Large Eddy Simulation of Turbulent Swirling Flows and Turbulent Premixed Combustion

University dissertation from Fluid Mechanics

Abstract: With the increasing concerns about our environment and human health, more and more stringent emission limits have been specified. These adopted and still coming emission limits require and promote more accurate, less experiential methods to design and develop the combustors. In this thesis, computation fluid dynamics (CFD) technology, based on large eddy simulation (LES) and level set G-equation methods, is applied to study the turbulent premixed combustion related problems. This thesis work consists of three parts. First, this thesis studies the flow structures of isothermal turbulent swirling flows in a dump combustor related geometry, and compared the simulation results with corresponding experimental data. The aims are to gain deeper understanding of the flow and turbulence structures in dump combustors and to examine the capability of LES for prediction of turbulent swirling flows. With appropriate inflow, outflow boundary conditions, and fine grid resolution, LES successfully simulates the vortex breakdown, and the anisotropic turbulence structures for all the swirl numbers considered. Large-scale motion of the vortex core center, i.e. the so-called precessing vortex core (PVC) phenomenon, is predicted. The PVC is found to rotate around the combustor axis at a frequency about 18?25 Hz (Strouhal number about 0.17?0.4). To model the turbulent premixed combustion processes, a level set G-equation is first derived in the LES framework. Some aspects of the level set G-equation have also been addressed in detail in this dissertation. This level set G-equation combined with the stationary flamelet library is used to simulate a lean premixed propane/air flame in a jet engine afterburner. The results indicate that the fluctuation of the flame surface, which is responsible for the broadening of the time averaged mean flame brush by turbulence, depends on the large resolved turbulence eddies. Time averaged mean flow velocity, temperature and major species concentrations also mainly depend on these large scale resolved eddies. Contrarily, the unresolved subgrid scale (SGS) eddies mainly contribute to the wrinkling at the SGS level and play an important role in the enhancement of the propagation speed of the resolved flame front. In addition, the spatially filtered intermediate species such as CO strongly depends on the small SGS eddies. Further, the effect of subgrid scale eddies on the wrinkling of premixed turbulent flame is studied using a so-called h-equation, in which the flow field is a prescribed shear flow field with eddies of different scales. It is shown that the effect of subgrid eddies on the wrinkling of flame surface decreases monotonically with the LES filter size; however, the decrease rate strongly depends on the filter size. This implies that, if one wants to resolve most of the flame wrinkling, rather fine grid resolution and filter size should be used.