PDF Calculations of Piloted Turbulent Flames with Inhomogeneous Inlet Conditions

Abstract

This thesis presents PDF calculations of piloted turbulent flames where the level of inhomogeneity at the inlet varies and different departures from blow-off exist. Five flames are investigated numerically to study their structure in the region where the interactions between turbulence and chemistry are high. These five flames are selected such that two comparable series exist. One series has three flames of the same bulk jet velocity but different compositional conditions at the exit plane, while the other series has the same compositional boundary conditions and increasing jet velocities such that blow-off is gradually approached. The studied burner has been modified from the standard configuration with the addition of a central tube carrying methane fuel that can slide within the outer tube carrying air to induce the compositional inhomogeneity at the burner exit plane. A transport equation for the composition PDF is solved using the Lagrangian particle-based Monte Carlo method in FLUENT 14.5 commercial code by ANSYS to facilitate the implementation of detailed chemical kinetics. The code solves the Reynolds Averaged Navier Stokes (RANS) equations for the mean conservation of mass, momentum, and energy. Numerically accurate solutions are obtained after significant testing. In this regard sensitivity studies for numerical and modeling parameters such as grid convergence, turbulence model and modeling constant, mixing model and mixing model constant, number of particles per cell, ISAT error tolerance, etc. have been conducted. According to the sensitivity studies further calculations were performed using k-ε turbulence model with standard constants except for c_ɛ1 of 1.6 since this is deemed to yield the appropriate rate of dissipation of kinetic energy and better spreading rates for circular jets. Species composition is accounted for using a transport equation for the composition PDF which is coupled and solved using the Lagrangian particle-based Monte Carlo method. Twenty particles per cell were used throughout the calculations. The reduced chemical mechanism DRM22 with 24 species and 104 reactions is employed in the calculation using the In-Situ-Adaptive Tabulation (ISAT) method with an error tolerance of 1e-5. The Euclidean Minimum Spanning Tree (EMST) mixing model is used with a mixing constant Cφ =1.5 which was found to be most appropriate for the piloted jet flames considered here. Extensive comparisons between the predictions and the experimental measurements are made. The flame structures are adequately predicted, but additional developments are needed to enable the computation of the mixed-modes of combustion present in these flames.