Effects of a Reacting Cross-Stream on Turbine Film Cooling

Abstract

Film cooling plays a critical role in providing effective thermal protection to components in modern gas turbine engines. A significant effort has been undertaken over the last 40 years to improve the distribution of coolant and to ensure that the airfoil is protected by this coolant from the hot gases in the freestream. This film, under conditions with high fuel-air ratios, may actually be detrimental to the underlying metal. The presence of unburned fuel from an upstream combustor may interact with this oxygen rich film coolant jet resulting in secondary combustion. The completion of the reactions can increase the gas temperature locally resulting in higher heat transfer to the airfoil directly along the path line of the film coolant jet. This secondary combustion could damage the turbine blade, resulting in costly repair, reduction in turbine life, or even engine failure. However, knowledge of film cooling in a reactive flow is very limited. The current study explores the interaction of cooling flow from typical cooling holes with the exhaust of a fuel-rich well-stirred reactor operating at high temperatures over a flat plate. Surface temperatures, heat flux, and heat transfer coefficients are calculated for a variety of reactor fuel-to-air ratios, cooling hole geometries, and blowing ratios. Emphasis is placed on the difference between a normal cylindrical hole, an inclined cylindrical hole, and a fanshaped cooling hole. When both air and nitrogen are injected through the cooling holes, the changes in surface temperature can be directly correlated with the presence of the reaction. Photographs of the localized burning are presented to verify the extent and locations of the reaction.

Analysis of Flow Migration in an Ultra-Compact Combustor

“…The difference in heat flux was increased slightly for higher blowing ratios as shown in Figure 6. 35. Additionally, there was a sizable and steady increase in the convective heat transfer coefficient with increases in blowing ratio.…”

“…The effect of heat release in cooling films due to secondary combustion was studied experimentally in [8], [34] and [35] all using the same test rig. The test rig, described in detail by Evans [8], used a WSR whose exhaust flowed over an The three previous studies report that the magnitude of the heat release due to combustion in the cooling film was controlled by the amount of fuel in the exhaust, the blowing ratio of the cooling scheme and the geometry of the cooling hole.…”

“…Previous research test rig (left)[35] and film cooling inserts (right)[8] shape had the largest area of negative effectiveness of all the shapes tested for the same blowing ratios. The negative effectiveness was due to a secondary combustion of the exhaust fuel streaks reacting with the cooling air resulting in an increased heat transfer rate to the surface.Figure 2.18 shows a secondary combustion for an equivalence ratio of 1.5 using an angled cooling hole and blowing ratios of 0.5, 1.0 and 1.5.…”

Chemiluminescence and High Speed Imaging of Reacting Film Cooling Layers

“…Additionally, incomplete combustion products enter the turbine if the residence time within the combustor is small, due to reduced combustor size, or rice fuel-to-air ratio (3,4). These incomplete combustion products in the turbine can react with the air used to cool the turbine blades, leading to destruction of turbine hardware.…”

“…4.58 and 4.59 show the temperature gauge trends for the normal holes. In both plots it is seen that the heat flux varies with both equivalence ratio and blowing ratio.…”