Entropy generation in turbulent syngas counter-flow diffusion flames

Abstract

Efficiency is one of the major objectives when designing energy systems. Irreversibilities of combustion processes can be characterized by analyzing entropy generation which is proportional to exergy destruction. In this paper, entropy generation is investigated in turbulent non-premixed counter-flow syngas flames at a high strain rate over a wide range of hydrogen percentage (H2/CO molar fraction from 0.4 to 2.0). The aim is to define the most efficient syngas composition to reduce irreversibilities. Irreversibilities involved in NO formation process are also examined. RANS (Reynolds Averaged Navier Stokes) technique including k-ε turbulence model is used for the flow field estimation. Flame structure is calculated using SLFM (Steady Laminar Flamelet Model) and EPFM (Eulerian Particle Flamelet Model) is applied for NOx predictions. Total entropy generation rate accounts for chemical, heat conduction, mixing and viscous effects. Computational results show that the total volumetric entropy generation decreases with H2 enrichment as well as its different contributing effects. Chemical effect is dominant, followed by heat conduction and mixing effects. Viscous effect is negligible. The maximum of both thermal and prompt NO formation routes are influenced by the three main entropy generation modes, with the predominance of the chemical effect. At high strain rates, H2-rich syngas flames are efficient in regards to irreversibilities and NO emissions reduction.