Axial Loss Development in Low Pressure Turbine Cascades

Abstract

The objective of this work presented in this paper is to study the performance of low pressure turbines in detail by extensive numerical simulations. The numerical flow simulations were conducted using the general purpose code ANSYS CFX. Particular attention is focused on the loss development in axial direction within the flow passage of the cascade. It is shown that modern CFD tools are able to break down the integral loss of the turbine profile into its components depending on attached and separated flow areas. In addition the numerical results allow to show the composition of the loss depending on the Reynolds number. The method of the analysis of axial loss development presented here allows for a much more comprehensive investigation and evaluation of the quality of the numerical results. For this reason the paper also demonstrates the capability of this method to quantify the influence of the axial velocity density ratio, the inflow turbulence level, the inflow angle and the Reynolds number on the loss configuration and the flow angle of the cascade as well as a comparison of steady state and transient results. The validation data of this LPT-Cascade have been obtained at the High Speed Cascade Wind Tunnel of the Institute of Jet Propulsion. For this purpose experiments were conducted within the range of Re2th = 40’000 to 400’000. To gather data at realistic engine operation conditions, the wind tunnel allows for an independent variation of Reynolds and Mach number. The experimental results presented herein contain detailed pressure measurements as well as measurements with 3-D-hot-wire anemometry. However, this paper shows only integral values of the experimental as well as the numerical results to protect the proprietary nature of the LPT-design.

Numerical investigations of secondary flow and loss development in a low-pressure turbine cascade with divergent endwalls

“…following the mixing loss region, which extends to approximately x/c ax = 1.3, there is a quasi linear loss increase as a result of the endwall flow in this region. This observation is consistent with the findings published by Muth and Niehuis (2013), which showed an rapid increase in mixing losses near the trailing edge, especially at low Reynolds numbers, followed by a decreased loss gradient once the flow is mixed out.…”

“…The ideal location of maximum velocity is depending on wake passing frequency, diffusion factor, and Reynolds number. Muth and Niehuis (2013) presented a method to deconstruct the integral loss of a low pressure turbine cascade, enabling an analysis of profile loss components at low Reynolds numbers. Experimental data and predicted results from RANS and U-RANS simulations were used to evaluate the axial loss development and to investigate the effect of incoming wakes among other influencing parameters.…”

Importance of the vane exit Mach number on the axial clearance-related losses

“…At subsonic Mach numbers, the mixing losses seem to have even higher importance at least with low pressure turbine designs. Muth and Niehuis 31 report that at low Reynolds numbers (Re = 40000), the mixing loss can contribute 50% of the overall loss share in a cascade, and at higher Reynolds numbers, they will drop to 35% (Re = 400,000). If the operating conditions are kept constant or nearly constant, the increase of the Reynolds number also increases the Mach number, and the conclusions of Muth and Niehuis 31 are valid in the current analysis.…”