2 - Theses

The present work deals with the simulation of turbulent particle--laden flows at high mass loadings. In order to achieve this goal, the fluid flow is described by means of the eddy--resolving concept known as Large--Eddy Simulation (LES) and the particles are described in a Lagrangian frame of reference. Special emphasis is placed on the inter--particle collisions and the impact of solid particles on rough walls. Both mechanisms are shown to be crucial for the correct description of the particle dynamics in wall--bounded flows. In order to distinguish the present methodology from the variety of methods available in the literature to treat turbulent flows laden with solid particles, the thesis starts with an overview of different simulation techniques to calculate this class of flows. In this overview special care is taken to underline the parameter space, where the different simulation methods are valid. After that, the governing equations and the boundary conditions applied for the continuous phase of the Euler--Lagrange approach used in the present thesis are given. In the subsequent section the governing equations for the solid particles and their interaction with smooth and rough walls are discussed. Here a new wall roughness model for the particles which incorporates an amplitude parameter used in technical applications such as the mean roughness height or the root--mean--squared roughness is presented. After that, the coupling mechanisms between the phases and the algorithmic realization are discussed. Furthermore, a new agglomeration model capable to treat inter--particle collisions with friction is presented. However, the agglomeration model is not evaluated in such a detail as the inter--particle collisions and the particle--wall collisions. The reason is that it does not represent a central aspect of this thesis. The numerical methods for the continuous and the disperse phase are presented in the subsequent section. The efficient algorithm to detect the inter--particle collisions is described in detail. With this efficient algorithm it is possible to detect the inter--particle collisions with computational costs which scale linearly with the number of particles present in the computational domain. The resulting methodology is validated based on a variety of test cases. The validation process starts with two turbulent channel flows at different Reynolds numbers and one turbulent pipe flow. Using this simple test cases possible error sources can be detected easily. After that, a turbulent pipe flow is simulated, where the gravity points vertical to the mean streamwise direction. The appearance of an interesting secondary flow of second kind, for which the particles are only indirectly responsible, is analyzed in detail. In order to demonstrate the applicability of the present methodology in practically relevant turbulent flow configurations, the particle--laden cold flow in a combustion chamber model and the flow in a cyclone separator are tackled. The results are discussed in detail, compared with experimental reference data and interpreted from a physical point of view. Regarding the combustion chamber model, good agreement is found with the reference experiment. Furthermore, it is shown that the present methodology is capable to reproduce in the high mass loading case the disappearance of two stagnation points present on the axis of the low mass loading configuration. Regarding the cyclone separator flow, in the core region still some differences with the reference experiment remain.