62 Project Results

In the framework of the collaborative investigation described in the original proposal, the Pennsylvania State University provided turbine airfoil details and the relative flow conditions that were carefully replicated at the linear cascade facility of the University of Minnesota.

During the initial stages of the program, the turbine facility (AFTRF) went through an extensive modification for the installation of the coolant delivery system that allowed the researchers to inject from the tip sections of rotating turbine blades (four blades). Since the rotor assembly was taken out of the rig for extensive cooling system modifications, a comprehensive evaluation of the rotor after the assembly was required. The rotor needed to be dynamically balanced after the re-assembly.

During that initial period the rotor instrumentation for heat transfer measurements was also completed. The slip-ring channels that transmit high current (up to 5 Amps) were connected to the electrodes imbedded in the hub endwall surface of the rotor. The Inconel heater foil that was cut in the form of the tip surface was connected to electrodes imbedded in the hub endwall surface via thick metallic foils that were non-intrusively attached to the blade surfaces.

The Penn State program focused on measuring the detailed aerodynamics field in the rotor frame of reference and in the stationary frame of reference when coolant injection near the inner hub surface and blade tip was performed.

6.2.1 Endwall coolant injection from the wheelspace cavity and its influence on turbine mainstream aerodynamics

Aerodynamic Measurements In The Stationary Frame

The relative aerodynamic and performance effects associated with rotor - NGV gap coolant injections were investigated in the Axial Flow Turbine Research Facility (AFTRF). This study quantifies the effects of the coolant injection on the aerodynamic performance of the turbine for radial cooling, impingement cooling in the wheelspace cavity and root injection. Overall, it was found that even a small quantity (1%) of cooling air can have significant effects on the performance character and exit conditions of the high pressure stage. Parameters such as the total-to-total efficiency, total pressure loss coefficient, and three-dimensional velocity field show local changes in excess of 5%, 2%, and 15% respectively. It is clear that the cooling air disturbs the inlet end-wall boundary layer to the rotor and modifies secondary flow development thereby resulting in large changes in turbine exit conditions. It was concluded that :

• The wheelspace coolant mixes with the mainstream flow and produces measurable changes in loss coefficient, velocity field, exit angle, and total-to-total efficiency. Local changes can be significant. The wheelspace coolant should not be neglected on the aerodynamic analysis of turbine blade stages. Overall, root injections showed the strongest effects although radial and impingement cooling showed measurable changes in loss coefficient, velocity, and efficiency.

• The cooling flow caused significant local perturbations in the pressure coefficients. Root injection showed the largest changes followed by radial cooling, and impingement cooling. In all cases the strongest effects were below midspan but dwindling effects exist out to the tip regions. All three cooling methods caused significant local changes and a general redistribution of the data over the entire passage in both radial and tangential directions. Maximum effects ranged from 1.70%, 0.86%, and 2.57% for radial cooling, impingement cooling, and root injection respectively. Although the local perturbations were quite high, when passage average data was evaluated the changes were found to be small for radial cooling and impingement cooling. Root injection was able to affect the overall pressure coefficient as well as cause a redistribution of pressure coefficient data. The amount of change was almost five times that of impingement cooling and 30 times that of radial cooling.

• The cooling flow was responsible for modifying the velocity profiles. Radial cooling and impingement cooling shifted the velocity profiles radialy outward while root injection was able to decrease the overturning and underturning. The point of maximum overturning was shifted radialy by 10% for radial cooling and 5% for impingement cooling. The radial cooling is injected normal to the mainstream flow and would energize and thicken the rotor inlet boundary layer more than impingement cooling. The thicker boundary layer displaces the core flow and the passage vortex towards the tip region. The root injection shows the most significant effects due to the coolant injection and is fundamentally different from radial cooling and impingement cooling. Root injection has the ability to affect the magnitude of the overturning and underturning. The amount of overturning and underturning is reduced.

• Overall root injection showed the strongest changes in total-to-total efficiency. The passage averaged efficiency increase was over 1.5% for root injection. The root injection efficiency increase was localized to the midspan region of the blade. With an injection hole size on the order of the trailing edge thickness and with the injection inclined at 45( to the hub wall, the root injection can significantly energize the nozzle wake region. Radial cooling showed irregular changes in total-to-total efficiency. For 1% coolant flow the change was positive, at 1.25% the change was negative and at 1.5% the change was positive again. Impingement cooling caused the passage averaged total-to-total efficiency to drop.

Detailed information and reduced data for this part of the study is presented in Appendix

"Mainstream Aerodynamic Effects Due to Wheelspace Coolant Injection in a High Pressure Turbine Stage: Part II- Aerodynamic Measurements in the Stationary Frame," (Christopher McLean, Cengiz Camci, Boris Glezer), The ASME Transactions, Journal of Turbomachinery, Vol.123, No.4 , October 2001, pp. 687-696.

Aerodynamic Measurements In The Rotating Frame

This part of the research program deals with the aerodynamic measurements in the rotational frame of reference of the Axial Flow Turbine Research Facility (AFTRF) at the Pennsylvania State University. Stationary frame measurements of "Mainstream Aerodynamic Effects Due to Wheelspace Coolant Injection in a High Pressure Turbine Stage" were presented in the previous page. The relative aerodynamic effects associated with rotor -nozzle guide vane (NGV) gap coolant injections were investigated in the rotating frame. Three-dimensional velocity vectors including exit flow angles were measured at the rotor exit. This study quantifies the secondary effects of the coolant injection on the aerodynamic and performance character of the stage main stream flow for root injection, radial cooling and impingement cooling. Current measurements show that even a small quantity (1%) of cooling air can have significant effects on the performance and exit conditions of the high pressure turbine stage. Parameters such as the total pressure coefficient, wake width, and three-dimensional velocity field show significant local changes. It is clear that the cooling air disturbs the inlet end-wall boundary layer to the rotor and modifies secondary flow development thereby resulting in large changes in turbine exit conditions. Effects are the strongest from the hub to midspan. Negligible effect of the cooling flow can be seen in the tip region.

• The effects of small (1%) coolant injection into the free-stream of a high pressure turbine stage can be very significant and should not be neglected on the aerodynamic analysis of turbine stages. The cooling air is affecting the structure of the three-dimensional secondary flow and inlet rotor boundary layer, which in turn has a large effect on the exit three-dimensional flow and stage performance.

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