Numerical Study Of Blade Surface Curvature Effects On Film Cooling Performance

Film Cooling is an important approach to improve gas turbine rotor inlettemperature level and achieve high efficiency, low cost gas turbine design. At present,flat plate film cooling model is widely used while its prediction is far different toexperiment’s result. As to the parts of blade whose curvature changes strongly, surfacecurvature should have significant influence on film cooling performance in this area.Along with curvature variation on turbine blade, analysis of the heat transfer parameters,demonstration of the difference in heat exchange and construction of film coolingprediction model in steady and unsteady flow field will have significant value to thedesign of gas turbine film cooling blades.With method of Non-uniform Rational B-spline, five curved surfaces with differentcurvature (R/D=±30、﹣75、120、∞) have been constructed in the fitting section ofCAD software Catia. All numerical simulation on fluid flow in these computationaldomains is finished with the help of RNG k-ε and LES turbulence models in softwareFluent. Effects of surface curvature R/D, blow ratio M and jet pulsation on film coolingperformance have been further studied. The main research tasks and results have beenlisted as follows:First of all, on the basis of RANS methods, RNG k-ε turbulent model withtemperature coupling fixed section is chosen to predict steady fluid flow field inGritsch’s test table-flat plate film cooling model. Compared with experimental results,RNG k-ε model in software Fluent is reliable for solving these problems. In addition, inorder to improve precision of data, all involved curved surface models’ calculationresults should be verified through grid irrelevance validation.Second, analytical research has been carried out to investigate the variation ofpressure gradient and surface heat transfer condition in steady flow field. Theinstallation of involved two-dimensional slot jet flow model (R/D=±30、∞) is asfollows: Coolant injection angle α is35°, density ratio between gas and coolant DR is1.5, and involved blow ratios M are0.5,1.2and2.0. The study suggests that role ofblow ratio is more important than curvature. The increase of blow ratio helps to improvethe heat transfer condition between coolant and gas, and reduce the sensitivity tocurvature of cooling film.Later on, analytical research has been carried out to investigate the variation of span-wise averaged film cooling efficiency and duct Nusselt number in steady flowfield. The installation of involved three-dimensional discrete holes jet flow model (R/D=﹣30、﹣75、120、∞) is as follows: Coolant injection angle α is35°, density ratiobetween gas and coolant DR is1.85, and involved blow ratios M are0.5,1.2and2.0.The study suggests that curvature’s impact on film cooling effect and heat transfercondition is related to the momentum of coolant. The optimal film cooling performancecan be achieved while the scheme has set up: M<1on leading edge, M=1on suction side,M>1on pressure side. Concave curvature can strengthen heat transfer at low blow ratio(M<1) while curvature has no influence on heat transfer at high blow ratio (M>1).Afterwards, choose the LES turbulent model to predict the unsteady flow field oftwo-dimensional slot jet flow model. The analysis on periodic changes of vortex oncurved surface suggests that the path of cooling film has something to do with jetfluctuation. Periodic changes of vortex determine the turbulence intensity, thusimproving the Nusselt number in this area. Period on convex surface is shorter than thaton concave surface at the same blow ratio.At last, choose the LES turbulent model to predict the unsteady flow field ofthree-dimensional discrete holes jet flow model. The analysis on structure changes ofcounter-rotating vortex pair–CVP suggests that jet fluctuation on flat plate leads to theemergency of unsymmetrical CVP. Due to the effect of recirculation region in far flowfield, coolant film rolls back to the surface. The area with intense vortex has high filmcooling effectiveness and Nusselt number. This change has periodic variation like jetfluctuation.