Large Eddy Simulation of Turbulent Swirling Flows in Combustor Related Geometries

Abstract

This thesis deals with the physics of turbulent swirling flows. Large eddy simulation (LES) method is used to investigate the vortex breakdown process, the precessing vortex core (PVC), and the effect of swirl number and flow field configuration on swirling flows. The study is based on an open source CFD code, \textit{OpenFoam}. Turbulent swirling flows are widely used in combustion devices such as internal combustion engines and gas turbine combustors to promote fuel/air/hot gas mixing so that a compact combustor can be designed and to provide combustion stabilization. The aims of the thesis work are to gain deeper understanding on the structures of the vortex breakdown and PVC and to develop and to evaluate simulation methods for predicting turbulent swirling flows. The work is focused on isothermal turbulent swirling flows in gas turbine related geometries.

The LES approach is first applied to simulate turbulent swirling flows in several experimental rigs where laser doppler velocimeter (LDV) and particle imaging velocimeter (PIV) data are available. The experimental data are used to evaluate the performance of LES models and the requirement of grid resolution and then they are used as baseline cases for further exploration of the flow physics with respect to variations in the flow conditions and geometrical configurations. In all cases it is found that the LES results are comparable to the experimental results and the sensitivity of LES results to the sub-grid models is not high provided that a sufficient grid resolution is used. Several criteria for the assessment of LES accuracy are examined.

It is found that the vortex breakdown structure in a combustor is sensitive to the swirl number in the inflow. In a given combustor geometry when swirl number increases above a critical level vortex breakdown is shown to occur. Further increase the swirl number leads to the onset of PVC and the drastic change in the structures of the recirculation zones. The coherent structures associated with vortex breakdown becomes unsteady. The onset of vortex breakdown and the unsteady mode are analysed using dynamic mode analysis (DMD) method. For a given swirl number it is found that when the combustor outlet geometry is changed the vortex breakdown structures can be significant affected. When the outlet area is gradually decreased a center bubble type vortex breakdown is replaced by an annular recirculation zone, and the center vortex core becomes more dynamic. This is expected to have a significant influence on the way the flame is stabilized in the combustor. Examination of the LES results indicates that the transition from the center bubble recirculation to the annular recirculation structures is a manifestation of the coupling of the tangential momentum and the pressure gradient.

Abstract (Swedish)

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.

• author: Wu, Yajing
• supervisor: Xue-Song Bai
• opponent: Proferssor Kempf, Andreas, University Duisburg-Essen, Germany
• organization: Fluid Mechanics
• publishing date: 2013
• type: Thesis
• publication status: published
• subject: Fluid Mechanics and Acoustics
• keywords: Swirling flow, swirler, vortex breakdown, precessing vortex core (PVC), large eddy simulation (LES).
• pages: 142 pages
• defense location: Lecture hall M:B, M-building, Ole Römersväg 1, Lund University Faculty of Engineering
• defense date: 2013-10-22 10:15:00
• external identifiers: other:ISRN LUTMDN/TMHP-13/1098-SE
• ISBN: 978-91-7473-605-2(print)
• ISBN: 978-91-7473-606-9(pdf)
• language: English
• LU publication?: yes
• id: e0ba7ff3-67e1-4287-b043-5f218c08d670 (old id 4058426)
• date added to LUP: 2016-04-04 09:19:07
• date last changed: 2018-11-21 20:52:16