Impact of Fuel Type on Toxic Emissions from a Non-premixed Boundary Layer Laminar Flame in Microgravity – A Numerical Study

Abstract

Examples from non-premixed boundary combustion representative of fire in microgravity environment are presented. A low forced flow of 0.25 m/s is present, which is typical of air circulation speeds in a spacecraft. The numerical simulations have been conducted to evaluate the modes of heat transfer with an emphasis on a discussion about the effects of various fuels on chemical emissions such as soot, CO, CO2 and unburnt hydrocarbons in reduced-gravity conditions. Typically, evaporation temperature is smaller for heptane than dodecane, and thus a lengthening of the heptane flame seems more pronounced as compared to that of dodecane flame with an increase in the visible flame zone by a factor of 2.3 times. A rapid regression rate of heptane due to the enhanced heat feedback contributes to an additional energy by a factor of 80% in heat release rate as compared to dodecane. In all the cases, only about 15% of the heat generated by an exothermic chemical reaction is supplied to the pyrolysis surface, and a large portion of energy is convected with the forward gas flow. The CO molar fraction reaches to a maximum of about 11% for dodecane and 8% for other fuels as ethylene, propane, propylene and heptane. The maximum mean value of CO concentration depends mainly on fuel injection rate, and appears practically insensitive to oxidizer flow velocity. Dodecane and heptane with a longer carbon chain produces more CO and less unburnt hydrocarbons than ethylene, propane and propylene. The existence of large unburnt toxic gas fuels even for a smaller flame size because of the absence of natural convection has important considerations for spacecraft fire safety due to smoke/gas incapacitation.