Large Eddy Simulation of Turbulent Swirling Flows in Combustor Related Geometries

Abstract: Popular Abstract in English The concern about pollutant emissions from combustion devices has stimulated the development of modern combustion devices. As one of the major power generation machines, modern gas turbine engine has been designed to run at fuel-lean and premixed mode of combustion in which the combustion temperature is moderate so that NOx emission is low. Combustion in premixed mode requires a method to stabilize. One popular way to stabilize premixed flames in a modern gas turbine engine is by use of swirling flows generated in a swirl burner. Swirling flow is commonly seen in nature as well. Tornado in the earth atmosphere is one example of swirling flow. One common feature of all swirling flows is its tendency of vortex breakdown, which is referred to as the process that the axial flow velocity in a swirling flow becomes reversed and the flow is re-circulated towards its original incoming flow direction. The aim of using swirling flows in gas turbine combustors is to generate such a recirculation flow region where hot combustion gas can be re-circulated so that steady transfer of heat from the recirculation zone to the incoming fresh fuel/air mixture can be achieved, which is essential to stabilize the flame. This thesis is about the physics of swirling flows. In particular, answers to the following questions are sought: (a) how is the recirculation zone in a swirling flow formed? (b) how to control the recirculation zone in swirling flows? (c) how to numerically simulate turbulent swirling flows? To answer these questions a sophisticated numerical method, large eddy simulation approach, is used and evaluated based on an open source numerical simulation code, \textit{OpenFoam}. The focus of this thesis is on isothermal turbulent swirling flows, although the engineering background of this work is combustion. The results form a basis for the development of combustors. The large eddy simulation approach is first validated using experimental data obtained in several laboratory rigs. The numerical results are found to agree very well with the experimental results. The method is then applied to systematically investigate the generation of swirling flows in swirlers, the development of swirling flows in combustors, and the impact of various flow and geometrical parameters on the structure of swirling flows. Major findings of this thesis work include the following: The appearance of recirculation zone in swirling flows is promoted by the increase in swirl intensity, i.e. the ratio of the tangential momentum to the axial momentum; when the swirl intensity increases from non-swirl (no tangential flow momentum) to high swirl, a center recirculation zone is often established; when further increasing the swirl intensity the recirculation zone becomes unsteady and the structure of the recirculation zone becomes significantly different from the low swirl ones. The downstream combustor geometry, namely, the area of the outlet of the combustor is shown to significantly affect the structure of the recirculation zones. One can conclude that large eddy simulation approach is a viable approach for design of swirlers and for analysis/prediction of swirling flows.