Buoyancy effects on smoldering combustion

Abstract

A theoretical and experimental study is carried out to determine the effect of buoyancy on the rate of spread of a cocurrent smolder reaction through a porous combustible material. Since buoyant forces are proportional to the product g( ϱ gi − ϱ g), they can be controlled experimentally by varying either the gravitational acceleration, g, or the density difference, ϱ gi − ϱ g. The latter approach was followed in the present work. Measurements are performed of the smolder spread rate through porous α-cellulose (0.83 void fraction) as a function of the ambient air pressure. The experiments are carried out in a pressure vessel for ambient pressures ranging from 0.5 to 1.2 atm. The rate of spread was obtained from the temperature histories of thermocouples placed at fixed intervals along the fuel centerline. The smolder velocity was found to increase as the ambient pressure was increased. Extinction was found to occur when the buoyancy forces could not overcome the drag forces, indicating that at least for the present experimental conditions transport by diffusion cannot, by itself, support the spread of a smolder reaction. This conclusion is particularly important for outer space conditions where gravity and consequently buoyancy could be negligible. In the analysis, which assumes one-dimensional processes, the transport equations are solved to give the smolder spread rate as a function of the inlet oxygen mass flux. This mass flux is then estimated by balancing buoyancy and drag forces. Assuming that the smolder chemical reaction is only weakly dependent on pressure, the analysis finally predicts a smolder velocity dependence of the form v ∼ Y oi gϱ 2 gi ∼ Pa 2, i.e. is proportional to the ambient pressure squared. Good qualitative agreement is found between the theoretical predictions and the experimental results.