Methodology for the Numerical Characterization of a Radial Turbine under Steady and Pulsating Flow

Doctoral Thesis / Dissertation from the year 2012 in the subject Engineering - Mechanical Engineering, Universidad Politécnica de Valencia (Departamento de M aquinas y Motores T ermicos), language: English, abstract: The increasing use of turbochargers is leading to an outstanding research to§understand the internal flow in turbomachines. In this frame, computational§fluid dynamics (CFD) is one of the tools that can be applied to contribute§to the analysis of the fluid-dynamic processes occurring in a turbine. The§objective of this thesis is the development of a methodology for performing§simulations of radial turbomachinery optimizing the available computational§resources. This methodology is used for the characterization of a vaned-nozzle§turbine under steady and pulsating flow conditions.§An important effort has been devoted in adjusting the case configuration§to maximize the accuracy achievable with a certain computational cost. Concerning§the cell size, a local mesh independence analysis is proposed as a§procedure to optimize the distribution of cells in the domain, thus allowing to§use a finer mesh in the most suitable places. Particularly important in turbomachinery§simulations is the influence of the approach for simulating rotor§motion. In this thesis two models have been compared: multiple reference§frame and sliding mesh. The differences obtained using both methods were§found to be significant in off-design regions. Steady flow CFD results have§been validated against global measurements taken on a gas-stand.§The modeling of a turbine, installed either on a turbocharger test rig or§an engine, requires the calculation of the flow in the ducts composing the§system. Those ducts could be simulated assuming a one-dimensional (1D)§approximation, and thus reducing the computational cost. In this frame of§ideas, two CFD boundary conditions have been developed. The first one allows§performing coupled 1D-3D simulations, communicating the flow variables from§each domain through the boundary. The second boundary condition is based§in a new formulation for a stand-alone anechoic end, which intends to represent§the flow behavior of an infinite duct.§Finally, the turbine was simulated under engine-like pulsating conditions.§The pulsating effects are analyzed separately in the different components of§the turbine and are compared with the results obtained in equivalent steady§simulations, in order to evaluate their quasi-steadiness. The information from§the computations is used to describe the unsteady flow in the turbine components,§and thus, contribute to engine modeling by the implementation of a§new turbine model for its use in 1D codes.