Effect of Sample Width on Flame Spread Rate over a Thin Material in Microgravity

Abstract

The effect of the sample width on the flame spread rate over a thin PMMA film in microgravity has been investigated by experiments and scale analysis. The oxygen level of the ambient gas is kept fixed at 30% while different diluents -N2, Ar, He, and CO2 – are used. The sample width is also used as a parameter, varying through 5mm, 10mm, and 20mm. When helium is used as the diluent, the conductive heat loss to the sides becomes large, and therefore, the flame is suppressed with a significant drop in the flame spread rate and flame size, especially at low oxygen level and for narrow sample widths. For other diluents, the radiative loss is the dominant factor for the suppression, and the sample width has small impact. These experimental findings can be explained with the scale analysis. Nomenclature a abs Absorption coefficiency of gas c g Specific heat of gas c s Specific heat of solid F Flame constant, Eq. (1) L g Gas-phase diffusion length scale L gx Gas-phase diffusion length scale in x-direction L gy Gas-phase diffusion length scale in y-direction L sx Length of the preheated solid phase L sy Thickness of the preheated layer R loss Total heat loss number, R loss =R rad +R side R rad Radiation loss number, Eq. (2) R side Side loss number, Eq. (3) T f Characteristic (adiabatic) flame temperature T v Constant vaporization temperature T ∞ Ambient temperature V g Ambient flow velocity V g,cr Critical ambient flow velocity V f Absolute spread rate V r Velocity relative to the flame, V r =V g +V f W Width of the fuel in z-direction α g Thermal diffusivity of gas, evaluated at T v α s Thermal diffusivity of solid ε Surface emissivity λ g Gas-phase conductivity evaluated at Tv λ s Solid-phase conductivity η Non-dimensional spread rate, Eq. (4) ρ g Gas density evaluated at Tv ρ s Solid density τ Fuel half-thickness