A numerical study of combustion in meso-scale vortex chambers

by Wang, Yanxing.

iii The present work numerically investigates the combustion and flow dynamics in meso- scale vortex combustors with continuum-based computational techniques. The primary objectives are to, (1) develop and implement a numerical approach based on the density-based, finite-volume method for the numerical treatment of nonpremixed combustion in a meso-scale vortex combustor using preconditioning technique; (2) numerically and systematically investigate the combustion dynamics in a meso-scale vortex combustor over a broad range of operating conditions; (3) extensively investigate the swirling flow in a cylindrical chamber, and analyze the underlying physics of some typical phenomena. The theoretical formulation is based on the conservation equations of mass, momentum, energy, and species concentration, with consideration of finite-rate chemical reactions and variable thermophysical properties. The governing system is discretized using a preconditioned, density-based, finite-volume method. A multiblock domain decomposition technique, along with static load balance, is used to facilitate the application of efficient parallel computation with message passing interfaces at the domain boundaries. A comprehensive numerical study is conducted first to investigate the swirling flow in a cylindrical chamber. Three-dimensional incompressible Navier-Stokes equations are solved using a finite element method. Three kinds of flow reversals are identified based on the variation of swirl level. At low swirl level, vortex breakdown occurs on the axis, which refers to the formation of a free stagnation point or a iv recirculation zone on the axis of flow with significant streamwise vorticity. At high swirl level, flow is characterized by a columnar flow reversal driven by centrifugal force. The connection between these two kinds of flow reversals is a critical issue. The transition process is analyzed with the introduction of Kelvin-Helmholtz instabilities in the free shear layer. The non-premixed combustion dynamics in the vortex combustor based on asymmetric whirl concept is investigated numerically. Both the stoichiometric and fuellean cases are considered. Three kinds of flow structures included in the combustor are identified: main flow, upstream and downstream recirculating flows. The interactions between flow and flame evolutions are analyzed. The unusual stability characteristics demonstrated by the asymmetric whirl concept in the experiments are verified numerically. Other than that, the behaviors of central recirculation zone associated with the chamber pressure and the magnitude of injection velocities in a whirl combustor are studied in detail.