Three Dimensional Computational Fluid Dynamic Simulation and Analysis of a Turbocharger Compressor

Abstract

This thesis constitutes detailed computational investigation on ow through the passages of a centrifugal compressor used for turbocharging applications. Given the dynamic nature of operation of the turbocharger, it becomes necessary to under- stand the ow that occurs within the blade passages and its e ect on performance. CFD is an established computational technique wherein the ow is dissected to fun- damental levels and a detailed picture is presented, application of this technique with limited and diverse sense towards understanding of ows through a turbocharger compressor has been successfully carried out by many before. This work presented attempts to address many of the lacuna reported and carries forward the work of several researchers to ll in the gaps. The complexity of the geometry of the blade shape poses many challenges in model- ing within the virtual space, an e ective way to overcome the obstacles is presented as a part of this work. Grid generation of the impeller and casing are discussed and adaptive approach is followed with generation of hexahedral grids for the impeller whereas tetrahedral for the casing. Since the grids of the impeller and its casing are di erent, ways of interfacing between the two domains in a CFD environment is discussed. An industry standard implicit 3D RANS solver was used to carry out the simula- tions. The importance of use of boundary conditions for the domain at unsteady operating points is presented in detail. On the choice made for turbulence model that governs the validity of the solution obtained, an extensive literature survey of the relevant topic as applicable for centrifugal compressors is presented and logic of the choice made for the present work is discussed. Menter's two equation SST-k! model emerges as the clear choice to be used even though the di erence in perfor- mance predictions by other turbulence models are insigni cant. Dynamics of ow at optimum design point, surge and choke of the compressor are presented in detail. With the geometry modeled with a tip clearance and the casing included within the simulation environment, it can be seen that the performance predicted is closer to actual at all operating points. A study of behavior of the compressor at extreme o design points is carried out and it can be seen that it depicts the trends that are seen in experimental works available in open literature. The distortion of pressure within the vaneless di user and the inviscid nature of the ow within the volute space are e ectively captured and an in depth analysis is carried out to uncover new patterns. A parametric study involving important geometric features such as the tip clearance and wrap angles are conducted leading to discovery of anomalies. The work summarizes to point out that the investigation carried out with the CFD simulations comprehensively leads to uncovering of ow dynamics within a complex system such as the centrifugal compressor within the limits of numerical analysis.