Analysis of non-adiabatic excess enthalpy spray flames

A steady, one dimensional, low speed flame propagating in a dilute, monodisperse, sufficiently off stoichiometric and weakly heterogeneous spray with external heat recirculation is analyzed using activation energy asymptotics. A completely prevaporized mode and a partially prevaporized mode of flame propagation are identified. Heat recirculation is achieved by transferring heat through a tube wall within a given distance L. The external heat transfer results in either globally external heat loss or excess enthalpy burning (which is globally adiabatic) to the spray system with increasing wall temperature. The influences of external heat recirculation and liquid fuel spray on the combustion characteristics of the spray flames are examined with five parameters, namely the heat transfer length for excess enthalpy burning, the heat transfer coefficient, the amount of external heat transfer, the liquid fuel loading and the droplet size. It is found that the extent of flammability is enlarged with increasing heat transfer length and heat transfer coefficient or decreasing external heat loss. The range of flammability is also enlarged with increasing liquid loading or decreasing droplet size for lean sprays, while the opposite holds for rich sprays.