Pressure change and transport process on flames formed in a stretched, rotating flow

Flame characteristics in a stretched, rotating flow have been investigated by numerical simulation of tubular laminar flames for lean hydrogen, methane, and propane/air mixtures. Twin planar flames in counterflow have been also simulated for comparison. A fixed inlet velocity at the porous wall of the burner was assumed in all cases, and the cylindrical containing tube (radius R = 9.5 mm) was either maintained stationary or rotated. Results showed that, within the range studied, the flame temperatures always increase monotonically with increasing fuel concentration, and at the same time the reaction zones move outwards. However, while the introduction of rotation also causes a monotonic temperature increase of hydrogen and methane/air mixtures, that of a propane/air mixture decreases. The temperature change with rotation becomes smaller with an increase of the fuel concentration. As a consequence of the centrifugal force, rho nu(theta)(2)/r, induced by the rotation, a pressure gradient is formed in the cylindrical containing tube, with low pressure along the axis. The pressure gradient at the outer, unburnt edge of the flame reaction zone becomes smaller as the fuel concentration increases. The resultant decreased mass transport by pressure diffusion provides an explanation for part of the above-mentioned temperature change associated with rotation. The remainder of the effect is associated with changed stretch characteristics of the flames. (C) 1999 by The Combustion Institute.