The loss of availability in turbulent nonpremixed flames due to internal irreversibility or local entropy generation is predicted using large eddy simulation (LES). Minimization of entropy generation (or exergy destruction) is an effective approach to increase the efficiency of energy and combustion systems in view of the second law of thermodynamics. The goal is to improve the work-producing capacity of the system by lowering the irreversible losses occurring during turbulent combustion processes. In this study, a transport equation for the filtered entropy is considered in LES. This equation includes entropy generation effects due to viscous dissipation, heat transfer, mass diffusion and chemical reaction, which appear as unclosed source terms. The closure of all these terms are provided by a novel methodology developed based on the filtered density function (FDF). This methodology, termed the entropy FDF (En-FDF), provides the joint probability density function of scalar and entropy fields. An exact transport equation is derived for the En-FDF which includes the effects of chemical reaction and its entropy generation contribution in closed forms. The unclosed terms are modeled by a system of stochastic differential equations, which is solved by a Lagrangian Monte Carlo method. The methodology is employed for LES of a nonpremixed turbulent methane jet flame. Predictions are validated by comparing with entropy statistics derived from experimental thermo-chemical data. All entropy statistics show favorable agreements with the data. The local entropy generation effects are obtained from the En-FDF and analyzed. The results illustrate the structure of irreversible losses in nonpremixed turbulent flames. It is shown that heat conduction and chemical reaction are the dominant entropy production mechanisms in this flame.
展开▼
