Spray-Flame Oscillations in Two-Fuel Lean Mixture

Abstract

We analytically and numerically analyse the stability of a spray-flame propagating through a homogeneous three-component lean premixture composed of fuel droplets (typically a heavy alkane with low vapour pressure), a second gaseous fuel (typically a light alkane “stimulating” combustion) and air in excess. The fuel vapour evaporating from the droplets has a Lewis number larger than one ( i.e. fuel-1 with Le1＞1), while the light gaseous fuel-2 has a Lewis number smaller than one ( Le2＜1 ). The initial mass fraction of fuel-1 (under liquid phase),(Yl1)u , expresses the spray liquid loading, while(Yg2)u is the initial mass fraction of gaseous fuel-2, the overall fuel amount in fresh mixture therefore being (Yl1)u＋(Yg2)u . For relative liquid loading ,δ=(Yl1)u /{(Yl1)u＋(Yg2)u} higher than 50%, an excellent agreement between both numerical and analytical　predictions on the intrinsic spray-flame instability is found. Conversely, poor agreement between numeric and asymptotic is found for the predicted threshold of diffusional-thermal pulsating instabilities in fully gaseous　premixed flames at large Lewis number. This difference comes from the fact that the mechanism leading to spray-flame oscillations is intrinsic, and not based on differential diffusional effects. When increasing (Yl1)u , spray-flame instability threshold decreases and for vanishing (Yg2)u pulsations are found to occur for conditions that can easily be met in experiments on large carbon number fuel sprays.