Flash-Fire Radiation Models

Because the Raj and Emmons (1975) expression for tv cannot be applied in a straightforward manner, the expression given here differs from that recommended by Raj and Emmons (1975). It should be emphasized that w, which represents the inverse of the volumetric expansion due to combustion in the plume, is highly 

The model gives no solution for the dynamics of a flash fire, and requires an input value for the burning speed S. From a few experimental observations, Raj and Emmons (1975) found that burning speed was roughly proportional to ambient wind speed U  

Radiation effects from a flash fire are now fully determined if vapor cloud composition, as well as the geometry of the flame front (dependent on time), is known. Vapor cloud composition is, of course, place- and time-dependent, and the shape of flame front will greatly depend on cloud shape and ignition site within the cloud. The total radiation intercepted by an object equals the surmnation of contributions by all successive flame positions during flame propagation. This is an impossible value to compute with the simplified approach just described. Because there are many uncertainties (e.g., cloud composition, location of ignition site) which greatly influence the final result, a conservative approach is recommended for practical applications  

The radiative power per unit area intercepted by some plane in the environment can now be computed from  

The fundamentals of thermal radiation modeling are treated in Chapter 3. The value for emissive power can be computed from flame temperature and emissivity. Emissivity is primarily determined by the presence of nonluminous soot within the flame. The only value for flash-fire emissive power ever published in the open literature is that observed in the Maplin Sands experiments reported by Blackmore 

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Hash-fire modeling is largely underdeveloped in the literature there are large gaps in the information base. Hardly any information is available concerning flash-fire radiation the only data available have resulted from experiments conducted to meet other objectives. Many items have not yet received sufficient attention. 

The only computational approach found in the literature to modeling flash-hre radiation is that of Raj and Emmons (1975), who modeled a flash fire as a two-dimensional, turbulent flame propagating at a constant speed. The model is based on the following experimental observations ... 

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. 

Atmospheric releases of flammable gases such as hydrogen may lead to major fires with extensive effects on the surroundings. In activities where hazards are associated with cloud fires, there is the need of societal risk assessment that involves the estimation of hazardous zones due to the resulting thermal radiation. However, till now only limited work has been done on modeling the effects of flash fires, in a way that available techniques may be judged insufficient [47],... 

A flash or vapour cloud fire is defined in [37] as the combustion of a flammable gas or vapour and air mixture in which the flame propagates through that mixture in a manner such that negligible or no damaging overpressure is generated . There are few models for treating flash fires. The objective of a model is to determine the heat radiation as a function of distance from the surface of the cloud. Whilst it may safely be assumed that a person inside the cloud dies, the amount of... 

Flash fire models—if based on flame radiation—are subject to large error if radiation is estimated incorrectly, because predicted radiation varies Avith the fourth power of temperature. 

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