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UNSTEADY METHODS (URANS AND LES) FOR SIMULATION OF COMBUSTION SYSTEMS

Amsini Sadiki
Institute of Energy and Power Plant Technology, Technische Universität Darmstadt, 64287 Darmstadt, Germany

Johannes Janicka
Institute of Energy and Power Plant Technology, TU Darmstadt, Jovanka-Bontschits-Strasse 2, 64287 Darmstadt, Germany; Darmstadt Graduate School of Excellence Energy Science and Engineering, TU Darmstadt, Jovanka-Bontschits-Strasse 2, 64287 Darmstadt, Germany

Abstract

A great variety of flows with importance to practical engineering applications are inherently unsteady, and virtually all of the Newtonian fluid flows in nature are turbulent. In order to better capture the dynamics of such complex flows using feasible computational costs, it is more appropriate to use unsteady methods. The present overview is confined to single-phase turbulent flows and discuses some basic issues related to unsteady modeling methods. The first part of this paper provides an evaluation of the performance of the unsteady RANS (URANS) method in a comparative manner to LES and experimental data. It could be confirmed that the U-RANS method employing a full Reynolds stress model is able to capture unsteady phenomena, such as the precessing vortex core phenomenon both qualitatively and in parts also quantitatively. In the second part, some important features of combustion LES are recollected and some results based on the conserved scalar method in connection with the concept of sub-grid scale pdf are presented. A flamelet approach is used to relate the filtered mixture fraction to density, temperature and species concentrations. It is shown that LES is able to deliver good results very close to available measured data, where the flow is governed by large, turbulent structures. Flamelet chemistry appears well able to reproduce experimental data for species, in particular with regard to kinetic effects prediction, whereas equilibrium chemistry strongly deviates. However, a good predictability could be achieved when appropriate choice of boundary and inflow conditions is made. The use of the technique of numerical inflow-generation appears to be very helpful.

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