(104e) A Flame Speed Correlation for Unconfined Gaseous Explosions

An approximate method for description of flame acceleration in congested areas filled with combustible gases is presented. The method takes into account both the flame folding due to interactions of the flame produced flow with obstacles and the increase of the burning rate due to turbulence generated in the flow ahead of the flame. A simple analytical expression for the flame speed is suggested. Coefficients in this correlation are determined by fitting the model predictions with a set of experimental data. This correlation is used then to evaluate the maximum flame speed that may be developed in vapor cloud explosions as a function of scale and obstacle density. As an example of applications, the flame speeds are evaluated for four typical fuels: methane, propane, ethylene, and hydrogen, and for three different levels of congestion. Both ideal stoichiometric mixtures of the four fuels with air and clouds with non-uniform concentration distributions are considered. The method is shown to give an appropriate account for mixture properties. In particular, the well known difference in combustion behavior of methane and propane despite of the similar values of the laminar burning velocities is well captured by the method. On the basis of the maximum flame speeds, the severity of the blast effect from unconfined gaseous explosions is evaluated. The results are compared with other methods for evaluation of the blast effect from unconfined vapor cloud explosions.