Heat and mass transfer at the ignition of a liquid substance by a single “Hot” particle

The results of a numerical solution to the problem of heat and mass transfer at the ignition of a liquid flammable substance by a single particle heated to a high temperature located on its surface are presented. The problem is solved within the framework of a gas phase model of ignition. A mathematical model is formulated. It describes the following processes in a two-dimensional statement: the heat conduction and evaporation of a flammable liquid and the diffusion and convection of the combustible vapors in the oxidizer medium in the system “particle heated to a high temperature-liquid flammable substance-air.” The numerical investigations established the relation between the ignition delay time, the particle temperature and sizes, and the particle minimum temperature and sizes at which ignition of a combustible liquid is possible.     

Ignition of a vapor-gas mixture by a moving small-size source

A theoretical analysis of the ignition of a liquid fuel vapor-air mixture by a moving small source of heating was performed. A gas-phase model of the ignition with consideration given to heat transfer, liquid fuel evaporation, diffusion and convective motion of fuel vapor in the oxidizer medium, crystallization of the heating source, kinetics of the vaporization and ignition processes, temperature dependence of the thermophysical characteristics of the interacting substances, and character of motion of the heating source in the vapor-gas mixture was developed. The values of the ignition delay time τ _d , the main characteristic of the process, were determined. It was established how τ _d depends on the initial temperature, heating source sizes, velocity and trajectory of the heating source, and ambient air temperature.     

Analyzing the characteristic times of physical-chemical processes running at ignition of a liquid condensed substance under local heating

The ranges of times of heat and mass transfer processes, phase transitions, formation of a reactive vapor-gas mixture, and abruptly exponential acceleration of oxidation at ignition of a liquid condensed substance by a typical source with a limited heat content, that is, by a small hot metal particle, are found. Results of the performed numerical and experimental investigations are used to find the limiting values of the main parameters (temperature, sizes) of a local energy source, which are sufficient for ignition of a typical liquid fuel.     

Simulation of the ignition of a liquid fuel with a hot particle

A two-dimensional gas-phase model of ignition of a flammable liquid by a single particle heated to a high temperature with consideration given to heat conduction, evaporation, diffusion, and convection of fuel vapor in an oxidizer medium was developed. Numerical simulations made it possible to determine the dependences of the ignition delay time for the liquid on the size and initial temperature of the particle. The minimum size and initial temperature of the particle at which ignition still occurs were estimated.     

Heat and mass transfer at gas-phase ignition of grinded coal layer by several metal particles heated to a high temperature

Characteristics of gas-phase ignition of grinded brown coal (brand 2B, Shive-Ovoos deposit in Mongolia) layer by single and several metal particles heated to a high temperature (above 1000 K) have been investigated numerically. The developed mathematical model of the process takes into account the heating and thermal decomposition of coal at the expense of the heat supplied from local heat sources, release of volatiles, formation and heating of gas mixture and its ignition. The conditions of the joint effect of several hot particles on the main characteristic of the process–ignition delay time are determined. The relation of the ignition zone position in the vicinity of local heat sources and the intensity of combustible gas mixture warming has been elucidated. It has been found that when the distance between neighboring particles exceeds 1.5 hot particle size, an analysis of characteristics and regularities of coal ignition by several local heat sources can be carried out within the framework of the model of “single metal particle / grinded coal / air”. Besides, it has been shown with the use of this model that the increase in the hot particle height leads, along with the ignition delay time reduction, to a reduction of the source initial temperatures required for solid fuel ignition. At an imperfect thermal contact at the interface hot particle / grinded coal due to the natural porosity of the solid fuel structure, the intensity of ignition reduces due to a less significant effect of radiation in the area of pores on the heat transfer conditions compared to heat transfer by conduction in the near-surface coal layer without regard to its heterogeneous structure.     

On peculiarities of heat and mass transfer in a hot metal particle-liquid fuel condensed substance-air system

Results of numerical modeling are used for validating the governing role of the ratio of areas of contact between the heater with the combustible liquid and the formed vapor-gas mixture in a complex of interrelated processes of heat and mass transfer at gas-phase ignition of a liquid condensed substance film by small-size hot metal particle. Critical values of the ratio at which the ignition conditions cannot be realized are marked out. Ranges of varying the main igniter parameters for which the influence of the parameter on the ignition parameters may be neglected are determined.     

Ignition conditions and characteristics for high-porosity condensed materials subjected to local conductive heating

An experimental study of the ignition conditions (limiting heat source temperature) and characteristics (delay time) for high-porosity condensed materials under local conductive heating is reported. The study has been carried out on dry pine needles, a typical high-porosity combustible forest material. The dependence of the ignition delay for the material on the initial temperature of the heat source—a single cylindrical particle preheated to a high temperature—has been elucidated. A hypothesis concerning the mechanism of ignition of high-porosity condensed materials under local conductive heating has been formulated: the effect of high open porosity on the intensity of heat and mass transfer in the boundary layer of the material in the induction period has been substantiated.     

Evolution of temperature of a droplet of liquid composite fuel interacting with heated airflow

The macroscopic patterns of a temperature change at the center of a droplet of three-component (coal, water, petroleum) composite liquid fuel (CLF) were studied using a low-inertia thermoelectric converter and system of high-speed (up to 10⁵ frames per second) video recording during the induction period at different heating intensity by the air flow with variable parameters: temperature of 670?870 K and motion velocity of 1?4 m/s. The studies were carried out for two groups of CLF compositions: fuel based on brown coal and coal cleaning rejects (filter cake). To assess the effect of liquid combustible component of CLF on characteristics of the ignition process, the corresponding composition of two-component coal-water fuel (CWF) was studied. The stages of inert heating of CLF and CWF droplets with characteristic size corresponding to radius of 0.75?1.5 mm, evaporation of moisture and liquid oil (for CLF), thermal decomposition of the organic part of coal, gas mixture ignition, and carbon burnout were identified. Regularities of changes in the temperature of CLF and CWF droplets at each of identified stages were identified for the cooccurrence of phase transitions and chemical reactions. Comparative analysis of the times of ignition delay and complete combustion of the droplets of examined fuel compositions was performed with varying droplet dimensions, temperatures, and oxidant flow velocity.     

Numerical estimation of the influence of natural convection in liquid on the conditions of ignition by a local heat source

Peculiarities of natural convection in a liquid condensed substance at ignition by a typical local energy source, that is, a small hot metal particle, are numerically investigated. The proposed model takes into account the whole complex of the main processes of heat and mass transfer with phase transitions, chemical reaction, and hydrodynamic processes during interaction between a liquid substance and a source with a limited energy capacity. The influence of convective streams in liquid during the ignition delay time on the process characteristics is analyzed.     

考虑Stefan影响的单颗粒硼着火过程研究

针对含硼推进剂固体火箭冲压发动机内单颗粒硼的着火过程展开了系统研究. 考虑硼颗粒周围气相流动以及硼颗粒与周围环境间的传热传质过程, 建立了考虑Stefan流作用的一维硼颗粒着火模型, 研究了硼颗粒实现着火和未能实现着火两种典型情形下硼颗粒及周围气相的参数变化规律, 对两种情形下Stefan流的变化规律及其成因展开了详细分析. 研究表明, 在硼颗粒实现着火的过程中, 液态B₂O₃的蒸发及硼的 氧化均能在硼颗粒的反应自加热作用下急剧加速, 硼颗粒表面附近的氧气和气相B₂O₃分布变化剧烈; 在未能实现着火的过程中, 液态B₂O₃的蒸发和氧气消耗的质量流率相对较小, 并逐渐趋于稳定, 硼颗粒表面附近的氧气和气相B₂O₃分布相对变化很小.在两种典型情形下, 硼颗粒外表面的Stefan流都会经历先由周围空间流向颗粒表面, 而后变为由颗粒表面流向周围空间的过程. 关键词： 固体火箭冲压发动机 硼颗粒 着火过程 Stefan流     

Numerical simulation of the ignition of liquid fuel with a limited-energy source under turbulent flow conditions

The ignition of a typical liquid fuel with a limited-energy source, a small metal particle heated to high temperature is numerically simulated with consideration given to the possible turbulization of the fuel vapor flow. The dependences of the integral ignition characteristics on the key parameters of the local heat source are established. The integral ignition characteristics, as well as the fields of fuel vapor concentrations and velocities predicted by models accounting for the laminar and turbulent modes of the vapor-oxidizer mixture flow are compared.     

镁颗粒群非稳态着火过程数值模拟

建立了镁颗粒群着火的一维非稳态有限影响体模型, 数值模拟颗粒群中镁颗粒的着火过程. 研究表明, 当镁颗粒表面反应加剧之后,颗粒相温度急剧上升, 迅速达到着火, 而其周围气相的温升速率却远小于颗粒的温升速率; 在着火过程中气相温度只在颗粒表面附近升高比较明显, 整体温度升高不大. 分析了颗粒群内部参数和环境参数对镁颗粒群着火的影响. 随颗粒浓度的增加, 颗 粒群变得易于着火, 其着火时间变短, 但颗粒浓度增大到一定程度后, 继续增大该值将对颗粒群的着火起消极作用. 环境压力对颗粒群着火的影响比较小,在1-5 atm范围内颗粒群的着火性能基本不变. 气相中氧气浓度对颗粒群的着火性能影响也不显著, 但当氧气浓度过小时, 对着火过程的影响将大大增强.颗粒粒径、气相/颗粒相初温、辐射源温度对颗粒 群着火的影响巨大,小粒径、高温度促使颗粒群快速着火.数值模拟与文献中试验 结果的变化趋势相一致.     

Numerical study of ignition of a metallized condensed substance by a source embedded into the subsurface layer

A numerical simulation of the ignition of structurally heterogeneous condensed material by a small single particle heated to high temperature, a typical limited heat content source of is performed within the framework of a solid-phase ignition model. The effect of the depth of embedment of the heated particle into the subsurface layer of the metallized material on the integral characteristics of the ignition is examined.     

A modified model of the ignition of a magnesium particle

A model of the ignition of an isolated magnesium particle in an oxidative atmosphere is suggested. The model takes into account the nonstationary character of the heat flux onto the surface of the particle and the nonuniformity of temperature distribution inside the particle. Calculations were performed to demonstrate the influence of these factors on the macrokinetic parameters of the empirical law of the oxidation of magnesium particles and the time characteristics of their ignition in hot air at atmospheric pressure.     

液体火箭发动机液氧箱与预冷回路的耦合计算模型

建立了液体火箭发动机的液氧贮箱与底部预冷回路的数值计算耦合模型,模拟了地面停放过程中贮箱与底部预冷回路的三维非稳态两相流动与传热过程,分析了自然循环预冷条件下液氧贮箱和底部预冷回路中的三维物理场分布及随时间变化规律。结果表明:随着停放时间的增加,液氧的蒸发量增加,停放中后期贮箱内的热传递基本趋于稳定。回流管内的气化导致回流口处的温度一直呈现波动。     

The influence of heat transfer conditions at the hot particle-liquid fuel interface on the ignition characteristics

The problem of ignition in the conditions of nonideal contact between liquid fuel and a single metallic particle heated to high temperatures is numerically solved. A gas-phase ignition model is created with regard to the heat-and-mass transfer processes in the gas region near the ignition source and the layer separating the particle and the fuel. The scale of the impact of the heat source surface roughness upon the ignition characteristics in a hot particle-liquid fuel-oxidant system is determined.     

Ignition model of boron particle based on the change of oxide layer structure

The oxide layer of boron particle is generally regarded as a two-layer structure the inner layer is B₂O₃ and the outer is (BO)_n. However, under lower temperature, a tiny layer of B₂O₃ can be generated at the surface of the (BO)_n and form a three-layer structure during the ignition process, which has been proven by experimental phenomenon. Accordingly, a parameter xo is adopted to represent the thickness of the outer B₂O₃(l) layer (outermost layer), and a simplified kinetic and diffusion model for the ignition process of boron particles in dry and wet atmosphere is developed with considering the generation and consumption process of the outermost layer. The ignition process is divided into two parts (ignition delay and first-stage combustion) by the parameter xo. The ignition temperature is defined as the particle temperature at the moment that xo reaches 0. When xo?>?0, the particle is under the ignition delay process, and the evaporation product is B₂O₃(g). When xo?=?0, the particle turns to the first-stage combustion process. (BO)_n is exposed to the environment, the evaporation products are B₂O₂(g) and B₂O₃(g), and the particle is under the two-layer structure. The oxygen diffusion inward is available during these two processes. The ignition time which is predicted by this model is in good agreement with published experimental data. Under a real ramjet condition, higher ambient temperature and concentration of water vapor can reduce both the maximum value of xo and the ignition time. The ignition temperature decreases with higher water vapor concentration, but increases with the higher ambient temperature.     

Evaporation waves in a metastable single-component liquid

In this paper, evaporation waves appearing at rapid depressurization of a vessel filled with liquid Freon R11 have been investigated experimentally. Regimes with a high-speed evaporation front have been revealed. It has been shown that the disintegration of a metastable liquid takes place in the form of a surface evaporation wave with a cellular structure of the front and anomalously high values of the coefficient of heat transfer from the liquid. The dependence of the evaporation front velocity on the initial temperature of the liquid has been obtained. A critical temperature value below which evaporation waves are not observed has been determined. Visualization of the evaporation front structure and dynamics with the use of a high-speed video camera has been performed. It has been found that the evaporation front motion has a pulsatory character. A method to calculate the surface evaporation wave with a quasi-plane interphase boundary has been developed. It is based on a model of nonequilibrium evaporation of the liquid and experimentally measured coefficients of the heat transfer from the liquid.     

Cyclic deflagration-to-detonation transition in the flow-type combustion chamber of a pulse-detonation burner

The possibility of realization of a rapid cyclic deflagration-to-detonation transition (DDT) with a frequency of up to 2 Hz under conditions of high-velocity flow (～10 m/s) and separate supply of the combustible mixture components (methane and air) in a tube, 5.5 m in length and 150 mm in diameter, with an open end at a low ignition energy (～1 J) is for the first time demonstrated. It is shown that such a tube with turbulizing obstacles of special shape and placement can ensure reliable DDT at a distance of 3–4 m from the ignition source within Δτ_DDT ≤ 20 ms after ignition. The results will be used in the development of a new type of industrial burner—a pulse-detonation burner for high-rate heating and fragmentation, combining thermal and shock-wave (mechanical) impacts on the treated object.     

Simulation of the ignition of liquid fuel with a local source of heating under conditions of fuel burnout

The processes of heat and mass transfer with phase transitions and chemical reactions in the ignition of liquid fuel by a local source of heating, a hot metal particle, under conditions of fuel burnout are studied. The influence of liquid fuel burnout on the ignition characteristics is analyzed, and the results of investigation of the extent of influence of this factor for solid and liquid condensed materials under conditions of local heating are compared.