THE EFFECT OF TRANSPIRATION ON DISCRETE INJECTION FOR FILM COOLING

A segment of permeable wall is installed near a row of cylindrical film holes, parallel to the flow and inclined at 35 degrees. Coolant is forced through both the permeable wall and the film holes resulting in a downstream film composed of both transpired and discretely injected coolant. The permeable wall extends 1.5 cylindrical hole diameters in the flow direction. The effects on the aerodynamic performance and cooling downstream of the row of cylindrical holes in the presence of transpiration is studied numerically with a procedure validated by hot-wire anemometer and temperature sensitive paint measurements. The hydrodynamic boundary layer in the presence of film and adiabatic film cooling effectiveness downstream of single and coupled film sources are compared with numerical predictions. The performance of the coolant film is predicted in order to understand the sensitivity of cooling and aerodynamic losses on the relative positioning of the two sources at each blowing ratio. The results indicate that a coupling of the two sources allows a more efficient use of coolant by generating a more uniform initial film. With careful optimization the discrete holes can be placed farther apart laterally and operate at a lower blowing ratio with a transpiration segment making the large deficits in cooling effectiveness mid-pitch less severe, overall minimizing coolant usage. Comparisons of linear superposition predictions of the two independent sources with the corresponding coupled scenario indicate the two films positively influence one another and surpass additive predictions of cooling. All relative placements have an overall beneficial effect on the cooling seen by the protected wall. Some cases show an increase in area-averaged film cooling effectiveness of 300% along with a 50% increase in aerodynamic loss coefficient by injecting an additional 10% coolant. In this study the downstream transpiration placement is found to perform best of the three geometries tested while considering cooling, aerodynamic losses, local uniformity and manufacturing feasibility. With further study and optimization this technique can potentially provide more effective thermal protection at a lower cost of aerodynamic losses and spent coolant.