Vaporisation controlled regime of flames propagating in fuel-lean sprays

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 para meter, the so-called vaporisation/reaction Damkoehler number (Da), which pe rmits 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 vapori sation occurs early in the pre-heating zone. In such a case and in agreemen t 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 lowe r and burning velocity slows down. For low vaporisation rate (Da > 100) the model puts forward a vaporisation controlled regime of spray combustion, a s it can be envisaged in presence of large droplets. This regime can be int erpreted as the mild transition between the classical picture of homogeneou s spray flame and individual (or group) droplet combustion. The coherence o f such a regime with our model, as well as its actual existence, are discus sed.