Thermodynamics of flame impingement heat transfer

Abstract

A theoretical model for entropy generation and utilization of work potential (exergy) in flame impingement (both premixed and diffusion) heat transfer has been developed in this article, to offer physical insights on the optimal operational regimes, depicting high values of the surface heat flux with minimal exergy destruction, within the practical constraints. The irreversibility components due to different physical processes have been evaluated from a general entropy transport equation. The velocity, temperature, and species concentration fields required for the solution of entropy transport equation have been determined from the numerical computation of flow-field in the flame. Global two-step chemical kinetics has been considered with methane (CH4) and air as fuel and oxidizer, respectively. The results have been predicted in terms of average nondimensional heat flux, expressed as Nusselt number at the target plate, the irreversibility components, and second law efficiency, as functions of the pertinent input parameters such as the jet Reynolds number and the ratio of plate separation distance to nozzle diameter (H∕d). The average Nusselt number has been found to increase with an increase in jet Reynolds number and a decrease in H∕d ratio, up to a value of 8. The dominant source of thermodynamic irreversibility in a premixed flame has been attributed to the thermal energy exchange whereas, in a diffusion flame, the same has been attributed to an uncontrolled exchange of electrons accompanying the reactive kinetics. The second law efficiency has been found to increase with an increase in jet Reynolds number and an increase in the H∕d ratio, up to a value of 20. Values of the jet Reynolds number greater than 10 000 and H∕d ratio in the tune of 15 have been observed to pertain to the regime of optimum flame impingement heat transfer, consistent with the energy and exergy balance constraints.