Advances and challenges of the Conditional Source-term Estimation model for turbulent reacting flows

Conditional Source-term Estimation (CSE) is a turbulence–chemistry interaction model to simulate reacting flows. This model is similar to the Conditional Moment Closure (CMC) approach in using the conditional scalar field to calculate the conditional reaction rates. However, unlike CMC, where transport equations are solved for the conditional scalars, an integral equation is inverted in CSE to estimate the conditional scalars. The model has been developed and applied to a wide range of combustion regimes, including diffusion, premixed, stratified premixed, mixed-mode combustion in lifted flames, spray combustion and MILD combustion in the past two decades. It has been tested against several Direct Numerical Simulation (DNS) databases in a priori analyses and also coupled with both Large-Eddy Simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) flow solvers to simulate benchmark burners. The CSE model has also been used in the simulation of practical combustion devices such as internal combustion engines and industrial furnaces. In this paper, the fundamental basis of the CSE model is first presented, and the model’s limitations and strengths are described. The challenges of the application of CSE to different combustion regimes are discussed through a comprehensive review of the past published works. Mathematical and numerical implementation techniques are presented, and future challenges in developing this turbulence–chemistry interaction model are also proposed.