Gravity Effects on the Minimum Explosive Concentrations in 1-D Dust Explosion

Abstract

ABSTRACT Theoretical predictions of dust-explosion characteristics, such as the minimum explosive concentration (MEC), can help improve the accuracy of quantitative risk assessment. This paper presents a continuum model to simulate one-dimensional dust-cloud combustion. It is found that the dilution of dust by thermal expansion reduces the propagation speed of a combustion wave through a dust cloud. At the same time, the delay in the increase of particle velocity due to relaxation time counteracts the dilution effect. Therefore, neglecting the relaxation time results in the underestimation of propagation speed. In the absence of heat loss, extinction occurs when the adiabatic combustion temperature decreases to the ignition temperature; MEC is independent of particle size in such cases. With the presence of heat loss, on the other hand, extinction occurs at a higher dust concentration; MEC increases with particle size. Accurate evaluation of ignition temperature is key to the quantitative prediction of MEC. A particular focus is given to gravity effects, reflecting recent experimental efforts to obtain fundamental insights into propagation mechanisms without being affected by gravity. In upward propagation, particles fall toward the reaction front, enhancing combustion and increasing the propagation speed. The situation in downward propagation is the opposite, and extinction occurs when the gravity level is beyond a critical value. In upward propagation, MEC can decrease with an increase in particle size, owing to the influence of gravity.