Secondary Air Interactions with Main Flow in Axial Turbines

Metodi Zlatinov
Advisor: Dr. Tan
In the past decade, industrial gas turbines have by far become the most popular type of plant for power generation due to their compactness, low emissions and potential for power-heat cogeneration. In the effort to increase energy conversion efficiency, engineers have raised turbine inlet temperatures to well above the metal melting point. Turbine blades are generally protected by expensive thermal barrier coatings and various forms of internal and film cooling. However, in order to prevent hot gasses from being ingested into the unprotected cavities between rotating and stationary components, cool air bled from the compressor is used to purge the gaps at the endwalls. MIT, in collaboration with Siemens Energy and Siemens Corporate Research, is developing a computational approach to identify and understand loss generating flow processes of purge air interacting with mainstream flow in axial turbines.

Contours of change in volumetric entropy generation rate relative to a baseline case with no purge flow bring out the regions in a rotor blade passage that have modified losses as a consequence of purge flow injection from the hub gap upstream of the rotor. We have identified a number of effects that result in these changes: mixing out of the velocity difference between purge and mainstream flows, the generation of radial velocity gradients as a consequence of purge flow interacting with the passage vortex structures, and increased wetted and tip clearance flow losses due to a change of reaction. There is also a positive effect of reduced tip clearance losses when purge flow is injected from the shroud. These effects have been rigorously quantified, and their drivers have been pinpointed. This new knowledge provides clear guidelines for better turbine designs.