Conditional Reynolds stress modeling in turbulent premixed jet flames

Conventional turbulence modeling approaches based on traditional unconditional averaging implicitly assume that combustion heat release does not affect turbulence. However, in turbulent premixed flames at low Karlovitz number, combustion-induced dilatation and flame motion significantly modify turbulence. Instead of relying on unconditional averaging, simultaneously solving momentum and scalar transport equations conditionally averaged on a flame structure variable could provide a superior framework for modeling combustion-affected turbulence since the flame dynamics are embedded into the flame-conditioning. The primary challenge in this approach is developing closure models for conditional terms, which evolve in both physical and conditional spaces. In this work, a new model for conditional Reynolds stresses, which appear in the conditionally averaged momentum equations, has been developed. The new model consists of a conditional Boussinesq-like term and a new term representing turbulent momentum transport in conditional space. A theoretical scaling of the new model has been investigated, and the model performance has been evaluated in a priori analyses using DNS databases of turbulent premixed jet flames at low and high Karlovitz numbers.