Gas Turbine Cooling Studies Prof. David G. Bogard Turbulence and Turbine Cooling Laboratory The University of Texas at Austin Presentation at UT-CEM Industrial Advisory Panel Meeting Nov. 14 , 2017 Acknowledgements: Sponsors: DOE National Energy Technology Laboratory Office of Naval Research Pratt & Whitney GE Aviation Siemens 1
Outline of presentation n Background on turbine cooling. n Explanation of film effectiveness. n Representative studies: q Internal cross-flow effects on film cooling q Coolant thermal field measurement and prediction n New high speed facility. 2
Heat transfer and cooling is important throughout the gas turbine engine Focus of this presentation Schematic from Bunker (ASME Turbo Expo 2013) 3
Cooling flows in the combustor and turbine section Schematic from Bunker (ASME Turbo Expo 2013) 4
Film cooling history in gas turbines Schematic from Bunker (ASME Turbo Expo 2013) 5
Turbine airfoil cooling: Internal cooling Film cooling Mainstream Flow Coolant Flow 6
Evaluation of the performance of film cooling 𝝌 ≡ 𝑼↓ 𝑼↓ ∞ − 𝑼↓𝒕𝒗 𝒕𝒗𝒔𝒈𝒃𝒅 Performance parameters: Overall ¡effec*veness: ¡ 𝜽 ≡ 𝑼↓ 𝑼↓ ∞ − 𝑼↓ 𝑼↓𝒃𝒙 /𝑼↓ 𝑼↓ Film ¡cooling ¡effec*veness: ¡ 𝒓 "= 𝒊↓ 𝒊↓𝒈 ( 𝑼↓ 𝑼↓𝒃𝒙 − 𝑼↓𝒕𝒗 𝒕𝒗𝒔𝒈𝒃𝒅𝒇 η , h f φ T c 𝒓 "= 𝒊↓ 𝒊↓𝒋 ( 𝑼↓ 𝑼↓𝒅 − 𝑼↓𝒋𝒐𝒖 𝒐𝒖 ¡ 𝒕𝒗 𝒕𝒗𝒔𝒈𝒃 7
Film cooling holes in gas turbine engines are often fed by an internal crossflow Figure from Han et al (2000) 8 November 20, 2017
Internal crossflow has been found to reduce film cooling effectiveness Crossflow Feed – VR c = 0.35 Quiescent Plenum Feed Mainstream Flow Coolant Jets Plenum Feed 0 5 10 20 15 0 5 10 20 15 x/d x/d 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 η Data is from Wilkes et al. (2016) and Anderson et al (2015) 9
Motivation and Objectives Film cooling holes are commonly fed by an internal crossflow, but the impact of the crossflow velocity is poorly understood, particularly for shaped holes. For this study, the performance of 7-7-7 shaped holes was studied with specific objectives of: • Understanding how crossflow velocity impacts film cooling effectiveness over a wide range of conditions • Determining which parameters govern how internal crossflow impacts film cooling performance • Providing insight into improved film cooling design 10 November 20, 2017
Low Speed Recirculating Wind Tunnel Facility Shaped hole geometry Engine condition coolant density ratios are achieved by using liquid nitrogen to cool film coolant feed to a density ratio of DR = 2.0 11 November 20, 2017
Internal cross-flow reduced film effectiveness, with larger decrease with increasing VR c Spatially average film effectiveness is the average of 4 pitches over x/d = 5-20 Internal cross- flow shown to cause substantial reduction in film effectiveness! Sensitivity to VR c was small at lower VR , but substantial for VR > 1. 12
The reduction in film effectiveness appeared to be due to jet asymmetry within the coolant hole. VR = 0.56 VR = 1.11 VR = 1.67 VR c = 0.2 VR c = 0.4 VR c = 0.6 0 5 10 15 20 25 0 5 10 15 20 25 0 5 10 15 20 25 x/d The jet trajectories deviated noticeably from the centerline of the hole This jet movement was the result of biasing within the diffuser 13
Model turbine vane test facility at the University of Texas Wind Tunnel Test Section Flow Bleed Turbulence Grid Mainstream Adjustable Wall Wax Sprayer Flow Location Bleed n Liquid nitrogen cooled secondary flow: DR = 1.5 Internal coolant n Mainstream turbulence 20% flow n Reynolds number R c = 7x10 5 14
Study to compare computational predictions with experimentally measured thermal fields for coolant jets Measurements were taken in a corner test section n with a simulated 3 vane linear cascade of an 11.6x scale C3X vane. Low (k = 0.043 W/mK) and high (k = 1.0 W/mK) n thermal conductivity vane models were tested, the high thermal conductivity model was designed to match the Biot number of an actual engine vane 2-D thermal fields were measured 0, 5 and 10 n hole diameters downstream of a single row of coolant holes on the suction side of the vane Schematic of vane model 15
Comparison of experimental and computational thermal fields for M =0.65 ADIABATIC ¡ CONDUCTING ¡ Experimental ¡ Computa1onal ¡ Experimental ¡ Computa1onal ¡ ϑ ¡ 2 ¡ 2 2 (a) ¡ (c) ¡ (e) ¡ (g) ¡ x/d =5 0.9 1.5 1.5 y/d ¡ 0.8 1 ¡ 1 1 0.7 0.5 0.5 0.6 0 ¡ 0 0 0 1 2 3 4 0 1 2 3 4 2 0.5 2 2 (b) ¡ (f) ¡ (h) ¡ (b) ¡ (d) ¡ x/d =10 0.4 y/d ¡ 1.5 1.5 0.3 1 1 1 0.2 0.5 0.5 0.1 0 0 0 0 1 2 3 4 0 1 2 3 4 1 3 1 3 1 2 3 2 2 1 2 3 z/d ¡ z/d ¡ z/d ¡ z/d ¡ CFD predicts a much colder core temperature n CFD thermal field predicted the jet core would be separated from the vane wall. n The conducting wall has only a small effect on the over-flowing coolant jets. n Experimental coolant jet has spread across the entire pitch by x/d = 10, CFD n coolant jet spread only 75% of the pitch 16
Comparison of experimental and computational thermal fields for M =1.11 ADIABATIC ¡ CONDUCTING ¡ ϑ ¡ Experimental ¡ Computa1onal ¡ Experimental ¡ Computa1onal ¡ 2 2 2 (a) ¡ (c) ¡ (e) ¡ (g) ¡ 0.9 x/d =5 1.5 1.5 y/d ¡ 0.8 1 1 1 0.7 0.5 0.5 0.6 0 0 0 0 1 2 3 4 0 1 2 3 4 2 0.5 2 2 (h) ¡ (b) ¡ (f) ¡ (d) ¡ x/d =10 1.5 0.4 1.5 y/d ¡ 1 1 0.3 1 0.5 0.2 0.5 0 0 0 0.1 0 1 2 3 4 0 1 2 3 4 1 2 3 1 2 3 1 2 3 1 2 3 z/d ¡ z/d ¡ z/d ¡ z/d ¡ CFD predicts a much colder core temperature. n Significant coolant jet separation seen both experimentally and computationally. n The CFD shows a split core due to counter rotating vortices– this was not observed n in the experiments. Again, the thermal effects on the high conductivity wall were minimal on the coolant n jets above the wall. 17
Schematic of Univ. of Texas high speed wind tunnel facility New facility will operate in the transonic Mach number, matching gas turbine engine operating conditions. Two wind tunnel facilities: flat plate test section and cascade test section. 18
Conclusions n Our lab has extensive experience in accurate simulation of gas turbine operating conditions in a laboratory environment. These simulations allow us to test large scale models to provide detailed spatial resolution of coolant flows. This is key to understanding the physical mechanisms involved. n This allows development of improved turbine cooling technologies. n It also allows development of and evaluation of improved computational models which are needed for engine designs. 19
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