A Comparison of the Flow Structures and Losses Within Vaned and Vaneless Stators for Radial Turbines

Abstract

This paper details the numerical analysis of different vaned and vaneless radial inflow turbine stators. Selected results are presented from a test program carried out to determine performance differences between the radial turbines with vaned stators and vaneless volutes under the same operating conditions. A commercial computational fluid dynamics code was used to develop numerical models of each of the turbine configurations, which were validated using the experimental results. From the numerical models, areas of loss generation in the different stators were identified and compared, and the stator losses were quantified. Predictions showed the vaneless turbine stators to incur lower losses than the corresponding vaned stator at matching operating conditions, in line with the trends in measured performance. Flow conditions at rotor inlet were studied and validated with internal static pressure measurements so as to judge the levels of circumferential nonuniformity for each stator design. In each case, the vaneless volutes were found to deliver a higher level of uniformity in the rotor inlet pressure field.

The influence of secondary flow structures in a turbocharger turbine housing in steady state and pulsating flow conditions

“…This is contrary to [9], where the secondary flow structure persisted throughout the volute, and the findings of [7] who found such structures up to the 270° plane. In the current study the cross section is of a circular shape and is symmetrical.…”

“…In the current study the cross section is of a circular shape and is symmetrical. As a result, the major secondary flow structures exist lower in the volute section compared with the trapezoidal shape tested by [9] where the vortices remained in the upper corners of the volute and hence were less effected by the strong radial pressure gradient into the wheel. It is thought that the reduction in volute cross sectional area with progression around the azimuth angle results in the pressure gradient influencing a larger proportion of the cross sectional area, therefore hindering secondary flow development beyond the first 90°.…”

“…Furthermore, low momentum energy was measured at the sharp corners around the volute exit, leading to high total pressure losses. [9] completed full stage CFD simulations and managed to capture detailed secondary flow structures within the volute. They showed that two counter rotating vortices formed within the trapezoid cross sectional shaped volute at approximately 90deg azimuth angle and continued to persist around the volute.…”

A Comparison of flow control devices for variable geometry turbocharger application

“…At the inlet, the flow direction was set to match the volute delivery flow angle of 68°, measured from the radial direction, by applying the assumption of free vortex flow to the scroll (Rajoo, 2007). For model numbers 6 to 10 the solution domain outlet had to be extended with a section of bladeless rotor (seen in Figure 6) as the significant wake which was being created by the sliding wall was reversed over the span of the boundary, thus detrimentally affecting the accuracy of the results (Tu et al 2008 [11] [7], the Shear Stress Transport (SST) turbulence model was adopted. For model numbers 6-10, the k-ε model had to be used as the extended solution domain made convergence more computationally demanding and could not be achieved with the criterion of 10 -5 for the RMS of the residuals used throughout.…”

“…The numerical model was validated by comparing the blade loading results to experimental pressure readings along the surface of the stator vanes. [7] noted the presence of horseshoe vortices emanating from the leading edge of the vane and propagating across the passage which drained a significant amount of low momentum boundary layer fluid into the centre of the passage. This low momentum flow and the wake coming off the trailing edge have a significant influence on the overall loss levels.…”