Unsteady methods (URANS and LES) for simulation of combustion systems

A great variety of flows in practical engineering applications are inherently unsteady, and virtually all of Newtonian fluid flows in nature are turbulent. In order to better capture the dynamics of such complex flows, it is appropriate to use unsteady methods. The present overview is confined to single-phase turbulent flows. The first part provides an evaluation of the performance of the unsteady RANS (URANS) method. It could be confirmed that the U-RANS method employing a full Reynolds stress model is able to capture unsteady phenomena, such as 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 for flames, that were already computed by RANS-based methods in the literature, are presented. Thereby 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 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.