Ignition of single coal particle in a hot furnace under normal- and micro-gravity condition

An experimental study on ignition and combustion of single particles was conducted at normal gravity (1-g) and microgravity (μ-g) for three high volatile coals with initial diameter of 1.5 and 2.0 mm, respectively. The non-intrusive twin-color pyrometry method was used to retrieve the surface temperature of the coal particle through processing the images taken by a color CCD camera. At the same time, a mathematical model considering thermal conduction inside the coal particle was developed to simulate the ignition process.Both experiments and modeling found that ignition occurred homogeneously at the beginning and then heterogeneously for the testing coal particles burning at μ-g. Experimental results confirmed that ignition temperature decreased with increasing volatile content and increasing particle size. However, contradicted to previous studies, this study found that for a given coal with certain particle size, ignition temperature was about 50–80 K lower at μ-g than that at 1-g.The model predictions agreed well with the μ-g experimental data on ignition temperature. The criterion that the temperature gradient in the space away from the particle surface equaled to zero was validated to determine the commence of homogeneous ignition. Thermal conduction inside the particle could have a noticeable effect for determining the ignition temperature. With the consideration of thermal conduction, the critical size for the phase transient from homogeneous to heterogeneous is about 700 μm at ambient temperature 1500 K and oxygen concentration 0.23.     

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.     

镁颗粒群着火和燃烧过程数值模拟

建立了一维非稳态球形镁颗粒群的着火燃烧模型, 数值模拟镁颗粒群的着火和燃烧过程, 研究表明, 颗粒群着火首先发生在颗粒群边界, 随后初始的燃烧火焰会分离为两个, 一个向颗粒群内部传播, 一个向外部传播, 最终内部火焰消失, 外部火焰维持并控制着整个颗粒群的燃烧; 内火焰向颗粒群内部传播过程中, 传播速度会逐渐加快, 且火焰温度值呈逐渐降低趋势. 分析了颗粒群内部参数和环境参数对镁颗粒群着火燃烧的影响. 随颗粒浓度的增大, 颗粒群着火时间略有增长, 但火焰传播速度更快, 燃烧稳定时火焰球尺寸也更大. 颗粒群初温越高, 则颗粒群着火时间越短, 火焰传播速度也会加快, 但燃烧稳定时火焰球尺寸基本不变. 环境温度对颗粒群着火燃烧的影响较复杂, 环境温度越高, 颗粒群着火时间越短, 但火焰传播速度却越慢, 燃烧稳定时火焰球尺寸变化很小. 颗粒粒径和辐射源温度对颗粒群着火燃烧的影响较显著, 颗粒粒径越小或辐射源温度越高, 则颗粒群着火时间越短, 火焰传播速度越快, 燃烧稳定时火焰球尺寸也越大. 数值模拟结果与文献中试验结果相一致. 关键词： 粉末燃料冲压发动机 镁着火燃烧 颗粒群     

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.     

3D problem of heat and mass transfer at the ignition of a combustible liquid by a heated metal particle

A nonlinear nonstationary 3D problem of heat and mass transfer at gas phase ignition of a combustible liquid spread on the surface of a solid body by a metal particle heated to a high temperature is solved. This is done within the framework of a model taking into account the heat conduction and evaporation of the liquid, the diffusion and convection of the combustible vapors in the oxidizer medium, the crystallization of the ignition source, the kinetics of the processes of evaporation and ignition of liquids, the dependence of the thermophysical characteristics of the interacting substances on the temperature, and the moisture content of the oxidizer—air. The dependences of the ignition delay time of the liquid on the temperature and sizes of the heating source are established. Limiting values of the temperature and particle sizes at which the ignition conditions take place are determined. The influence of the air humidity on the inertia of the process being investigated is analyzed. A comparison of numerical values of typical parameters of the process under investigation for 2D and 3D models is performed.     

Prediction improvements of ignition characteristics of isolated coal particles with a one-dimensional transient model

A modified 1-D transient model considering intra-particle thermal conduction is adopted to improve the predictions of the ignition characteristics of isolated coal particles. The study aims at resolving the incorrect prediction on the variation trend of ignition temperature T_i with the change of oxygen concentration X_O2, interpreting the contradictory dependencies on coal particle size and furnace temperature and clarifying the conditions when the intra-particle thermal conduction should be considered. The predictions are compared with microgravity data in which the buoyancy effect is minimized. The results reveal that the previous ignition model with transient adiabatic criterion fails to predict the T_i variation with X_O2, since it cannot accurately predict T_i and delay time in the low X_O2 region. Instead, the ignition model with flammability limit ignition criterion can well predict T_i in a wide range of X_O2. Intra-particle thermal conduction causes remarkable temperature differences for large coal particles, and moreover, the variation trends of surface and center temperatures with particle size are very different. The center temperature at ignition drops remarkably with increasing particle size, while the surface temperature barely changes or slightly increases with particle size. At the same particle size, the variation trends of surface and center temperatures with furnace temperature are also opposite. The ignition mode and variation trend of ignition surface temperature with particle size depends on the heating rate and particle size itself. The contradictory experimental results reported by different researchers are attributed to the particle size and temperature measurement location. The conditions necessary to consider the intra-particle thermal conduction are discussed. Lastly, the effect of the intraparticle thermal conduction is shown on an ignition mode diagram.     

Effect of the shape of a particle heated to a high temperature on the gas-phase ignition of a liquid film

The process of gas-phase ignition of a liquid fuel film with incandescent small metal particles in the form of a parallelepiped, disk, or hemisphere was numerically simulated. The magnitude of influence of the particle shape on the delay time of ignition of a liquid fuel was determined. The range of parameters of the particle at which the effect of its shape on the ignition delay time is unimportant was established.     

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.     

Numerical simulation of solid-phase ignition of metallized condensed matter by a particle heated to a high temperature

Numerical simulation of the ignition of a composite propellant by a single “hot” particle of metal is carried out in the framework of the solid-phase model of ignition. The dependences of the ignition lag time for a metallized condensed matter on the initial temperature of a local energy source are determined. Close agreement of the obtained theoretical results with the known experimental data is found.     

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.     

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.     

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.     

Spontaneous ignition temperature for the compacted mixture of Ni–Al system: Experiment,theory, and comparisons

Spontaneous ignition of compacted mixture has been examined not only experimentally but also theoretically, relevant to materials synthesis for Ni–Al system. Spontaneous ignition temperature, determined from the inflection-point of the temporal variation of surface temperature, is found to decrease at first, reach the minimum, and then increase gradually, with increasing size ratio, being defined as the ratio of compact and particle diameters and reported to be useful in correlating experimental results. The lowest temperature observed is as low as that, more than 200 K below the melting point 934 K of Al, at the size ratio of c.a. 700. While its decrease is attributed to the increase in the particle surface per unit spatial volume of compacted mixture, as reported in the literature, its increase found in the course of the present study can fairly be correlated to an enhancement of heat loss from each particle in the compacted mixture, by conducting a theoretical consideration. As for the dependence of spontaneous ignition temperature on the mixture ratio, a shifting occurs from a decreasing trend with increasing mixture ratio at small size ratios, less than about 700, to an insensitive trend to the mixture ratio, had not been reported in the literature. In addition, a fair degree of agreement, shown in experimental comparisons with theoretical results, indicates that the present formulation has captured the essential features of the spontaneous ignition of compacted mixture, especially for size ratios with large values. Since this kind of particle size effects, relevant to the spontaneous ignition of the compacted mixtures, has not been captured in the previous studies, its elucidation can be considered not only notable but also useful, in manipulating combustion process in materials synthesis, especially, in choosing a specific condition for the lowest spontaneous ignition temperature by varying the size ratio.     

Spontaneous ignition temperature for the compacted mixture of Ti–Al system: Theory and experimental comparisons

Spontaneous ignition of compacted mixture has been examined not only experimentally but also theoretically, relevant to materials synthesis by combustion for Ti–Al system. By varying compact and particle sizes, mixture ratio, and degree of dilution, spontaneous ignition temperature has been measured, which is determined from the inflection-point of the temporal variations of the surface temperature. It is found that the spontaneous ignition temperature decreases with increasing aspect ratio, defined as the ratio of the sample height and compact diameter, due to an establishment of the stationary temperature distribution in the radial direction in the compacted mixture, as the sample height becomes tall. It is also found that the spontaneous ignition temperature decreases with increasing size ratio, defined as the ratio of compact and particle diameters, due to an increase in the particle surface per unit spatial volume of the compacted mixture, caused by a decrease in the particle diameter and/or an increase in the compact diameter. By further examining its dependence on mixture ratio and/or degree of dilution, it is confirmed that the limit of flammability also exerts influences on the spontaneous ignition temperature. In addition, a fair degree of agreement, shown in experimental comparisons with theoretical results, indicates that the present formulation has captured the essential features of the spontaneous ignition of compacted mixture. Since this kind of particle size effect, especially, relevant to the spontaneous ignition of compacted mixtures, has not been captured in the previous studies, its elucidation can be considered not only notable but also useful, especially, in manipulating combustion process in materials synthesis.     

Effect of the thermophysical properties of the material of a local energy source on conditions and characteristics of ignition of metallized composite propellants

Solid-phase ignition of metallized composite propellants by a single particle heated to a high temperature under conditions of an ideal thermal contact has been numerically studied. The effect of the thermophysical properties of the material of a local energy source on the conditions and characteristics of ignition of composite propellants has been analyzed. It has been found that sources with a high heat storage capacity exhibit shorter ignition delay times for metallized propellants (by 10–60%) and lower initial temperatures required to initiate the combustion process (by 170 K). In addition, it has been found that the presence of particles of metals (boron, aluminum, magnesium, lithium) in the propellant composition leads to an increase in the effective thermal conductivity of the propellant. The cumulative effect of the thermophysical properties of the materials of the “particle heated to a high temperature–metallized composite propellant” system leads to an increase in the ignition delay times (by 25–65%) and the heat penetration depth of the near-surface layer of the propellant (by 25–40%) at the time of combustion initiation compared with metal-free compounds.     

Heat and mass transfer at ignition of liquid fuel droplets spreading over the surface of massive hot bodies

Processes of heat and mass transfer with phase transitions and chemical reactions at the ignition of a liquid fuel droplet colliding with the surface of a hot metal substrate are numerically investigated. The droplet ignition delay times are found. The scale of the influence of the temperature of the substrate, droplet, and oxidizer, and also the droplet size and spreading rate on the ignition inertia is determined. Conditions in which the liquid fuel droplet spread plays an important role in the ignition process are found.     

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.     

α粒子的慢化过程对D-T等离子体聚变燃烧的影响

在双温聚变燃烧点模型框架下，对比D-T等离子体聚变燃烧过程中α粒子能量逐步沉积与瞬时沉积两种描述的等离子体温度、离子数密度随时间的变化，在不同的密度条件下作了计算，考察了α粒子的慢化过程对D-T聚变点火的影响.发现考虑α粒子的慢化过程后，D-T等离子体峰值温度的出现将会推迟若干皮秒甚至几十皮秒，在较低的初始温度密度条件下，时间推迟得更多些.等离子体的峰值温度比α粒子能量瞬时沉积描述也会下降13keV左右. 关键词： α粒子 聚变燃烧 能量沉积 慢化过程     

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.     

Analysis of possible reasons for macroscopic differences in the characteristics of the ignition of a model liquid fuel by a local heat source and a massive heated body

A numerical study of the ignition of the liquid fuel drop-massive heat source-air and liquid fuel-small-size heat source-air systems was performed. It was established how the ignition delay times of single drops and large amounts of liquid fuel depend on the temperature of the heated body. Possible modes of ignition of a typical fuel by small and extensive heat sources were identified.