Interpretation of Low-Pressure Turbine Profile Loss Generation Mechanisms From a Viewpoint of Blade Drag Forces

This paper describes the interpretation of a generation mechanism of profile loss of low pressure turbine (LPT) blades from a viewpoint of blade drag forces. On the analogy of profile drag of an isolated body, the profile loss of a cascade blade is subdivided into two components, the loss due to friction drag and the loss due to pressure drag. The friction drag is equal to the integral of all axial component of shearing stresses taken over the surface of the blade. The pressure drag, which does not exist in an inviscid flow, is due to the fact that the presence of the boundary later modifies the pressure distribution on the blade. The losses due to friction drag and pressure drag are evaluated for two kinds of blade profiles using the results of steady incompressible Reynolds Averaged Navier-Stokes (RANS) simulations at three different Reynolds numbers (Re), 57,000, 100,000 and 147,000. It is found that the trend of the total profile loss with Reynolds number is mainly determined by the trend of the loss due to pressure drag with Reynolds number. A rise in the total profile loss of the blade with a laminar separation bubble on the suction surface at low Reynolds number is mainly attributed to the increase in the pressure drag due to thickened suction surface boundary layer by the enlarged separation bubble. The friction drag and the pressure drag are also estimated for the measured data of low speed linear cascade tests with a moving-bar mechanism. In the estimation, the pressure drag is derived from the estimated total profile loss and the estimated friction drag by using boundary layer integral equations. It is found that the trend of total profile loss with incoming wake passing frequency is almost determined by the trend of the loss due to pressure drag with the wake passing frequency.

Recognition of Structures Leading to Transition in a Low Pressure Turbine Cascade: Effect of Reduced Frequency

The boundary layer developing over the suction side of a low pressure turbine cascade operating under unsteady inflow conditions has been experimentally investigated. Time-resolved Particle Image Velocimetry (PIV) measurements have been performed in two orthogonal planes, the blade to blade and a wall parallel plane embedded within the boundary layer, for two different wake reduced frequencies. Proper Orthogonal Decomposition (POD) has been used to analyze the data and to provide an interpretation of the most significant flow structures for each phase of the wake passing cycle. To this purpose, a POD based procedure that sorts the data synchronizing the measurements of the two planes has been developed. Phase averaged data are then obtained for both cases. Moreover, once properly sorted, POD has been applied to sub-ensembles of data at the same relative phase within the wake passing cycle. Detailed information on the most energetic turbulent structures at a particular phase are obtained with this procedure (called phased POD), overcoming the limit of classical phase average that just provides a statistical representation of the turbulence field. Furthermore, the synchronization of the measurements in the two planes allows the computation of the characteristic dimension of boundary layer structures that are responsible for transition. These structures are often identified as vortical filaments parallel to the wall, typically referred to as boundary layer streaks. The largest and most energetic structures are observed when the wake centerline passes over the rear part of the suction side, and they appear practically the same for both reduced frequencies. The passing wake forces transition leading to the breakdown of the boundary layer streaks. Otherwise, the largest differences between the low and high reduced frequency are observed in the calmed region. The post-processing of these two planes further allowed us to compute the spacing of the streaks and make it non-dimensional by the boundary layer displacement thickness observed for each phase. The non-dimensional value of the streaks spacing is about constant, irrespective of the reduced frequency.

Effects of Unsteady Wakes on Flow Through High Pressure Turbine Passages With and Without Tip Gaps

Experiments were conducted in a linear high pressure turbine cascade with wakes generated by moving upstream rods. The cascade included an adjustable top endwall that could be raised and lowered above the airfoils to change the tip gap. Conditions were considered with no tip gap, and gaps of 1.5% and 3.8% of axial chord. For each of these, cases were documented both with and without upstream wakes. The pressure distributions on the airfoils were acquired at the midspan and near the tip for each case. The total pressure loss was measured in the endwall region. Velocity fields were acquired in two planes normal to the flow direction using particle image velocimetry (PIV). For the case with no tip gap, the passage vortex and other vortices were clearly visible in the velocity fields. For the cases with a tip gap, the tip leakage vortex was the dominant flow feature, and it became stronger as the gap size increased. The other vortices were still present, but were moved by the tip leakage vortex. For the cases with unsteady wakes, the PIV data were ensemble-averaged based on phase within the wake passing cycle, to show the motion and change in strength of the vortices in response to the wake passing. The regions of high total pressure loss can be explained in terms of the secondary velocity field.

Effect of Endwall Boundary Layer Thickness on Losses in a LPT Cascade With Unsteady Wakes

The effects of inlet endwall boundary layer thickness and up-stream unsteady wakes are investigated experimentally in a low-speed linear cascade. The examined airfoil is the front-loaded L2F, a high-lift low-pressure turbine profile with high resistance to separation even in the low Reynolds number regime. Cases are documented with and without incoming wakes for two inlet endwall boundary layers of different thickness at a Reynolds number of Re = 30,000. Periodic incoming wakes are simulated with moving bars upstream of the cascade. The inlet endwall boundary layer is conditioned with a two-part splitter plate, one part downstream and one part upstream of the wake generator. By the documentation of pressure distributions on the blades, velocity profiles in the cascade inlet as well as total pressure loss and phase-locked velocity data in the outlet, this work attempts to show that varying the inlet endwall boundary layer thickness combined with the effect of incoming wakes has significant influence on the performance of blades with relatively low aspect ratio in cascade experiments. Depending on boundary layer thickness, wakes are shown to have either a stronger impact on midspan or on endwall performance. Time-resolved velocity and vorticity plots additionally show the motion of the vortex and loss core at the blade trailing edge during the event of wake passing.

Investigation of Wake Induced Transition in Low-Pressure Turbines Using Large Eddy Simulation

Modern ‘high-lift’ blade designs incorporated into the low pressure turbine (LPT) of aero-engines typically exhibit a separation bubble on the suction surface of the airfoil. The size of the bubble and the loss it generates is governed by the transition process in the separated shear layer. However, the wakes shed by the upstream blade rows, the turbulent fluctuations in the free-stream and the roughness over the blade complicates the transition process. The current paper numerically investigates the transition of a separated shear layer over a flat plate with an elliptic leading edge using large eddy simulations (LES). The upper wall of the test section is inviscid and specifically contoured to impose a streamwise pressure distribution over the flat plate to simulate the suction surface of a LPT blade. The influences of free-stream turbulence (FST), periodic wake passing and streamwise pressure distribution (blade loading) are considered. The simulations were carried out at a Reynolds number of 83,000 based on the length of the flat plate (S0 = 0.5m) and the velocity at the nominal trailing edge (UTE ∼ 2.55 m/s). A high turbulence intensity of 4% and a dimensionless wake passing frequency (fr = fwakeS0/UTE, where fwake is the dimensional wake frequency) of 0.84 is chosen for the study. Two different distributions representative of a ‘high-lift’ and an ‘ultra-high-lift’ turbine blade are examined. An in-house, high order, flow solver is used for the Large Eddy Simulations (LES). The Variational Multi-scale approach is used to account for the sub-grid scale stresses. Results obtained from the current LES compare favorably with the extensive experimental data previously obtained for the test cases considered. The LES results are then used to further explore the flow physics involved in the transition process, in particular the role of Klebanoff streaks and their influence on performance. The additional effect of surface roughness of the blade has also been studied for one of the blade loadings. The benefit that roughness can offer for highly loaded turbine blades is demonstrated.