International Journal of Gas Turbine, Propulsion and Power Systems December 2014, Volume 6, Number 3 Conjugate Heat Transfer Analysis of Convection-cooled Turbine Vanes Using γ-Reθ Transition Model Gang Lin1, Karsten Kusterer1, Anis Haj Ayed1, Dieter Bohn2, Takao Sugimoto3 1 B&B-AGEMA GmbH Jülicher Strasse 338, 52070 Aachen, Germany E-mail: [email protected] 2 RWTH Aachen University 3 University of Hyogo conduction, it is obvious that a coupled calculation of the fluid, the heat transfer and the heat conductivity in the solid body can lead to a higher accuracy in the design process. A little bit more than 20 years ago, in the early 1990s, the Institute of Steam and Gas Turbines at RWTH Aachen University headed by Prof. Dieter Bohn started a long-term development of a most sophisticated approach for the coupled calculation of fluid flows and heat transfer with focus on the hot gas components in a gas turbine. The numerical group of the institute developed the homogeneous method for the conjugate calculation technique (CCT). The method involves the direct coupling of the fluid flow and the solid body using the same discretization and numerical principle for both zones. This makes it possible to have an interpolation-free crossing of the heat fluxes between the neighboring cell faces. Thus, additional information on the boundary conditions at the blade walls, such as the distribution of the heat transfer coefficient, becomes redundant, and the wall temperatures as well as the temperatures in the blade walls are a direct result of this simulation. First results and validation cases have been published in the 1990s for convection-cooled cases (e.g. [3-6]) as well as for film-cooled configurations (e.g. [7-9]). The detailed description of the conjugate calculation technique and its validation is provided by Bohn et al. in [10]. ABSTRACT In order to achieve high process efficiencies for the economic operation of stationary gas turbines and aero engines, extremely high turbine inlet temperatures at adjusted pressure ratios are applied. The allowable hot gas temperature is limited by the material temperature of the hot gas path components, in particular the vanes and blades of the turbine. Thus, intensive cooling is required to guarantee an acceptable life span of these components. Modern computational tools as well as advanced calculation methods support essentially on the successful design of these thermally high-loaded components. The homogeneous, or sometimes also mentioned as “full”, conjugate calculation technique for the coupled calculation of fluid flows, heat transfer and heat conduction is such an advanced numerical approach in the design process and huge experiences on validation and application have been collected throughout the years. This paper summarizes the numerical approach for this method as well as provides a collection of numerical results obtained by the authors for validation cases for a convection-cooled turbine vane test case as well as comparison to calculation data for this test case provided in open literature. Furthermore, systematic studies on the impact of calculation parameters, e.g. hot gas fluid properties, and numerical models for turbulence calculation are performed and the numerical results are compared to the experimental results of the test case. NOMENCLATURE INTRODUCTION Due to the necessity of cooling technologies in modern gas turbines, turbulent heat transfer is of significant importance in the thermal design process of the cooled components. Tremendous efforts have been put into the determination of empirical correlations for the internal and external heat transfer, which are necessary for the conventional design process. Here, analysis of turbine blade cooling and heat transfer consists of three areas (Patankar, [1]): (a) prediction of the heat transfer coefficients on the external surface of the airfoil (Kays and Crawford, [2]), (b) prediction of heat transfer in the internal cooling passages (Kays and Crawford, [2]) and (c) calculation of the temperature distribution in the blade material. However, the accuracy of the approach based on heat transfer coefficients is very much limited by the uncertainties of the correlations if applied to the real gas turbine geometric configurations and conditions. With regard to the inter-relations between the external fluid flow, the internal fluid flow and the heat Flength Fonset H H0 L Ma k Re Rec Ret ~ Ret S T Tref X y+ γ μ μt Manuscript Received on November 20, 2013 Review Completed on November 26, 2014 = function to control transition length = function to control transition onset location = heat transfer coefficient (W/m2/K) = reference heat transfer coefficient (1135 W/m2/K ) = axial chord length (m) = Mach number = turbulent kinetic energy (m2/s2) = momentum thickness Reynolds number = momentum thickness Reynolds number, where the intermittency starts to increase = momentum thickness Reynolds number, where the skin friction starts to increase = transported variable for Ret = streamwise distance = temperature (K) = reference temperature (811K) = axiale chord position (m) = non-dimensional wall-normal distance, = intermittency = dynamic viscosity (kg/m/s) = turbulent viscosity (kg/m/s) Copyright © 2014 Gas Turbine Society of Japan 9 JGPP Vol.6, No. 3 ρ ω = density (kg/m3) = specific turbulence dissipation rate (s-1) mentioned, presented results from the CCM+ solver have been obtained by application of this model. TEST CASE DESCRIPTION The famous Mark II test case for a convection-cooled vane has been chosen for comparative calculations of the thermal load by application of the CCT. The vane has been investigated extensively by Hylton et al. [11] over a wide range of operating conditions in a hot gas duct. Mark II is a high-pressure turbine nozzle guide vane, which is convectively cooled with air by ten radial cooling channels. Figure 1 shows the vane geometry and the arrangement of the cooling passages. The test case no. 5411 has been chosen for the numerical investigations. The numerical validations have been carried out in both 2-D and 3-D cases. In 2-D case the heat transfer coefficients and cooling air temperatures have been defined as boundary conditions for calculation of heat transfer from internal cooling air. In 3-D case the heat transfer from internal cooling air to solid is calculated by given cooling air mass flow and cooling air inlet temperature. After Hylton et al. [11] the inlet turbulence intensity was 6.5% in the experiment, which has also be used in the numerical investigations. Instead of turbulence length scale the turbulent viscosity ratio has been applied as another inlet boundary condition for turbulence conditions. The inlet turbulent viscosity ratio has been set at 10. In the study by Mansour et al. [12] the inlet turbulent viscosity ratio has insignificant impact on the heat transfer coefficient distribution prediction at suction side in the case Mark II. Fig. 2 Mid-section of 3D polyhedral mesh for CCM+ application The SST turbulence model [15] is a two-equation turbulence model, which combines the advantages of both k-ω and k-ε model. With the help of a blending function F1, the turbulence model can be transformed between k-ε model in the freestream zone and κ-ω model in the boundary layer zone. With this method, the advantages of near-wall performance of k-ω model can be implemented without any influence on sensitivity of any boundary condition in free stream, which can lead to errors in the original k-ω model. The equations of two transport models, one for turbulent kinetic energy k and the other for specific turbulence dissipation rate ω, are given as follows: u j Dk k 2 k S k t Dt x j xj x j eff t min max eff ,0.1 ,1 k The own-developed solver, CHTflow, is based on the homogeneous method as it is described in [10]. Conjugate calculations have been performed with structured hexahedral grids for 2-D case. The y+ in the boundary layer is approximately y+=1. The Baldwin-Lomax algebraic turbulence model [13] has been applied. The model has been established during the validation procedure of the CHTflow code in the mid 1990s and the results have been presented in [4, 10]. These results are taken as the reference for the present calculations with commercial CFD software. In the recent years, this kind of homogeneous conjugate methodology found its way also into the commercial CFD codes and, thus, the Star CCM+ solver [14] has been applied for the new calculation and comparison to the CHTflow results. Figure 2 shows the mid-section of the 3-D polyhedral mesh, which has been used for the CCM+ application. In order to establish an appropriate calculation of the local heat transfer, the y+ value of the first cell in the boundary layer is less than y+=1. The suitable cell number of the grid after mesh independency study is 2.86 million including the prism layers on the fluid-solid boundaries. Best results have been obtained with the SST k-ω turbulence model and if not otherwise uj D 2 2 S k t t Dt x j x j x j 2 1 F 1 2 Fig. 1 Geometry and boundary conditions for Mark II test case 1 k xj xj Usually the γ-Reθ transition model is applied in the SST turbulence model [15]. This transition model is build up by two transport equations: one is for the intermittency γ and the other is for the transported transition momentum thickness Reynolds ~ . number R et D t S Dt x j x j F length ca1 ca 2 F turb 1 ce 2 10 F 1 c 0.5 onset e1 JGPP Vol.6, No. 3 ~ ~ D Ret Ret U 2 ( t ct t Dt xj 500 Ret x j ~ Ret )1 F t The intermittency γ determines the production of turbulent kinetic energy in the boundary layer. It is 0 in the laminar boundary ~ is used as the criterion layer and 1 in the fully turbulent layer. R et of transition onset position, which transforms non-local free stream information into a local quantity. So that the function Fonset (shown in eq. 3), which is used to control transition onset location, can be expressed. Open literature numerical data by other authors for this test configuration can be found in [16-19], for example, as well as numerical data [20-24] for the similar C3X test case from Hylton et al. [11]. The same test configuration Mark II has been studied by Yan et al. [16] using a structured mesh in the commercial solvers CFX and Fluent with different turbulence models. The results of SST Gamma Theta calculation in CFX solver have shown a good agreement with the experimental data in temperature distribution. The temperature at the reattachment point on suction side was over predicted by about 6% to 7%. Lin et al. [17] have done the analysis of this test case also in the CFX solver but using unstructured mesh with the SST-γ-Reθ model. The numerical results capture the trends of temperature along the surface with an under prediction by about 5% to 8% on the pressure side. Zeng and Qing [18] have completed another 2D and 3D conjugate heat transfer simulation of Mark II with unstructured mesh. The grid in the 3D calculation was obtained by extruding of the 2D grid. The boundary conditions in the cooling channels, for both 2D and 3D simulation, were applied with cooling temperature and heat transfer coefficient. The SSTγ-Reθ model again showed the best accuracy among all of the used turbulence models. The maximum difference between prediction and test data occurred at the reattachment point with about 3% to 4% over prediction. a) CHT flow result RESULTS Both applied numerical codes show reasonable results for the Mach-number distribution in the mid-section of the vane as it is compared in Fig. 3. There is a strong and sharp compression shock on the front part of the vane suction side. In front of the shock the flow is accelerated up a Mach-number of slightly over Ma=1.6. Downstream the strong shock, the flow is accelerated again, before a second, weaker shock can be observed shortly in front of the trailing edge. b) CCM+ result Fig. 3 Mach-number distribution (mid-section) The temperature distribution in the mid-section for the fluid region as well as for the solid body of the vane is shown in Fig. 4. The lowest temperature inside the vane appears near the leading edge between the 2nd and the 3rd cooling hole. The hottest region can be found in the trailing edge. As shown in the distribution of isotherms in the vane, the temperature gradients are high at two positions: One near the stagnation point at the leading edge and the other at the suction side after the first shock. Such a high density of the isotherms indicates locally a high heat transfer between the fluid and solid region. 11 JGPP Vol.6, No. 3 Fig. 5 Comparison of numerical and experimental results of the vane surface temperature a) CHT flow result The effect of turbulence models As shown in Fig. 6, the results obtained by application of CCM+ depend on the chosen turbulence model in the 2-D cases. The SST k-ω model provided by the CCM+ code performs the best for the conjugate application. Applying other turbulence models, the calculated temperature distributions in the front part of the vane suction side, i.e. the region with a laminar boundary layer, show a significantly reduced accuracy. The major reason for this is the fact, that those turbulence models provide no possibility to prescribe or influence the turbulence onset location. The results have shown that for all tested turbulence models the onset of turbulence production is located to far upstream. Figure 7 illustrates this for the Realizable k-ε model. Compared to the calculation with the SST-γ-Reθ model, the Realizable κ-ε model starts much earlier (by X/L=0.044) to over predict the turbulence at the suction side, which results in too much heat flux in this laminar region. By the calculation with the SST-γ-Reθ model, the turbulence is firstly produced at position by X/L=0.45, which is the same as in the experiment. Although the SST-γ-Reθ model can predict a quite well transition at the suction side, the temperature distribution at the pressure side is even worse predicted than the Realizable k-ε model. As shown in Fig. 7, the boundary layer at the pressure side, near the trailing edge, is still laminar in the calculation with SST-γ-Reθ model, while the other calculation results are turbulent. The main reason behind this effect is that the value of Reθt,min in the transition model, which is suitable for the suction side, is too high for the pressure side. A value of 20 is recommended as the lower bound for Reθt,min, and this leads also to minimum reachable Reθc. It is clear from the model equations that decreasing Reθc results in an earlier transition onset. Figure 8 shows the surface temperature prediction with variation of Reθt,min. The result with prescribed Reθt,min, which equals 130, agrees very well with the test data at the suction side before the first shock. However, a smaller Reθt,min shows better agreement for the pressure side. The increase of Reθt,min results in a later transition at both sides of the vane, which in turn leads to further under prediction at suction side before the first shock and also at the pressure side. b) CCM+ result Fig. 4 Temperature distribution (mid-section) Figure 5 gives a comparison of the obtained numerical surface temperature results with the experimental results. The CHTflow results for 2-D case show excellent agreement with the experimental results. The CCM+ results for the suction side are also very good in agreement with the experimental results, but on the pressure side the surface temperatures are calculated consistently lower than the experimental results. The 3-D CCM+ results have improved significantly with only small deviations compared to the experimental results along the whole surface. Here, the deviation between 2-D and 3-D results comes from the different internal cooling air boundary conditions and also the influence of secondary flow. 12 JGPP Vol.6, No. 3 0,9 while the measured surface temperatures have been used as boundary conditions. Thus, the uncertainties regarding the accuracy of the experimental heat transfer coefficients are much higher than for the experimental surface temperatures. Experiment 0,85 V2F Realizable k-epsilon SST_GammaReTheta T/Tref [-] 0,8 CHTflow 0,9 0,75 0,85 0,7 0,6 0,55 Re_theta=100 0,75 Re_theta=200 T/Tref [-] 0,65 Experiment 0,8 Re_theta=130 0,7 0,65 -1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1 0,6 X/L [-] 0,55 Fig. 6 Comparison of numerical results with different turbulence models and experimental results of the vane surface temperature 0,5 -1 -0,8 -0,6 -0,4 -0,2 0 X/L [-] 0,2 0,4 0,6 0,8 1 Fig. 8 Temperature distribution under different Reθt,min (2-D result) The y axis in Fig. 9 shows the heat transfer coefficient normalized with respect to H0, which is 1135 W/m2/K. Similar to the temperature prediction, the results of Realizable κ-ε model and V2F model also show very well agreement with the test data at the pressure side, but much higher heat transfer coefficient at the laminar region of suction side. In contrast, the result of SST-γ-Reθ model agrees very well before the transition at suction side but an under prediction at pressure side. Comparing the test data, there is another large error of all turbulence models at the suction side after the first shock. It is reattachment of the separated flow after the transition, which causes a high heat flux in this reattachment point on the blade. Although the SST-γ-Reθ model can predict the right onset location of transition, the temperature error of calculation is still around 20K (2.5%) at this point. Some possible reasons for this error in the SST-γ-Reθ model are as follows: (1) at present transition model, the Mach number effect is still not been implemented [25]; (2) in this transition model the local intermittency can exceed 1, when the laminar boundary layer separates. This leads to a large production of turbulent kinetic energy to get an earlier reattachment, but at the same time also results in a locally high heat flux. a) Realizable k-ε model 1,4 Test Data V2F 1,2 Realizable k-epsilon SST-GammaReTheta H/H0 [-] 1 CHTflow 0,8 0,6 0,4 b) SST-γ-Reθ model 0,2 Fig. 7 The midspan turbulence viscosity ratio distribution under different turbulence models (2-D result) 0 -1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 X/L [-] In Fig. 9, the vane surface heat transfer coefficient distributions at midspan taken from calculation with different turbulence models are compared to heat transfer coefficient published by Hylton et al. [11]. The latter values have not been obtained by measured, but by application of an FEM tool solving the energy equation in the solid, Fig. 9 Heat transfer coefficient distribution under different turbulence models (2-D result) 13 1 JGPP Vol.6, No. 3 References [1] Patankar, SV., 1994, “Advances in Numerical Prediction of Turbine Blade Heat Transfer,” In Heat Transfer in Turbomachinery, RJ. Goldstein, DE. Metzger and AI Leotiev (eds). Begell House, Inc., New York and Wallingford, pp. 501-510. [2] Kays, W. M., and Crawford, M. E., 1980, Convective Heat and Mass Transfer, McGraw-Hill Book Company: 2nd edition. [3] Bohn, D., Bonhoff, B., Schönenborn, H., 1995, “Combined Aerodynamic and Thermal Analysis of a High-pressure Turbine Nozzle Guide Vane,” IGTC-108, Yokohama, Japan. [4] Bohn, D., and Schönenborn, H., 1996, “3-D Coupled Aerodynamic and Thermal Numerical Analysis of a Turbine Nozzle Guide Vane,” 4th ISHT-conference, Beijing, China. [5] Bohn, D., Heuer, T., Kusterer, K., and Lang, G., 1997, “Application of a Conjugate Fluid Flow and Heat Transfer Method in the Thermal Design Process of a Convection-Cooled Turbine Nozzle Guide Vane,” 99-AA-5, ASME Asia 1997, Singapore. [6] Bohn, D., Becker, V., Kusterer, K., Otsuki, Y., Sugimoto, T., and Tanaka, R., 1999, “3-D International Flow and Conjugate Calculations of a Convective Cooled Turbine Blade with Serpentine-Shaped and Ribbed Channels,” ASME Turbo Expo 1999, 99-GT-220, Indianapolis, USA. [7] Bohn, D., and Bonhoff, B., 1994, “Berechnung der Kühl- und Störwirkung eines filmgekühlten transsonisch durchströmten Turbinengitters mit diabaten Wänden, ” VDI-Berichte 1109, S.261 – 275. [8] Bohn, D., Bonhoff, B., Schönenborn, H., and Wilhelmi, H., 1995, “Validation of a Numerical Model for the Coupled Simulation of Fluid Flow and Diabatic Walls with Application to Film-cooled Gas Turbine Blades,” VDI-Berichte-Nr. 1186, pp. 259-272. [9] Bohn, D., and Kusterer, K., 1998, “3-D Numerical Aerodynamic and Combined Heat Transfer Analysis of a Turbine Guide Vane with Showerhead Ejection,” Heat Transfer 1998,Proceedings of 11th IHTC, Vol. 4, pp. 313 - 318, Kyongju, Korea. [10] Bohn, D., Krüger, U., and Kusterer, K., 2001, “Conjugate Heat Transfer: An Advanced Computational Method for the Cooling Design of Modern Gas Turbine Blades and Vanes,” In Heat Transfer in Gas Turbines, B. Sundén and M. Faghri (eds). WIT Press, Southampton, pp. 58-108. [11] Hylton, L. D., Milhec, M. S., Turner, E. R., Nealy, D. A., and York, R. E., 1983, “Analytical and Exp. Evaluation of the Heat Transfer Distribution Over the Surfaces of Turbine Vanes”, NASA-Report. CR 168015, NASA Lewis Research Center, Cleveland, Ohio. [12] Mansour, M. L., Hosseini, K. M., Liu, J. S., Goswami, S., 2006, “Assessment of the Impact of Laminar-Turbulent Transition on the Accuracy of Heat Transfer Coefficient Prediction in high Pressure Turbines,” Proceedings of ASME Turbo Expo 2006, GT2006-90273, Barcelona, Spain. [13] Baldwin, B. S., and Lomax, H., 1978, “The Layer Approximation and Algebraic Model for Seperated Flows,” AIAA-paper 78-257. [14] STAR-CCM+ Software Package, Version 6.04, CD-adapco, Melbillr NY. [15] Malan, P., Suluksna, K., and Juntasaro, E., 2009, “Calibrating the γ-Reθ Transition Model for Commercial CFD,” AIAA-092407. [16] Yan, P. G., and Wang, Z. F., 2010, “Conjuate Heat Transfer Numerical Validation and PSE Analysis of Transonic Internally-Cooled Turbine Cascade,” Proceedings of ASME Turbo Expo 2010, GT2010-23251, Glasgow, UK. [17] Lin, L., Ren, J., and Jiang, H. D., 2011, “Application of γ-Reθ Transition Model for Internal Cooling Simulation,” Heat Transfer—Asian Research, 40 (1). [18] Zeng, J., and Qing, X. J., 2011, “Conjugate Flow and Heat Transfer of Turbine Cascades,” In Heat Transfer - Theoretical Analysis, Experimental Investigations and Industrial Systems, InTech, 2011; pp. 635-654. The impact of gas properties The effect of gas properties on predicted surface temperatures has been studied with two kinds of gases: (1) ideal gas with molecular weight of 28.9664 kg/kmol and constant specific heat of 1003.62 J/kg/K; (2) real gas with molecular weight of 28.5705 kg/kmol and specific heat based on a temperature polynomial. Figure 10 shows the surface temperature distributions at these two conditions. The trends of temperature along the surface are consistent with these two calculations, and the values are almost identical. Only the results of real gas calculation show slightly further over prediction than ideal gas calculation at the suction side before the transition and at the reattachment point. Fig. 10 Temperature distribution under different gas properties (3-D result) CONCLUSIONS In recent years the conjugate heat transfer method has been implemented in several commercial CFD codes. In the present study, the solver Star CCM+ has been applied for calculation of a convection-cooled high pressure test vane, Mark II, and the results have been compared to experimental results and numerical results, which have been obtained by application of the solver CHTflow. Three different turbulence models have been chosen for the calculations with Star CCM+, showing significant variations in the surface temperature prediction, especially in laminar region at the suction side. Due to the integrated transition model, only the SST-γ-Reθ model tends to predict a right onset location of the transition, which results in a good agreement with the test data in this laminar region. For SST-γ-Reθ model, the maximum temperature error between calculation results and test data occurs in the reattachment point after the transition (2.5%), where the heat flux is over estimated. Compared with the 2D calculation, 3D calculation shows overall better agreement with test data except at the reattachment point. Other authors also published numerical results for the same test case, using different numerical codes. Those results show larger deviations to the experimental temperature distributions. Most of their studies also found that the SST-γ-Reθ model presents the best agreement for prediction of temperature distribution in the test case Mark II. But none of them has investigated the influence of parameter Reθt,min on the transition at airfoil suction side. The CHTflow results of 2D case show overall a very good agreement with the experimental data, which is comparable with the results of SST-γ-Reθ model. The maximal error for calculation with CHTflow is approximately 2%. ACKNOWLEDGMENTS The numerical simulations presented in this paper have been carried out with the STAR CCM+ Software of CD-adapco. Their support is gratefully acknowledged. 14 JGPP Vol.6, No. 3 [19] Wang, Z. F., Yan, P. G., Huang, H. Y., and Han, W. J., 2010, “Conjugate Heat Transfer Analysis of a High Pressure Air-Cooled Gas Turbine Vane,” Proceedings of ASME Turbo Expo 2010, GT2010-23247, Glasgow, UK. [20] Luo, J., and Razinsky, E. H., 2006, “Conjugate Heat Transfer Analysis of a Cooled Turbine Vane Using the V2F Turbulence Model,” ASME Turbo Expo 2006, GT2006-91109, Barcelona, Spain. [21] Ledezma, G. A., Laskowski, G. M., and Tolpadi, A. K., 2008, “Turbulence Model Assessment for Conjugate Heat Transfer in a High Pressure Turbine Vane Model,” Proceedings of ASME Turbo Expo 2008, GT2008-50498, Berlin, Germany. [22] Nowak, G., and Wroblewski, W., 2009, “Application of Conjugate Heat Transfer for Cooling Optimization of a Turbine Arifoil,” Proceedings of ASME Turbo Expo 2009, GT2009-59818, Orlando, Florida, USA. [23] Sasaki, D., Yoshiara, T., Yasuda, S., and Nakahashi, K., 2011, “Conjugate Heat Transfer Simulation of Cooled Turbine Vanes Using Unstructured-Mesh CFD Solver,” IGTC2011-ABS-0204. [24] Bamba, T., Yamane, T., and Fukuyama, Y., 2007, “Turbulence Model Dependencies on Conjugate Simulation of Flow and Heat Conduction,” ASME Turbo Expo 2007, GT2007-27824, Montreal, Canada. [25] Langry, R. B., 2006, “A Correlation-based Transition Model Using Local Variables for Unstructured Parallized CFD Codes,” Dr.-Ing thesis, Institute of Thermal Turbomachinery and Machinery Laboratory, University of Stuttgart. 15
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