ENTROPY BALANCE AND EXERGY ANALYSIS OF THE PROCESS OF DROPLET COMBUSTION

An entropy balance and subsequent exergy analysis of the process of co mbustion of a liquid fuel droplet in a quiescent gaseous surrounding a t high temperature has been performed in order to determine the second -law efficiency of the process. Velocity and species concentration fie lds for the gas phase and the temperature field both for the gas and f or the droplet phases have been evaluated from the numerical solution of the equations of conservation of mass, momentum and heat, according ly. The rate of generation of entropy due to transport processes and c hemical reaction in the gas phase has been determined from the general ized entropy transport equation. A theoretical model for exergy analys is of the process of droplet combustion has been developed in order to predict the second-law efficiency in terms of the pertinent controlli ng parameters, namely, the ratio of free stream to initial droplet tem peratures and the initial Damkohler number. It has been observed that, in a typical diffusion-controlled droplet combustion process, in whic h the rate of chemical reaction is much faster than the rates of diffu sion of heat, mass and momentum, the irreversibility rate has, in cont rast, a lower value due to chemical reaction than that due to diffusio n processes taken together. A low value of the initial Damkohler numbe r las close as possible to its limiting value for initiation of igniti on) and a high value of free stream temperature should be preferred fo r the process of droplet combustion from the viewpoint of energy econo my in relation to thermodynamic utilization of available energy.