Burning cloud velocity

Before the size of the flammable portion of a vapor cloud can be calculated, the flammability limits of the fuel must be known. Flanunability limits of flammable gases and vapors in air have been published elsewhere, for example, Nabert and Schon (1963), Coward and Jones (1952), Zabetakis (1965), and Kuchta (1985). A summary of results is presented in Table 3.1, which also presents autoignition temperatures and laminar burning velocities referred to during the discussion of the basic concepts of ignition and deflagration. 

In-cloud overpressure is dependent on outflow velocity, orifice diameter, and the fuel s laminar burning velocity. 

This concept can be generalized for more arbitrarily shaped clouds, provided that a reasonable estimate can be made of combustion process development in terms of burning velocity and flame surface area. According to Strehlow (1981), a conservative estimate of source strength is made by... 

These results were analytically reproduced by Taylor (1985), who employed a velocity potential function for a convected monopole. This concept makes it possible to model an elongated vapor cloud explosion by one single volume source which is convected along the main axis at burning velocity, and whose strength varies proportionally to the local cross-sectional cloud area. 

Rosenblatt, M., and P. J. Hassig. 1986. Numerical simulation of the combustion of an unconfined LNG vapor cloud at a high constant burning velocity. Combust. Science and Tech. 45 245-259. 

When a vapor cloud bums, there is always a leading flame front propagating with uniform velocity into the unbumed cloud. The leading flame front is followed by a burning zone. 

The subject of flash fires is a highly underdeveloped area in the literature. Only one mathematical model describing the dynamics of a flash fire has been published. This model, which relates flame height to burning velocity, dependent on cloud depth and composition, is the basis for heat-radiation calculations. Consequently, the calculation of heat radiation from flash fires consists of determination of the flash-fire dynamics, then calculation of heat radiation. 

The linear burning rate of a propellant is the velocity with which a chemical reaction progresses as a result of thermal conduction and radiation (at right angles to the current surface of the propellant). It depends on the chemical composition, the pressure, temperature and physical state of the propellant (porosity particle size distribution of the components compression). The gas (fume) cloud that is formed flows in a direction opposite to the direction of burning. 

Deflagration is the type of combustion with a subsonic burning velocity level. It occurs under non-adiabatic conditions and at densities lower than that of the unbumt mixture. Its propagation mechanism is conduction heating and free radical diffusion. Only fuel-air gas clouds at around stoichiometric mixture imply high flame front velocities and maximum expansion upon ignition, i.e., the ratio of the density of unbumt mixture to that of the reaction products, which is about 7 - 8 for hydrocarbons (in reality smaller by up to 50 % because of non-ideal micro-mixture), but < 1 for hydrogen. 

In the case of a vapor cloud formed by a turbulent jet the vapor concentration is rather inhomogeneous. The laminar flame speed depends on mean values of the volume expansion and the burning velocity ... 

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