Reduce tip leakage flow using squealer tip in an axial turbine = giảm xoáy đầu mút cánh turbine dọc trục sử dụng đầu mút lõm

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY THESIS REDUCE TIP LEAKAGE FLOW USING SQUEALER TIP IN AN AXIAL TURBINE DO DINH CHINH ID: 20202603M CLASS: 20BKTHK Advisors: PhD Dinh Cong Truong Department: Faculty: Department of Aerospace Engineering School of Transportation Engineering Hanoi, 05/2021 Advisor’s sign CỘNG HÒA XÃ HỘI CHỦ NGHĨA VIỆT NAM Độc lập – Tự – Hạnh phúc BẢN XÁC NHẬN CHỈNH SỬA LUẬN VĂN THẠC SĨ Họ tên tác giả luận văn: Đỗ Đình Chinh Đề tài luận văn: Giảm xoáy đầu mút cánh turbin dọc trục sử dụng đầu mút lõm Chuyên ngành: Kỹ thuật Hàng không Mã số SV: 20202603M Tác giả, Người hướng dẫn khoa học Hội đồng chấm luận văn xác nhận tác giả sửa chữa, bổ sung luận văn theo biên họp Hội đồng ngày 06/09/2021 với nội dung sau: Chỉnh sửa hình thức, lỗi chế (lỗi đánh máy, phần mục lục thêm phần kết luận tài liệu tham khảo) Bổ sung danh mục từ viết tắt, kí hiệu Đánh số phương trình dẫn nguồn phương trình Đổi tên tiêu đề mục 2.1 Làm rõ thêm ý nghĩa giá trị y + Thêm biểu diễn w hình 33 Ngày 10 tháng năm 2021 Giáo viên hướng dẫn Tác giả luận văn CHỦ TỊCH HỘI ĐỒNG Student’s information: Full name: Do Dinh Chinh ID: 20202603M Email: chinh.dd150370@sis.hust.edu.vn Class: 20BKTHK The project was done at Department of Aerospace Engineering Date of assignment: 14/12/2020 Date of completion: 15/07/2021 Missions of the thesis: Investigate the effect of tip clearance, squealer tip on aerodynamic performance in an axial turbine Student’s statement: I assure that this thesis was my independent research under the instructions of my advisor PhD Dinh Cong Truong This research is not a copy from any previous research paper Hanoi, 15 July 2021 Do Dinh Chinh Acknowledgements This research is conducted as a fulfillment to pursue a Master degree at the Hanoi University of Science and Technology, Department of Aerospace Engineering First I would like to thank PhD Dinh Cong Truong for his enthusiastic and dedicated assistance through each stage of the process Without his dedicated instructions, I could have not completed this project I would like to express my sincere thanks to my friends and my colleges for advises and encouragements in this time I am grateful to my parents who taught me to cherish excellence Without their support, this work would have not been completed This is the first time I did a project in this field of study so it is inevitable that there are some shortcomings Finally, I would like to thank the commitee for take the time in reading this research work and I look forward to receiving the comments and corrections to complete this study Student Do Dinh Chinh TABLE OF CONTENTS CHAPTER INTRODUCTION 1.1 Introduction 1.2 Previous research 1.3 Tip clearance of rotor blade 1.4 Tip leakage flow 1.5 Flat tip and squealer tip CHAPTER NUMERICAL ANALYSIS 10 2.1 Turbine model Error! Bookmark not defined 2.1.1 Turbine “LISA” 10 2.1.2 Stator and rotor blade geometry 11 2.2 Numerical method 16 2.2.1 Turbine performance curves 16 2.2.2 The fundamental equations of fluid dynamics 17 2.2.3 Simulation procedure 21 2.3 Meshing 22 2.4 Boundary conditions 25 2.5 Convergence criteria 27 CHAPTER RESULTS AND DISCUSSIONS 28 3.1 Grid dependency test and validation 28 3.2 Pressure, velocity and temperature contours 32 3.3 Effect of tip clearance 36 3.4 Reduce tip leakage flow using squealer tip 38 3.5 Effects of the squealer tip on aerothermal performance …………… 38 CONCLUSION AND FUTURE WORK REFERENCES TABLE OF FIGURES Figure Location of turbine in aircraft engine Figure High pressure shrouded (left) and unsrhouded (right) turbine rotor blades Figure Illustration of tip leakage flow over a flat tip Figure Outline of the flow in the region of an unshrouded turbine rotor blade Figure 5: Rotor tip with flat and squealer show clearly the cavity squealer tip with a small figure at the tip region Figure 6: Schematic view of LEC’s LISA research axial turbine 10 Figure 7: Sketch of the turbine first stage with the relevant dimensions 11 Figure 8: Rotor blade (left) and stator blade (right) 12 Figure 9: Stator blade geometric parameters and profile pressure distribution 12 Figure 10: Rotor blade geometric parameters and profile pressure distribution 12 Figure 11: Stator and rotor blade design in Ansys Design Modeler 14 Figure 12: Conceptual view of rotor blade without squealer tip (WST) and with cavity squealer tip (CST) 15 Figure 13: Rotor blade with cavity squealer tip and fillet radius at the hub 15 Figure 14: 3D mesh of the stator blade 22 Figure 15: 3D mesh of rotor blade 23 Figure 16: Diffuser computational domain 23 Figure 17: The computational domain of turbine with WST 24 Figure 18: 3D mesh of the computational domain with CST 25 Figure 19: Complete computational domain when mirrored around the rotational axis 25 Figure 20: Mesh dependency test results 29 Figure 21: 𝑦 + contours on stator and rotor blade for Mesh to Mesh 29 Figure 22: Measured point and computed total pressure ratio compare two interface cases and Blanco’s computed results 30 Figure 23: Measured point and computed adiabatic efficiency compare two interface cases 32 Figure 24: Static pressure contour of the stator and rotor at the mid-span plane 33 Figure 25: Relative Mach number of the stator and rotor at the mid-span plane 33 Figure 26: Total pressure and Static entropy at the outlet plane of the stator 34 Figure 27: Total pressure and Static entropy at the outlet plane of the rotor 34 Figure 28: Temperature contour on the surface of stator and rotor blades 35 Figure 29: Static pressure contour at the end wall of the rotor 35 Figure 30: Flow visualization of the recirculation bubble over the rotor tip surface 36 Figure 31: Peak efficiency at different rotor blade tip clearance 37 Figure 32: Efficiency at mass flow rate of 11.7 kg/s 37 Figure 33: Squealer tip parameters 38 Figure 34: Pressure and Static entropy contour at the rotor outlet plane 39 Figure 35: Aerothermal performance of LISA turbine with cavity squealer tip 42 Figure 36 Distribution of temperature [K] on the stator blade 43 Figure 37: Pressure [Pa] contours on the shroud casing of rotor blade without squealer and with w/τ = 100% 43 Figure 38: Streamline through the tip clearance of rotor blade with w/τ =100% 45 Figure 39: Static entropy contours on the blade with w/τ = 100% 45 Figure 40: Nu contours on the blade with h/τ= 150% 47 Figure 41: Nu contours on the blade with w/τ = 200% 47 LIST OF TABLES Table LISA research turbine facility controlling parameters 11 Table Design parameters of the first stage blades 13 Table 3: Design specifications of cavity squealer tip 16 Table Measured operating condition at turbine design 16 Table Thermodynamic properties of the gas used in the CFD analysis 26 Table Boundary conditions in CFD analysis 26 Table 7: Mesh dependency test results 28 Table Pressure ratio compared to Blanco’s results and measured point 31 Table Maximum efficiency and stall point of turbine stage 32 Table 10 Flow angle at the inlet and outlet locations 36 Table 11 Values of tip clearance investigated and computed adiabatic efficiency 37 Table 12 Aerodynamic performance at different cases 39 Table 13: Effect of cavity squealer on aerodynamic and aerothermal performances for LISA turbine 41 CHAPTER INTRODUCTION 1.1 Introduction In the aviation industry, increasing the performance of aircraft is the most important thing to improve aircraft operating cost and reduce emissions In addition to improvements in aerodynamics and materials of the structure, engine improvement is the top concern of many studies Turbines are always mentioned as the essential part in the engine, directly affecting the performance of the engine Research to improve turbine efficiency plays an important role in increasing overall engine performance Turbine in general and turbine blades in particular are the parts operating under extreme conditions: continuous high temperatures, aerodynamic loads and large centrifugal forces Therefore, experimenting with turbines in particular and aviation engines in general faces many difficulties in terms of cost and equipment The methods of numerical simulation have been created to solve this difficulty thanks to the use of calculation models based on complex solving equations In this project, I focus on analyzing the performance of a turbine stage using CFD simulation The location of turbine is shown in the following Fig Figure Location of turbine in aircraft engine Improving turbine performance is an issue of great concern in the jet engine and power sector Researches in turbine blade technology can be categorized as reducing the tip clearance, casing grooves, airflow injection and so on One of these methods is the use of squealer tip to reduce tip leakage losses This paper presents an analysis of the squealer tip configuration, in which the leakage flow through the tip gap was extensively investigated using computational fluid dynamics (CFD) methods The effects on aerothermal performance of the axial turbine were evaluated based on efficiency and Nusselt number The turbine studied in this investigation is an axial annular turbine named “LISA”, which was experimentally tested at the Laboratory for Energy Conversion (LEC), Institute of the ETH Zürich, Switzerland Numerical calculations have been performed using 3-D Reynolds Averaged Navier-Stokes (RANS) equations with the shear stress transport (SST) turbulence model and “total energy” option with “mixingplane” option between rotor and stator interfaces The impact on aerothermal performance and leakage loss of various geometric parameters related to the height and width of cavity on tip are also discussed The numerical results showed that the created vortex directly affects the turbine’s aerothermal performance and most of the different sizes of the cavity gave higher performance than the original case without squealer tip with a maximum of 0.88% and 9.64% increase in efficiency and averaged Nusselt number This research work used the CFD simulation to investigate the effect of tip clearance on aerodynamic performance of an axial turbine Then apply two methods which are using squealer tip to improve the performance of the turbine 1.2 Previous research Flow structure in turbomachinery passages is extremely complex In turbine, rotor is a rotational part therefore there is always a small space between the rotor tip and casing called tip clearance or tip gap Some research declared that this tip clearance produces a lot of losses and vortices, so it reduces the performance of the turbine As we can see the curved passages, the clearance between the blades and the end walls give rise to non-uniform velocity profiles, pressure gradients and temperature gradients These unsteady flows generate leakage flows therefore reduce efficiency of the turbine Currently, there are many studies on aerodynamic enhancing methods to limit the influence of the tip leakage flow One of the most popular methods is to design the squealer tip for the blade In one study, Heyes et al [19] showed that blade tip geometry had a positive effect on the aerodynamic performance of axial turbine cascades by limiting the undesirable effects of the tip leakage flow In terms of thermodynamics, Ameri et al [20] showed that a squealer tip directly slowed down the leakage flow, while also increasing the total heat transfer Table 13: Effect of cavity squealer on aerodynamic and aerothermal performances for LISA turbine PR η [%] 𝑁𝑢 𝑟𝑜𝑡𝑜𝑟 𝑏𝑙𝑎𝑑𝑒 𝑚 𝑙 /𝑚 𝑖 [%] WST 1.3663 84.2045 734.198 3.0244 w50h50 1.3656 84.7400 804.996 2.6774 w50h100 1.3647 84.8551 781.063 2.4112 w50h150 1.3642 84.9447 778.828 2.3052 w50h200 1.3645 84.9293 777.057 2.2392 w100h50 1.3671 84.6000 800.212 2.7316 w100h100 1.3654 84.7121 779.722 2.5106 w100h150 1.3648 84.8290 780.805 2.3991 w100h200 1.3646 84.8516 774.867 2.3270 w150h50 1.3669 84.4582 792.701 2.7658 w150h100 1.3660 84.6546 788.759 2.5753 w150h150 1.3652 84.7950 787.124 2.4578 w150h200 1.3647 84.8319 786.955 2.3811 w200h50 1.3668 84.4250 798.279 2.7946 w200h100 1.3657 84.6546 795.884 2.6177 w200h150 1.3648 84.7888 792.216 2.4937 w200h200 1.3644 84.8505 787.343 2.4184 (a) Total pressure ratio 41 (b) Adiabatic efficiency (c) Averaged Nusselt number on the rotor blade (d) Tip leakage mass flow rate Figure 35: Aerothermal performance of LISA turbine with cavity squealer tip 42 Figure 36 Distribution of temperature [K] on the stator blade a) WST b) w100h50 c) w100h100 d) w100h150 e) w100h200 Figure 37: Pressure [Pa] contours on the shroud casing of rotor blade without squealer and with w/τ = 100% Because of the relatively small variation in the geometry of the cavity on the tip of the rotor, the effects on the stator blade are similar for all cases As shown in Fig 36, due to the airflow hitting the LE of the stator blade, the temperature in these regions is the highest and then the temperature will decrease gradually from LE to TE In the case without squealer tip, it can be observed from Fig 37 that the low-pressure areas appear on the shroud casing and as the height of cavity increases, the static pressure on shroud casing also increases These low-pressure areas are gradually narrowed due to the squealer cavity compensating for the lack 43 of this air shortage This drop in pressure can be contributed to the resulted fast velocity of the air flow through the gap Fig 38 shows the streamlines of the leakage flow with a fixed squealer width at w/τ= 100% and h/τ = 50%; 100%; 150% and 200%, respectively The incoming flow enters from the leading edge, passes over the squealer rim and is directed to the cavity Increasing the squealer height h increases cavity volume, the cavity vortex becomes larger and more pronounced before exiting the tip gap as shown in Fig 38 And after leaving the gap, the leakage flow splits into two main flows, with a large vortex that is quite similar for all cases, characterized by a high turbulence, which is represented by the pink areas in Fig 39 Increasing the height and reducing the width of squealer in general makes the cavity vortices relatively larger Because the volume of the cavity is expanded, the air flow is blocked by creating multiple eddy structures, this blockage obstructs the air flow from pressure side to suction side and tip leakage mass flow rate is effectively reduced a) WST c) w100h100 b) w100h50 d) w100h150 44 e) w100h200 Figure 38: Streamline through the tip clearance of rotor blade with w/τ =100% a) w100h50 c) w100h150 b) w100h100 d) w100h200 Figure 39: Static entropy contours on the blade with w/τ = 100% 45 The tip leakage flow that causes a significant amount of aerodynamic loss, and also a significant source of enhanced convective heat transfer on the blade tip platform exposed to the hot gas stream The leakage flow between the stationary casing of the turbine and the exposed rotor surface is a significant source of aerothermal performance degradation Fig 40 illustrates the Nusselt number distribution on the blade tip for a fixed squealer height Because of the intrusion, a local high heat transfer region emerges Expanding the squealer height makes the cavity vortex larger so that thermal transport in the encroachment zone and high heat transfer region is decreased It is seen that extending the cavity vortex reduces the boundaries of the high heat transfer region beneath Results are plotted in Fig 41 for variety of the Nusselt number distribution on the blade tip in the function of squealer height Reducing squealer height enhances the low heat transfer on blade tip where leakage flow leaves the tip gap without reattachment The Nu contours shown in Fig 41 demonstrate a high heat transfer region on the top surface of the squealer rim, particularly near the leading-edge zone This zone has the most elevated number of around 950 This high thermal zone involves a more extended zone close to the leading edge as the rim width is progressively increased from 50 to 200% It very well may be that wider squealer rims suffer from thermal loads and oxidation in these high heat transfer zones The average heat/mass transfer rate on the cavity floor reduces with expanding squealer height as Kang and Lee [26] observed in their linear cascade experiment Besides, Zhou and Hodson [27] performed a numerical study and found that decreasing the width and expanding the height of the squealers diminish the average heat transfer coefficient In our numerical study, the thermal performance of squealer rims has been researched in a wide width and height range Results are shown in Tab and Fig for the variety of the Nu number on the rotor blade Diminishing squealer width and expanding squealer height remarkably decreased the Nu number This outcome is in good concurrence with the investigation of Kang and Lee [26] and Zhou and Hodson [27] 46 a) w50h150 c) w150h150 b) w100h150 d) w200h150 Figure 40: Nu contours on the blade with h/τ= 150% a) w200h50 b) w200h100 c) w200h150 d) w200h200 Figure 41: Nu contours on the blade with w/τ = 200% 47 Because of the limited time and computer ability, the number of cases that can be calculated is not much, we need to calculate more cases to have a clearer overview of the value of this change 48 Conclusion and future work Conclusion The investigation studied the effect of the height and width of the squealer tip on the aerodynamic and aerothermal characteristics of an axial turbine using numerical analysis The stator and rotor blade in this numerical calculation are similar to the profile of blades of the axial annular turbine The numerical results were consistent with the initial prediction and experimental results This passive control system has also achieved improvements in aerodynamic and aerothermal performance Increasing the height of the squealer tip raises the adiabatic performance Decreasing the width of the squealers reduces the adiabatic performance, which is higher than that of the case without squealer tip Numerical methods have been developed to simulate and evaluate the performance of axial turbines The study has achieved some results as follows: Determine the effect of the blade tip gap on the efficiency of an axial turbine With increasing gap, the greater the loss at the tip, the more the turbine's efficiency decreases Specifically, in the case of the smallest gap of 0.5%, the largest efficiency is 93.46%, which is 1.81% larger than the case of standard clearance (91.65%) Turbine’s vortex zones are also determined by pressure, static entropy and temperature distributions Secondly, the research investigated the effect of the recessed tip on turbine performance For all cavity profiles surveyed, the turbine efficiency increased In which the largest case is w/τ = 100 and h/τ [%]= 25 However, with this increase, we have not investigated its law The pressure and static entropy distribution shows that the over the tip vortex has decreased, leading to reduce losses compared to the flat tip Finally, the study investigated the method of adding a current to the tip of the rotor blades and evaluating its effect on turbine’s performance In both cases with a single-tube and three-tube flow, we achieve higher efficiency results than flat tip However, the performance is reduced compared to the case of squealer tip Future work 49 Firstly, the optimal survey of the tip configuration was carried out not only on the "LISA" turbine but also directly on a compressor or an aircraft turbine From analyzing the effect of the squealer tip on the turbine's efficiency, we study more parameters of the cavity squealer design or investigate more new shapes of the cavity Secondly, more cool air flow may be supplied to the cavity squealer tip from the compressor through the pipes inside the rotor blades to increase the thermal effectiveness of the axial turbine In addition to the efficiency improvement, the turbine's other performances also changed, the study should consider the change of these values so that the change in efficiency does not significantly affect the turbine overall performance 50 Nomenclature A total area of rotor blades [m2] p pressure [Pa] C chord length of rotor blades [m] pt total pressure [Pa] Ca ′′ axial chord length of rotor blades [m] 𝑞𝑤 wall heat flux [W/m2] b rotor blade span [m] h squealer height or cavity depth [m] h0 local heat transfer coefficient T temperature [K] ℎ0 averaged coefficient local heat transfer Ti averaged temperature at inlet [K] Tw wall temperature [K] kt thermal conductivity [W/(m.K)] w squealer width [m] 𝑚̇l leakage mass flow rate [kg/s] 𝑦+ dimensionless wall distance 𝑚̇i inlet mass flow rate [kg/s] + 𝑦𝑚𝑎𝑥 Nu Nusselt number maximum distance 𝑁𝑢 averaged Nusselt number on rotor η τ blade PR dimensionless wall adiabatic efficiency [%] tip clearance rotor blade [m] pressure ratio Abbreviations CFD computational fluid dynamics SST shear stress transport WST CST CV without squealer tip (or flat tip) cavity squealer tip cavity vortex 51 REFERENCES [1] Rains, D A., “Tip Clearance Flows in Axial Flow Compressors and Pumps,” Hydrodynamics and Mechanical Engineering Labs., California Inst Of Technology Report 5, Pasadena, CA, 1954 [2] Moore, J., Moore, J G., Herny, G S., and Chaudhry, U., “Flow and Heat Transfer in Turbine Tip Gaps,” American Society of Mechanical Engineers Paper GT-88-135, 1988 [3] Bindon, J P., “The Measurement and Formation of Tip Clearance Loss,” American Society of Mechanical Engineers Paper 88-GT-203, 1988 [4] Yamamoto, A., “Endwall Flow/Loss Mechanisms in a Linear Turbine Cascade with Blade Tip Clearance,” Journal of Turbomachinery, Vol 111, No 3, 1989, pp 264-275 [5] Tallman, J., and Lakshminarayana, B., “Numerical Simulation of Tip Leakage Flows in Axial Flow Turbines, with Emphasis on Flow Physics Part I— Effect of Tip Clearance Height,” 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height near a HP turbine blade", International Journal of Heat and Mass Transfer, Vol 120, pp.18-32 DOI: 10.1016/j.ijheatmasstransfer.2017.12.017 [30] Anderson, John D., Jr., Fundamentals of Aerodynamics, 2nd Edition McGraw-Hill, New York, 1991 55 ... turbine efficiency plays an important role in increasing overall engine performance Turbine in general and turbine blades in particular are the parts operating under extreme conditions: continuous... shown in the following Fig Figure Location of turbine in aircraft engine Improving turbine performance is an issue of great concern in the jet engine and power sector Researches in turbine blade... the study CHAPTER NUMERICAL ANALYSIS 2.1 Turbine model 2.1.1 Turbine “LISA” The turbine studied in this investigation is an axial annular turbine named “LISA” This turbine was tested at the Laboratory