Effusion-Cooling Performance at Gas Turbine Combustor Representative Flow Conditions

Abstract

The thermal management of aero gas turbine engine combustion systems commonly employs effusion-cooling in combination with various cold-side convective cooling schemes. The combustor liner incorporates many small holes which are usually set in staggered arrays and at a shallow angle to the cooled surface; relatively cold compressor delivery air is then allowed to flow through these holes to provide the full-coverage film-cooling effect. The efficient design of such systems requires robust correlations of film-cooling effectiveness and heat transfer coefficient at a range of aero-thermal conditions, and the use of appropriately validated computational models. However, the flow conditions within a combustor are characterised by particularly high turbulence levels and relatively large length scales. The experimental evidence for performance of effusion-cooling under such flow conditions is currently sparse. The work reported here is aimed at quantifying typical effusion-cooling performance at a range of combustor relevant free-stream conditions (high turbulence), and also to assess the importance of modeling the coolant to free-stream density ratio. Details of a new laboratory wind-tunnel facility for the investigation of film-cooling at high turbulence levels are reported. For a typical combustor effusion geometry that uses cylindrical holes, spatially resolved measurements of adiabatic effectiveness, heat transfer coefficient and net heat flux reduction are presented for a range of blowing ratios (0.48 to 2), free-stream turbulence conditions (4 and 22%) and density ratios (0.97 and 1.47). The measurements reveal that elevated free-stream turbulence impacts on both the adiabatic effectiveness and heat transfer coefficient, although this is dependent upon the blowing ratio being employed and particularly the extent to which the coolant jets detach from the surface. At low blowing ratios the presence of high turbulence levels causes increased lateral spreading of the coolant adjacent to the injection points, but more rapid degradation in the downstream direction. At high blowing ratios, high turbulence levels cause a modest increase in effectiveness due to turbulent transport of the detached coolant fluid. Additionally, the augmentation of heat transfer coefficient caused by the coolant injection is seen to be increased at high free-stream turbulence levels.

Time-resolved temperature and velocity field measurements in gas turbine film cooling flows with mainstream turbulence

“…They found that the increased freestream turbulence increases dilution and the lateral spreading of the jet. They further concluded that, at high blowing ratios, higher turbulence can also have a positive effect on the adiabatic effectiveness, which is consistent with the results of Kakade et al (2012). Since the heat transfer coefficient was not measured in this study, the influence of mainstream turbulence on the net heat flux was not evaluated.…”

“…These values are highly dependent on the geometry, the flow conditions and the location relative to the wall. Kakade et al (2012), for instance, measured turbulence conditions in an enginescale annular combustor at isothermal conditions. They found near-wall turbulence intensities larger than u ′ /u m = 25% (here and throughout this article, the main flow velocity and the time-average velocity fluctuation are denoted by u m and u ′ , respectively); and turbulent length scales (normalised by the combustor radius) larger than ~ 15%.…”

“…Several prior experimental studies have considered higher turbulence main flow conditions for film cooling flows. Kakade et al (2012), for example, measured adiabatic effectiveness, heat transfer coefficients and net heat flux reductions downstream of effusion holes with 20° injection angle and main flow turbulence intensities up to 22%. They conclude that higher turbulence causes a greater lateral distribution of the coolant.…”

Effects of Effusion Cooling Pattern Near the Dilution Hole for a Double-Walled Combustor Liner—Part 1: Overall Effectiveness Measurements

“…This phenomenon was attributed to the turbulent mixing that brought separated coolant back toward the surface. Kakade et al [4] also found similar effects of high freestream turbulence to Martin and Thorpe for blowing ratios of the effusion jets up to M eff,in ¼ 1.5. However, Kakade et al found that high freestream turbulence had an adverse effect on adiabatic effectiveness at the highest evaluated blowing ratio (M eff,in ¼ 2).…”

“…The conclusion that high freestream turbulence decreased the overall effectiveness is contradictory to previous studies that look at adiabatic effectiveness of effusion liners. As mentioned before, Martin and Thorpe [3] and Kakade et al [4] found that high freestream turbulence, compared to low freestream turbulence, resulted in a higher adiabatic effectiveness for effusion blowing ratios in the range of M eff,in ¼ 0.5-1.5. The reason given by each author for the increased effectiveness was that when there was jet lift off, the turbulent mixing brought some of the coolant back to the surface.…”