Numerical study of effect of oxygen fraction on local entropy generation in a methane-air burner

Abstract

This study considers numerical simulation of the combustion of methane with air, including oxygen and nitrogen, in a burner and the numerical solution of local entropy generation rate due to high temperature and velocity gradients in the combustion chamber. The effects of equivalence ratio (φ) and oxygen percentage (γ) on combustion and entropy generation rates are investigated for different φ (from 0.5 to 1.0) and γ values (from 10 to 30%). Combustion is simulated for the fuel mass flow rate resulting in the same heat transfer rate (Q) to the combustion chamber in each case. Numerical calculation of combustion is performed individually for all cases with the use of the Fluent CFD code. Furthermore, a computer program has been developed to calculate the volumetric entropy generation rate and the other thermodynamic parameters numerically by using the results of the calculations performed with the FLUENT code. The predictions show that the increase of φ (or the decrease of λ) significantly reduces the reaction rate levels. Average temperature in the combustion chamber increases by about 70 and 35% with increase of γ (from 10 to 30%) and φ (from 0.5 to 1.0) respectively. With increase of γ from 10 to 30%, volumetric local entropy generation rate decreases by about 9 and 4% for φ = 0.5 and 1.0 respectively, while total entropy generation rate decreases exponentially and the merit numbers increase. The ratio of the rates useful energy transfer to irreversibility therefore improves as the oxygen percent-age increases.