Effect of radiation loss on flame spread over a thin PMMA sheet in microgravity

Abstract

Flame spread over a thin polymethylmethacrylate sheet in microgravity is investigated experimentally and analytically. A scale analysis yields a simple equation, η+ R /ζ=1, which states the radiation loss is a function of relative flow velocity and the fuel thickness. The prediction with the scale analysis states that the reduction in the relative flow velocity enlarges the size of the preheat zone, which increases radiation loss, and that the presence of radiation loss reduces the spread rate and also may cause extinction. To confirm the prediction, drop experiments are carried out with a 4.5 s drop tower in MGLAB, in Gifu, Japan. Flame-spread rates are measured with varying ambient flow velocity, fuel thickness, and oxygen level. Additionally, the temperature fields near the flame front are measured in quiescent conditions with a Michelson interferometer system. The experimental result shows that the spread rate is actually a function of sample thickness and ambient flow velocity, and that the spread rate becomes the minimum when the relative flow velocity is close to zero. It is also found that steady flame spread is established even in quiescent conditions if R is sufficiently low. The interferometer measurement shows the enlargement of the preheat zone in quiescent microgravity conditions as predicted by the scale analysis. It also shows that the steady heat balance is established when R is small, whereas it is not established in near extinction conditions. These results support the scale analysis and clarify the extinction mechanism via radiation loss.