Cooling performance limits of a turbine blade with potassium evaporative cooling

A new method of turbine blade cooling, the Return Flow Cascade, has been developed in which vaporization of a liquid metal such as potassium is used to maintain the blade surface at a nearly uniform temperature. Turbine blades cooled using this technology have lower blade temperature levels compared to that available with conventional air cooling, potentially resulting in higher firing temperatures or a choice of a wider range of materials for the hot gas path. The detailed operation of the Return Flow Cascade is described including fluid mechanics and heat transfer phenomena that occur at high heat flux and radial acceleration levels characteristic of modem gas turbine engines. The performance limits of the Return Flow Cascade are identified by the development of a theoretical model that estimates the performance of the system for a range of operating conditions found in the experimental test rig and in an actual gas turbine engine. These limits include vapor choking in the internal blade passages, pool boiling limits in the blade, and surface tension restriction of liquid flow. Cascade initiation limits predicted by the internal vapor choking model are in good agreement with experimental results from testing performed at the Massachusetts Institute of Technology.