Manifold assumptions in modeling radiation heat losses in turbulent nonpremixed combustion

Abstract

Accurate predictions of heat losses in turbulent combustion are critical for accurate predictions of pollutant emissions such as nitrogen oxides and soot due to their extreme sensitivity to the underlying gas-phase temperature and composition. Reduced-order manifold approaches to modeling turbulent combustion require an additional enthalpy or enthalpy-like parameter to account for these heat losses. The particular focus of this work is radiation heat losses in turbulent nonpremixed combustion. A general and mathematically consistent two-dimensional reduced-order manifold model is developed that can account for arbitrary radiation heat losses in nonpremixed combustion. From this model, two further assumptions are made, considering the specific characteristics of radiation heat losses in turbulent nonpremixed combustion, resulting in two models. First, the radiation heat loss is presumed to be sufficiently weak as to prevent flame quenching. In this case, the two-dimensional manifold equation is shown to be reduced to a pseudo-unsteady one-dimensional manifold model. Second, the radiation heat loss is presumed to be much slower than the gas-phase chemistry governing the flame structure. In this case, the pseudo-unsteady one-dimensional manifold model is reduced to a quasi-steady one-dimensional manifold model. These two models are implemented within a Large Eddy Simulation (LES) context and applied to Sandia Flame D to determine whether or not these assumptions are physically correct. The results indicate that major species and temperature are relatively insensitive to these assumptions. However, results for nitrogen oxides indicate that the quasi-steady assumption is invalid and worsens with increasing downstream distance from the nozzle, with differences between the quasi-steady model and pseudo-unsteady model as large as the differences with an adiabatic model. Further analysis reveals that the chemical time scales rapidly slow with increasing downstream distance, and the assumption that the radiation heat losses are much slower than the chemistry breaks down.