Modeling wall influence on solid-fuel flame spread in a flow tunnel

Concurrent-flow flame spread over a thin solid in a low-speed flow tunnel is investigated theoretically to support a proposed space experiment. By modifying a previous flame-spreading model, valid for unbounded domain, the effects of flow confinement due to finite tunnel height and the radiative interaction between the tunnel wall and the flame and solid fuel are studied. Computed results show that, as tunnel height is decreased, the flow is accelerated to a higher velocity in the downstream due to thermal expansion. This presses the flame closer to the solid fuel, and increases the heat conduction rate to the solid, the flame length, and the spread rate. When the channel height became too small, however, conductive heat loss to the wall became substantial, which reverses the trend and decreases the flame length. Radiative interaction between the tunnel wall and the flame system is found to be a strong function of the wall radiation reflectivity. When the wall reflectivity is increased, the total radiative loss from the flame system (including the solid fuel) is decreased. This substantially increases the flame length and spread rate and hence becomes an important parameter in the experimental design.