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The combined effects of pulsed film cooling and upstream wakes were studied. In film cooling, compressed air is routed around the combustion chamber of a gas turbine engine and bled through holes on the surface of the turbine blades. This compressed air creates a protective film of relatively cool air that reduces the heat transfer between the combustion gases and the blades. Diverting air from the combustor reduces the power and efficiency of the turbine; however, pulsing the air may provide equivalent or acceptable protection for the turbine blades with less cooling air. Previous pulsed film cooling studies have been completed with a simplified, continuous freestream flow. In an actual turbine, the combustion gases pass through a cascade of rotor blades and stator vanes, which interrupt the flow, sending wakes downstream to subsequent rows of turbine blades. In this study, periodic wakes were added to the mainstream flow. A large test plate was constructed with a row of holes through which film cooling air could be pulsed. A wind tunnel provided a wall jet at a controlled velocity across the test plate. A wake generator was located upstream of the test plate to simulate the effect of upstream turbine blades, so that the resulting flow field, film cooling effectiveness, and heat transfer could be studied. Continuous film cooling resulted in better blade protection than pulsed film cooling at equivalent wake frequencies. For the cases with a continuous freestream and the cases with lower wake frequencies, continuous film cooling jets blowing at half the freestream velocity provided the best protection. For the highest wake frequency tested, continuous film cooling jets blowing at a velocity equal to the freestream velocity provided the best protection. Finally, when comparing pulse timing relative to the wake passing, there was some improvement in blade protection when the cooling jet was on as the wake passed over the cooling holes; however in most cases, differences were small. This study suggests that, for the geometry tested, continuous film cooling provides better protection for gas turbine blades for the same amount of cooling air.
A comprehensive reference for engineers and researchers, Gas Turbine Heat Transfer and Cooling Technology, Second Edition has been completely revised and updated to reflect advances in the field made during the past ten years. The second edition retains the format that made the first edition so popular and adds new information mainly based on selected published papers in the open literature. See What’s New in the Second Edition: State-of-the-art cooling technologies such as advanced turbine blade film cooling and internal cooling Modern experimental methods for gas turbine heat transfer and cooling research Advanced computational models for gas turbine heat transfer and cooling performance predictions Suggestions for future research in this critical technology The book discusses the need for turbine cooling, gas turbine heat-transfer problems, and cooling methodology and covers turbine rotor and stator heat-transfer issues, including endwall and blade tip regions under engine conditions, as well as under simulated engine conditions. It then examines turbine rotor and stator blade film cooling and discusses the unsteady high free-stream turbulence effect on simulated cascade airfoils. From here, the book explores impingement cooling, rib-turbulent cooling, pin-fin cooling, and compound and new cooling techniques. It also highlights the effect of rotation on rotor coolant passage heat transfer. Coverage of experimental methods includes heat-transfer and mass-transfer techniques, liquid crystal thermography, optical techniques, as well as flow and thermal measurement techniques. The book concludes with discussions of governing equations and turbulence models and their applications for predicting turbine blade heat transfer and film cooling, and turbine blade internal cooling.
Pulsed film cooling jets subject to periodic wakes were studied experimentally. The wakes were generated with a spoked wheel upstream of a flat plate. Cases with a single row of cylindrical film cooling holes inclined at 35 deg to the surface were considered at blowing ratios B of 0.50 and 1.0 with jet pulsing and wake Strouhal numbers of 0.15, 0.30, and 0.60. Wake timing was varied with respect to the pulsing. Temperature measurements were made using an infrared camera, thermocouples, and constant current (cold wire) anemometry. The local film cooling effectiveness and heat transfer coefficient were determined from the measured temperatures. Phase locked flow temperature fields were determined from cold-wire surveys. With B0.5, wakes and pulsing both lead to a reduction in film cooling effectiveness, and the reduction is larger when wakes and pulsing are combined. With B1.0, pulsing again causes a reduction in effectiveness, but wakes tend to counteract this effect somewhat by reducing jet lift-off. At low Strouhal numbers, wake timing had a significant effect on the instantaneous film cooling effectiveness, but wakes in general had very little effect on the time averaged effectiveness. At high Strouhal numbers, the wake effect was stronger, but the wake timing was less important. Wakes increased the heat transfer coefficient strongly and similarly in cases with and without film cooling, regardless of wake timing. Heat transfer coefficient ratios, similar to the time averaged film cooling effectiveness, did not depend strongly on wake timing for the cases considered.
Abstract: An experimental film cooling study was performed to understand heat transfer effects due to upstream synfuel deposition. A scaled up model incorporating actual synfuel deposition topography was used. The model contained six cylindrical holes inclined at 300. A flat model with the same film cooling hole geometry was used for comparison. An infrared camera was used to obtain both the film cooling effectiveness from a steady state test and heat transfer coefficient from a transient test. The roughness model had deposition valleys along the centerline of the holes, which caused the flow to accelerate around the peaks and helped to keep the coolant jet down on the surface. This flow acceleration caused large film cooling effectiveness improvements for the 1 and 1.5 blowing ratio cases. The heat transfer coefficient showed both cooling benefits as well as cooling losses due to deposit roughness. The roughness model showed no kidney vortices as opposed to the flat model, which produced a beneficial localized reduction in heat transfer coefficient around the film cooling holes. Although a sharp detrimental rise in heat transfer coefficient occurred on the rough model for the 1.5 blowing ratio case. The heat flux ratio shows the overall cooling effect and a substantial cooling benefit compared to the flat model occurred at blowing ratios of 1 and 1.5. It is concluded that the film cooling effects of upstream deposition on actual turbine blades can be highly beneficial.
Partial Contents: Rotating Heat Transfer on a Multipass Cooling Geometry; Pressure Drop and Heat Transfer Characteristics of Circular and Oblong Low Aspect Ratio Pin Fins; External Heat Transfer Study on a HP Turbine Rotor Blade; Effects of Wakes on the Heat Transfer in Gas Turbine Cascades; Effect of Hole Geometry, Wall Curvature and Pressure Gradient on Film Cooling Downstream of a Single Row; The Effect of Density Ratio on the Film-Cooling of a Flat Plate; Shroud Segments for Unshrouded Blade Turbines; Heat Transfer Test Evaluation of the Shell-Spar Blade Cooling Concept Applied to Industrial Gas Turbines; Heat-Flux Measurements and Analysis for a Rotating Turbine Stage; Calculation of Laminar-Turbulent Boundary Layer Transition on Turbine Blades; A Model for Correlating Flat Plate Film Cooling Effectiveness for Rows of Round Holes; Heat Transfer in Gas Turbine Combustors; Effectiveness Measurements for a Cooling Film Disrupted by a Single Jet with Wall Plunging; Full Coverage Impingement Heat Transfer: The Variation in Pitch to Diameter Ratio at a Constant Gap; The Measurement of Local Heat Transfer Coefficients in Blade Cooling Geometries; High Frequency Response Heat Flux Gauge for Metal Blading; Transient Thermal Behavior of a Compressor Rotor with Ventilation - Test Results under Simulated Engine Conditions; Heat Exchangers in Regenerative Gas Turbine Cycles.
The report documents accomplishments made toward understanding the fluid flow and heat transfer processes in gas turbines at the University of Minnesota over the past two years. The research is divided into three subtopics: studies of film cooling, airfoil surface heat transfer and endwall flow and heat transfer. Film cooling experiments show the effects of interaction among jets on curved surfaces and calculations show that parabolic techniques give accurate effectiveness predictions in regions away from injection holes. The surface heat transfer program showed that tripping the flow or roughening the wall has a clear effect near airfoil transition and separation points and that recovery from concave curvature is surprisingly slow. Endwall studies show flow visualization on the cascade endwall and the value of a fence on the endwall for rerouting the horseshoe vortex away from the suction wall.
Research results on curved surface heat transfer, airfoil heat transfer, film cooling and end-wall heat transfer are presented. In particular these studies focus on the recovery process of a turbulent boundary layer from curvature, heat transfer measurements and numerical prediction techniques of film-cooling on an adiabatic flat plate by injection through a single row of holes. Keywords: Heat transfer, Turbulence.