Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors: Part 2 — Near Wall Reaction Effects on Film-Cooled Heat Transfer

As commercial and military aircraft engines approach higher total temperatures and increasing overall fuel-to-air ratios, the potential for significant chemical reactions on a film-cooled surface is enhanced. Currently there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. A shock tube experiment was employed to generate short duration, high temperature (1000–2800 K) and pressure (6 atm.) flows over a film-cooled flat plate. The test plate contained two sets of 35° film cooling holes that could be supplied with different gases, one side using air and the other nitrogen. A mixture of ethylene and argon provided a fuel rich freestream that reacted with the air film resulting in near wall reactions. The relative increase in surface heat flux due to near wall reactions was investigated over a range of fuel levels, momentum blowing ratios (0.5–2.0), and Damko¨hler numbers (ratio of flow to chemical time scales) from near zero to 30. For high Damko¨hler numbers, reactions had sufficient time to occur and increased the surface heat flux by 30 percent over the inert cooling side. When these results are appropriately scaled, it is shown that in some situations of interest for gas turbine engine environments significant increases in surface heat flux can be produced due to chemical reactions in the film-cooling layer. It is also shown that the non-dimensional parameters Damko¨hler number (Da), blowing ratio (B), heat release potential (H^*), and scaled heat flux (Q_s) are the appropriate quantities to predict the augmentation in surface heat flux that arises due to secondary reactions.