The Effects of Inlet Boundary Layer Condition and Periodically Incoming Wakes on Secondary Flow in a Low Pressure Turbine Cascade

Abstract

A particular turbine cascade design is presented with the goal of providing a basis for high quality investigations of endwall flow at high-speed flow conditions and unsteady inflow. The key feature of the design is an integrated two-part flat plate serving as a cascade endwall at part-span, which enables a variation of the inlet endwall boundary layer conditions. The new design is applied to the T106A low pressure turbine cascade for endwall flow investigations in the High-Speed Cascade Wind Tunnel of the Institute of Jet Propulsion at the Bundeswehr University Munich. Measurements are conducted at realistic flow conditions (M2th = 0.59, Re2th = 2·105) in three cases of different endwall boundary layer conditions with and without periodically incoming wakes. The endwall boundary layer is characterized by 1D-CTA measurements upstream of the blade passage. Secondary flow is evaluated by Five-hole-probe measurements in the turbine exit flow. A strong similarity is found between the time-averaged effects of unsteady inflow conditions and the effects of changing inlet endwall boundary layer conditions regarding the attenuation of secondary flow. Furthermore, the experimental investigations show, that all design goals for the improved T106A cascade are met.

Characterization of the Endwall Flow in a Low-Pressure Turbine Cascade Perturbed by Periodically Incoming Wakes, Part 2: Unsteady Blade Surface Measurements Using Pressure-Sensitive Paint

“…While the pressure amplitudes and the basic periodic sequence can be reproduced in the numerical simulations, the predictions are not as accurate as in the case of the velocity field in the front part of the blade passage (x/C x = 0.3). Here, a phase shift between 0°and 36°was presented in part 1 [14]. The main difference at P1 is an exaggeration of the low-pressure region before the wake passing, and thus a severe increase in the pressure gradient of 188%.…”

“…However, more relevant for the present investigation is the amount of time relative to one bar passing period ∆τ/t BP , which is determined by the wake orientation inside the blade passage and thus the flow coefficient. Due to the relatively high flow coefficient in the present test case, the bowed wake has a very acute angle with respect to the rear suction surface (see Figure 10 and part 1 [14] for further detail). Therefore, the incline of the diagonal wake-lane in Figure 11 is quite flat.…”

“…In order to enable an investigation of the time-dependent effects of periodically incoming wakes, a phase-locking method according to Bitter and Niehuis [36] was applied to the PSP measurements as well as both PIV setups presented in part 1 [14]. Here, it is critical that the PSP sampling frequency and the dominant bar passing frequency (502 Hz) are decoupled so that the pitchwise bar positions are randomly distributed during the measurements.…”

“…The normalized pressure fluctuations are given by p ′ /q 2th = ( p(τ) − ⟨p⟩)/q 2th , where p(τ) is the mean surface pressure of a τ-bin, ⟨p⟩ is the ensemble average over the entire time series, and q 2th is the theoretical dynamic pressure of the turbine exit flow. The indicated mean bar wake positions were determined by phase-locked PIV measurements in the blade-to-blade plane of the passage presented in part 1 [14]. Since the PIV data did not fully extend to the blade surface due to background reflections, the wake surface position had to be extrapolated (see Figure 10).…”

“…Thereby, the present work will demonstrate how the classic approach to secondary flow investigations can be extended and enhanced to form a more comprehensive picture. While the discussion of phase-locked PIV results was the focus of the first part [14] of this two-part publication, the second part will present unsteady pressure-sensitive paint (i-PSP) measurements on the blade suction surface.…”