ISSN:0010-2202    

DOI:10.1080/00102209508907748

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

This paper presents an investigation of the behavior of rich hydrogen-air flames near the flammability limit. Ignition processes arc simulated using a computational model which involves the solution of the governing equations (for one-dimensional geometries) by implicit methods on an adaptive non-uniform grid, using detailed chemistry and a multi-species transport model. The reaction mechanism consists of 37 elementary reactions and 9 species. Calculations have been performed for different one-dimensional geometries. An investigation of the fundamental flammability limit {intrinsic to the combustion system itself), which is governed only by the physical and chemical processes in the gaseous mixture, is carried out by eliminating external factors such as heat loss, buoyancy, etc, in the calculations. Compulations have been performed for hydrogen-air mixtures of varying hydrogen content. Mixtures containing 75% or less hydrogen are found to be steadily propagating, whereas mixtures containing 82% or more hydrogen extinguish. Inbetween these bounds, an oscillatory flame propagation is observed, which is characterized by an oscillation between a fast and a slow flame propagation. Furthermore, the effect of the pressure on the flammability limit is studied

ISSN:0010-2202    

DOI:10.1080/00102209508907749

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Measurements of the composition of stable and unstable species and temperature in laminar, premixed, flat flames of H2-NO2-Ar have been made and compared to the structure calculated with a flame code including detailed chemical kinetics. No previous detailed flame structure measurements and chemical kinetic modeling of this flame have been presented. Similar measurements and calculations are reported for a companion H2-O2-Ar flame in order to provide a comparison to a previously well characterized flame. Measurements were made of stable species concentration by probe sampling and mass spectrometric sample analysis. Measurements of relative OH concentration and of temperature were made by laser-induced fluorescence. Agreement between the measurements and calculations of stable species concentrations in the H2-NO2-Ar flame using existing chemical kinetic mechanisms with no adjustments to the kinetic parameters is discussed. The reaction mechanism is dominated by the three elementary reaction steps given below:

ISSN:0010-2202    

DOI:10.1080/00102209508907750

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The motivation for the present study is the attempt to enhance the understanding of the influence of inhomogeneous turbulence influence upon autoignition processes, which are of interest for practical applications such as the Diesel engine. Special attention is paid to the modelling of chemical source term closure and its coupling with inhomogeneous turbulence. Detailed two-dimensional simulations of turbulent autoignition with flamelet source term closure have been performed using a laminar flamelet presumed pdf model (pdf== probability density function). The transient autoignition calculations of the laminar flamelet are based on a reduced 4-step chemical kinetic mechanism of heptane combustion in air. New ways of modelling turbulent chemical source term closure are investigated. Specifically, an ignition time variable is introduced to characterize the aspect of transient chemical reaction during the turbulent autoignition process. The model successfully covers the period from early autoignition to flame propagation

ISSN:0010-2202    

DOI:10.1080/00102209508907751

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

This paper presents the formulation and results of numerical simulation of autoignition in gaseous, nonpremixed, turbulent flows. Two specific problems are considered, (a) a homogeneous, isotropic turbulence with chemical reactions and mixing, and (b) an axisymmetric, fuel jet released into an oxidiser environment. For the first problem the fluid mechanics and combustion are decoupled, while the second problem fully incorporates such a coupling. A hybrid computational approach is adopted wherein the governing mean flow and the κ — ϵ equations are solved through a finite-volume, predictor-corrector, pressure implicit code TURBO-2D and the joint scalar pdf-transport equation (pdf= probability density function) is simulated via a Monte Carlo technique. The flow code and the pdf code togelher run in tandem with each other. The fluid dynamics code supplies the mean flow and turbulence quantities to the pdf code. The pdf code, in turn, provides the mean mass density via the mean thermochemical scalars to the fluid dynamics code.

ISSN:0010-2202    

DOI:10.1080/00102209508907752

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Autoignition histories are considered under conditions such that the compression and heat release occur sufficiently rapidly that molecular transport is negligible during the ignition and propagation processes. The objective is to account for arbitrary spatial variations of temperature and composition that may be present when ignition begins. A one-step, Arrhenius reaction of large activation energy and arbitrary orders with respect to both fuel and oxidizer describes the.chemistry. Expressions are obtained for the ignition time and for the rate of pressure buildup after ignition in terms of the temperature and species concentrations that exist in the nonreacting, multiphase, turbulent flow just prior to or shortly after ignition

ISSN:0010-2202    

DOI:10.1080/00102209508907753

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The autoignition process of fuel sprays has relevance to compression-ignition engines. The present study focusses on a detailed description of the processes involved. In particular, variable liquid and gas properties, detailed transport in the gas phase, and detailed reaction mechanisms are considered. The model uses a simplified geometry and assumes a uniform spray. Results of numerical calculations of the autoignition of methanol, ethanol, n-heptane, and n-octane are presented, where special attention is focussed on differences in liquid properties. It appears that the vaporization rate is dominated by the ratio MF/φ, the molecular weight of the fuel over the molar ratio of oxygen to fuel, and by the latent heat of vaporization, Lv. For the fuel methanol. both MF/φand Lvare largest, so that in spite of the high volatility of methanol. both spray lifetime and ignition delay are largest for a methanol spray compared to ethanol, n-heplane and n-octane. Ignition delay increases with initial droplet size if pure air is used as the oxidizer. There is a minimum ignition delay time when the air is premixed with fuel vapor

ISSN:0010-2202    

DOI:10.1080/00102209508907754

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A new theoretical model for the low-temperature ignition process of magnesium particles is proposed. The model is based on physical-mechanical effects resulting from the buildup of oxide layers on the particle surface and their possible destruction as a result of mechanical stresses attributed to thermal and pressure expansion which are due to temperature elevation, melting and, in some instances, liberation of dissolved hydrogen and generation of magnesium vapors. The model attempts to describe and predict peculiar and unexplained phenomena, which have been observed experimentally during the low temperature ignition of magnesium particles, such as increase or pulsations in particle size, formation of hollow spheres, and fragmentation of particles

ISSN:0010-2202    

DOI:10.1080/00102209508907755

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

An interesting phenomenon, later on termed solid flame, was discovered in 1967 by the author in collaboration with I. P. Borovinskaya and V. M. Shkiro. At that time, it was considered as an unconventional burning mode of solids yielding only solid reaction products with no gas or liquid. In the solid flames, the predominant transport processes are heat conduction and diffusion. Studies of solid flames resulted in the development of the method of self-propagating high-temperature synthesis (SHS) of inorganic compounds and materials. The method has found wide practical applications and substantially extended the scope of scientific enquiry. Studies then were initiated on the processes of the solid flame type involving liquids and gases as reactants. Attention was paid to a wide class of reactions, in which solid products were formed via liquid or gaseous intermediates. In these processes, of importance are the more complicated modes of mass transfer, such as convection and filtration. Novel phenomena of the fluid dynamics nature have been found to occur in SHS waves. This paper is the first attempt to thoroughly consider and systematize the fluid dynamics phenomena taking place in the SHS waves of various types, such as gasless burning and the filtration- and condensation-controlled SHS processes. Also discussed are the phenomena of impurity degassing and self-purification, capillary spreading, gravitational impregnation, convective phase separation in multicomponent melts, surface and layer-by-layer burning, bifurcation and reflection of combustion fronts, filtration extinction, superadiabatic temperature rise, etc

ISSN:0010-2202    

DOI:10.1080/00102209508907756

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The behavior of an array of liquid oxygen droplets vaporizing and burning in a gaseous mixture of hydrogen and water vapor at conditions supercritical for both propellants is investigated. The main objective is to investigate the effect of species and temperature non-uniformity (due to the presence of other droplets) on a LOX droplet transcritical behavior. Previously validated high-pressure phase equilibria computation algorithms and appropriate methods for the evaluation of thermophysical properties over wide temperature and pressure ranges are used. A simplified “parallel stream” configuration is considered. However, the contribution of the mass vaporized to the overall mass flux, is included in a quasi-steady manner. Hence the longitudinal velocity is non-uniform. Finite-rate chemical kinetics is used with a reduced four-step mechanism, three of which are assumed to be at partial equilibrium. The droplets are followed in a Lagrangian manner. The model used to predict the droplet behavior includes transient liquid heating, internal circulation effects and allows for critical interface regression if the critical mixing conditions are reached and secondary atomization in the stripping mode if it is found to occur. It is shown that, high temperatures from the reaction zone, and decreasing relative velocity caused by both the droplet drag and the accelerating gas flow concur to limit the effect of stripping. In most cases investigated, the critical mixing temperature was reached at the droplet surface. However, the total gasification rate is controlled by the stripping rate and is more than one order of magnitude larger than the primary vaporization rate. The predicted droplet lifetime is consequently reduced by at least one order of magnitude

ISSN:0010-2202    

DOI:10.1080/00102209508907757

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor