J.P. Roberts

Combustion of two-phase hydrocarbon fuel clouds released into the atmosphere

Abstract Numerical modeling of the evolution, behavior, and combustion of two-phase hydrocarbon clouds released into the open atmosphere is presented. A Eulerian-Lagrangian model for transient flows of fuel vapor–droplet mixtures is formulated taking into account heat, mass, and momentum exchange between the gaseous and dispersed phases, soot formation, and radiative heat transfer. The calculations are performed for releases of pressure-liquefied propane; the total mass of fuel released varied in a wide range from 1 g up to 1000 kg and prerelease temperature 268–351 K. Formation and evolution of a two-phase cloud following a short-duration release of pressure-liquefied gas is first considered without ignition. Parameter ranges corresponding to mixing-controlled and diffusion-controlled regimes of evaporation are obtained. The time for total evaporation of liquid fuel droplets is determined and the structure of the cloud is analyzed. Fireball development upon ignition of the fuel cloud is studied and the main stages of its evolution from reaction initiation until total fuel burnout are considered in detail. The calculated fireball shape and dynamics of ascent are shown to correlate quite well with the data from the Hasegawa-Sato experiments. The role of scale effects is studied by comparing the structure and gross characteristics of fireballs calculated for different fuel masses and storage conditions. The calculated dependence of the nondimensional fireball burning time on the Froude number agrees well with the experimental data. Radiation field distributions in fireballs of different scales are obtained and differences between optically thin and thick clouds are demonstrated. The radiative fraction of total combustion energy is shown to correlate well with available experimental data on turbulent propane flames.

Numerical modelling of fireballs from vertical releases of fuel gases

Abstract Evolution and combustion of fuel gas discharged into the atmosphere is simulated numerically. Finite-duration vertical releases are considered, the ignition occurring on the axis at some elevation above the source. The main stages of the release and burning are discussed, spatial distributions of concentrations, temperature and reaction rate in the fireball are presented at various stages of evolution, from ignition up to total burnout. Influence of the release parameters (fuel mass, injection velocity, orifice size, ignition source height) on the lifetime of the fireball is studied for methane and propane releases. The dependence of the fireball burning time on the Froude number (defined as the square of the ratio of the release velocity and the characteristic buoyant velocity) is obtained. The effect of the source size and ignition source location is shown to be much less significant. The results of calculations are compared with the experimental data on small-scale methane and propane fireball...

Effect of Lewis number on flame front fragmentation in narrow closed channels

This work is concerned with the numerical investigation of the influence of the ratio of thermal conductivity to diffusivity of a combustible gas mixture on the dynamics of adiabatic flame propagation in narrow closed planar channels under zero gravity. Reynolds numbers are less than 100. A low Mach number limit of the Navier-Stokes system for the A → B reaction was used. The appearance of a flame front bifurcation, resulting in flame inversion, was obtained numerically for low Lewis numbers. The combined effect of a specific hydrodynamic flow field in a closed channel and of the domination of diffusivity over thermal conductivity, similar to the mechanism of thermal-diffusive instability, was found to be the main driving force of the bifurcation. It was concluded that, both numerically and experimentally, the inverted flame front has the same general shape. However, this shape is unlikely to be attributed to the same physical mechanism of its formation over the entire range of regimes. Fragmentation of inverted flames into two segments is clearly observed for low Lewis numbers.

Fireball during combustion of hydrocarbon fueld releases II. Thermal radiation

The processes of radiative heat transfer in a fireball which develops upon ignition of a cloud of hydrocarbon fuel near the Earth’s surface are simulated numerically. The emissive characteristics of combustion products (mixtures of nitrogen dioxide, water vapor, and soot) are described using the weighted-sum-of-gray-gases model with temperature-dependent weighting coefficients. The radiation field in the fireball for individual gray gases is calculated in a diffusion approximation (gases for which the fireball is optically thick) or in a volume emission approximation (gases for which the fireball is optically thin). Results of calculations for propane fireballs with fuel masses of 1 g to 103 kg are presented. The role of scale effects is analyzed by comparing the spatial distributions of the radiative source term for fireballs of different dimensions. It is shown that the radiation of burning clouds of small scale proceeds uniformly over the volume, whereas fireballs of large scale radiate predominantly from the surface. The calculated fraction of energy converting to radiation is in good agreement with literature data. The radiation field outside fireballs and the fluxes on the surface are calculated by the Monte Carlo method. The dose of energy incident on the surface during burning of a fireball is calculated.

Fireball during combustion of hydrocarbon fuel releases. I. Structure and lift dynamics

The formation, combustion, and thermal interaction of the fireballs which develop upon ignition of a cloud of hydrocarbon fuel near the Earth’s surface are simulated numerically. The axisymmetric nonstationary flow is described by a system of Favre averaged conservation equations invoking a (k−ε)-turbulence model, a model for turbulent combustion, and a global-kinetic scheme for formation and burnup of soot particles. The optical properties of the mixture of combustion products and soot are modeled by a weighted sum of gray gases. The radiation field is calculated using a combination of a volume emission approximation and a diffusion approximation. Calculations are done for fireballs formed during vertical releases of gaseous propane masses of 1 g to 103 kg with ignition near the release point. The internal structure of a fireball is analyzed in detail at various stages of its evolution. The lift dynamics of a fireball is illustrated for release velocities corresponding to Froude numbers (defined as the square of the ratio of the linear outflow velocity to the characteristic velocity owing to buoyancy forces) ranging from 5–250. The temperature, concentrations, and reaction rates in the fireball are determined as functions of time. It is shown that for these ranges of fuel mass and release velocity, the dimensionless parameters introduced here can be used for scaling the results and using the calculated dependences obtained here in a unified fashion.

Numerical analysis of laminar combustion of fuel gas clouds

Combustion of finite amounts of fuel gas in an oxidizer atmosphere is studied numerically using one-dimensional spherical geometry. Calculations are performed for nonpremixed octane fuel clouds ignited at the periphery and for premixed clouds ignited at the center or at the periphery, with the reaction rate described by a one-step global chemical kinetic scheme. Flame front coordinates are obtained as function of time for different initial fuel concentrations. The numerical results obtained are compared with the analytical solutions available. For initially unmixed fuel clouds the effects of variation of gas properties with the temperature are studied. Solutions obtained for constant density and transport coefficients are compared with the results obtained in the calculations where density and transport coefficient variations were alternatively taken into account. Gas expansion is shown to change the cloud size up to several times while increase in the transport coefficients results in higher burning rates. Both factors make the characteristics of cloud combustion different from those studied previously assuming constant gas properties. Combustion of fuel-rich premixed clouds is featured through the existence of two reaction zones, of which one propagates as a premixed flame, while the other one is a diffusion flame burning out the remaining fuel. The trajectories of both flames are studied in detail for the cases of central and peripheral ignition. The dependencies of the burnup time on the initial fuel concentration obtained for the unmixed and premixed clouds are compared.

Smoke Coagulation in Compartment Fire Modelling

Abstract The effect of coagulation on the total smoke particles concentration and mean particles volume fields is studied under the conditions of a compartment fire. An approximate analytical consideration of aerosol temporal behaviour accounting for the simultaneous action of aerosol generation, coagulation, and dilution is given, and two regimes of fast and slow coagulation are considered. A CFD model of compartment fire, including smoke aerosol generation and coagulation, is presented. The results of CFD simulation for a hydrocarbon burner sooting fire with smoke movement and coagulation in a two room compartment are analysed, where the regimes of coagulation and their key features predicted by means of analytical consideration are illustrated by numerical simulations. For a given geometry of the compartment, fuel type, and volumetric smoke generation rate, the coagulation rate was shown to be determined by the mean volume of the particles generated in the flame zone. In the limit of small particles generated (fast coagulation regime), the aerosol parameters far away the flame zone were found to be weakly dependent on those inside the flame zone due to a strong coagulation effect. In the opposite limit of large particles generated (slow coagulation regime), coagulation has no effect on the aerosol parameters. The approximate boundary between the two regimes has been defined.

Turbulent buoyant thermal in a densitystratified atmosphere

A model of a turbulent axisymmetric thermal rising in an atmosphere with altitude-dependent density is proposed. A numerical study of the thermal is performed for an isothermal atmosphere with exponentially decreasing density. The structure of the thermal corresponding to the square-root law of the ascent is obtained in similarity coordinates. An analytical solution is found for a wide class of the density variation functions and studied in detail for an inverse-square law of density diminution with height. It is shown that as the thermal penetrates the low-density atmospheric layers, its top edge becomes less sharp and the width of the thermal increases. However, the self-similar coordinate of the cloud top remains almost constant and the square-root law of the ascent still holds.

Modelling of atmospheric pollution by explosions

Abstract A mathematical model of air pollution by gaseous and particulate substances caused by near-ground explosions is developed. The model describes the evolution of a hot cloud of explosion products and the transport of the contaminants by upward convective flows. A software package APEX ( A tmospheric P ollution by EX plosions) is developed for IBM PC/386/486 computers. The capability of the model is demonstrated on two problems - pollution of the atmosphere by fine dispersed particles carried aloft by a nuclear burst cloud (a large-scale thermal) and air pollution after an explosion and release of a hazardous particulate contaminant (a small-scale cloud). The calculated time-dependent evolution of a hot cloud and the transport of the pollutants by the rising thermal are demonstrated. Final distributions of the particulate matter in the atmosphere after the rise of the hot cloud are presented.

Formation and combustion of gas clouds in accidental discharge to the atmosphere

The formation and combustion of a fire-hazardous cloud arising in accidental discharge of a flammable gas is analyzed. A criterion is obtained which makes it possible to identify instantaneous, continuous, and intermediate types of discharge from the known physicochemical properties of the material, vessel and hole configurations, and storage conditions. Relations for the critical hole dimensions corresponding to the boundaries between the different discharge regimes are given. Ranges of parameters for which ignition of the outflowing gas can give rise to a flame or a fireball are established. The most probable limits for the coefficient of fuel participation in the burning fireball for various ignition delays are determined. The results are compared with the available experimental data.