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.