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The Generation of Free Stream Turbulence Intensity and its Effects on Flow Separation Occurring on a Highly Loaded, Low-Pressure Gas Turbine Blade at Low Reynolds Numbers

Current gas turbine engines experience internal fluid temperatures that are beyond the melting point of many components in the engine. The high-pressure turbine solves this problem by cooling the turbine blades with internal air channels and film cooling; however, the low-pressure turbine, which also experiences high temperatures, does not utilize cooling technologies. The higher temperatures now seen by the low-pressure turbine blade are combined with flow separating from the surface of the blades, resulting in thermal fatigue. At the high Reynolds numbers seen at takeoff, the flow over low-pressure turbine blades tends to stay attached. However, at lower Reynolds numbers (25,000 to 200,000), such as those seen during cruise at high altitudes, the flow on the suction surface of the low-pressure turbine blades can separate. This paper examines how the flow field over the L1A turbine blade varies with a change in low Reynolds numbers (60,000, 108,000, and 165,000) and turbulence intensities (1.89% to 19.87%) within the Baylor University Cascade Wind Tunnel (BUC). Quantifying the change in separated flow seen due to varying Reynolds numbers and turbulence intensities will help to characterize the different flow conditions in the low-pressure turbine and how these changes affect the efficiencies of the engine. The ultimate goal of this research is to improve blade design in the low-pressure turbine for all commercial and military aircraft. Introduction and Theory In a turbofan engine, the low-pressure turbine (Fig. 1) is used to extract energy from the flow of a high-pressure, high energy fluid and convert the extracted energy into work. The engine then uses that work to spin the fan and low-pressure compressor (Fig. 1). Any excess energy not extracted can then be converted into thrust. Therefore, using less energy from the flow to spin the fan and compressor will result in more thrust from the same amount of fuel, thus, producing a more efficient engine. Fig. 1: High Bypass Ratio Turbofan with blue box denoting the low-pressure turbine [1] Research conducted at NASA Glenn Research Center showed a 2% decrease in the relative efficiency of aircraft engines operating at cruising altitudes when compared to the same engines running at sea level [2]. This decrease in relative efficiency at altitude was found to be mainly caused by flow separation occurring in the low-pressure turbine. When flying at high altitudes, flow separation, which causes pressure losses and lowers efficiencies, can be seen on the suction side of the low-pressure turbine blades. Flow separation occurs when a viscous boundary layer lifts off of the surface of an airfoil due to adverse pressure gradients or sudden turns, both of which are present in the low-pressure turbine. As the flow reaches the maximum thickness of the turbine blade, the air will meet an adverse pressure gradient which causes the air to decelerate. When the airflow does not have the momentum needed to overcome the adverse pressure gradient, the boundary layer will separate, resulting in less usable work. Effectively, flow separation reduces the overall efficiency of the engine. Flow separation is very common at high altitudes because there is a lower air density than at sea level. For example, at a cruise altitude of 60,000 feet, the density of air drops to one-tenth of that seen at sea level. Such a low air density along with a low cruising speed can result in a reduced Reynolds number (Re) typically between 25,000 and 200,000. Reynolds number (Equation 1) is defined as the product of the density of the fluid, ρ, velocity of the fluid, V, and the characteristic length of the airfoil, c, all divided by the dynamic viscosity of the fluid, μ: