A Theoretical Study of Supercritical Liquid Fuel Combustion

Abstract

In Diesel engines the state of compressed air in the combustion chamber is usually beyond the thermodynamic critical point of the injected liquid fuel (Fig.1) [1]. The evaporation/cmbustion characteristics of the spray may by different from those at pressures below the critical value of the fuel. Under such conditions, it is possible for the atomized liquid fuel to lose its sharp liquid-gas interface and change into a puff of fuel wherein the phase change is spatially continuous. These have been a number of studies of single droplet combustion [1–9] to gain insight into the elementary evaporation / combustion characteristics of Diesel spray. Previous theories, however, do not satisfactorily describe the phenomenon at high pressures, in that they fail to approach the critical situation in which the droplet surface disappears. Major difficulties encountered are the following. (1) The phenomenon is inevitably unsteady [10] so that the simplifying gas-phase quasi-steadiness assumption employed in the analyses of droplet combustion at low pressure [11] is not applicable. (2) Some of the material properties involved change abruptly in the vicinity of a thermodynamic critical point of the mixture [12] and there is a serious lack of precise knowledge about their behavior.