Experimental studies of the role of chemical kinetics in turbulent flames

Flames of di-t-butyl-peroxide (DTBP) decomposition in a 0.376DTBP + 1.0N₂ mixture are studied in laminar and turbulent media. The observed values of unstretched laminar burning velocity are in reasonable agreement with the value obtained from the Zel’dovich-Semenov-Frank-Kamenetsky theory. Turbulent explosions in this particular mixture are characterized by a number of features that are believed to be common for all developing turbulent flames and have relevance to spark-ignition engine combustion of lean mixtures. Flame propagation is unsteady and is characterized by a mass burning rate that increases in time. The rate of the flame acceleration varies from one explosion to another. If the burning rate is related to the average flame radius, however, it exhibits much smaller variations. This phenomenon bears a striking resemblance to cycle-to-cycle variations in a spark-ignition engine. Comparisons of the present results with mixtures of significantly different composition, chemical kinetics, and exothermicity, but with similar laminar flame speed and Lewis number show that the data obtained in closed-volume explosions are in good agreement if the unsteady character of the flame is taken into account. The differences in details of the kinetic mechanisms and thermochemistry appear to be responsible for the flame behaviour only near the limit of extinction by turbulence. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 43–52, July–August, 2009.     

片状变燃速发射药燃烧性能的数值计算

为研究包覆方式对片状发射药燃烧性能的影响,建立了不同包覆方式下多层片状发射药的物理模型,并推导了形状函数和燃气生成猛度表达式,利用Matlab软件对不同的宽厚比、长厚比及多层结构的片状发射药进行了数值计算;制备了不同长厚比的片状变燃速发射药,并进行了密闭爆发器实验。结果表明,四面包覆和全包覆可以很好地消除临界宽厚比对片状发射药燃烧性能的影响;与两面包覆的片状发射药相比,四面包覆和全包覆的片状发射药能够延缓内层药减面燃烧的时间,其燃气生成猛度的阶跃程度分别提高了1.17%和1.23%,呈现出良好的燃烧渐增性。     

Flame structure and laminar burning speeds of JP-8/air premixed mixtures at high temperatures and pressures

Jet propellant 8 (JP-8)/air laminar burning speed was experimentally measured and its flame structure was studied at high temperatures and pressures using a high-speed camera. The experimental facilities included a spherical vessel, used for the measurement of burning speed, and a cylindrical vessel, used in a shadowgraph system to study flame shape and structure and to measure burning speed. A thermodynamic model was developed to calculate burning speeds using the dynamic pressure rise in the vessel due to the combustion process. The model consists of a central burned gas core of variable temperature surrounded first by a reaction sheet, then by an unburned gas shell with uniform temperature and lastly by thermal boundary layers at the wall and electrodes. Radiation from burned gases to the walls was also included in the model. Burning speeds of laminar flames of JP-8/air were calculated for a wide range of conditions. A Power law correlation was developed to calculate laminar burning speed at temperatures ranging from 500-700 K, pressures of 1-6 atm and equivalence ratios of 0.8-1. Flame structure and cell formations were observed using an optical system. Experimental results showed that pressure and the fuel-air equivalence ratio have a strong influence on flame structure.     

预测生物质颗粒热解时热损失影响的分析模型(英文)

This paper presents the combined influence of heat-loss and radiation on the pyrolysis of biomass parti-cles by considering the structure of one-dimensional, laminar and steady state flame propagation in uniformly pre-mixed wood particles. The assumed flame structure consists of a broad preheat-vaporization zone where the rate of gas-phase chemical reaction is small, a thin reaction zone composed of three regions：gas, tar and char combustion where convection and the vaporization rate of the fuel particles are small, and a broad convection zone. The analy-sis is performed in the asymptotic limit, where the value of the characteristic Zeldovich number is large and the equivalence ratio is larger than unity （i.e. u 1？ ≥ ）. The principal attention is made on the determination of a non-linear burning velocity correlation. Consequently, the impacts of radiation, heat loss and particle size as the de-termining factors on the flame temperature and burning velocity of biomass particles are declared in this research.     

Effect of constant and pulsed-periodic electric fields on combustion of a propane-air mixture

The effect of constant and pulsed periodic electric fields on combustion of a propane-air mixture is studied experimentally. The experiments are performed for the laminar and turbulent combustion modes with flame stabilization by a separation region behind a central body. The constant and pulsed-periodic electric fields are found to exert different effects on the burning rate. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 1, pp. 22–25, January–February, 2008.     

Analytical calculation of the negative erosive burning rate

The rate of negative erosive combustion is calculated using analytical methods and a simple model of the gas-phase chemical reactions A → B. The conversion of part of the thermal energy into the kinetic energy of motion of gaseous combustion products along the propellant gasification surface is taken into account within the model. Solutions are obtained for the cases where the thickness of the laminar sublayer is larger or smaller than the width of the combustion zone in the gas phase. The calculation results confirm the author’s previous conclusion: manifestation reduction in the negative erosive effect with decreasing initial temperature of the propellant is caused by narrowing of the region of its occurrence.     

多层管状发射药燃烧性能的数值计算

为了模拟和改善层状发射药的内弹道性能和燃烧性能,建立了一种多层管状发射药的燃烧模型.在服从几何燃烧定律条件下,通过多层管状发射药的形状函数推导出其Γ-Ψ表达式.用Γ-Ψ曲线研究了多层管状发射药的缓燃层厚度与药片初始总厚度比、内外层的燃速比以及药柱厚度与药柱长度之比对其燃烧性能的影响.结果表明,(1)多层管状发射药缓燃层...     

Numerical Simulation of Quenched Combustion Model for AP/HTPB Propellant under Transient Depressurization

In order to study the out‐of‐phase blowing effect of ammonium perchlorate/hydroxyl terminated polybutadiene (AP/HTPB) under transient depressurization, a two‐dimensional periodic sandwich model of AP/HTPB sandwich unsteady combustion was established. The gas‐solid two‐step total reaction was used to couple the gas‐solid boundary layer, and the AP/HTPB step‐down temperature criterion was applied. The numerical simulation comparative analysis under the initial combustion pressure 3.5 MPa∼10 MPa and the initial depressurization rate of 1000 MPa/s∼2000 MPa/s was experimented. The results show that under the condition of initial combustion pressure of 3.5 MPa and pressure reduction rate of 1000 MPa/s, the formation of narrow diffusion chemical reaction zone appears in the initial stage of depressurization; when the combustion pressure drops to about 1.7 MPa, the flame appears dual characteristics: diffusion and premixed combustion; when the pressure dropped to 0.1 MPa, the flame is premixed combustion. AP(g) is the main factor causing the out‐of‐phase blowing effect. During the initial stage of combustion, AP(g) decomposes rapidly and its decomposition product is close to the combustion surface; with the rapid decrease of pressure, the gas‐phase heat feedback decreases, leading to the decrease of AP(g) decomposition rate and AP(g) diffusion to the gas phase, which is the initial phase of out‐of‐phase blowing effect; when the gas phase heat feedback reduced and the temperature is below 750 K, the decomposition of AP(g) is stopped and the undecomposed AP(g) is further diffused to the gas phase region, which exacerbates the out‐of‐phase blowing effect, leading to the extinguishment of AP/HTPB. The unsteady combustion flame extinction time increases with the initial combustion pressure, and decreases with the initial depressurization rate.     

Boundary-Layer Structure with Hydrogen Combustion with Different Injection Intensities

Results of numerical simulation of the influence of intensity of hydrogen injection through a porous surface in the case of hydrogen burning in the boundary layer are presented. Turbulent characteristics of the flow were simulated using the k–epsiv; turbulence model with Chien's modification for low Reynolds numbers. The diffusion model (infinitely large burning rate) was used to describe the chemical reaction process, but the difference in diffusion coefficients of different substances was taken into account. A comparison of injection with and without combustion shows that the presence of a heat-release front delays the laminar–turbulent transition and significantly deforms the profiles of density and viscosity of the gas mixture. As the injection velocity increases, the flame front is shifted from the porous surface toward the outer edge of the boundary layer. The contributions of injection itself and combustion to reduction of skin friction are analyzed. Key wrds: boundary layer, combustion, porous injection, heat and mass transfer, friction.     

Erosive burning. Modeling problems

Existing models for the erosive burning of homogeneous energetic materials ignore the fact that the burning regime can be changed radically by exposure to a hot gas blowing over the burning surface. In other words, transition can occur from the gasification regime at very high blowing rates (where the burning rate is determined primarily by the heat transfer from the flow core) to the self-heating regime of the condensed phase at low or zero blowing if the heat release in the condensed phase is sufficient to heat it to the surface temperature. A possible method for solving this problem is proposed. The approach proposed provides a plausible explanation for the experimentally observed singular (kinked) dependence of the magnitude of the negative erosion effect on the initial temperature. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 3, pp. 47–58, May–June, 2007.     

包覆层可靠性对高燃速火药燃速测试的影响及改进

利用恒压燃速测试法,测试了经硅橡胶包覆的高燃速药柱的燃速.结果表明,在30～80MPa时,样品燃速出现骤增现象.经分析得出,由于在高压下包覆层与高燃速药柱之间的黏结力降低,导致燃烧过程出现传火现象而使测得的燃速在高压下反常升高.通过采用新的包覆材料及包覆工艺后对药柱进行测试,结果表明,在高压下包覆可靠、燃烧稳定.指出选择传热系数小、黏接可靠的包覆材料是保障高压高燃速火药测试准确性的关键技术.     

Is turbulent burning velocity a meaningful parameter?

The concept of turbulent burning velocity arises from an analogy with that of the laminar burning velocity. Experimental measurement or theoretical computation of the latter must take account of the effects of flame stretch. This is also necessary for turbulent flames, and when flame propagation originates at a point source, there is a temporal development of the turbulence acting on the flame and also of the thickness of the flame brush. Under these conditions, while the turbulent burning velocity is a measure of the propagation rate of the front relative to unburnt mixture, it cannot be a direct measure of the mass rate of burning.Department of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, U. K. Published in Fizika Goreniya i Vzryva, Vol. 29, No. 3, pp. 5–7, May–June, 1993.     

DAGQ发射药膛内静态和动态燃烧性能

为了研究DAGQ发射药在膛内的燃烧性能,以经典内弹道理论为基础,建立了发射药膛内燃烧测试系统和处理方法,通过密闭爆发器燃烧试验和微波干涉法发射药膛内动态燃烧性能试验,研究了DAGQ发射药的静动态燃烧规律及不同温度下的动态燃烧特性。结果表明,所建立的试验系统和处理方法,能够很好地获得弹丸在膛内的运动过程。DAGQ发射药的静动态燃速都存在转折现象,静态燃速在转折点前压力指数大于1,转折点后压力指数都远小于1,动态燃速压力指数基本都小于1。在膛内燃烧过程中,由于高速气流对发射药的燃面冲刷,使得膛内的动态燃速要大于密闭爆发器内的静态燃速,并且随着膛内压力的增大,燃速相差越来越大。     

甲烷/乙烷/空气预混火焰层流燃烧速度与火焰稳定性的数值研究(英文)

A numerical study on premixed methane/ethylene/air flames with various ethylene fractions and equivalence ratios was conducted at room temperature and atmospheric pressure. The effects of ethylene addition on laminar burning velocity, flame structure and flame stability under the condition of lean burning were investigated. The results show that the laminar burning velocity increases with ethylene fraction, especially at a large equivalence ratio. More ethylene addition gives rise to higher concentrations of H, O and OH radicals in the flame, which significantly promotes chemical reactions, and a linear correlation exists between the laminar burning velocity and the maximum H + OH concentration in the reaction zone. With the increase of ethylene fraction, the adiabatic flame temperature is raised, while the inner layer temperature becomes lower, contributing to the enhancement of combustion. Markstein length and Markstein number, representative of the flame stability, increase as more ethylene is added, indicating the tendency of flame stability to improve with ethylene addition.     

Ammonium perchlorate-based composite solid propellant formulations with plateau burning rate trends

This paper presents burning rates as a function of pressure of several propellant formulations based on ammonium perchlorate (AP) and hydroxyl-terminated polybutadiene cured by isophorone diisocyanate, many of which exhibit significantly low (nearly zero or negative) values of the pressure exponent of the burning rate in distinct pressure ranges, termed as plateau burning rate trends. The propellants contain a bimodal distribution of AP particles with the size of the coarse and fine particles within narrow ranges whose mean values are widely separated. Two mean sizes of fine particles were considered for the propellant formulations in the present work, namely, 5 and 20 μm. These choices are based on the mid-pressure extinction behavior exhibited by the matrix of fine AP and binder contained in the propellants but when tested alone over a wide range of fine AP size and pressure. The propellants that include the fine AP/binder matrixes exhibiting a mid-pressure extinction, in turn, exhibit the plateau burning rate trends within the corresponding pressure ranges. A plateau is also observed at elevated pressures in the burning rates of some formulations, which is related to the diminishing relative importance of the near-surface leading-edge region of the oxidizer/fuel diffusion flame in the gas-phase combustion zone. The choice of the coarse AP size influences the exact pressure range within the mid-pressure extinction domain of the matrix where the propellant exhibits the plateau burning rate trends. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 4, pp. 73–81, July–August, 2007.     

Selective Diffusion during Flame Propagation and Quenching in a Porous Medium

The propagation of propane-air flames in an inert high-porosity medium with nitrogen dilution and oxygen enrichment of the mixture was studied experimentally. It is shown that variation in the nitrogen or oxygen concentration (in the gas phase) leads to a more significant variation in the flame propagation velocity than in the laminar burning velocity; with the addition of nitrogen, the rate of increase in the flame velocity with the initial pressure becomes lower and the concentration range of flame propagation becomes narrower. At the flame propagation limit, the Peclet number obtained from the laminar burning velocity of the initial mixture is not constant but depends on the fuel-to-oxidizer ratio and the nitrogen content in the mixture. The results are interpreted from a physical point of view based on the hypothesis of selective diffusion. It is shown that accounting for the effects of the Lewis numbers of the fuel and oxidizer allows flame propagation in inert porous media to be described quantitatively over wide parameter ranges using a unified relation. At the flame propagation limit, the Peclet number constructed from the laminar burning velocity taking into account these effects is a constant.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 50–59, July–August, 2005.     

Combustion Wave Structure of AP Composite Propellants

The combustion mechanism of ammonium perchlorate (AP) composite propellants were studied. The oxidizer-rich propellants tested were made with excess concentrations of AP particles. The pressure deflagration limit of propellant decreases with increasing the concentration of binder. The combustion wave consists of two reaction regions I and II: the region I is the zone of AP monopropellant flame and the region II is the zone of diffusion flame. The heat flux feedback from the gas phase to the burning surface increases as pressure increases, and the heat flux is responsible for the burning rate characteristics.     

Some Features of Gas‐Flame Propagation in Narrow Tubes

The dependence of the flamefront curvature on the composition of the mixture in tubes of various diameters is considered. The combustionwave velocities are measured for different fractions of fuel in the mixture. It is shown that the dependence of the burning velocity on the composition of the mixture has two maximums, one of them being related to the transition of the laminar combustion mode to turbulent combustion. Combustion turbulization is caused by flame instability to acoustic oscillations. Variations of the combustionwave shape due to acoustic oscillations are observed.     

Flames with Realistic Thermal Expansion in a Time-Dependent Turbulent Flow

The propagation velocity of turbulent premixed flames with real thermal expansion in a time-dependent external flow is studied for an infinitely thin flame front and for flames of small but finite thickness. It is shown that the influence of temporal pulsations is usually small and can be neglected for reasonable values of the pulsation frequency. The role of pulsations is even smaller for realistic thermal expansion in comparison with the artificial case of zero thermal expansion studied previously. The results obtained indicate that the Taylor hypothesis of “ stationary” turbulence is a good approximation for turbulent flames. The role of pulsations is substantial only if the integral turbulent length scale is close to the cut-off wavelength of the Darrieus-Landau instability. In this particular case, temporal pulsations can be important for explaining recent experiments on turbulent burning.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 3–17, July–August, 2005.     

Simulation of the Erosion Burning of a Granular Propellant

For combustion of axisymmetric propellant grains under blowing conditions, a mathematical model is proposed and numerical simulation is performed. The effects of incoming–flow parameters (velocity, pressure, and temperature) and surface dimensions and geometry on grain–burning rate are studied. Physical patterns of flow around burning propellant grains are presented.