An Experimental Investigation of Adiabatic Film Cooling Effectiveness and Heat Transfer Coefficient on a Transonic Squealer Tip

Abstract

The effect of film cooling on a high pressure turbine blade with an open squealer tip has been examined in a high speed linear cascade. The cascade operates at engine realistic Mach and Reynolds numbers, producing transonic flow conditions over the blade tip. Tests have been performed on two uncooled tip geometries with differing pressure side rim edge radii, and a cooled tip matching one of the uncooled cases. The pressure sensitive paint technique has been used to measure adiabatic film cooling effectiveness on the blade tip at a range of tip gaps and coolant mass flow rates. Complementary tip heat transfer coefficients (HTC) have been measured using transient infrared thermography, and the effects of the coolant film on the tip heat transfer and engine heat flux examined. The uncooled data show that the tip heat transfer coefficient distribution is governed by the nature of flow reattachments and impingements. The squealer tip can be broken down into three regions, each exhibiting a distinct response to a change in the tip gap, depending on the local behaviour of the overtip leakage flow. The edge radius of the pressure side rim causes the overtip leakage flow to change dramatically at low clearance. Complementary CFD shows that the addition of casing motion causes no further change on the pressure side rim. Injected coolant interacts with the overtip leakage flow, which can locally enhance the tip heat transfer coefficient compared to the uncooled tip. The film effectiveness is dependent on both the coolant mass flow rate and tip clearance. At increased coolant mass flow, areas of high film effectiveness on the pressure side rim coincide strongly with a net heat flux reduction and in the subsonic tip region with low heat transfer coefficient.

Aerothermal Effect of Cavity Welding Beads on a Transonic Squealer Tip

“…The baseline geometry (left-hand side of figure 4) is a squealer tip with open TE and slight overhangs on the TE section of the Pressure Side (PS) and Suction Side (SS) rims, similar to that studied by Saul in [17]. Near-tip cooling holes are present in the exterior of the squealer rim on both PS and SS.…”

“…The impulse response model described by Oldfield in [22] is employed to measure Heat Transfer Coefficient (HTC), which utilises discrete deconvolution and Fourier transforms to derive filter impulse responses of linear, time intransient, systems with aaaa aaaa Uncertainty of the impulse response method depends on the variance in tip material properties and individual uncertainties of: the greyscale IR camera reading, the in-situ calibration and the surface thermocouple. The measurement uncertainty for the configuration used in this publication was calculated to be approximately 7% by Saul et al [17].…”

“…Examples of this are shroud cavities and vane-pack gaps on the platforms of stators [16]. Similarly, tips are tested with uninterrupted cavities and without rim abrasives [17].…”

“…However, the cascade does not reproduce the scrapping flow aaaaa caused by the relative rotation of the casing. Saul et al [17] have shown that, for a similar tip geometry, the scrapping flow only has a significant impact on the thermal properties of the cavity, whilst the rims are largely unaffected. The effect of welds on Nusselt number, Film Cooling Effectiveness (FCE or 𝜂) and aerodynamic loss is studied at two tip clearances (0.65% span and 1.15% span) and different coolant mass flow rates (𝑚̇𝑐).…”

“…The tips were manufactured in both epoxy resin and Aluminium, with the former being used for Pressure Sensitive Paint (PSP) tests and the latter in IR and traverses. The difference between the tested baseline and the geometry in [17] is the presence of a larger number of cooling holes with smaller hole diameters, resulting in the same total coolant ejection area. To evaluate the effect of weld beads, 5 small inserts were attached to the cavity floor, as per the right-hand side of figure 4.…”