Near-wall reaction effects on film-cooled surface heat transfer

Abstract

As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, there exists a potential for significant heat release to occur in the turbine if energetic species emitted from the combustor are further oxidized during interaction with film-cooling flows. Currently there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. To study surface heat flux augmentation due to near-wall reactions, a shock tube experiment was employed to generate short duration, high temperature (1000-2800 K) and pressure (6 atm.) fuel-rich flows over a film-cooled flat plate. The relative increase in surface heat flux due to near-wall reactions was investigated over a range of fuel levels, mass blowing ratios (0.5-2.0), and Damkohler numbers (ratio of flow to chemical time scales) from near zero to 30. It was shown that significant increases in surface heat flux can be produced due to chemical reactions in the film-cooling layer. Under some conditions, the heat flux exceeded that obtained when no film-cooling layer was present on the surface. A numerical tool was developed and showed good agreement with the experimental results for predicting changes in surface heat flux and film effectiveness in the presence of local reactions. Off-surface effects and changes in convective heat transfer coefficient were also evaluated. Realistic turbine and cooling flows were examined to ascertain the robustness of various cooling configurations to near-wall reactions. The result of this work is a set of tools based on a group of parameters that can be used to assess changes in heat load due to near-wall reactions. The non-dimensional parameters are the Damkohler

(cont.) number (Da), mass and momentum blowing ratios (B and I), freestream energetic heat release potential (H*), and scaled heat flux ratio (Qs). The scaled heat flux ratio always increases with Damk6hler number and depends on the structure of the cooling jet, but is not a function of the freestream fuel energy content.