Investigations at the Over-Tip Casing of a High Pressure Turbine Including Off-Design Conditions and Heat Transfer Correlations

Turbine inlet temperature is reaching values far above acceptable material temperatures. Thus the thermally highly loaded components have to be cooled intensively. One of the most thermally loaded components is the over-tip casing. In recent years research on heat transfer at the over-tip casing received a big boost by availability of faster and more accurate measurement techniques. So far the investigations are limited to design condition, no information is available at off-design conditions. This paper aims to contribute to a better understanding of the major phenomena governing heat transfer in the over-tip casing region, including the influence of different operating conditions. Using an HP-turbine at different flow conditions, extended measurements were performed at DLR’s wind tunnel for Rotating Cascades (RGG), Go¨ttingen, including the flow field in the tip gap and close to the over-tip casing. The Nusselt correlation for a flat plate was investigated for it’s applicability to the over-tip casing. The influence of the incidence angle on the heat transfer levels at the upstream part of the over-tip casing is investigated. Also the influence of the rotational speed on tip-leakage flow and casing heat transfer will be discussed. These results are compared to flat plate heat transfer correlations for design and off-design conditions.