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Article

Assessment of the 3D Flow in a Centrifugal Compressor Using Steady-State and Unsteady Flow Solvers

Authors: Sundström, E.S., Semlitsch, B., Mihaescu, M
Document Type: Conference
Pubstate: Published
Journal: SAE 2014 International Powertrain, Fuels & Lubricants Meeting
Volume: 2014-01-2856  
Year: 2014

Abstract

This paper focuses on numerical analysis of the flow in a ported-shroud centrifugal compressor at stable and unstable near-surge operating conditions. A production turbocharger compressor which is widely used in the heavy automotive sector is considered. A systematic approach is taken to assess the flow solver employed and to characterize the flow field under steady-state and unsteady conditions. The computational data are compared with available experimental measurements, which represent an idealized installation. The Reynolds Averaged Navier-Stokes (RANS) method is used initially on successively refined grids and cell types to assess the dependency of the computed flow solution to the mesh resolution used. The RANS method used is based on the integral formulation of the Navier-Stokes equation with the time-dependent part set to zero. Convection and diffusion terms are approximated in finite volumes with averaged values over cell faces. The SST k-omega two-equation model is used, assuming isotropy of turbulence with no curvature correction option. In areas with flow reversal and separation off walls, unsteady flow is expected, and hence these approximations will result in poor predictions. However, for a stable operating condition near by the maximum efficiency, most part of the compressor flow is steady so the RANS approximation can give a converged steady-state solution, capturing in reasonable limits the performance parameters of the compressor. It is expected however that steady-state RANS may lead to erroneous results when approaching the surge-line. The Large Eddy Simulation (LES) approach is employed to capture the unsteady features in the compressor flow and the possible flow instabilities, which develop under unstable operating conditions (near surge). The sliding mesh technique is used for treating the rotation of the impeller. Under stable operating condition the Power Spectral Density is broad-banded with no distinct unsteady flow-structure. For low mass flow rates, close to the surge line severe flow reversal may occur in the impeller. The aim with this study is to predict with a high resolution the unsteady flow under stable and unstable (near-surge) compressor operating conditions. The unsteady features in the flow field are quantified by means of FFT analysis and transient visualization of fluctuating velocity and pressure fields. The limitations as well as the benefits associated with the two numerical formulation used are also exposed.