Gas Turbine Engine Durability Impacts of High Fuel-Air Ratio Combustors: Near Wall Reaction Effects on Film-Cooled Backward-Facing Step 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 to occur downstream of the combustor is increased. This may take place when partially reacted species leave the combustor and encounter film-cooled surfaces. One common feature on turbine endwalls is a step between various engine components and seals. Such step features produce recirculating flows which when in the vicinity of film-cooled surfaces may lead to particularly severe reaction zones due to long fluid residence times. The objective of this paper is to study and quantify the surface heat transfer implications of such reacting regions. A shock tube experiment was employed to generate short duration, high temperature (1000–2800 K) and pressure (6 atm) flows over a film-cooled backward-facing step. The test article contained two sets of 35 deg film cooling holes located downstream of a step. The film-cooling holes could be supplied with different gases, one side using air and the other nitrogen allowing for simultaneous testing of reacting and inert cooling gases. A mixture of ethylene and argon provided a fuel-rich free stream 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 Damköhler numbers (ratio of characteristic flow time to chemical time) from near zero to 30. The experimental results show that for conditions relevant for future engine technology, adiabatic flame temperatures can be approached along the wall downstream of the step leading to potentially significant increases in surface heat flux. A computational study was also performed to investigate the effects of cooling-jet blowing ratio on chemical reactions behind the film-cooled step. The blowing ratio was found to be an important parameter governing the flow structure behind the backward-facing step, and controlling the characteristics of chemical-reactions by altering the local equivalence ratio.