Vaporisation controlled regime of flames propagating in fuel-lean sprays

Abstract
We present a minimal model which allows us to study some of the fundamental features of fuel-lean spray flames ; the model exhibits a main control parameter, the so-called vaporisation/reaction Damkoehler number (Da), which permits of studying a wide range of spray combustion configurations. For fast vaporisation rate (compared with reaction rate, i.e. for Da<1), the model recovers the classical picture of a premixed spray flame, for which vaporisation occurs early in the pre-heating zone. In such a case and in agreement with literature, spray flame is comparable to gaseous premixed flame, as far as plane propagation is concerned. For intermediate vaporisation rate (1<Da<100), evaporation interacts with chemical reaction : vaporisation occurs up to the reaction zone, in such way that gaseous fuel available for combustion is less than in the premixed case, reaction temperature is lower and burning velocity slows down. For low vaporisation rate (Da>100) the model puts forward a vaporisation controlled regime of spray combustion, as it can be envisaged in presence of large droplets. This regime can be interpreted as the mild transition between the classical picture of homogeneous spray flame and individual (or group) droplet combustion. The coherence of such a regime with our model, as well as its actual existence, are discussed.