Numerical Investigation of the Performance Impact of Stator Tilting Endwall Designs on a Mixed Flow Turbine

Yang Gao,Stephen Spence,Jens Fridh,Pangbo Ren,Richard Morrison

doi:10.3390/ijtpp6020014

Abstract

This paper numerically investigates stator endwall designs for a mixed flow turbine. One key design parameter studied is the tilting angle of the stator endwall. By examining stator designs with different tilting angles, the aim of this paper is to improve the efficiency of the studied mixed flow turbine at low velocity ratio working conditions. The performance curve at the design speed was chosen for the comparison between the baseline design and the tilted endwall designs. First, the numerical predictions for the baseline design were validated with experimental data. Then, to understand the mechanism of the performance variation between the different designs, the internal flow field was analyzed in detail. It was found that the tilting stator endwall could form a geometric “kink” in the endwall profiles. On the shroud side, certain designs with such kink caused local flow separations upstream the rotor leading edge. This separation could have the effect of reducing the intensity of the tip leakage vortex and the exit kinetic energy losses at the rotor outlet and may also improve the performance of the exhaust diffuser. As a result, the peak of the efficiency curve shifted toward lower velocity ratio. If the turbine stage incorporated a downstream exhaust diffuser, the optimal design in this study showed a shift of the velocity ratio of the peak efficiency point from 0.62 to 0.60 compared with the baseline. The maximum efficiency improvement was 1.3% points, which occurred at low velocity ratio. Meanwhile, the peak efficiency was 0.2% points higher than the baseline. If the exhaust diffuser was removed, a similar shift of the efficiency curve was observed but less efficiency gain was achieved at the low velocity ratio condition. A preliminary unsteady simulation was also conducted for the optimal design in this study.

Highlights

A mixed flow turbine (MFT) is a promising alternative design to a radial flow turbine (RFT) in turbocharger applications
A well-designed MFT can have the advantage of lower rotor inertia than an RFT, which will improve the transient performance of the coupled engineturbocharger system [1]
To evaluate the performance of the stator tilting endwall designs in such installations, the exhaust diffuser was modified by replacing the divergence shroud with a constant-radius annular one

Summary

Introduction

A mixed flow turbine (MFT) is a promising alternative design to a radial flow turbine (RFT) in turbocharger applications. It introduces several potential benefits to the turbocharging system. A well-designed MFT can have the advantage of lower rotor inertia than an RFT, which will improve the transient performance of the coupled engineturbocharger system [1]. For high specific speed designs, higher efficiency can be achieved from MFT [2]. The forward-curved leading edge of an MFT leads to a better incidence condition and a better performance for high-stage-loading designs [3]. An MFT can improve the energy recovery from the exhaust gas of the engine in a pulse turbocharging because of the potentially better efficiency at low velocity ratios [4]

Objectives

Methods

Results