THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 8$ -GT-493 345 E. 47th St., New York, N.Y. 10017 E S The Society shad not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or TQ Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME Journal. Authorization to photocopy for internal or personal use is granted to libraries and other users registered with the Copyright Clearance Center ICCC) provided $3/article or $4/page is paid to CCC, 222 Rosewood Dr., Danvers, MA 01923. Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright 0 1998 by ASME All Rights Reserved Printed in U.SA. MODELING OF TURBULENT SWIRLING FLAME STABILIZATION IN LPP COMBUSTORS A. Smirnov, A. Lipatnikov and J. Chomiak Department of Thermo and Fluid Dynamics Chalmers University of Technology 412 96 Gothenburg, SWEDEN ABSTRACT Combustion flows with swirl are investigated in the context of a lean premixed pre-vaporized combustor. The turbu- lent flame speed closure model implemented into the Kiva program appears to be efficient in utilizing computing time and memory. It also predicts a larger flame spread than a distributed reactor model. The effect of different turbulence and combustion models on flow recirculation patterns and heat release is reported. INTRODUCTION The main goal of LPP combustion concept is to reduce the NOX emissions. For that purpose a pre-mixing duct has to be incorporated into the design. This brings into focus the phenomena of spray evaporation, turbulent mixing and pre- mixed and partially premixed combustion. Flame stabiliza- tion by swirl is beneficial as compared to the flame holder stabilization from the point of view of combustion efficiency and emissions (Gupta et al., 1984). As for the turbulent mixing process, swirl may either enhance or dampen it de- pending on the configuration of the swirler. It is therefore of practical interest to investigate the above processes in the swirling flow regime. The unsteady three-dimensional character of spray evaporation, combustion and swirl dy- namics require a complete 3-d model of the whole system. The goal of this work is three-dimensional numerical simu- lation of an LPP combustor with the special emphasis on the comparison of the various turbulence and combustion models. METHOD Turbulent swirl flows are inherently unstable and difficult to predict (Sloan et al., 1986). Both turbulence and combus- tion models are important for the accurate representation of the flow-field in and around the central recirculation zone. In this work we consider the two-equation models because the constants of the second moment closure models are still not well defined for the reacting flows. This choice was also made from the considerations of efficiency and robustness. The standard k — e (KE) turbulence model (Launder and Spalding, 1972) and the method of Hirsch (Hirsch, 1995), based on streamline coordinate correction (SCC), were used to account for turbulence. The SCC model was selected because of its simplicity and special features that make it more suitable for describing swirl flows. The tech- nique is based on the exact solution of the fluid dynam- ics equations for a one-dimensional vortex. This solution is used to compute the shear stress components in local streamline coordinate system and then to transform these components back into the laboratory frame of reference. Theoretically, the procedure allows the two equation models to approach the accuracy of second-moment closure models in swirl flow cases. Two combustion models were used. The first is based on a distributed reactor (DR) concept (Libby and Williams, 1994) discussed below. The second one is the turbulent flame speed closure (TFSC) model put forward by Zimont (1977) and discussed in detail in the papers of Zimont and Lipatnikov (1995) and Karpov et al. (1996). The TFSC model is based on the following balance equation for the progress variable c: at (p c) + ax; (P u1 E) _ a aE 3 a^ i/a =a xi (P Dc a xi ) + pu Ut E (a x ) 2 1 (1) where pu is the density of the unburnt mixture and Dt is the Presented at the International Gas Turbine & Aeroengine Congress & Exhibition Stockholm, Sweden — June 2-June 5, 1998 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 09/11/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use