Entropy balance and exergy analysis of the process of droplet combustion

Abstract

An entropy balance and subsequent exergy analysis of the process of combustion of a liquid fuel droplet in a quiescent gaseous surrounding at high temperature has been performed in order to determine the second-law efficiency of the process. Velocity and species concentration fields for the gas phase and the temperature field both for the gas and for the droplet phases have been evaluated from the numerical solution of the equations of conservation of mass, momentum and heat, accordingly. The rate of generation of entropy due to transport processes and chemical reaction in the gas phase has been determined from the generalized entropy transport equation. A theoretical model for exergy analysis of the process of droplet combustion has been developed in order to predict the second-law efficiency in terms of the pertinent controlling parameters, namely, the ratio of free stream to initial droplet temperatures and the initial Damkohler number. It has been observed that, in a typical diffusion-controlled droplet combustion process, in which the rate of chemical reaction is much faster than the rates of diffusion of heat, mass and momentum, the irreversibility rate has, in contrast, a lower value due to chemical reaction than that due to diffusion processes taken together. A low value of the initial Damkohler number (as close as possible to its limiting value for initiation of ignition) and a high value of free stream temperature should be preferred for the process of droplet combustion from the viewpoint of energy economy in relation to thermodynamic utilization of available energy.