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.     

Numerical simulation of laser ignition of a liquid fuel film

Numerical simulations were used to examine a set of interrelated physicochemical processes involved in the ignition of a liquid fuel film by a low-power laser beam. The delay time of ignition of a liquid fuel film and the ignition zone location were determined. The scale of influence of the power and radius of the laser beam on the ignition characteristics was determined. The ignition criteria of ignition were identified.     

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.     

The influence of radiative-convective heat transfer on ignition of the drops of coal-water fuel

Simulation results are presented for thermal treatment and ignition of coal-water fuel drops under conditions of radiative-convective heating. The data demonstrate reasonbble compliance between theory and experiment for the integral parameter of ignition process — the delay time of ignition. The radiative component of heat transfer is significant for parameters and conditions of ignition. The increase in the fuel particle size makes this influence bigger. Prognostic potential was evaluated for differnet models of radiative heat tarnsfer. The delay time of ignition obtained from radiative heat transfer model “grey wall” is in good agreement with experimental data. Meanwhile, the method based on radiation diffusion approximation gives the simulation data for delay time much higher than experimental data. It is confirmed that while the process of inflammation of a coal-water particle, the key impotance belongs not to fuel-oxidizer reactions, but rather to a chain of heat treatment events, such as radiative-convective heating, water evaporation, and thermal decomposition of fuel.     

Numerical investigation of pulverized coal particle group combustion using tabulated chemistry

The ignition and combustion of coal particle groups are investigated numerically in a laminar flow reactor. The Flamelet Generated Manifold method is extended to account for the complex mixture of gases being released during devolatilization, which is calculated with a competing two-step model. A second mixture fraction is introduced to include the mixing with the second methane fuel stream. The interactions of the gas phase with particles are modeled within a fully coupled Euler-Lagrange framework. To investigate the influence of particle groups on ignition and combustion, successively increasing densities of particle streams have been analyzed. The ignition delay time is increased significantly by higher particle densities. This delay is validated successfully with the available measurements. Moreover, the shape of the volatile flame was found to be strongly influenced by the particle number density inside the flame. A transition from spherical flames around single particles to a conical flame around the particle cloud could be found in numerical results as well as in experiments. As the primary mechanism for the substantial ignition delay and the formation of the flame, the increased heat transfer from the gas-phase to the particle group, resulting in lower gas-phase temperatures, was identified.     

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.     

Effect of the shape of an organic water–coal fuel particle on the condition and characteristics of its ignition in a hot air flow

The results of experimental studies of the effect of the shape of an organic water–coal fuel (OCWF) particle on its ignition delay time and the time of its complete burnout in a hot air flow are reported. Three most common shapes of real particles, such as spherical, ellipsoidal, and irregular-polyhedron-like, are considered. It is shown that the shortest ignition delay time and the time of complete burnout correspond to polyhedron- shaped OCWF particles. Conditions are identified under which this factor significantly influences the ignition characteristics. The experiments were carried out at initial particle sizes (averaged maximum values) of 0.5–5 mm and temperatures and velocities of the oxidant flow of 600–900 K and 0.5–5 m/s, respectively. The main components of the studied fuels were coal processing wastes and waste motor, turbine, and transformer oils.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

轻组份燃料对乳化油蒸发与着火影响的研究

本文研究了较组份添加剂对单滴乳化油着火规律的影响。实验采用挂滴法。实验中,测量了体积比为十二烷：正庚烷：水6：0：4,5：1：4,4：2：4,3：3：4的着火延迟时间。实验结果和数值计算表明,在乳化油中加入易挥发添加剂能有效缩短乳化油的着火延迟时间,着火延迟时间随着添加剂加入量的增加而缩短,当易挥发添加剂含量较大时,着火延迟随着添加剂加入量的增加,变化越来越缓慢。这一结果对解决在掺水量大的条件下保证柴油机中乳化燃料的正常点火与启动具有实用价值。     

Effects of fuel blending on first stage and overall ignition processes

The effects of blending ratio on mixtures of an alcohol-to-jet (ATJ) fuel and a conventional petroleum-derived fuel on first stage ignition and overall ignition delay are examined at engine-relevant ambient conditions. Experiments are conducted in a high-temperature pressure vessel that maintains a small flow of dry air at the desired temperature (825 K and 900 K) and pressure (6 MPa and 9 MPa) for fuel injections from a custom single-hole, axially-oriented injector, representing medium (7.5 mg) and high (10 mg) engine loading. Formaldehyde, imaged using planar laser-induced fluorescence, is measured at discrete time steps throughout the first and second stage ignition process and is used as a marker of unburned short-chain hydrocarbons formed after the initial breakdown of the fuel. The formaldehyde images are used to calculate the first stage ignition delay for each ambient and fuel loading condition. Chemiluminescence imaging of excited hydroxyl radical at 75 kHz is used to determine the overall ignition delay. At all conditions, increased volume fraction of ATJ resulted in longer, but non-linearly increasing, overall ignition delay. Across all of the blends, first stage ignition delay accounted for about 15% of the increase in overall ignition delay compared to the military's aviation kerosene, F-24, which is Jet A with additives, while extended first stage ignition duration accounted for 85% of the increase. It is observed that blends consisting of 0–60% by volume of the low cetane number ATJ fuel produced nearly identical first stage ignition delays. These results will inform the development of ignition models that can capture the non-linear effects of fuel blending on ignition processes.     

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.     

Characteristics of heat and mass transfer at ignition of a thin film of condensed liquid substance by hot particles of different configuration

Characteristics of a complex of heat-mass transfer processes with phase transitions and chemical reaction are numerically investigated at ignition of a thin film of typical liquid condensed substance by single hot metal particles shaped as a hemisphere, disc, and parallelepiped. The influence of a source configuration with a limited energy content on the ignition delay time of liquid is estimated. The ranges of heat source parameters at which the influence of the source configuration on the ignition delay time is minimal are determined.     

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.