Second Law Analysis of Hydrogen-Enriched Methane-Air Flames

Abstract

A theoretical -numerical an alysis based on the Second law of thermodynamics is used for laminar H2-enriched CH 4-air flames. The analysis is based on computing the various entropy generation terms in reacting flow field. The objective is to examine the effect of flame configuration a nd fuel type on entropy generation and the Second law efficiency. A comprehensive, time -dependent computational model, which employs a detailed description of chemistry and transport, is used to simulate the transient ignition and flame propagation in a tw o-dimensional reacting flow field . Because this propagating flame exhibits a triple flame structure containing premixed and nonpremixed reaction zones, one -dimensional propagating premixed flames and counterflow nonpremixed flames are also analyzed and com pared with two -dimensional propagating triple flames. For latter flames, the triple point exhibits the maximum entropy generation, indicating that this point is characterized by high chemical reactivity, and large temperature and mass fraction gradients. T he volumetric entropy generation is the highest in the two premixed reaction zones, and the lowest in the nonpremixed reaction zone. In the premixed zones, the volumetric entropy generation due to chemical reaction is the highest, followed by heat conducti on, and then mixing. The converse is true for the nonpremixed zone. However, the integrated entropy generation rate indicates that heat conduction is the major contributor, followed by chemical reactivity, and then mixing. As H2 addition to methane fuel is increased, while the integrated entropy generation increases, the contributions of heat conduction, chemical reactivity, and mixing to total entropy generation weakly depend on the fuel being burned. The Second law efficiency of the system also remains ne arly constant with H 2 addition, since the increased irreversibilities due to H 2 addition are compensated by the increase in the flow availability in the fuel blend. Simulations of freely propagating premixed flames and counterflow nonpremixed flames also i ndicate that the dominant entropy generation mode depends on flame configuration rather than on the fuel blend being used. As is the case with the propagating triple flames, the Second law efficiency for these flames remains nearly constant with H 2 additio n.