Direct numerical simulation and radiation Monte Carlo for turbulence--radiation interactions in combustion systems

Turbulent combustion is encountered in many industrial applications such as combustors, nozzles, turbines, engines and furnaces. The increasing concern for pollutant emissions to the environment has led to a wide interest in studying the process in detail using numerical simulation with an aim to develop predictive models that would minimize expensive experiments. Three main approaches in this direction are direct numerical simulation (DNS), large eddy simulation (LES) and Reynolds average simulation (RAS). Moreover, the presence of high temperatures in turbulent combustion results in substantial heat transfer by radiation. Most work until today has either simplified or neglected radiation. This leads to neglect of turbulence-radiation interactions (TRI).;In this work, DNS is coupled with a photon Monte Carlo method to solve the radiative transfer equation (RTE) to isolate and quantify TRI. A canonical statistically one-dimensional turbulent premixed combustion system is studied. The inflow boundary condition is improved to introduce turbulence and to help simulate a quasi-stationary flow. Both emission TRI and absorption TRI were found to be significant. A statistical analysis showed that using a moving average over time of the radiative source term reduces statistical noise and saves valuable computational resources without affecting the overall solution.;For the first time, a third-order spherical harmonics method, P3 approximation is coupled with DNS to replace the photon Monte Carlo method. A canonical turbulent premixed combustion system is simulated and TRI are studied at large-to-small optical thicknesses. Emission TRI was found to be significant at all optical thickness, while absorption TRI was important at large and intermediate optical thickness and negligible at the optically thin limit. The implementation of the P3 approximation is seen as a viable alternative to the costly photon Monte Carlo method. Elliptic equations are typically solved in RAS-based and LES-based modeling and the P3 approximation consists of six elliptical partial differential equations (PDEs). It is hoped that the demonstration of use of the P3 approximation will generate interest for it to be included in future LES and RAS approaches and developments.;A low-order model for LES using a beta-PDF approach is also developed for a canonical statistically one-dimensional turbulent nonpremixed system. The mixture fraction and its variance are inputs to the model to obtain a PDF of mixture fraction. With the knowledge of the PDF of mixture fraction, the quantities that are functions of mixture fraction are convoluted with the mixture fraction PDF to obtain the chemical source term and the radiative emission terms. The mean profiles are predicted correctly but the nonlinear terms of radiative emission are not captured accurately, as the beta-PDF cannot provide information for the higher-order terms. Still, as a one-equation model to be used as a first pass for LES simulation, this model has its advantages in that it is easy to implement and uses negligible additional computational resources over what is required for LES without any models.