Second-law thermodynamic analysis for premixed hydrogen flames with diluents of argon/nitrogen/carbon dioxide

Exergy losses from hydrogen premixed flames with different diluents (argon, nitrogen, carbon dioxide and nitrogen/carbon dioxide mixtures) were numerically studied. The sources causing exergy losses in premixed flames were divided into four parts, namely heat conduction, mass diffusion, chemical reactions and incomplete combustion, respectively. The chemical, thermal and transport effects of different diluents were isolated to evaluate their contributions to the exergy loss from each source. The results revealed that the total exergy loss increased as the diluent changed from nitrogen to argon or carbon dioxide while slightly decreased as the nitrogen was partially replaced by carbon dioxide. The thermal effect of diluents was the primary factor influencing the exergy loss, followed by the chemical effect and the transport effect. Comparing with the nitrogen-diluted flame, the thermal effect of argon decreased the exergy losses by heat conduction, mass diffusion and chemical reactions while increased the exergy loss by incomplete combustion. The thermal effect of carbon dioxide on exergy loss differed from that of argon due to the increased specific heat capacity. Moreover, comparing with the nitrogen-diluted flame, the chemical effect of carbon dioxide decreased the exergy loss by chemical reactions because of the reduced flame thickness, while the chemical effect of argon had negligible effects on the exergy loss from each source. Finally, transport properties of different diluents slightly changed the exergy loss from each source. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.