Second-Law Thermodynamic Analysis in Premixed Flames of Ammonia and Hydrogen Binary Fuels

A theoretical analysis based on the second law of thermodynamics was conducted for the ammonia/hydrogen/air premixed flames at different initial pressures. The irreversibility causing exergy losses in premixed flames was divided into five parts, namely, heat conduction, mass diffusion, viscous dissipation, chemical reaction, and incomplete combustion, respectively. The results revealed that as the hydrogen percentage in fuel blends increased from 0% to 100%, the total exergy losses decreased. Specifically, the exergy destructions induced by heat conduction and mass diffusion decreased with the increasing hydrogen percentage. The exergy loss induced by incomplete combustion increased with hydrogen addition, as more incomplete combustion products such as H₂, H, and OH were generated with the increasing hydrogen percentage. The exergy destruction by chemical reactions first decreased and then increased with the increasing hydrogen percentage, which was attributed to the combination effects of the increased entropy generation rate and reduced flame thickness. Compared to the other four sources, the exergy destruction induced by viscous dissipation was negligible. Furthermore, at the elevated pressure of 5 atm, the effects of hydrogen blending were similar to those at the atmospheric condition. However, the exergy destructions by heat conduction and mass diffusion increased while the exergy destruction by the chemical reaction and the exergy loss by incomplete combustion were both reduced, with the overall exergy loss decreased by 1–2% as the pressure increased from 1 atm to 5 atm.