NUMERICAL SIMULATION OF INTERNAL COOLING EFFECT OF GAS TURBINE BLADES USING V SHAPED RIBS

Transcription

1 NUMERICAL SIMULATION OF INTERNAL COOLING EFFECT OF GAS TURBINE BLADES USING V SHAPED RIBS Harishkumar Kamat Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal University, India C Raghavendra Kamath Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology, Manipal University, India Abstract Thermal efficiency and power output of gas turbines increase with increasing turbine rotor inlet temperatures. The rotor inlet temperatures in most of the gas turbines are far higher than the melting point of the blade material hence, the turbine blades need to be cooled. The leading edge of the turbine is internally cooled by coolant passages having V-shaped ribs of rib angles of 300, 450 and 600 for each aspect ratio of 1:1, 1:2 and 2:3. Numerical analysis is carried out for each configuration and results are compared. The conclusions made were as follows: [1] the passage with V-shaped ribs with aspect ratio of 2:3 with 450 rib angle provides the best cooling in the leading edge [2] Temperature of the blade is reduced by approx. 30 % i.e. from 1561 K to approx.1100 K at the leading edge and approx. 17% at the trailing edge of the gas turbine blade Keywords Turbine blade cooling; ribs; leading edge; trailing edge I. INTRODUCTION Thermodynamic study of gas turbine shows that plant efficiency and energy output can be enhanced with higher turbine inlet temperatures. Modern gas turbines try to approach these high temperatures (1500 C) to improve performance but are limited by the maximum allowable thermal stresses for the blade material. To enhance fatigue life of gas turbine blade many cooling techniques can be used on the blade exterior such as internal convective cooling and film cooling. One of the toughest regions to cool is the trailing edge as it must be thin to reduce aerodynamic losses. As the trailing edge is thin, cooling of this region is a challenging task as enough coolant can t be guided. Additional constraints due to structural integrity and manufacturing difficulty for internal cooling passage geometry in this thin section also arise. One of the cooling techniques frequently used by turbine designers for the trailing edge is using twisted tape inserts. Among the different heat transfer techniques, twisted tape is widely used due to their simple configuration and easy installation. Twisted tapes generates swirl in tubes which enhance the heat transfer by generating swirl within the tube [1-3]. Date and Saha [4, 5] used a uniform heat flux tube fitted with regularly spaced twisted tapes and predicted the heat transfer and fluid flow behaviour with help of Navier stokes and energy equations. Chang [6] has compared the results of serrated and broken tape inserts with smooth twisted inserts. Results showed that thermal energy transfer can be improved with usage of serrated twisted tape. It is also shown that 14

2 heat transfer coefficient, Fanning friction factor and heat assessment factor of the tube were increased in a particular Re range. Kini et al. [7-15] in earlier research work established from CFD analysis that helicoidal cooling duct and buttress shaped grooved configuration provided a significant improvement in turbine blade cooling. II. PHYSICAL MODEL AND NUMERICAL METHOD A. Governing Equation The essential governing equations for flow and heat transfer in a flow passage are Navier-Stokes energy and continuity equations along with the equations for modelling the turbulence magnitudes are incorporated. B. Physical model A schematic diagram of the gas turbine blade with different ribs and fins configurations are shown in figure 1-3. Modeling of gas turbine blade is carried out by measuring coordinates of gas turbine blade by CMM in CATIA software. Cooling passages are designed using CAD software and then imported into CFD tool. Fig. 1 shows a gas turbine blade model. and 600 are used for the analysis as shown in fig. 3. Fig 3(a), 3(b) and 3(c) shows different rib angles. Fig. 2 (a) Aspect ratio: 1:1 Fig. 2 Ribs with three different aspect ratios Fig. 1 Blade design C. Leading edge with V Shaped ribs: Ribs with aspect ratios (W/H) 1:1, 1:2 and 2:3 are modelled using CAD software and placed in the leading edge of the turbine blade as shown in fig. 2. Fig. 2(a), 2(b) & 2(c) shows the ribs with different aspect ratios. Similarly ribs with rib angles 300, 450 Fig. 3 Ribs with three different rib angles D. Numerical model and Boundary conditions The computational domain was modelled using CFD tool and Meshing is done by using the tetrahedral grid with Tgrid type. 0.7 spacing is maintained for the element. The standard pressure and first order upwind discretization schemes for momentum and energy equations are employed in the numerical model. Simulation needs to be conducted under high pressure and high temperature conditions to understand the physics of cooling of gas turbine working under real time operating environment. The parameters simulating gas turbine operating environment are 15

3 listed in Table 1 & 2. Energy equation and k-epsilon model (second order equation) with enhanced wall treatment for turbulent flow model is used for the analysis. The turbulence model has been explained in [14-15] and is summarized in table 2 without further explanations. Boundary conditions and material properties are specified as derived from CK et. Al. [7-15]. The through flow of hot gases has a convective heat transfer coefficient of 2028 W/m2K and a free stream temperature of 1561 K. Inlet of the coolant has a pressure of 1.6 MPa and a temperature of 644 K. Table I Thermo-physical properties of air at 644 K configuration with 450 has best cooling effect as compared with other configuration due to its effective swirling effect. Table II Thermal properties of blade material III. RESULTS AND DISCUSSION A. Thermal performance of V-shaped ribs at leading edge: The cooling passages for the leading edge with an aspect ratios of 1:1, 1:2 and 2:3, are used for the analysis and angle between the V in the ribs is varied from 300, 450 and 600. The thickness between the passages and the blade surface is maintained as 3 mm, for structural stability and to get maximum possible cooling effect near the edges. Numerical analysis is carried out for different angles of V-shaped ribs with different aspect ratios. Figure 4 illustrates temperature distribution plot for three different angles of V shaped ribs with aspect ratio of 1:1, 1:2 and 2:3. From the graphs it is clearly seen that Fig. 4 Temperature distribution plot for different aspect ratio From the above results and comparisons, we can conclude that V-shaped ribs with aspect ratio of 2:3 and rib angle of 450 offers more cooling effect when compared to other rib configurations. Fig. 5 Temperature contour for Aspect ratio of 2:3, rib angle= 450 Temperature contour plot for aspect ratio of 2:3 with rib angle of 450 is shown in figure 5. Using V shaped ribs with an aspect ratio of 2:3 and a rib angle of 450 the temperature of the blade is decreased by approx. 30 % i.e. from 1561 K to approx.1100 K at the leading 16

4 edge and approx. 17% at the trailing edge of the gas turbine blade. IV. CONCLUSIONS It was found from the results that ribs and fins configurations are promising technique for heat transfer enhancement. It was also found that heat transfer can be enhanced with suitable ribs and fins configurations. CFD analysis is carried out for ribs with different aspect ratios and rib angle configurations at the leading edge of the gas turbine blade. The results showed that ribs of 2:3 Aspect ratio with 450 rib angle provides better cooling in the leading edge compared to others and blade temperature is reduced by approx. 30 % i.e. from 1561 K to approx.1100 K at the leading edge and approx. 17% at the trailing edge of the gas turbine blade. Hence it can be concluded that cooling effect of gas turbine blade can be enhanced with suitable ribs configurations. NOMENCLATURE ACKNOWLEDGMENTS The Corresponding author wishes to gratefully acknowledge the financial support extended by the Manipal University, Manipal, India for sponsoring to this conference. The computational facilities were extended by Department of Mechanical Engineering, Manipal Institute of Technology, Manipal which is thankfully acknowledged REFERENCES [1] R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twisted tape inserts in isothermal tubes: part I e laminar flows, ASME J. Heat Transfer 115 (1993) [2] R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twisted tape inserts in isothermal tubes: part II e transition and turbulent flows, ASME J. Heat Transfer 115 (1993) [3] S. Martemianov, V.L. Okulov, On heat transfer enhancement in swirl pipe flows, Int. J. Heat Mass Transfer 47 (2004) [4] A.W. Date, Prediction of fully developed flow in a tube containing a twisted tape, International Journal of Heat and Mass Transfer 17 (1974) [5] A.W. Date, S.K. Saha, Numerical prediction of laminar flow and heat transfer characteristics in a tube fitted with regularly spaced twisted-tape elements, International Journal of Heat and Fluid Flow 11 (1990) [6] S.W. Chang, Y.J. Jan, J.S. Liou, Turbulent heat transfer and pressure drop in tube fitted with serrated twisted tape, Int. J. Therm. Sci. 46 (2007) 506e518. [7] Chandrakant R Kini, Satish Shenoy B, and N Yagnesh Sharma, A Computational Conjugate Thermal Analysis of HP Stage Turbine Blade Cooling with Innovative Cooling Passage Geometries, Journal of Lecture Notes in Engineering and Computer Science, Volume 2192 Issue 1, July 2011, pp , [8] Chandrakant R Kini, Satish Shenoy B, and N Yagnesh Sharma, Computational Conjugate Heat Transfer Analysis of HP Stage Turbine Blade Cooling: Effect of Turbulator Geometry in Helicoidal Cooling Duct, Proceedings of World Academy of Science 17

5 Engineering and Technology Special Journal Issue, Issue No: 0070, pp , [9] Chandrakant R Kini, Satish Shenoy B, and N Yagnesh Sharma, Numerical Analysis of Gas Turbine HP Stage Blade Cooling with New Cooling Duct Geometries, International Journal of Earth Sciences and Engineering, Volume 05 No 04 (02), pp , [10] Chandrakant R Kini,, Sai Sharan Yalamarty, Royston Marlon Mendonca, N Yagnesh Sharma and Satish Shenoy B, CHT Analysis of Trailing Edge Region Cooling In HP Stage Turbine Blade, Indian Journal of Science and Technology, Volume 9, Issue 6, pp. 1-6, [11] Chandrakant R Kini, Satish Shenoy B, and N Yagnesh Sharma, Thermo-structural analysis of HP stage gas turbine blades having helicoidal cooling ducts, International Journal of Advancements in Mechanical and Aeronautical Engineering, Volume 1: Issue 2, pp , 2014 [12] Chandrakant R Kini, N Yagnesh Sharma and Satish Shenoy B, Thermo-Structural Investigation of Gas Turbine Blade Provided with Helicoidal Passages, Indian Journal of Science and Technology, Volume 9, Issue 6, pp. 1-6, [13] Chandrakant R Kini, Satish Shenoy B, and N Yagnesh Sharma, (in press), Effect of grooved cooling passage near the trailing edge region for HP stage gas turbine blade -A numerical investigation, Progress in Computational Fluid Dynamics, An International Journal. [14] Chandrakant R Kini, Harishkumar kamat, Satish Shenoy B, Numerical simulation of internal cooling Effect of Gas Turbine Blades using Twisted Tape Inserts, Indian Journal of Science and Technology, Volume 9, Issue 31, 2016 [15] Chandrakant R Kini, Harishkumar kamat, Satish Shenoy B, Effect of Twisted Tape Inserts and Stacks on Internal Cooling of Gas Turbine Blades, Indian Journal of Science and Technology, Volume 9, Issue 31, pp [16] Xianchang Li, T. Wang, Two phase flow simulation of mist film cooling on Turbine blades with conjugate internal cooling, Journal of Heat Transfer, October 2008, Vol. 130 / [17] J. Guo, A. Fan, X. Zhang, W. Liu, A numerical study on heat transfer and friction factor characteristics of laminar flow in a circular tube fitted with centre-cleared twisted tape, Int. J. of Thermal Sciences 50 (2011) [18] J. Choi, S. Mhetras and J. Han, Film Cooling and Heat Transfer on Two Cutback Trailing Edge Models With Internal Perforated Blockages, Journal of heat transfer by ASME,Vol