Irreversible entropy production rate in high-pressure turbulent reactive flows

Abstract

A Direct Numerical Simulation (DNS) database is created describing high-pressure reactive flows for studying the flow characteristics and the irreversible entropy production rate that must be modeled by Subgrid-Scale (SGS) models in Large Eddy Simulation. The governing equations are the continuity, momentum, total energy and species transport equations complemented by a real-gas equation of state. The molecular transport model is based on complete mass-diffusion and thermal-diffusion matrices having elements computed according to all-pressure mixing rules. The mixture viscosity and thermal conductivity are calculated from the individual species values, valid at high pressures, by using all-pressure mixing rules. The reaction is a one-step process and the values of different coefficients in the reaction rate ensure that it gives physically-correct trends for autoignition. The DNS is performed for a temporal mixing layer. Three realizations are computed and examined to reveal the influence of the initial pressure p0 and of exhaust gas recirculation (EGR). It is found that the main flame is of diffusion type, flanked by premixed flames. As p0 increases, the most intensive premixed-flame regions draw closer to the diffusion flame. Additionally to the well-known advantage of EGR, we found that it promotes the development of uphill diffusion which is a molecular process inducing the formation of strong species gradients that in turn induce turbulence production, i.e. the formation of dynamic small scales. Analysis of the irreversible entropy production rate revealed that its four modes – due to viscosity, mass diffusivity, thermal conductivity and reaction – operate in different spatial regions of the flow where different phenomena occur. Increasing p0 and lack of EGR both result in an increase in the magnitude of the irreversible entropy-production rate. For the Reynolds number values achievable in DNS, the reaction mode dominates in magnitude all other modes of the irreversible entropy-production rate.