Effect of Wakes and Secondary Flow on Re-attachment of Turbine Exit Annular Diffuser Flow

Abstract

In this paper numerical results of wake and secondary flow interaction in diffuser flow fields are discussed. The wake and secondary flow are generated by a rotating wheel equipped with 30 cylindrical spokes with a diameter of 10 mm as a first approach to the turbine exit flow environment. The apex angle of the diffuser is chosen such that the flow is strongly separated according to the well-known performance charts of Sovran and Klomp (1967, “Experimentally Determined Optimum Geometries for Rectilinear Diffusers With Rectangular, Conical or Annular Cross-Section,” in Fluid Mechanics of Internal Flow, Elsevier, New York, pp. 272–319). This configuration has been tested in an experimental test rig at the Leibniz University Hannover (Sieker and Seume 2007, “Influence of Rotating Wakes on Separation in Turbine Exhaust Diffusers,” Paper No. ISAIF8-54). According to these experiments, the flow in the diffuser separates as free jet for low rotational speeds of the spoke-wheel, as expected by theory. However, if the 30 spokes of the upstream wheel rotate beyond the value of 500 rpm the measurements indicate that the flow remains attached to the outer diffuser wall. It will be shown by the present numerical analysis with the commercial solver ANSYS CFX-10.0 that only an unsteady approach using the elaborate scale adaptive simulation with the shear stress transport turbulence model is capable of predicting the stabilizing effect of the rotating wheel to the diffuser flow at larger rotational speeds. The favorable comparison with the experimental data suggests that the mixing effect of wakes and secondary flow pattern is responsible for the reattachment. As a result of our studies, it can be stated that the considerably higher numerical costs associated with unsteady calculations must be accepted in order to increase the understanding of the physical flow phenomena in turbine exit flow and its interaction with the downstream diffuser.

Total Pressure Loss Reduction in Annular Diffusers

“…The ongoing increase in affordable computing power, made the analysis of unsteady, heterogeneous inflow conditions viable. Kluß et al showed by means of numerical simulations that unsteady wakes and secondary flow can lead to a rise of the pressure recovery of an annular diffuser above the values predicted by empirical design charts [5]. These findings were confirmed by simulations and measurements [6][7][8][9].…”

Correlation between total pressure losses of highly loaded annular diffusers and integral stage design parameters

“…In addition to the static inlet conditions Kluß et al (2009) numerically investigated the effects of unsteady wakes and secondary flows shed from rotating cylindrical spokes upstream of the diffuser on the pressure recovery. In contrast to predictions made using diffuser design charts assuming static inlet conditions, the unsteady inflow caused a re-attachment of the boundary layer.…”