Aerothermal Investigation of an Intermediate Turbine Duct

Doktorsavhandling, 2009

Intermediate turbine ducts are used to guide the flow in turbofan engines from the short-radius high-pressure turbine to the large-radius low-pressure turbine. The demand for more efficient and silent jet engines and with a reduced environmental impact requires turbofans with increasingly higher by-pass ratios. Intermediate turbine ducts that could provide a greater radial offset in shorter length would contribute greatly to achieving this goal, especially if they maintained low pressure losses and avoided non-uniformities in the outlet flow that might affect the downstream low-pressure turbine. This thesis presents an experimental study of the flowfield and the heat transfer in an aggressive intermediate turbine duct. The goals of this research were to obtain an understanding of the mechanisms governing the heat transfer in intermediate turbine ducts; and as well, to provide detailed high-quality flow and heat transfer experimental data for CFD validation purposes. The experiments were carried out in a state-of-the-art aggressive intermediate turbine duct with structural struts in a large-scale low-speed turbine facility at realistic engine Reynolds numbers and inlet conditions. For the flow measurements, oil-film visualization, static pressure measurements, 5-hole pressure probe and hot-wire anemometry were used. The heat transfer measurements were performed using a technique based on IR-thermography. The experiments were performed for three different turbine operating points in order to evaluate the effect of inlet swirl on the flow and heat transfer in the duct. The results showed that the stationary flow features arising from the upstream turbine had a large effect on both the flowfield and the heat transfer in the duct. The combination of tip leakage flow and structural struts gave rise to a large vortex with changing relative strength for different inlet swirl. This vortex locally dominated the heat transfer in the neighborhood of the suction side of the vane. Also, counter-rotating vortex pairs near the hub created streaks of low and high heat transfer coefficient that varied ±20% on the hub. For a lower inlet swirl angle, flow separation occurred at the corner between the shroud endwall and the strut pressure side due to the mismatch of the strut and flow angles in the tip leakage region. These flow separations created large circumferential variations in the duct outflow, and significantly modified the flowfield and heat transfer on the endwalls where an increase in the heat transfer coefficient of up to 25% was seen. These and other important effects of the flowfield on the heat transfer distributions have been identified. The results provide valuable insights into duct flow phenomena and how they are coupled to the heat transfer. Together they represent a first step towards understanding the heat transfer phenomena in the duct. The results are already being used to improve design methods in industry. Keywords: Intermediate turbine duct, flowfield measurements, heat transfer measurements.