Analysis of entropy generation in a hydrogen-enriched turbulent non-premixed flame

Abstract

The analysis of local entropy generation and exergy loss was performed in a turbulent non-premixed H2-enriched CH4–air bluff-body flame. Detailed chemical kinetic, transport properties, and turbulence-chemistry interaction were taken into account in using laminar flamelet model for the simulation of combustion process via an in-house, finite volume code. The analysis was based on local entropy generation calculation. Results showed that thermal conduction made the most contribution to entropy generation followed by chemical reaction and mass diffusion, while the contribution of viscous dissipation was negligible. Entropy generation resulting from thermal conduction occurs in a large volume of the domain, while entropy generation resulting from chemical reaction and mass diffusion occurs only near the bluff surface. The effect of H2 addition to fuel and air preheating on the entropy generation rate was investigated. It was observed that entropy generation and exergy loss were decreased by H2 addition, mainly due to a decrease in the chemical reaction component of entropy generation, while entropy generation resulting from thermal conduction slightly increased and entropy generation resulting from mass diffusion remained almost constant. Entropy generation resulting from heat conduction by preheating combustion air decreased, while entropy generation resulting from chemical reaction and mass diffusion remained almost constant. The decrease of thermal conduction contribution in entropy generation is so significant that, by preheating air up to 750 K in the case of pure CH4, chemical reaction becomes the main source of irreversibility. These investigations show that H2 addition and preheating the combustion air both lead to the improvement of the second law efficiency, although the second law efficiency is more sensitive to flame structure and air temperature.