G.M. Makhviladze

On the theory of flashover development

The characteristics of hot smoke layer development from ignition to flashover are obtained for a fire in a compartment with one opening. Flashover itself as a final stage of development is analysed using heat balance considerations for the developed hot layer (in direct contact with the external atmosphere through the opening) on the basis of a two-zone model and classical thermal explosion theory. Flashover is reasonably assumed to take place during the early development of the fire when the density and temperature of the lower zone may be taken to be close to their initial values. Critical conditions for flashover to occur are found in a general form in terms of the determining similarity criteria. The adiabatic flashover induction period in the absence of heat losses, which is a useful qualitative and quantitative characteristic, is given. Calculation of the flashover induction time allows for an evaluation of the total time for fire growth from ignition to flashover, with which we place the study of critical conditions in the context of the full fire scenario. The results obtained give simple but general enough formulae for representation of the flashover phenomenon.

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.

The effects of the thermal inertia of the walls upon flashover development

In recent studies by the authors of the early development of a compartment fire simple criteria have been found using the zone model approach to determine if flashover would occur, and to find the temperature/time characteristics of the fire development. The cases of compartment walls of low/high thermal inertia were examined and the temporal characteristics found. Analysis was performed in terms of dimensionless groups. In this follow-up paper the case for walls of arbitrary thermal inertia, which is shown to have a significant effect is addressed. More general formulae are developed and the validity of using the special cases described in the previous study as useful approximations is examined.

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...

The effect of particle coagulation and fractal structure on the optical properties and detection of smoke

Numerical simulation and analysis of the effect of coagulation on the optical properties of smoke and its detectability are given for a burner fire in a multi-room environment. The specific extinction coefficients are calculated for smoke composed of polydisperse spherical particles and soot fractal-like aggregates. Qualitatively different dependencies of specific extinction on mean-volume diameter and, due to coagulation, smoke residence time are illustrated for these kinds of smoke by 2-D and 3-D simulations of fire-driven smoke flow. The light-scattering and ionisation detector response and time needed to trigger an alarm are compared in terms of the coagulation effect. Calculations without consideration of coagulation underestimate the response of the light-scattering detector and overestimate the response of the ionisation detector. The inclusion of coagulation and the fractal-aggregate description reduces the sensitivity of modelling results to uncertainty in input data concerning the size of the smoke aerosol particles inside the flame zone.

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.

Turbulent diffusion combustion under conditions of limited ventilation: flame projection through an opening

Development of turbulent diffuse flame in a compartment with a vertical opening is studied experimentally and numerically. Flame projection through the opening observed under conditions of limited natural‐convective ventilation is considered. The measurements are performed in a laboratory box designed for compartment‐fire simulation. The critical (minimum) flow rate of the fuel sufficient for flame projection is determined, as well as the delay between fuel ignition and flame projection with subsequent establishment of external combustion. Dimensionless variables for processing experimental data are proposed. A generic empirical dependence of the dimensionless time of flame projection on the dimensionless flow rate of the fuel is obtained for various opening sizes, burner positions, and box sizes. The dimensionless critical flow rate of the fuel obtained is in agreement with the previously published measurements performed for gaseous and condensed fuels. Unsteady stages of flame evolution before the projection and scenarios of flame projection through the opening are identified and analyzed. A three‐dimensional numerical model is developed for calculating turbulent diffusion combustion in a compartment with an opening. The model takes into account the conjugate radiative‐convective heat transfer on solid surfaces and the thermal conductivity of the wall and floor materials. The experimentally observed stages of flame development, flame projection through the opening, and stabilization of external combustion are reproduced in numerical calculations. The numerical values of flame‐projection time are in good agreement with the measurement results and proposed empirical relation.

An implicit finite element solution of thermal flows at low Mach number

Thermal flows at low Mach numbers are a basic problem in combustion, environmental pollution prediction and atmospheric physics areas. Most of the existing schemes for solving this problem treat convection explicitly, which confines time step width due to the CFL condition. In this paper, based on the pseudo residual-free bubble approach [F. Brezzi, L.P. Franca, T.J.R. Hughes, A. Russo, b=@!g, Methods Appl. Mech. Eng. 145 (1997) 329-339; T.J.R. Hughes, Multiscale phenomena: Greens functions, the Dirichlet-to-Neumann formulation, subgrid scale models, bubbles and the origins of stabilised methods, Method. Appl. Mech. Eng. 127 (1995) 387-401], we introduce an implicit finite element scheme for the thermal flow problem. We firstly give a low Mach number asymptotics of compressible Navier-Stokes equations for the thermal flows and then derive the numerical scheme for them in detail. Three representative case studies are used to investigate and to test the numerical performances of the proposed scheme.

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.