrogen-rich mixtures cannot be ignited when C2HF5 content reaches 10 %. Numerical simulations have confirmed that both CHF3 and C2HF5 can effectively reduce the concentration of active radicals with the decrease order of H > OH > O. C2HF5 is more effective in suppressing explosions than CHF3 by converting more H atoms to HF formation through the elementary reactions CHF2 + H <=> CHF + HF, CF2:O + H <=> CF:O + HF, and CF2 + H = CF + HF.

Flame acceleration and deflagration-to-detonation transition in hydrogen-air mixture in a channel with an array of obstacles of different shapes

Numerical simulations were conducted to study the flame acceleration and deflagration-to-detonation transition (DDT) in a channel with a bank of aligned obstacles. The multidimensional, fully compressible reactive Navier–Stokes equations coupled to a calibrated chemical-diffusion model for stoichiometric hydrogen-air mixture were solved using a high-order numerical algorithm. The results in the case with circular obstacles show that occurrence of DDT arises from shock focusing at flame front. Higher blockage ratio leads to faster flame acceleration and facilitates the formation of choked flame. Heat losses can significantly attenuate flame acceleration and delay detonation initiation in case of DDT, and even hinder occurrence of DDT for high blockage ratio. To understand the effect of shape of obstacle on the flame acceleration and DDT, circular, square, left triangular, and right triangular shapes, always maintaining the same formal blockage ratio, were simulated. The simulations indicate that there are significant differences in flame acceleration and DDT among these differently shaped obstacles, although the basic mechanism for detonation initiation is similar in all of the cases studied and involves interactions of shock wave with flame front and reactivity gradient. Square obstacles produce the largest growth rate of flame surface area in the flame-acceleration stage and thus result in the fastest flame acceleration and shortest detonation onset time compared to circular or triangular obstacles. Circular obstacles produce the weakest flame-surface growth, and thus they have the least effect on promoting flame acceleration and transition to detonation. A closer analysis shows that the sharp angles of the triangular obstacles, when compared to the circular obstacles, are more conducive to flame stretching and convolution and thus facilitate flame acceleration and DDT. The results also show that the effect of obstacle shape does not correlate with obstacle area.

Control of a Detonation Wave in a Channel with Obstacles Using Preliminary Gas Mixture Preparation

Using the detailed kinetic mechanism of chemical interaction, the effect of preliminary preparation of the stoichiometric hydrogen-air mixture (dissociation of a part of molecular hydrogen and oxygen on atomic gases) on the detonation wave characteristics is investigated numerically with the aim to control detonation combustion in plane channels with obstacles. It is found that the above-mentioned partial initial dissociation can be used to prevent quenching of detonation in channels with both a single obstacle and multiple barriers. It is revealed that the mixture, obtained as a result of preparation, differs from the initial hydrogen-air mixture by the qualitatively different type of detonation re-initiation after interaction with obstacles.

Analytical and numerical study of near-field ignition of H2/air by injection of hot gas

Ignition initiation by a turbulent hot jet involves complex transport and chemical processes with disparate and sensitive time scales. Its understanding is important for improved ignition in advanced and novel combustion engines and in ignition avoidance for explosion safety measures. This study is aimed at the modeling of turbulent hot jet ignition of stoichiometric hydrogen-air mixture with fast chemistry and short chemical time scales relative to the large-scale mixing time scales. The evolution of jet mixture fraction in the near-field shear layer of a suddenly-starting turbulent jet is analytically modeled and calibrated by large-eddy simulation of the reacting fluid. When integrated with a correlation for instantaneous chemical induction time, the model estimates the radial location and the timing of ignition as the jet travels in the streamwise direction. The estimation includes accounting for the role of the scalar dissipation rate in the suppression of ignition. The radial location of ignition migrates towards the jet centerline with time, migrating faster for higher injection temperatures and much slower for leaner ambient fuel-air ratios. The limitations of the present model are investigated by assessing the regions of mixing layer where strong diffusive transport collocates with the high production rate of active radicals. To complete the perspective into analytic modeling of transient jet ignition, other limitations associated with turbulent fluctuations and jet composition are discussed, as well as the role of near-field ignition relative to ignition at the jet tip or head vortex ring.