Heat Transfer Investigations at the Over-Tip Casing of a High Pressure Turbine Stage – Development of Heat Transfer Correlations

Kurzfassung

The development of gas turbines aims to an increase of power and efficiency accounting environmental and economical aspects. Meanwhile combustion temperatures reaches values far above of the acceptable material temperatures. Thus the thermal highly loaded components have to be cooled intensive. These parts includes besides the blades, the platform and the casing of the first stages. Up to now the aero-thermal loading of the over-tip casing above the rotor was investigated poorly. This report contributes to a better understanding of the major phenomena governing the heat flux in the over-tip casing region. Using a HP-turbine at different flow conditions, extended measurements were performed at DLR’s wind tunnel for rotating cascades, Göttingen, concerning the flow field above the rotor and close to the over-tip casing. The data collected include time-averaged as well as time-resolved values as i.e. static pressure at the over-tip casing, wall temperatures and calculated heat flux coefficients, hot-film measurements. Additionally the passage near the casing was investigated using probes (steady and unsteady ones). The sensors and inserts necessary to perform the measurements had to be developed and manufactured; the necessary procedures to evaluate the gained data had to be developed, too. From these investigations an extended data basis was generated and used to proceed in getting information about work process within the tip gap and about the heat flux into linear segments. The Nusselt correlation for a flat plate is investigated for it’s applicability at the over-tip casing. The correlation is enhanced with operating parameters to take the elevated heat transfer levels observed at off-design conditions into account. Additionally a tip leakage model is introduced to examine the difference between heat transfer in the passage and in the gap. Finally differences between time-resolved and pitchwise averaged input data with the flat plate correlation are discussed.