Influence of oxidizer flow speed on the toxic species composition in laminar diffusion flame under weightless conditions

The molecular species composition of major toxic species, such as soot, CO and unburnt hydrocarbons from a boundary layer diffusion flame over heptane or dodecane surface at microgravity is numerically investigated. A two-step global reaction model for gas-phase chemistry and a simplified soot model consisting of laminar smoke point type for soot inception are used. Thermal radiation is calculated using the discrete-ordinates method coupled with a non-grey model for the radiative properties of CO, CO₂, H₂O and soot. The numerical results provide further insights into the intimate coupling between burning rate, flame length, thermal radiation, and toxic products at reduced gravity level. The importance of oxidizer flow speed to the flame structure, soot formation and thermal radiation at microgravity is demonstrated. The amount of soot from the microgravity heptane/dodecane flames augments with an increase of oxidizer flow velocity from 0.1 to 0.3 m/s due to an enhancement of burning rate. This finding is contrasted to the case with porous gas burners relative to toxic species production from microgravity flames. For the burning of various liquid fuels, the radiative loss fraction in general increase in a range of 0.5 to 0.7 due to enhanced soot formation with augmentation of the oxidizer flow velocity.