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.

A Direct Numerical Simulation-Based Analysis of Entropy Generation in Turbulent Premixed Flames

“…It is worth noting that Soret and Dufor effects are ignored here following several previous analyses on entropy generation in turbulent reacting flows [4][5][6][7][8][10][11][12][13][14][15]17]. Moreover, there have been several previous DNS based computational analyses [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], which ignored Dufor and Sorret effects without much loss of generality.…”

“…Puri [4] analyzed entropy generation in the context of droplet combustion and identified the optimum transfer number for the purpose of minimization of entropy generation. Hiwase et al [5] subsequently proposed a theoretical model for exergy analysis for droplet-laden flows where the exergy loss is expressed as a function of Damköhler number and initial droplet temperature. Exergy analysis in spray combustion was analyzed by Datta and Som [6] for different conditions in terms of pressure, temperature, swirl number, droplet diameter and spray cone angle.…”

Effects of gravity on structure and entropy generation of confined laminar diffusion flames

“…Therefore, the coupling term is not taken into consideration while calculating the entropy generation rate in the flame and the four other terms of Eq. (15) are only considered.…”

“…Puri [14] and Hiwase et al [15] studied the second law analysis for single droplet burning. Puri [14] obtained an optimum transfer number for minimizing entropy generation in droplet combustion.…”

“…The optimum transfer number is observed to be directly proportional to the square of the relative velocity and inversely proportional to the heat release rate and the temperature difference between the droplet and the surrounding flow. Hiwase et al [15] developed a theoretical model of exergy analysis of droplet combustion as a function of Damkohler number and initial droplet temperature. Datta and Som [16] evaluated the rate of exergy loss and the second law efficiency in a spray combustion process under different operating conditions, like inlet pressure, temperature, swirl and drop characteristics in the atomized fuel spray.…”