The Effect of Pulsed Blowing on the Boundary Layer of a Highly Loaded Low Pressure Turbine Blade

Abstract

The performance of the low pressure turbine (LPT) can vary appreciably, because this component operates under a wide range of Reynolds numbers. At higher Reynolds numbers, mid and aft loaded profiles have the advantage that transition of suction side boundary layer happens further downstream than at front loaded profiles, resulting in lower profile loss. At lower Reynolds numbers, aft loading of the blade can mean that if a suction side separation exists, it may remain open up to the trailing edge. This is especially the case when blade lift is increased via increased pitch to chord ratio. There is a trend in research towards exploring the effect of coupling boundary layer control with highly loaded turbine blades, in order to maximize performance over the full relevant Reynolds number range. In an earlier work, pulsed blowing with fluidic oscillators was shown to be effective in reducing the extent of the separated flow region and to significantly decrease the profile losses caused by separation over a wide range of Reynolds numbers. These experiments were carried out in the High-Speed Cascade Wind Tunnel of the German Federal Armed Forces University Munich, Germany, which allows to capture the effects of pulsed blowing at engine relevant conditions. The assumed control mechanism was the triggering of boundary layer transition by excitation of the Tollmien-Schlichting waves. The current work aims to gain further insight into the effects of pulsed blowing. It investigates the effect of a highly efficient configuration of pulsed blowing at a frequency of 9.5 kHz on the boundary layer at a Reynolds number of 70000 and exit Mach number of 0.6. The boundary layer profiles were measured at five positions between peak Mach number and the trailing edge with hot wire anemometry and pneumatic probes. Experiments were conducted with and without actuation under steady as well as periodically unsteady inflow conditions. The results show the development of the boundary layer and its interaction with incoming wakes. It is shown that pulsed blowing accelerates transition over the separation bubble and drastically reduces the boundary layer thickness.

High Frequency Boundary Layer Actuation by Fluidic Oscillators at High Speed Test Conditions

“…The suction side flow is imposed on a flat plate in 1:1 scale of the surface length. With matching in-and outlet conditions and without actuation, an excellent correspondence with former studies is shown, compare [9]. The Mach number distribution within the area of interest (white shaded) is shown in Fig.…”

“…1 left). This design, which has a direct dependency between mass flow rate and frequency, was already used in former studies (see [2,9,13,14]. In contrast to the former applications, the pulsed blowing at the outlet of the feedback oscillator (master) is not directly used for flow control, but for frequency tuning of the oscillator without feedback loops (slave) through the control ports ( Fig.…”

“…Compared to the T161 profile, the size of the flat plate is a 1:1 suction side surface length scale. It is equipped with 33 static pressure taps within s/s tot = 0 and 1, ensuring twice the resolution of static pressure taps in relation to the T161 cascade, compare [2,9,13,14]. The positions s/s tot = 0 and 1 correspond to the leading and trailing edge, respectively.…”

“…Considering mass flow investment, Mack et al [9] used for AFC on the T161, an aerodynamically highly loaded low pressure turbine (LPT) research profile, fluidic oscillators with a mass flow rate related to the passage mass flow ofṁ osc.,T 161 ≈ 1.14% •ṁ pas.,T 161 . To improve the efficiency, the latest investigations at the Institute of Jet Propulsion (ISA) prove for an aerodynamically highly loaded LPT exit casing profile that the mass flow investment of the oscillator can be decreased as low aṡ m osc.,T EC = 2.1 • 10 −4 •ṁ pas.,T EC .…”

“…Modern LPTs are aerodynamically highly loaded, usually featuring flow separation on the suction side towards low Reynolds number operating points. Since the high speed investigations at ISA showed many promising results [2,9,13,14] for AFC applied on the T161, this paper focuses on a T161-like suction side flow of a flat plate with pressure gradient. The massive flow separation is controlled with high frequency periodic excitation [15], induced by a newly developed coupled fluidic oscillator [16].…”