Impact of an Upstream Film-Cooling Row on Mitigation of Secondary Combustion in a Fuel Rich Environment

Abstract

In advanced gas turbine engines that feature very short combustor sections, an issue of fuel-rich gases interacting with the downstream turbine components can exist. Specifically, in combustors with high fuel-to-air ratios, there are regions downstream of the primary combustion section that will require the use of film-cooling in the presence of incompletely reacted exhaust. Additional combustion reactions resulting from the combination of unburnt fuel and oxygen-rich cooling films can cause significant damage to the turbine. Research has been accomplished to understand this secondary reaction process. This experimental film-cooling study expands the previous investigations by attempting to reduce or mitigate the increase in heat flux that results from secondary combustion in the coolant film. Two different upstream cooling schemes were used to attempt to protect a downstream fan-shaped cooling row. The heat flux downstream was measured and compared between ejection with air compared to nitrogen in the form of a heat flux augmentation. Planar Laser Induced Fluorescence (PLIF) was used to measure relative OH concentration in the combustion zones to understand where the reactions occurred. A double row of staggered normal holes was unsuccessful at reducing the downstream heat load. The coolant separated from the surface generating a high mixing regime and allowed the hot unreacted gases to penetrate underneath the jets. Conversely, an upstream slot row was able to generate a spanwise film of coolant that buffered the reactive gases off the surface. Essentially no secondary reactions were observed aft of the shaped coolant hole ejection with the protective slot upstream. A slight increase in heat transfer was attributed to the elevated freestream temperature resulting from reactions above the slot coolant. Creating this full sheet of coolant will be a key toward future designs attempting to control secondary reactions in the turbine.