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.     

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.     

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.     

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.     

Spreading of a viscous liquid film over the corrugated surface and the heat and mass transfer calculations

There are a lot of industrial applications of structured packing. Distillation columns are one of the examples where the liquid flows over the corrugated surface as a thin film to provide a good mass-transfer surface between the liquid and vapor phase. The purpose of the present paper is to study the hydrodynamics and the heat-mass transfer of the liquid film spreading down the corrugated surfaces when the corrugation amplitude is comparable with Nusselt’s film thickness (the amplitude corresponds to a small texture of the structured packing). As a result, a nonlinear type diffusion equation is obtained to describe the evolution of the film thickness profile. The nonlinear diffusion coefficient is obtained for three cases: a smooth inclined plate, a corrugated plate with large ribs, and an inclined corrugated plate with small ribs. The equations are solved numerically. As a result, it has been obtained that the small texture significantly increases the rate of the film thickness evolution in comparison with a smooth plate. To obtain the nonlinear diffusion coefficient in the case of a small texture, the hydrodynamics of the film flow over an inclined corrugated surface are studied. The viscosity, inertia, and surface tension forces are taken into account. The calculations were carried out on the basis of the Navier-Stokes equations. The influence of the microcorrugations on both the heat transfer from the wall and the mass transfer through the free surface was investigated.     

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.     

Heat and mass transfer at ignition of solid condensed substance with relatively low calorific power by a local energy source

Macroscopic laws of heat and mass transfer at gas-phase ignition of solid condensed substance with relatively low calorific power by a typical local energy source, namely, a small hot metal particle shaped as a parallelepiped, are investigated. The proposed model takes into account a group of interrelated processes of heat and mass transfer with thermal decomposition and chemical reaction in the interaction of solid and a source with limited energy content. The influence of the heat content of a local energy source on the characteristics of the process 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.     

On the theory of heat transfer in massive bodies

It has been shown that the allowance for the interactions of quasi-static gravity waves with density fluctuations makes thermal conductivity κ independent of temperature T; according to numerical estimates, the mechanism of this interaction is the most effective mechanism of interaction in bulk solids. The κ(T) dependence is calculated in a wide range of variations in the argument.     

On calculation of the rate of mass transfer by periodic waves propagating over the surface of a viscous liquid

A generalization of the Stokes classical drift model describing mass transfer by periodic waves propagating over the surface of an infinitely deep incompressible liquid to the case of a viscous liquid is constructed. An analytic expression is proposed for the velocity of an additional flow into which the liquid is involved by horizontal viscous shear stresses emerging between adjacent horizontal layers of the liquid participating in the drift.     

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < d_o < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < T_c < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.     

Heat and mass transfer on MFD flow through porous media over an accelerating surface in the presence of suction and blowing

The heat and mass transfer of electrically conducting fluid through porous media over an accelerating surface subject both to power law surface temperature and power law heat flux variations with a temperature-dependent heat source in the presence of a transverse uniform magnetic field is studied. A series solution to the energy and species concentration equation in terms of Kummer’s function is studied. The effect of Prandtl number and Schmidt number is studied with the help of graphs.     

Heat and mass transfer at adiabatic evaporation of binary zeotropic solutions

Results of numerical simulation of heat and mass transfer in a laminar flow of three-component gas at adiabatic evaporation of binary solutions from a flat plate are presented. The studies were carried out for the perfect solution of ethanol/methanol and zeotrope solutions of water/acetone, benzene/acetone, and ethanol/acetone. The liquid-vapor equilibrium is described by the Raoult law for the ideal solution and Carlson–Colburn model for real solutions. The effect of gas temperature and liquid composition on the heat and diffusion flows, and temperature of vapor-gas mixture at the interface is analyzed. The formula for calculating the temperature of the evaporation surface for the binary liquid mixtures using the similarity of heat and mass transfer was proposed. Data of numerical simulations are in a good agreement with the results of calculations based on the proposed dependence for all examined liquid mixtures in the considered range of temperatures and pressures.     

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.     

Conjugate heat and mass transfer at gas-phase ignition of a cable line under current overload

The nonlinear nonstationary problem of heat and mass transfer at ignition of an overloaded cable line that penetrates from one room to another through a partition is numerically solved. Dependences of the ignition delay time of a mixture of oxidizer and thermal decomposition components of a cable sheath on the current load and also on the geometrical sizes and the partition material are found.     

Monte Carlo study of the surface area of liquid droplets

Computer simulation of droplets containingl molecules (l 1000) in a lattice gas shows that the average surface area is proportional tol ; 0.6 in two and = 0.825 in three dimensions for small droplets. These exponents agree approximately with those in Kadanoff's modification of Fisher's droplet model near critical points [= (1 + )/; ourT/T _c is 0.4, 0.7, and 0.9]. For larger droplets, these exponents change to 1/2 (d = 2) and 2/3 (d = 3), the transition occurring for droplet diameters larger than the coherence length and smaller than the critical diameter in the nucleation of supersaturated vapors. This latter result rises some doubts on a recent nucleation theory of Eggingtonet al.     

Characteristics of conjugate heat and mass transfer in the flow of supersonic and hypersonic streams over blunt bodies

Problems associated with the formulation and solution of problems of conjugate heat and mass transfer in the supersonic and hypersonic motion of bodies are considered. The influence of the blunting geometry in streamline flow at different angles of attack, with gas injection into the shock layer, on the aerodynamic characteristics and the characteristics of heat and mass transfer is analyzed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 82–95, August, 1992.     

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.     

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.