FSI ANALYSIS OF TRAILING EDGE REGION COOLING IN HP STAGE TURBINE BLADE Harishkumar Kamat Chandrakant R Kini Engineering,Manipal Institute of Technology, Engineering,Manipal Institute of Technology, N Yagnesh Sharma Engineering, Manipal Institute of Technology, Abstract - Gas turbines play a vital role in the today s industrialized society, and as the demands for power increase, the power output and thermal efficiency of gas turbines must also increase. Modern high-speed aero-engines operate at elevated temperatures about 2000 K to achieve better cycle efficiencies. The internal cooling techniques of the gas turbine blade includes: jet impingement, rib turbulated cooling, and pin-fin cooling which have been developed to maintain the metal temperature of turbine blades within acceptable limits. Since FSI is the objective of this analysis, the blade loading corresponding to the static pressure as well as temperature field on the blades surfaces are obtained using CFD run. The output results are then used as structural boundary condition to solve FSI, using finite element method. The present work brings out thermal and structural deformation of the HP stage gas turbine blade. A parametric approach is used for varying the cooling duct geometry to optimize the cooling process. It is found from the FSI analysis that cooling passages having pin fins and aerofoil fins in the trailing edge region achieve maximum thermal performance in terms of cooling and in turn reduce structural distortion. Keywords - FSI analysis, Fins, Trailing edge cooling, blade deformation I. INTRODUCTION Gas turbines are highly effective engineered prime movers for converting energy from thermal form (combustion stage) to mechanical form, and are widely used for propulsion and power generation systems. One method of increasing both the power output and thermal efficiency is to increase the temperature of the gas entering the turbine section. In the advanced gas turbines of today, the turbine inlet temperature can be as high as 1500 C; however, this temperature exceeds the melting temperature of the metal blades. From Brayton cycle it is known that the increase in pressure ratio and turbine inlet temperature increases the gas turbine thermal efficiency. However, increasing the pressure ratio beyond a certain value at any given firing temperature can actually result in lowering the overall cycle efficiency. As TIT increases, the heat transferred to the blades in the 41
turbine also increases. The temperature level and variations on the turbine blade cause thermal stresses which must be limited to achieve reasonable durability goals [1]. II. COMPUTATIONAL DOMAIN FOR THE ANALYSIS In the trailing edge, fins are introduced into the cooling passage to improve cooling. Three shapes are analysed to determine which would provide maximum and efficient cooling and lower deformation [1]. A line is chosen along the blade surface which is at 70 % chord length to compare each design based on blade deformation along the line. A. Pin fin Three rows of pin fins are arranged in two patterns. The first includes fins arranged in an array, and in the second, fins are arranged in a staggered pattern as shown in Figure 1. Analysis is performed to determine the most effective pattern which would be followed for the other shapes. Based on the concept of increased surface area resulting in increased heat transfer, diameters of the fins are varied to determine the effect on thermal performance and blade deformation. Figure 1 (a) array arrangement of fins, (b) staggered arrangement of fins B. Triangular Fins Triangular fins are modelled by maintaining the crosssectional area constant. Additionally, the fins are arranged in four orientations to optimize airflow and provide the best thermal and structural performance. Study of orientation of fins is important since airflow can be studied which helps to analyse flow pattern, obstructions in flow, and the variation of turbulence. The orientations are shown in Figure 2. Figure 2 triangular fins - (a) orientation 1, (b) orientation 2, (c) orientation 3, (d) orientation 4 C. Airfoil fins Figure 3 airfoil fins - (a) orientation 1, (b) orientation 2,(c) orientation 3, (d) orientation 4 Airfoils are well known for their aerodynamics. Similar to the triangular fins, the airfoil fins are arranged in the same four orientations. The orientations are shown in Figure 3. Fins are created in the trailing edge to increase heat transfer, thus improving cooling. Three shapes and four orientations are analysed. Different models are compared based on blade deformation and airflow around the fins and within the blade. III.NUMERICAL MODEL AND SOLUTION PROCEDURE At the outset, a CFD analysis was performed on gas turbine cooling duct to capture temperature and pressure field related to cooling air medium present within the duct. This was followed by a finite element analysis of the gas turbine blade structure for steady state thermal cooling of the blade for which the results of CFD was used as thermal boundary condition to capture temperature stresses that are developed due to relatively hot blade [2].The steady state thermal analysis was carried out by importing the 3D gas turbine blade geometry into Workbench [3]. The blade is re-meshed with an unstructured mesh containing 3D tetrahedral elements. Surface thermal loads 42
corresponding to ambient convective boundary condition of the hot gas surrounding the blade as well as heat flux dissipated by the cooling ducts on the inside blade surface were imported from fluid flow analysis computed from CFD run. A convective boundary condition of hot gas with free stream temperature of 1561 K and convective heat transfer coefficient of 2028 W/m 2 K is applied to the blade surface [3-9].Further for the FSI analysis, a structural analysis was carried out by re-meshing the blade domain with an unstructured mesh containing 3D tetrahedral elements. The pressure loading over the blade surface due to ambient hot combustion gases were applied over the outer blade surface, whereas pressure loads at the interface from the cooling duct were imported from CFD run. The steady state blade temperatures were also imported in order to carry out static structural analysis. The hub being fixed to rotor disc is assumed to be fully constrained and the rated speed of the turbine blade was taken as 3600 rpm [2]. IV. RESULTS AND DISCUSSIONS A. Pin fin From Table II, it is seen that staggered arrangement of fins is better than that of array. This can be clearly justified by the path lines of air in the cooling channel. In an array arrangement of fins, when the incoming air comes in the contact with the first few rows of fins, velocity reduces and is unable to cool the fins which are further ahead. This causes higher temperatures at the top of the blade resulting in majority of the blade being hotter. However, in a staggered arrangement, fins are placed in a triangular manner which causes air to come in contact with the first few rows and divert outward (towards the adjacent columns of fins). This results in a cooler blade surface resulting in lower deformation of 3.125 mm as seen in figure 5. Table II Comparison of blade deformation between array and staggered arrangement of fins Figure 5 Deformation of blade along the span of the blade Diameter Analysis: A diameter analysis is performed and from Table III it is found that 5mm provides the best thermal performance resulting in lower deformation. While 7 mm seems to be the best due to highest surface area, it resulted in the restriction of airflow due to narrow gaps between the fins resulting in marginal increase in total deformation. Hence, 5mm is better due to high surface area as well as smooth airflow around the fins resulting in marginal lower deformation as shown in figure 6. Table III Diameter analysis of pin fins 43
Figure 6 Deformation of blade along the span of the blade for different diameters of pin fins Figure 7 Contours of deformation for pin fins having diameter 1 mm and 3 mm Figure 10 Contours of deformation for triangular fins having orientation 1 and 2 Figure 8 Contours of deformation for pin fins having diameter 5 mm and 7 mm Figure 7 and 8 shows contours of total deformation for pin fins having different diameters. B. Triangular Fins Table IV Comparison of blade deformation between orientations of triangular fins Figure 9 Deformation of blade along the span of the blade for different orientations of triangular fins Figure 11 Contours of deformation for triangular fins having orientation 3 and 4 The airflow patterns play a crucial role in determining the optimum orientation. While the aim is to reduce the highest temperature on the blade, it s also important to cool the entire blade surface. Therefore, orientation 3 proves to be the most effective orientation since it has resulted in lower deformation of 3.389 as shown in table IV. It can be noted there is very negligible change in deformation for the different orientations of triangular fins as seen in figure 9.Figure 10 and 11 shows contours of total deformation for triangular fins having different orientations. C. Airfoil Fins Similar to triangular fins, it s difficult to decide the optimum orientation from pathline plots [1]. While orientation 3 was the best for triangular fins, it s not necessary that the same orientation would have the same effect on different shapes. This is because every shape diverts and directs flow in different patterns, thus making it important to analyse all orientations for every shape. For airfoil fins, from Table V it can be seen that orientation 4 provides the least deformation of the blade as seen in figure 12, hence orientation 4 is chosen as the optimum one. 44
Figure 12 Deformation of blade along the span of the blade for different orientations of aerofoil fins Table V Comparison of blade deformation between orientations of airfoil fins V. CONCLUSIONS It is seen that fins provide an augmented convective area for better heat dissipation and result in lower deformation. The fins also act as hindrances which disrupts the smooth flow of air, hence increasing turbulence. This increased turbulence improves heat dissipation rate resulting reduction in deformation. The shape and orientation of fins plays a major role in air flow patterns. This can greatly affect the heat dissipation rate as well as total deformation of the blade. The deformation plots have a wavy nature because of the presence of equally spaced fins within the blade. REFERENCES [1] Kini CR, Yalamarty SS, Mendonca RM, Sharma NY, Satish Shenoy B. CHT Analysis of Trailing Edge Region Cooling In HP Stage Turbine Blade, Indian Journal of Science and Technology,2016,9(6), pp. 1-6. [2] Kini CR, Sharma NY, Satish Shenoy B. Thermo- Structural Investigation of Gas Turbine Blade Provided with Helicoidal Passages, Indian Journal of Science and Technology, 2006, 9(6), pp. 1-6. [3] Kini CR, Shenoy B S, Sharma NY. Thermo-structural analysis of HP stage gas turbine blades having helicoidal cooling ducts, International Journal of Advancements in Mechanical and Aeronautical Engineering, 2014, 1(2), pp. 57-60. [4] Kini CR, Satish Shenoy B, Sharma NY.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, 2011 Jul, 2192 (1), pp. 2168-73 [5] Kini CR, Satish Shenoy B, Sharma NY. 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 Engineering and Technology Special Journal Issue, UK,2011, pp. 645-52. [6] Kini CR, Satish Shenoy B, Sharma NY, Numerical Analysis of Gas Turbine HP Stage Blade Cooling with New Cooling Duct Geometries, International Journal of Earth Sciences and Engineering, 2012,5(2), pp. 1057-1062. [7] Kini CR, Shenoy B S, Sharma NY. (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, 2016 [8] Choi J, Mhetras S, Han J. Film Cooling and Heat Transfer on Two Cutback Trailing Edge Models with Internal Perforated Blockages, Journal of heat transfer by ASME, 2008, 130(1), pp.13. [9] Li X, Wang T. Two phase flow simulation of mist film cooling on Turbine blades with conjugate internal cooling, Journal of Heat Transfer, 2008 Oct, 130(1), pp. 13. [10] Material Properties [Online]. Available: www.specialmetals.com/documents/nimonic% 20alloy%20105.pdf 45