The uniform conditional state model for turbulent reacting flows

Abstract

A novel model for the source term in RANS/LES of turbulent combustion, the Uniform Conditional State (UCS) model, is derived. The UCS model can be viewed as both a generalization and a specialization of Conditional Moment Closure (CMC): a generalization in that the number of conditioning variables is arbitrary, but a specialization in that conditionally-filtered fields are assumed to be independent of position. As a consequence of the latter property, distinct physical and conditional domains exist, and fields defined on the two domains are only loosely coupled. The central conditional transport equation in UCS takes the same form as that in the Multidimensional Flamelet Manifold (MFM) model, but is based on different starting assumptions and therefore does not require a “flamelet” assumption in turbulent flows. Like the MFM, the model is compatible with any diffusive flux law, and remains theoretically valid for any flame on the premixed-non-premixed continuum.The UCS model is tested on the Sandia flame series using mixture fraction and progress variable as conditioning variables and an experimental PDF for post-processing. Predicted scalar profiles show good agreement with experiment, with the notable exception of NO. Scalar profiles generally show modest sensitivity to the conditional scalar dissipation model, but the source of progress variable and mass fraction of the soot precursor C2H2 show greater sensitivity.