Effects of finite sample width on transition and flame spread in microgravity

Abstract

In most microgravity studies of flame spread, the flame is assumed to be two-dimensional, and two-dimensional models are used to aid data interpretation. However, since limited space is available in microgravity facilities, the flames are limited in size. It is important, therefore, to investigate the significance of three-dimensional effects. Three-dimensional and two-dimensional simulations of ignition and subsequent transition to flame spread were performed on a thermally thin cellulosic sample, Ignition occurred by applying a radiant heat flux in a strip across the center of the sample. The sample was bounded by an inert sample holder. Heat loss effects at the interface of the sample and the sample holder were tested by varying the thermal-physical properties of the sample holder. Simulations were also conducted with samples of different widths and with different ambient wind speeds (i.e., different levels of oxygen supply). The width of the sample affected both the duration of the flame transition period and the post-transition flame spread rate. Finite width effects were most significant when the ambient wind was relatively small (limited oxygen supply). In such environments, the velocity due to thermal expansion reduced the net inflow of oxygen enough to significantly affect flame behavior. For a given sample width, the influence of thermal expansion on the net incoming oxygen supply decreased as the ambient wind speed increased. Thus, both the transition and flame spread behavior of the three-dimensional flame (along the centerline) tended to that of the two-dimensional flame with increasing ambient wind speed. Heat losses to the sample holder were found to affect the flame spread rate in the case of the narrowest sample with the slowest ambient wind.