Burning rates in turbulent fine dust-air explosions

Turbulent burning velocities of a cornflour-air premixture (mean diameter 10.6 μm) have been measured by high speed schlieren cine photography, during the pre-pressure period in a fan-stirred explosion bomb. Measurements were at different fan speeds, to ascertain the effects of turbulence. It was found that the correlation of the ratio of turbulent to laminar burning velocities with the ratio of effective r.m.s. turbulent velocity to laminar burning velocity and the Karlovitz flame stretch factor is similar to that with gaseous premixtures. This suggests the importance of gas phase reactions in driving the flame propagation. Both compensated thermocouple measurements of gas temperature and those of mean density behind the flame front, derived from the flame speed and the turbulent burning velocity, showed dust flames to be thicker than gaseous flames. This is attributed to the time lag in the heating and delolatilising of the particles. Radiative loss also is significant. Values of the burning velocity based upon the actual mass rate of burning are presented and these are lower not only than those in gaseous explosions but also than the turbulent burning velocity. This mass burning velocity is related to the scaled pressure rise parameter, K st , much used in hazard analysis, and comparisons are made with previously reported dust explosion data on the basis of this relationship.