扇形孔气膜冷却效果的数值模拟

不同孔型气膜冷却流场的大涡模拟
张玲;李少华;郭婷婷
【期刊名称】《华北电力大学学报(自然科学版)》
【年(卷),期】2010(037)002
【摘要】采用大涡模型(LES)模拟了圆形气膜孔和锥形角γ=30°扇形孔在吹风比M:1.0时不同截面上的涡量等值线以及分布特征随时间的变化过程.结果表明:对称面的正反旋涡和垂直截面的马蹄形涡的两翼交替周期性地脱落成新的涡,同一截面扇形气膜孔旋涡生成和脱落的时间比圆形气膜孔的短,冷气射流与主气流掺混剧烈,带走的能量较多,所以壁面的冷却效率高;反向旋涡对气膜冷却流场有重要影响,在同一截面圆形孔的反向涡旋对(CVP)要比扇形孔的大,且涡心位置要比扇形孔的高,因此圆孔对壁面的冷却效果要比扇形孔的差.
【总页数】6页(P79-84)
【作者】张玲;李少华;郭婷婷
【作者单位】华北电力大学,能源与动力工程学院,北京,102206;东北电力大学,能源与机械工程学院,吉林,吉林,132012;东北电力大学,能源与机械工程学院,吉林,吉林,132012
【正文语种】中文
【中图分类】TK474.71
【相关文献】
1.扇形喷孔气膜冷却流场的大涡模拟 [J], 郭婷婷;邹晓辉;刘建红;李少华
2.不同孔型对气膜冷却效果影响的数值模拟 [J], 张晓东;董若凌;施红辉;陈伟;沈伟杰;张苹
3.不同孔型平板气膜冷却特性的数值模拟 [J], 王春娟;董若凌;施红辉;陈伟;张晓东
4.开槽圆柱孔气膜冷却流场的大涡模拟 [J], 张玲;杨鹤;刘琳琳
5.气膜冷却流场的大涡模拟 [J], 郭婷婷;刘建红;李少华;徐忠
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Chinese Journal of TurbomachineryA Numerical Investigation on Cooling Effectiveness forAdvanced Fan-shaped Film CoolingXu Li Yan-jing Qu Liu-li Song(AECC Shenyang Engine Research Institute,Shenyang,China)Abstract：As one of the advanced film cooling scheme,fan-shaped hole exhibits significant improvement in film cooling effectiveness compared with cylindrical holes.In this paper,numerical simulations are applied to predict the cooling effectiveness for fan-shaped hole at various flow conditions.The flow conditions are evaluated at three blowing ratios(0.5,1,1.5)and four mainstream Mach numbers(0.3,0.45,0.6,0.75).CFD simulations are performed in three RANS turbulence models namely realizable k-εmodel,SST k-ωmodel and standard k-εmodel.Simulation results are compared with experimental data in terms of centerline adiabatic cooling effectiveness,the realizable k-εmodel showsa better agreement in predicting the film cooling performance.Although some agreements are obtained,all the threeturbulence models tend to overpredict the cooling effectiveness compared to experimental data.Keywords：Film Cooling,CFD,Turbulence Models,Adiabatic Cooling EffectivenessDOI:10.16492/j.fjjs.2019.03.00110IntroductionDuring past70years,there has been a very significant increase in turbine entry temperature(TET)in order to im-prove gas turbine performance,currently TET has reached to about2000°C around year2010for the advanced gas tur-bine,but at the same time,the temperature limit of current Nickel based super-alloy for turbine blade is about1100°C [1].Currently,blade cooling technology is a complex combi-nation of multiple different cooling techniques,among these, film cooling[2]is one of the most common ways to provide cooled protective layer between the hot gas and the external surfaces of gas turbine blades through discrete film holes by blowing of cooling air extracted from compressor.Researches on film cooling have revealed that film hole shapes have significant impacts on film cooling effective-ness.Goldstein et al.(1974)firstly used a cylindrical hole with a conical diffusing section and measured effectiveness downstream of the hole[3].They found that the jet remained near the surface with mainstream flow applied,and the cen-terline effectiveness was comparable with slot cooling,the lateral variation in effectiveness was significantly reduced compared with conventional cylindrical holes.Haven et al. (1997)found the conical diffuser could also improve film cooling coverage[4].Gritsch et al.(1998)and Thole et al. (1998)present adiabatic cooling effectiveness and flow field measurements for two types shaped holes:fan-shaped hole and laid-back fan-shaped hole[5-6],the exit expansion in these shaped holes reduces the velocity and hence the mo-mentum of the coolant flow,so that the jet penetration is de-creased,leads to improve the cooling effectiveness.Sargison et al.(2001)presented that a converging slot hole(console), which showed same cooling performance compared with fan-shaped holes,but significantly reduced aerodynamic loss[7]. Besides that,investigations are performed for many other shaped film holes[8-11],and numerical simulations are also widely applied to investigate film cooling besides experimen-tal studies[12-15],generally numerical simulations are able to capture visible detailed flow and temperature filed despite of the measurement limitations in the experiments.1Fan-shaped Film Cooling Simulations 1.1Geometry of fan-shaped holeGritsch et al(1998)provided measurements on flow field and cooling effectiveness on three different film hole geometries:a cylindrical hole,a fan-shaped hole and a laid-back fan-shaped hole[5][16].In this paper,numerical study concerning fan-shaped film hole cooling performance is based on these experiments.The detailed schematic of fan-shaped film hole is shown as Fig.1.Single,scale-up fan-shaped hole with a30°inclination angleαwas applied.The fan-shaped film hole in-cluded two sections:a cylindrical inlet section and an ex-A Numerical Investigation on Cooling Effectiveness for Advanced Fan-shaped Film Cooling··72第61卷，2019年第3期 Vol.61，2019,No.3Chinese Journal of Turbomachinerypanded outlet section.At the inlet,the diameter of the cylin-drical hole was 10mm with a length-to-diameter ratio of 2.The lateral expansion angle βfor the fan-shaped film hole was 14°,leading to a width of 30mm at the expanded outlet-section,the length of the expanded section is 40mm,result-ing an outlet -to-inlet area ratio of 3.0.All the hole geometry parameters were well balanced to enable an increased flow expansion before the coolant flow entering the expansion sec-tion hence encourage flow diffusion,and it also limited the flow separation at the outlet so as to improve the coolant flow coverage.1.2Computational domainThe computational domain in this numerical investiga-tion matches the film cooling test section,according to the test condition,the computational domain mainly consists of three parts:a primary channel which simulates the main-stream,a secondary channel which is adapted to deliver the coolant,the coolant passes through the fan-shaped film hole between the primary channel and secondary channel and then injects into the mainstream.As shown in Fig.2and Fig.3the width and height for the primary channel are 90mm and 41mm,respectively,and the secondary channel is 60mm in width and 20mm in height.The diameter of the fan-shaped film hole at the inlet section is 10mm,resulting in a 3.0owclet -to-entry area ratio of the fan-shaped film hole.The outlet plane of the mainstream is located at 15D downstream the centre of the hole outlet .1.3GridIn this study,commercial software ICEM CFD was ap-plied to generate multi-block structured grid.Grid indepen-dence was obtained through solution-based adaption,the me-dium mesh with a total element number of 3288744was se-lected for all the simulations,for the whole computational do-main,the mesh quality for all the cells are above 0.3,which indicates the mesh quality are reasonable for the simulation,the first point above the bottom wall of the primary channel is about 0.004mm,which results in the average Y plus value around 1at this surface.Fig.4shows the overview of the fi-nal grid used in this study,boundary layer refinement are al-so detailed in the near wall regions of the side walls of the primary channel,the side walls of the film coolant channel,and the walls of the fan-shaped film hole.1.4Turbulence models and test casesThe simulations were carried out by applying the com-mercial CFD codes Fluent 14.0software.The particular solv-er was pressure correction to achieve the pressure-velocity coupling by multi-grid acceleration.In the three dimensional computational domain with structured grid,steady,time-aver-aged Navier-Stokes equations were processed and pressure-based SIMPLEC solver with second-order upwind discretiza-tion schemes were used.The flow parameters investigated in a matrix in this study are shown in Table 1,in order to obtain the basic flow characters and film cooling mechanism for fan-shaped film hole,a baseline case is set at selected flow condition (Ma c =0.6,Ma c =0,M =1),here Ma c and Ma c are main stream and coolant flow Mach number respectively ，and M refers to blowing ratio.Besides that,seven other test cases are classi-fied into three groups to investigate the effects of flow pa-rameters on film cooling effectiveness.In order to evaluate the performance from different tur-bulence models to predict fan-shaped hole coolingperfor-Fig.1Schematic of fan-shaped holegeometryFig.2Schematic structure of computationaldomainFig.3Solid model of computational domainFig.4Mesh overview of the computational domain and mesh de-tails in the film hole region··73Chinese Journal of Turbomachinerymance,three RANS turbulence models,namely the standard k-ε(SKE)with enhanced wall treatment ,the SST k −ωmodel and realizable k-ε(RKE)model are examined in various flow conditions in this study and compared with published experimental data [5].1.5Boundary ConditionsAll the other surfaces of the computational domain were set to isothermal no-slip wall condition where the heat flux through the wall was specified to be zero.The inlet tur-bulence intensities for the mainstream and coolant channel are specified to 1.5%and 1%respectively.In order to achieve the expected flow conditions include and blowing ratio M,pressure inlets are adopted both for the primary channel(mainstream)and secondary channel (cool-ant),likewise,pressure outlets were specified at the outlet of the both channels,static pressures and total temperatureswere given at the outlets.In the baseline case,mainstream in-let total pressure is 93800Pa and static pressure at the outlet is 68000Pa,coolant flow inlet total pressure is 100520Pa,for the other test cases,both the primary channel and secondary channel pressure at the inlets varies to satisfy the determined flow conditions.In all test cases,the coolant T c and the main-stream total temperatures T ∞are selected to be 290K and 540K respectively and consistent with experiment condition [5],hence the temperature ratio (T c /T ∞)is set to 0.54and kept constant,which represents for typical gas turbine air cooled blade operational condition.2Results and Discussion2.1Baseline caseSince the flow field directly affects the interaction be-tween the mainstream and coolant and hence influences film cooling performance,flow field hence is a very important is-sue in film cooling simulation.In the baseline condition (Ma m =0.6,Ma c =0,M =1),three turbulence models are applied to run the simulations.Results show complicated flow structure in the film hole and the near hole region.It is clear that all the three turbulence mod-els are able to predict the jetting region with high momentum at the leading edge within film hole,the relatively high mo-mentum jet does not pass through the film hole with fully ex-pansion.Besides that,the flow separation due to the large turning at the trailing edge of the inlet is observed and hence a low momentum region appears along the trailing edge (See Fig.5).Tab.1Test case matrixCaseBaseline Case Group 1Group 2Group 3Description and flow conditionsMa m =0.6,Ma c =0,M =1.0The effects of blowing ratios on film cooling effectiveness (Ma m =0.6,Ma c =0,M =0.5,1.0,1.5)The effects of coolant flow mach number on film cooling effectiveness(Ma m =0.6,Ma c =0,0.3,0.6,M =1.0)The effects of mainstream mach number on film cooling Effectiveness(Ma m =0.3,0.45,0.6,0.79,Ma c =0,M=1.0)(a)RKE (b)SST (c)SKEFig.6and Fig.7clearly reveal that the coolant jet interac-tion with the mainstream along the stream-wise distance.Thanks to the laterally diffusion within the fan-shaped hole,the jet lift-off effect is not as pronounced as conventional cy-lindrical film holes,the counter-rotating vortex pair (CVP)is restrained by the anti-CVP and hence delays jet lift-off and penetration into the mainstream.The effect of CVP on film cooling effectiveness was reported by Haven et al(1997)[17].Firstly,increased lateral separation reduces the mutual induction between the counter-rotating vortices and delays the jet lift-off.Second,fan-shaped holes are found to gener-ate anti-counter-rotating vortex pair (anti-CVP with an oppo-site rotation sense relative to CVP.The anti-CVP,the pres-ence and the formation of anti-CVP can cancel the adverse effect of the CVP so as to prevent the jet lift-off.The centerline and laterally averaged film cooling effec-tiveness over the downstream surfaces are calculated in three different turbulence models:a)RKE,b)SST,c)SKE model.These computed results are compared with experimental data in this section.As it is shown in Fig.8,the two dimensional local effectiveness predicted by RKE and SKE are very simi-lar to each other,results from all three model are different from the experimental data to some extent,where adiabatic cooling effectiveness is relatively high along the centrelineFig.5Near hole region velocity magnitude contour (m/s)for the central plane (Y =0)at baseline case (Ma m =0.6,Ma c =0,M =1)A Numerical Investigation on Cooling Effectiveness for Advanced Fan-shaped Film Cooling··74第61卷，2019年第3期 Vol.61，2019,No.3Chinese Journal of Turbomachineryin the experiment,the difference possibly because of they arenot conjugate simulations and only fluid domains are solved [15].Besides that,on average,the predicted cooling effec-tiveness distributions in present study are higher than the ex-perimental data.The centerline cooling effectiveness results predicted by three turbulence models are shown in Fig.9.All the three models show the same tendency of centerline cooling effec-tiveness,it decreases steadily along the stream-wise distance,due to the jet lifts off the wall and mixes with the main-stream gradually.But compared with the experimental data,all the three turbulence models over-predict the centerline cooling effectiveness,especially at the near film hole region (x /D <3).That may be due to the fact that the conduction inthe experiment cannot be neglected.2.2Effect of blowing ratio on cooling effectivenessThe effects of blowing ratio on the film cooling perfor-mance are simulated at three different blowing ratios (M =0.5,1.0,1.5),where the mainstream and secondary channel flow condition remain the same (Ma m =0.6,Ma c =0).Three tur-bulence models (RKE,SKE,and SST)are applied to predict the cooling effectiveness and compared with experimental data.As shown in Fig.10to Fig.12,at all three blowing ra-tios,the same as the experimental data,three turbulence mod-els predict a consistence reduce in the cooling effectiveness downstream the coolant ejection.Moreover,with the in-crease of the blowing ratio from 0.5to 1.5,the coolant tends to concentrate on the centerline,causing larger cooling effec-tiveness gradient at lateral directions.Generally,fan-shaped hole provides a better coverage and lateral spreading than the cylindrical hole at all blowing ratios,mainly because only limited jet separation happens near the film hole compared with conventional cylindrical hole.According to the experiment conditions,for fan-shaped hole,the centerline cooling effectiveness ηis influenced by the blowing ratio,at low blowing ratio (M =0.5),the center-line cooling effectiveness decreases dramatically,increasing the blowing ratio from 0.5to 1.0results in improved cooling effectiveness,but further increasing the blowing ratio from 1.0to 1.5slightly reduces the effectiveness in the x /D <8re-gion.Generally,the predictions of RKE provide better agree-ments with the experimental data relative to other two turbu-lence models.Although the predicted values are much higher than the experiment results,the main tendency of the RKE simulation results match the experimental data well,the ef-fect of blowing ratio on the centerline cooling effectiveness is revealed clearly in the simulation.The overall deviation of cooling effectiveness is about 0.1at higher blowing ratio (M =1.0,1.5)and about 0.2at lower blowing ratio (M =0.5).Fig.6Velocity vector in Y direction at x /D =0plane obtained by RKE model at baseline caseFig.7Total temperature contour predicted by RKE model at baselinecase(a)RKE(c)SKE(b)SST(d)Gritsch(1998)Fig.8Local cooling effectiveness predicted by three different turbulence and comparisons with experimental data··75Chinese Journal of Turbomachinery2.3Effect of mainstream on cooling effectivenessTo evaluate the effect of mainstream Mach number on the film cooling performance,four representative main-stream flow conditions (Ma m =0.3,0.45,0.6,0.79)are select-ed in present study.Three turbulence models are applied ateach mainstream flow condition with the same coolant chan-nel flow condition and blowing ratio (Ma c =0,M =1),the per-formance in predicting cooling effectiveness for three turbu-lence models are compared.Theoretically,as the mainstream flow Mach number in-creases,the coolant-to-mainstream pressure ratio needs to be increase correspondingly,resulting in that more coolant is in-jected along the centerline of the fan-shaped hole,which means lateral expansion of the coolant jet is decreased,con-sequently,the lateral cooling effectiveness is reduced as the mainstream Mach number increases.Generally,higher coolant-to-mainstream pressure ratio is needed to maintain the same blowing ratio as the main-stream flow Mach number increases from 0.3to 0.79,as a re-sult,much more coolant is injected along centerline of the film hole.Hence,the centerline cooling effectiveness is im-proved gradually as the mainstream flow Mach number in-creases.The simulation results of centerline cooling effec-tiveness obtained by both RKE model and SKE model match the trends well.3ConclusionsThe effects of flow parameters on fan-shaped film cool-ing effectiveness have been numerically investigated based on three turbulence models:RKE,SST and SKE.The perfor-mances to predict film cooling effectiveness for three models are also evaluated by comparing with experimental data.1)Simulation results show that all the three turbulence models are able to predict the main trends of the film cooling effectiveness along the streamwise distance,and the effects of evaluated blowing ratios,mainstream Mach numbers and coolant flow Mach numbers are not pronounced as conven-tional cylindrical hole.2)Generally,the RKE model has a better performance in predicting cooling effectiveness at evaluated test cases.3)Although a few agreements are obtained between the simulation results and the experimental data,all the three tur-bulence models tend to overpredict the cooling effectiveness downstream the film hole exit and the deviation is about 0.1～0.2.References[1]Rolls-Royce.Gas turbine technology[M].Introduction to a jet engine,Rolls-Royce plc,2007.Fig.9Centreline local adiabatic cooling effectiveness for three turbulence models at baseline caseFig.12Centreline adiabatic cooling effectiveness for SKE simu-lation at different blowingratiosFig.10Centreline adiabatic cooling effectiveness for RKE simulation at 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Turbomachinery基于大涡模拟和声类比方法的平板冲击射流噪声指向性数值研究/刘鑫谢军龙谢晴王淼摘要：利用大涡模拟（LES）和FW-H声类比方法对平板冲击射流的噪声指向性分布规律进行了数值研究。