Comparative analysis of methods for heat losses in physically-derived reduced-order manifolds

Heat loss substantially modifies turbulent combustion processes, especially the formation of pollutants such as nitrogen oxides, which are strongly temperature sensitive. To account for the effects of heat loss in Large Eddy Simulation (LES) using a reduced-order manifold approach, thermochemical states are computed via a priori 1-D premixed flame calculations over a range of reduced enthalpy states. Two basic approaches are explored for generating these reduced enthalpy states, which are compared to assess any effects on turbulent flame structure and emissions. In the first approach, a variable heat loss source term is introduced into the 1-D flame solutions by mimicking a real heat loss to reduce the post-flame enthalpy. In the second approach, fuel and oxidizer are converted to products in the unburned gases at a constant temperature to produce reduced enthalpy in the entire 1-D flame solution. The two approaches are compared in methane-air piloted turbulent premixed planar jet flames that maintain a constant adiabatic flame temperature but experience differing radiation heat losses. The results indicate that the manner in which the heat loss is accounted for in the manifold is of secondary importance compared to other model uncertainties such as the chemical mechanism.