Detonation propagation in narrow gaps with various configurations

In general all detonation waves have cellular structure formed by the trajectory of the triple points.This paperaims to investigate experimentally the propagation of detonation in narrow gaps for hydrogen-oxygen-argonmixtures in terms of various gap heights and gap widths.The gap of total length 1500 mm was constructed bythree pair of stainless plates,each of them was 500 mm in length,which were inserted in a detonation tube.Thegap heights were varied from 1.2 mm to 3.0 mm while the gap widths were varied from 10 mm to 40 mm.Variousargon dilution rates were tested in the present experiments to change the size of cellular structure.Attempts havebeen made by means of reaction front velocity,shock front velocity,and smoked foil to record variations of cel-lular structure inside the gaps.A combination probe composed of a pressure and an ion probe detected the arrivalof the shock and the reaction front individually at one measurement point.Experimental results show that thenumber of the triple points contained in detonation front decreases with decrease in the gap heights and gapwidths,which lead to larger cellular structures.For mixtures with low detonability,cell size is affected by a cer-tain gap width although conversely cell size is almost independent of gap width.From the present result it wasfound that detonation propagation inside the gaps is strongly governed by the gap height and effects of gap widthis dependent on detonability of mixtures.     

Mitigation of H2/air gaseous detonation via utilization of PAN-based carbon fibre felt

This paper first studied the mitigation of the hydrogen/air mixture detonation wave by tuning the tube inner wall with the absorbing material of polyacrylonitrile (PAN)-based carbon fibre felt. The experimental tests were performed in a single-trial circular cross-section tube filled with premixed hydrogen and air detonative mixture. The pressure values and flame front propagation were measured by means of pressure transducers and photodiodes respectively. The attenuation regimes of detonation wave in walled tubes with different thicknesses and layouts of absorption material were compared. The PAN-based carbon fibre felt makes a significant attenuation on the detonation propagation. The decoupling of leading shock wave and flame front can be observed under the effect of this absorbing layer. The ultimate strength close to the tube end and propagation velocity of the combustion wave decrease with the increase of felt thickness. When the interval layout felt is adopted, the spacing distance has almost no impact on the attenuation effectiveness. When the sectional layout is adopted, the effectiveness of detonation mitigation is however improved for a higher proportion of the absorbing material.     

Deflagration to detonation transition in aluminum dust-air mixture under weak ignition condition

Deflagration to detonation transition (DDT) in flake aluminum dust-air mixture was studied in a 199 mm inner diameter and 29.6 m long horizontal tube. 40 sets of dust dispersion system were used to disperse flake aluminum into the experimental tube. An electric spark of 40 J was used to ignite the aluminum-air mixture. Self-sustained detonation was observed and the characteristics of deflagration to detonation transition process were studied. Contributed to the transverse wave and cellular structure of detonation wave front in aluminum-air mixture, the propagation velocity of detonation wave ranges from 1480 m/s to 1820 m/s and the maximum overpressure oscillates between 40 and 102 bar. Single head spinning detonation wave in aluminum dust-air mixture was observed and the cell size was evaluated.     

Modeling of attenuation and suppression of cellular detonation in the hydrogen-air mixture by circular obstacles

The results of numerical study of the interaction of the gaseous detonation with the regular obstacle consisting of checkered elements with circular cross section are presented. The aim of this work is to identify and generalize the parameters affected the attenuation of a cellular detonation propagating in a premixed stoichiometric hydrogen-air mixture. Studies of such problems are aimed at studying issues related to explosion and fire safety in the operation of highly efficient gaseous fuels, which are currently very widespread. As a result, the dependencies of the leading shock wave propagation velocity on the obstacle geometry are obtained. The contribution of each of the considered parameters to the detonation wave velocity deficit is estimated. Maps of detonation suppression and re-initiation modes for varying barrier parameters are obtained.     

Self-organized generation of transverse waves in diverging cylindrical detonations

Self-organized generation of transverse waves associated with the transverse wave instabilities at a diverging cylindrical detonation front was numerically studied by solving two-dimensional Euler equations implemented with an improved two-step chemical kinetic model. After solution validation, four mechanisms of the transverse wave generation were identified from numerical simulations, and referred to as the concave front focusing, the kinked front evolution, the wrinkled front evolution and the transverse wave merging, respectively. The propagation of the cylindrical detonation is maintained by the growth of the transverse waves that match the rate of increase in surface area of the detonation front to asymptotically approach a constant average number of transverse waves per unit length along the circumference of the detonation front. This cell bifurcation phenomenon of cellular detonations is discussed in detail to gain better understanding on detonation physics.     

双点激光点火直接起爆的数值模拟

针对双点激光点火直接起爆过程中爆轰波的形成、发展和传播问题,采用高精度数值模拟方法求解带化学反应的二维欧拉方程组,研究了不同环境压力情况对流场结构与波系变化的影响.结果表明,环境压力会影响激波强度与爆轰波的传播速度,是决定双点激光点火形成的火核在碰撞过程中能否实现爆轰并维持爆轰波传播的重要因素,利用双激光点相互作用形成...     

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.     

Cellular pattern evolution in gaseous detonation diffraction in a 90°-branched channel

This paper presents recent results of an experimental investigation on gaseous detonation diffraction in a 90°-branched channel. The entire process of diffraction is demonstrated by cellular patterns and the analysis is mainly based on their evolution. Detonation pressure history and velocity are measured and the corresponding cellular patterns are recorded on soot foils around the branched segment. Results show that detonation propagation is notably disturbed by the branched wall geometry and that a complex wave configuration appears in both channels. Cellular patterns show that an expansion fan appears at the T-junction area with a Mach reflection taking place in the horizontal channel, while regular reflection takes place in the vertical channel. Subsequently, it appears that there is a transition from a regular reflection to a Mach reflection in the vertical channel. Details of the cellular pattern indicate that from the early stage to the end of diffraction, the detonation wave sequentially experiences attenuation, front decoupling, and degradation into deflagration, reinitiation, and recuperation. According to cellular pattern evolution and velocity measurement, a recuperated detonation with nearly the same velocity as the undisturbed incoming wave finally develops downstream in both channels, at a distance of about four times the channel height (160 mm). The mechanism of diffraction is explored based on the ZND (Zel'dovich-von Neumann-Döring) model, and the soot foils in both channels show a pattern consistent with air shock-wave diffraction in a 90°-branched channel.     

Numerical study of acoustic coupling in spinning detonation propagating in a circular tube

Spinning detonations propagating in a circular tube were numerically investigated with a two-step reaction model by Korobeinikov et al. The time evolutions of the simulation results were utilized to reveal the propagation behavior of single-headed spinning detonation. Three distinct propagation modes, steady, unstable, and pulsating modes, are observed in a circular tube. The track angles on a wall were numerically reproduced with various initial pressures and diameters, and the simulated track angles of steady and unstable modes showed good agreement with those of the previous reports. In the case of steady mode, transverse detonation always couples with an acoustic wave at the contact surface of burned and unburned gas and maintains stable rotation without changing the detonation front structure. The detonation velocity maintains almost a CJ value. We analyze the effect of acoustic coupling in the radial direction using the acoustic theory and the extent of Mach leg. Acoustic theory states that in the radial direction transverse wave and Mach leg can rotate in the circumferential direction when Mach number of unburned gas behind the incident shock wave in the transverse detonation attached coordinate is larger than 1.841. Unstable mode shows periodical change in the shock front structure and repeats decoupling and coupling with transverse detonation and acoustic wave. Spinning detonation maintains its propagation with periodic generation of sub-transverse detonation (new reaction front at transverse wave). Corresponding to its cycle, whisker is periodically generated, and complex Mach interaction periodically appears at shock front. Its velocity history shows the fluctuation whose behavior agrees well with that of rapid fluctuation mode by Lee et al. In the case of pulsating mode, as acoustic coupling between transverse detonation and acoustic wave is not satisfied, shock structure of spinning detonation is disturbed, which causes failure of spinning detonation.     

Numerical studies on flame inclination in porous media combustors

Inclinational instability developing during propagation of a filtration combustion wave in an inert porous medium is studied using two-dimensional numerical model. Stable and unstable combustion waves are generated by varying combustion parameters such as pressure, equivalence ratio, filtration velocity, effective conductivity of porous media, pellet diameter and combustor scale. The wave propagation velocity of inclinational flame is studied and compared with flat flame. The growth and reduction of inclinational instability are analyzed at different conditions. The numerical results show that a development of inclinational instability causes essential flow non-uniformity and can result in a separation of the flame front in the multiple flame zones. The limited conductive and radiant heat transfer in the solid phase, small pellet diameter of packed bed, high inlet velocity, large combustor scale and low equivalence ratio promote the instability growth. The inclinational instability is suppressed in a reciprocal combustor.     

A universal model of opposed flow combustion of solid fuel over an inert porous medium

In this study, a universal model is developed to examine the behavior of combustion wave observed in porous solid matters (e.g., smoldering, self-propagating high-temperature synthesis (SHS), diesel particulate filter (DPF) regeneration process). Analytical expressions of the combustion characters of solid combustible (e.g., diesel particulate matters trapped in a DPF) deposited over an inert porous medium are obtained employing large activation energy asymptotic taking into account the sensible transport processes; namely, heat transfer between the porous medium and gas phases, radiation heat transfer from the porous medium, heat loss from the porous medium to the environment, mass transfer of oxygen from the gas stream to the surface of solid fuel and the effective diffusion in modeling the species diffusion. Then it has been validated that the present model is applicable and adaptable for predicting the characteristics of smoldering combustion and thus SHS process. As a result, the features of combustion wave of the present phenomena would be useful to other processes. From practical point of view and for deep understanding of the behavior of combustion wave of these processes, we investigate the effects of various physical parameters over a wide range of conditions. We observe that the moving speed of the reaction front increases with the increase of porosity of the porous medium, mass transfer coefficient and initial fuel mass fraction; while it decreases owing to the increase of heat transfer rate from the porous medium to the gas, heat loss to the environment and radiative heat transfer. Furthermore, the results reveal that extinction tends to occur due to lower porosity of the porous medium, higher radiative heat transfer from the porous medium, higher heat transfer rate from the porous medium to the gas and higher heat losses from the porous medium to the environment. Even the observed near-extinction behavior in reaction front speed versus heat loss diagram is found to be similar what we got in gaseous premixed flame propagating through the porous medium. An extinction limit diagram has been presented as a function of radiation-conduction parameter and the gas flow velocity. In addition to, the impact of radiation and the combined effect of the inclusion of Knudsen diffusion and tortuosity are demonstrated in terms of the spatial temperature and species profiles to examine how these two parameters modify the reaction front structure. Furthermore, the governing equations have been solved numerically and it is observed that asymptotic analysis gives a good agreement with the numerical solution.     

Direct observations of reaction zone structure in propagating detonations

We report experimental observations of the reaction zone structure of self-sustaining, cellular detonations propagating near the Chapman-Jouguet state in hydrogen-oxygen-argon/nitrogen mixtures. Two-dimensional cross sections perpendicular to the propagation direction were imaged using the technique of planar laser induced fluorescence (PLIF) and, in some cases, compared to simultaneously acquired schlieren images. Images are obtained which clearly show the nature of the disturbances in an intermediate chemical species (OH) created by the variations in the strength of the leading shock front associated with the transverse wave instability of a propagating detonation. The images are compared to 2-D, unsteady simulations with a reduced model of the chemical reaction processes in the hydrogen-oxygen-argon system. We interpret the experimental and numerical images using simple models of the detonation front structure based on the “weak” version of the flow near the triple point or intersection of three shock waves, two of which make up the shock front and the third corresponding to the wave propagating transversely to the front. Both the unsteady simulations and the triple point calculations are consistent with the creation of keystone-shaped regions of low reactivity behind the incident shock near the end of the oscillation cycle within the “cell.”     

Effects of confinement on detonation in H2–O2 mixture with transverse concentration gradient

Since hydrogen has wide flammability limit and low ignition energy, it could be easily ignited and be easy for the transition to a detonation, leading to extremely serious impacts in explosion accidents or extremely high combustion effeciency in the propulsion. In the field of explosion accidents and the combustion chamber of propulsion systems, hydrogen mixtures are more likely to be highly non-uniform and a detonation usually propagates in a non-homogeneous medium. The work studies behaviors of detonations in non-homogenous medium by a high-resolution simulations. We widen computational domain and steepen the gradient to weaken the role of transverse wave on cellular detonations propagating in the medium with transverse concentration gradient and to reveal the interaction of longitudinal shock with reaction wave. The results show that characteristics of detonations in nonuniform medium is controlled by coupling role of gradient and confinement. As a domain is sufficient wide, the reflection wave is rather weak so that the detonation takes on galloping propagation, with a single-head mode. As the width is mediate, detonation cell takes on highly irregularity, similar to that of highly unstable detonation. However, in the narrow domain, steepening gradient plays a key role while confinement becomes minor in detonation propagation.     

Studies in the transition from deflagration to detonation in granular explosives—II. Transitional characteristics and mechanisms observed in 91/9 RDX/Wax

The apparatus and instrumentation described in the previous paper have been used to observe the transitional behavior of burning to detonation for a 91/9 RDX/wax mixture. Transitions to detonation were observed for all densities between 67 and 95% TMD. The sequence of observed events following ignition of the explosives charge was: stable propagation of a convective flame front, compaction of the more porous burning charges, formation of a postconvective (compressive) wave in the ignition region, subsequent coalescence of compressive waves some distance beyond the ignition region, and the formation of a detonation wave 1 to 2 cm beyond the intersection of the convective and postconvective fronts. It is proposed that the shock to detonation sequence starts in the region of coalescence of compressive waves. The velocity of the convective flame front was observed to increase with increasing charge density in agreement with earlier results from ammonium picrate. The predetonation column length as a function of charge compaction exhibited a minimum; such minima have been reported by other workers for PETN, RDX, and HMX.     

Numerical simulations of the cellular structure of detonations in liquid nitromethane—regularity of the cell structure

The detailed structure of planar detonation waves in liquid nitromethane was studied using time-dependent two-dimensional numerical simulations. The walls are assumed to confine heavily the liquid explosive and boundary layer effects are neglected. The solution thus simulates the detonation structure near the center of a wide channel. Chemical decomposition of nitromethane is described by a two-step model composed of an induction time followed by energy release. A simplified equation of state based on the Walsh and Christian technique for condensed phases and the BKW equation of state for gas phases us used. When mixtures of both phases are present, pressure and temperature equilibrium between them is assumed. The simulations show a cellular pattern traced by a system of triple points dividing the detonation front into sections. However, a substructure of weaker triple points also traces out a nonuniform pattern within the main pattern, resulting in an irregular cellular structure. A correlation exists between the regularity of the cellular pattern and both the curvature of the front and the change in induction zone thickness at the triple points. If the induction time is a stronger function of temperature, the weaker triple points disappear and a more regular structure is produced. When the structures are regular, the detonation front is more curved and there is a larger change in induction zone thickness at the triple points. However, the large change in induction zone thickness also leads to the formation of unburned pockets that eventually disturb the symmetry and uniformity of the structure. We conclude that the regularity of the cellular pattern is strongly influenced by the temperature-dependence of the induction time.     

Numerical simulation of interaction of cellular detonation wave with systems of inert porous filters

Numerical modeling of the interaction of cellular detonation in a hydrogen-air mixture with several systems of porous filters covering part of the channel was carried out. The main regimes of detonation propagation and the critical conditions for detonation failure in the filter systems were obtained for each system. A map of detonation regimes was constructed, from which it follows that with an increase in the concentration of particles in the filters, it is possible to increase the distance between the filters or reduce the number of filters in the system necessary to successfully suppress detonation. A comparison of various filter systems in terms of blockage ratio and the total surface area of particles was made, from which it was concluded that the system of two filters was the most efficient to suppress detonation.     

基于电-力-声多物理场耦合数值模型的含水合物多孔介质声速和衰减特性研究

在物理模拟实验中对水合物微观赋存模式和饱和度进行准确控制和评价尚存在技术困难,仅依赖实验技术研究含水合物沉积物声学特性、建立储层参数解释模型存在局限性。采用基于有限元的数字岩石物理技术,针对悬浮、接触和胶结三种典型的水合物微观赋存模式分别建立多孔介质的三维电-力-声多物理场耦合模型,考察了微观赋存模式和水合物饱和度对多孔介质声速和衰减的影响规律,对比了声速数值模拟与理论模型计算结果,建立了声波衰减参数与水合物饱和度之间的关系式。研究结果表明：（1）对于三种水合物赋存模式,由于水合物相比孔隙水具有更高的弹性模量,多孔介质的声速随着水合物饱和度的增大而增大;水合物的存在导致声波在传播过程中遇到更多不连续的声阻抗界面,声衰减随着水合物饱和度的增大而近似线性增大;（2）悬浮和接触赋存模式条件下,水合物饱和度对多孔介质的声速和衰减影响规律基本一致;对于相同的水合物饱和度,胶结模式条件下含水合物多孔介质具有更高的声速和更小的声衰减;（3）通过合理选择参数值,利用权重方程与Lee改进的Biot-Gassmann Theory(BGTL)模型估算的含悬浮和接触模式水合物多孔介质的声速较为准确;通过等效...     

Effect of uniform suction on laminar filmwise condensation on a finite-size horizontal flat surface in a porous medium

The theoretical analysis of filmwise condensation outside a finite-size horizontal flat surface embedded in a porous medium filled with a dry saturated vapor has been solved by a boundary layer treatment. The Newton-Raphson scheme was employed to solve the finite-size horizontal flat plate in porous medium. Results turns out that the average Nusselt number for condensation heat transfer is expressed in terms of Darcy number, Jakob number, film liquid Prandtl number, Darcy-modified Rayleigh number and the parameter of suction, as well as are given for the condensate layer thickness profiles.     

Plane waves in nonlocal thermoelastic solid with voids

This work is concerned with the propagation of time harmonic plane waves in an infinite nonlocal thermoelastic solid having void pores. Three sets of coupled dilatational waves and an independent transverse wave may travel with distinct speeds in the medium. All these waves are found to be dispersive in nature, but the coupled dilatational waves are attenuating, while transverse wave is nonattenuating. Coupled dilatational waves are found to be influenced by the presence of voids, thermal field and elastic nonlocal parameter. While the transverse wave is found to be influenced by the nonlocal parameter, but independent of void and thermal parameters. For a particular model, the effects of frequency, void parameters, thermal parameter and nonlocality have been studied numerically on the phase speeds, attenuation coefficients and specific losses of all the propagating waves. All the computed results obtained have been depicted graphically and explained.     

Numerical simulation of detonation failure and re-initiation in bifurcated tubes

A numerical approach is developed to simulate detonation propagation, attenuation, failure and re-initiation in hydrogen–air mixture. The aim is to study the condition under which detonations may fail or re-initiate in bifurcated tubes which is important for risk assessment in industrial accidents. A code is developed to solve compressible, multidimensional, transient, reactive Navier–Stokes equations. An Implicit Large Eddy Simulation approach is used to model the turbulence. The code is developed and tested to ensure both deflagrations (when detonation fails) and detonations are simulated correctly. The code can correctly predict the flame properties as well as detonation dynamic parameters. The detonation propagation predictions in bifurcated tubes are validated against the experimental work of Wang et al. [1,2] and found to be in good agreement with experimental observations.