Sensitivity analysis based reduction of complex reaction mechanisms in turbulent non-premixed combustion

Abstract

A turbulent combustion model without simplifying assumptions about the chemical reactions is evaluated by means of a sensitivity analysis. The turbulence model is conventional and based on gradient transport assumptions and an eddy viscosity concept. In addition to the equations of the turbulent flow field the species conservation equations are solved. The chemical reactions are described by a detailed, elementary kinetics reaction mechanism. The source terms in the species conservation equations are treated by means of a presumed-shape pfd-closure relaxing the assumption of statistically independent variables. The sensitivity analysis for the turbulent combustion model is performed in terms of the local first order sensitivity coefficients. The turbulent combustion model and sensitivity analysis are adopted to a turbulent hydrogen air diffusion flame in a simple flow geometry. Temperature, major chemical species and OH radical concentrations are reasonable well predicted. The sensitivity analysis shows the relative importance of single elementary steps of the reaction mechanism compared with turbulent transport. Only few rate coefficients of the entire chemical mechanism influence the main response of the turbulent combustion model, viz. temperature and mass fractions of the major chemical components. From this simplifications of the closure for the mean chemical reaction rates are derived. Furthermore, for the turbulent hydrogen air flame under consideration sensitivity analysis shows control of turbulent transport throughout the largest part of the flame. Therefore, the substantial properties of the flame, viz. temperatures, mass fractions of H2, O2 and H2O are predicted reasonably by any reduced chemical model providing sufficiently large conversion rates of the fuel.