The impact of additive manufacturing on micro-turbine ports

CRANFIELD UNIVERSITY ORIOL LÁZARO CASTELL THE IMPACT OF ADDITIVE MANUFACTURING ON MICROTURBINE PARTS SCHOOL OF ENERGY AND POWER MSc in Advanced Mechanical Engineering MASTER OF SCIENCE Academic Year: 2016 - 1017 Supervisors: Prof. John Oakey and Dr. Nigel Simms September 2017 CRANFIELD UNIVERSITY SCHOOL OF ENERGY AND POWER MSc in Advanced Mechanical Engineering MASTER OF SCIENCE Academic Year 2016 - 2017 ORIOL LÁZARO CASTELL THE IMPACT OF ADDITIVE MANUFACTURING ON MICROTURBINE PARTS Supervisor: Prof. John Oakey and Dr. Nigel Simms September 2017 © Cranfield University 2017. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright owner. i ABSTRACT In the coming years, Additive Manufacturing will play an increasingly role in microturbine production, therefore the importance to know the impact of this type of manufacturing are crucial for the optimum design and performance of the device and the right definition of the stress fatigue generated. Is for this reason that the aim of this project is to study the impact that Additive Manufacturing puts on the design and the performance of a micro-turbine wheel, regarding the stresses generated on it. To study the final stresses properly, a model of the micro-turbine has been developed with ANSYS Workbench. It includes an initial CFD study with ANSYS CFX to find the temperatures and pressures under which the micro-turbine works. Then, these temperatures are imported to ANSYS Thermal to calculate the temperature distribution in the geometry and after, CFX and ANSYS Thermal results are imported to ANSYS Structural to carry out a stress analysis. The results of the project are taken from the execution of this model with the properties of Inconel 625, manufactured by Selective Laser Melting and manufactured by Casting, and a deep study of the stress generated at the junction between the blade and the hub is carried out at the end. Keywords: Inconel 625, Micro-turbine, Selective Laser Melting, Casting. ii ACKNOWLEDGEMENTS I would like to thank the sponsor of this project, HIETA Technologies. Without its support and information provided, the work presented here would not be possible. I am thoroughly grateful to my academic supervisor Prof. John Oakey for all his support, help and guidance and throughout this MSc thesis, I could not have done this without him. I would also like to thank Dr. Nigel Simms for his help as a second supervisor. Finally, thank you my family who has supported me unconditionally not only during my MSc thesis, but also my university years, with understanding and patience. iii TABLE OF CONTENTS ABSTRACT ......................................................................................................... i ACKNOWLEDGEMENTS.................................................................................... ii LIST OF FIGURES ............................................................................................. v LIST OF TABLES ............................................................................................. viii LIST OF EQUATIONS ........................................................................................ ix LIST OF ABBREVIATIONS ................................................................................ x 1 INTRODUCTION ............................................................................................. 1 2 LITERATURE REVIEW ................................................................................... 3 2.1 Gas turbine ............................................................................................... 3 2.2 Micro-turbine ............................................................................................. 3 2.2.1 Breakdown ......................................................................................... 4 2.2.2 Working cycle ..................................................................................... 5 2.2.3 Turbine wheel working conditions ...................................................... 6 2.2.4 Performance ....................................................................................... 7 2.2.5 Turbine wheel materials ..................................................................... 8 2.2.6 Turbine wheel manufacture .............................................................. 10 2.2.7 Microstructure characteristics of Inconel 625 ................................... 17 3 METHODOLOGY .......................................................................................... 20 3.1 Geometry ................................................................................................ 20 3.2 Material properties .................................................................................. 21 3.2.1 Conventional manufacturing: Casting............................................... 21 3.2.2 Additive manufacturing: Selective Laser Melting .............................. 23 3.3 CFX ......................................................................................................... 24 3.3.1 Fluid domain ..................................................................................... 25 3.3.2 CFX Mesh ........................................................................................ 25 3.3.3 Boundary conditions ......................................................................... 28 3.4 Thermal analysis ..................................................................................... 31 3.4.1 ANSYS Thermal mesh ..................................................................... 31 3.4.2 Boundary Conditions ........................................................................ 34 3.5 Structural analysis................................................................................... 35 3.5.1 ANSYS Structural mesh ................................................................... 36 3.5.2 Boundary Conditions ........................................................................ 37 4 RESULTS AND ANALYSIS ........................................................................... 40 4.1 CFX ......................................................................................................... 40 4.2 Thermal analysis ..................................................................................... 45 4.3 Structural analysis................................................................................... 46 4.3.1 Structural analysis results for a rotation speed of 90,000rpm ........... 48 4.3.2 Structural analysis results for a rotation speed of 100,000rpm ......... 51 4.3.3 Structural analysis results for a rotation speed of 110,000rpm ......... 53 4.3.4 Comparison ...................................................................................... 55 iv 5 DISCUSSION ................................................................................................ 59 6 CONCLUSIONS AND FUTURE RESEARCH ............................................... 61 REFERENCES ................................................................................................. 62 APPENDICES .................................................................................................. 65 Appendix A : Data sheet of Inconel 625 manufactured by SLM from CRP Meccanica ..................................................................................................... 65 Appendix B : Properties of Inconel 625 manufactured by casting from MatWeb ........................................................................................................ 67 Appendix C : Inconel 625 properties ............................................................. 68 Appendix D Mesh sensitivity results ............................................................. 69 Appendix E : CFX results .............................................................................. 70 Appendix F : Structural data results for 90,000rpm from ANSYS Structural (example) ...................................................................................................... 71 v LIST OF FIGURES Figure 1: Sectional view of a micro gas turbine. ................................................. 4 Figure 2: Micro-turbine working cycle. ................................................................ 5 Figure 3: The electrical efficiency of the competitive offerings in the micro-turbine size range. ................................................................................................... 7 Figure 4: Investment casting steps. .................................................................. 10 Figure 5: Conventional manufacture post-processing. ..................................... 11 Figure 6: Additive manufacturing testing cycle. ................................................ 12 Figure 7: SLM process. .................................................................................... 13 Figure 8: Post-processing................................................................................. 13 Figure 9: The stress-strain curves of SLM produced IN625 (1), MWCNT-IN625 (2) and heat-treated MWCNT-IN625 (3) (left) and their mechanical properties (right). ........................................................................................................ 15 Figure 10: HIETA Technologies Wheel turbine. ............................................... 15 Figure 11: EBM process. .................................................................................. 16 Figure 12: EBM post-processing. ..................................................................... 17 Figure 13: Casting at 1289°C/30 (left) minutes and at 1288°C /24 minutes (right). .................................................................................................................. 18 Figure 14: Bidirectional scan mode of the laser for SLM process. .................... 19 Figure 15: Microstructure of the top surface of an Inconel 625 sample manufactured with SLM (left) and “scales” shape of YZ direction for an Inconel 625 sample manufactured with SLM (right). .................................. 19 Figure 16: ANSYS project interface. ................................................................. 20 Figure 17: Isometric (left) and frontal (right) view of the radial turbine analysed. .................................................................................................................. 21 Figure 18: Approximated curve of the Young Modulus of Inconel 625 manufactured by casting versus the Temperature. .................................... 22 Figure 19: Thermal conductivity of Inconel 625 versus the Temperature. ........ 23 Figure 20 Young Modulus of Inconel 625 manufactured by SLM versus the Temperature. ............................................................................................. 24 Figure 21: CFX fluid domain for the turbine wheel geometry (left) and sectional view of the turbine and the fluid domain (right). ......................................... 25 Figure 22: CFX fluid domain mesh for the turbine wheel geometry. ................. 27 vi Figure 23: Mesh sensitivity for the CFX model. ................................................ 28 Figure 24: Inlet and Outlet domain and the direction of rotation of the turbine. 29 Figure 25: Inflow of the turbine. ........................................................................ 30 Figure 26: Thermal analysis mesh for the turbine geometry (left) and refinement zone (right). ............................................................................................... 32 Figure 27: Thermal analysis controlled mesh for the shaft zone (left) and for the top zone (right). ......................................................................................... 32 Figure 28: Control points for the mesh sensitivity. ............................................ 33 Figure 29: Maximum temperature of the control points of the mesh sensitivity. 34 Figure 30: Minimum temperature of the control points of the mesh sensitivity. 34 Figure 31: Hub zone (left) and blades zone (right). .......................................... 35 Figure 32: Imported temperature of the Hub zone (left) and the Blades zone (right). ........................................................................................................ 35 Figure 33: Micro-turbine mesh for the Structural analysis. ............................... 36 Figure 34: Mesh sensitivity of the Structural analysis. ...................................... 37 Figure 35: Imported pressure of the hub (left) and the blades (right). .............. 38 Figure 36: Imported temperature from Thermal analysis. ................................. 38 Figure 37: Subjection point of the turbine. ........................................................ 39 Figure 38: Efficiency versus Inlet pressure for a rotation speed of 90,000 rpm. 40 Figure 39: Power versus Inlet pressure for a rotation speed of 90,000rpm. ..... 41 Figure 40: Efficiency versus Inlet pressure for a rotation speed of 100,000 rpm. .................................................................................................................. 42 Figure 41: Power versus Inlet pressure for a rotation speed of 100,000rpm. ... 42 Figure 42: Efficiency versus Inlet pressure for a rotation speed of 110,000rpm. .................................................................................................................. 43 Figure 43: Power versus Inlet pressure for a rotation speed of 110,000rpm. ... 43 Figure 44: Velocity flow without vortex generation under operating conditions of Pin = 3bar, Tin = 650°C and rotation speed 110,000rpm. ........................... 44 Figure 45: Velocity flow with vortex generation under operating conditions of Pin = 2bar, Tin = 550°C and rotation speed 110,000rpm. ................................. 44 Figure 46: Flow temperature distribution on the turbine surface under operating conditions of Pin = 3bar, Tin = 650°C and rotation speed 110,000rpm. ...... 45 vii Figure 47: Turbine body distribution under the operating conditions of Pin = 3bar, Tin = 650°C and rotation speed 110,000rpm. ............................................. 46 Figure 48: Stress distribution on a turbine manufactured with SLM and under the operating conditions of Pin = 3bar, Tin = 650°C and rotation speed 110,000rpm. .............................................................................................. 47 Figure 49: Stress distribution on a turbine manufactured with Casting and under the operating conditions of Pin = 3bar, Tin = 650°C and rotation speed 110,000rpm. .............................................................................................. 47 Figure 50: Path to calculate the stresses generated. ....................................... 48 Figure 51: Stress vs X under different operating conditions, 90,000rpm and Inconel 625 (Casting)................................................................................. 50 Figure 52: Stress vs X under different operating conditions, 90,000rpm and Inconel 625 (SLM). .................................................................................... 50 Figure 53: Difference between the values for Casting and SLM versus the inlet pressure for 90,000rpm. ............................................................................ 51 Figure 54: Stress vs X under different operating conditions, 100,000rpm and Inconel 625 (Casting)................................................................................. 52 Figure 55: Stress vs X under different operating conditions, 100,000rpm and Inconel 625 (SLM). .................................................................................... 52 Figure 56: Difference between the values for Casting and SLM versus the pressure with 100,000rpm. ........................................................................ 53 Figure 57: Stress vs X under different operating conditions, 110,000rpm and Inconel 625 (Casting)................................................................................. 54 Figure 58: Stress vs X under different operating conditions, 110,000rpm and Inconel 625 (SLM). .................................................................................... 54 Figure 59: Difference between the values for Casting and SLM versus the pressure with 100,000rpm. ........................................................................ 55 Figure 60: Stress versus rotation speed for σmax (middle zone). ...................... 56 Figure 61: Stress versus rotation speed for σmin. .............................................. 57 Figure 62: Stress versus rotation speed for σmax. ............................................. 57 Figure 63: Difference between casting and SLM results versus Inlet pressure comparison. ............................................................................................... 58 4 Concerning the environmental impact, micro-turbines have a high noise levels and require a specific acoustic system. However, the air cooling bearings avoid lubricants contamination by combustion products and prolongs the equipment useful life. Moreover, micro-turbine manufacturers usually uses heat exchangers to take profit of the combustion gases exhaust and preheat the air intake of the combustion chamber, achieving a thermal efficiency of 30% (Rosa Do Nascimento et al., 2013). The commercial customer requirements for micro-turbines are that they need to be very clean, with low NOx, CO and unburned hydrocarbons, have high efficiency and low maintenance, very low forced outage rate and low installation cost to provide fast payback to the owners (Goli, Kondi and Timmanpalli, 2015). 2.2.1 Breakdown This type of turbines below few hundreds of kilowatts in size usually uses centrifugal machinery (Epstein, 2004), and consists of a supersonic radial flow compressor and turbine, connected by the same shaft. Radial-flow turbomachinery, can handle the very small volumetric flows of air and combustion products, with higher component efficiency and with simpler construction than axial-flow components (Soares, 2007). The electrical generator is also mounted on the same shaft. Hence there are only one rotating part, avoiding the needed of a gearbox and the problems due to considerable number of moving parts. This simplicity, makes this part relatively easy to manufacture and maintain, and presents a great potential for inexpensive and large scale manufacturing (Rosa Do Nascimento et al., 2013). Although, the low mechanical inertia of the shaft, makes micro-turbines difficult to control. The main parts are showed in Figure 1 (Traverso, Calzolari and Massardo, 2017). Figure 1: Sectional view of a micro gas turbine. 5 The inner bearing holding the shaft is hydrodynamic bearing, which contains grooves for lubrication, making the bearing smaller, cheaper and more efficient, and providing the bearing with low maintenance and high service life (Kingsbury, n.d.). The outer bearing utilizes a ceramic ball race. The heat exchanger has importance to increase the cycle efficiency of the micro turbine (Goli, Kondi and Timmanpalli, 2015). 2.2.2 Working cycle The exhaust gases coming from the combustor with high velocity and temperature rotate the turbine wheel. The turbine wheel, the compressor and the generator are mounted on the same shaft, in consequence, the generator rotates with the same speed as the turbine and generates electricity. This electricity is conditioned through power electronics devices and supplied to the required areas. The fuel is injected to the combustor in the gaseous form and fresh and compressed air pass through the heat exchanger and is also introduced. The schematic process is showed in Figure 2. The role of the heat exchanger is to take profit from the hot turbine exhaust gases, with a temperature about 500 - 600ºC, and increases the temperature of the air coming from the compressor (typically around 150ºC), increasing the overall efficiency of the micro gas turbine (U.S. Enviromend Protection Agency, 2015). This allows the cycle efficiency to take values as much as 30% while the average net efficiency of unrecovered micro-turbines is 17 % (Rosa Do Nascimento et al., 2013). Figure 2: Micro-turbine working cycle. 6 2.2.3 Turbine wheel working conditions This thesis focuses only on the turbine wheel, then the operating conditions are explained only for this part. The following information is extracted from an experimental test of the operating conditions of a micro gas turbine device (Roberto Capata, 2015) which shows the working conditions of the turbine wheel from a Graupner/JetCat turbo-prop engine (this information can be extrapolated as an initial estimation to the performance of a micro gas turbine). As well as these values are contrasted with the experimentation of a Turbec T100 radial micro-turbine (Hohloch et al., 2017). The experiment shows that the inlet temperature range for the turbine is from 500 to 800ºC and for the outlet temperature is from 500 to 600ºC (Cadorin et al., 2012). The turbine pressure ratio is the ratio of turbine pressure outlet to inlet pressure and decreases with higher turbine speed. 𝛽𝑡𝑢𝑟𝑏𝑖𝑛𝑒 =𝑃𝑜𝑢𝑡𝑝𝑢𝑡 𝑃𝑖𝑛𝑝𝑢𝑡 Equation 1 The range for pressure ratio on a micro-turbine is from 0.5 for low turbine speed to 0.25 for high turbine speed, and the outlet pressure goes from 0.95 bar to 1 bar respectively, giving an inlet pressure range from 2 to 4bar approximately. The increase in pressure with higher turbine speed is due to the rising pressure losses caused by the subsequent components (Hohloch et al., 2017). Concerning the inlet turbine flow, the value varies from 0.15 kg/s at low turbine speeds to 0.8 kg/s at high speeds. The inlet turbine flow is the result of air inlet plus inlet fuel mass flow (Cadorin et al., 2012). 𝑚𝑡𝑢𝑟𝑏𝑖𝑛𝑒,𝑖𝑛𝑙𝑒𝑡 =𝑚𝑎𝑖𝑟,𝑖𝑛𝑙𝑒𝑡 +𝑚𝑓𝑢𝑒𝑙,𝑖𝑛𝑙𝑒𝑡 Equation 2 The fuel inlet flow goes from 0.001 kg/s to 0.010 kg/s demonstrating that most of the inlet turbine flow is composed by air. Another interesting parameter is the rotational speed of the turbine wheel, which delimit the speed of the compressor and the generator (except some manufacturers who include a gearbox between the turbine-compressor shaft and 7 the generator shaft to reduce the speed on the generator). The rotational speed goes from 30,000 to 120,000rpm (Rosa Do Nascimento et al., 2013). 2.2.4 Performance Commercial micro turbines from 25 to 500kW used for power generation produce both heat and electricity on a relatively small scale. Un-recuperated microturbines have an electrical efficiency around 15%. If they have recuperators, the electrical efficiency will be around 20 - 30%, and if it has heat recovery, the efficiency will arise up to 85% (Goli, Kondi and Timmanpalli, 2015). The performance of micro gas turbines is strongly affected by temperature conditions. The decline observed at higher temperatures is explained by the lower air density and consequently lower mass flow rate through the power unit (Caresana et al., 2010). The electrical efficiency of the current micro-turbines in the market are shown in Figure 3 (Gillette, 2010). Figure 3: The electrical efficiency of the competitive offerings in the micro-turbine size range. 8 2.2.5 Turbine wheel materials Current micro-turbine turbine wheels are manufactured with nickel-based alloys, providing them with lightweight and good resistance. However, as the turbine wheel is exposed to high inlet temperatures, normal light weighted materials cannot be used. These turbines are subjected to a long-term exposure to high temperature up to 800ºC, so heat resistance is an essential prerequisite. Therefore, light common materials like aluminium or titan-based alloys cannot be used. Thermal creep, which is a time-dependent deformation under high levels of stress below the Yield Strength of the material, is also a problem when choosing the material of the turbine. This phenomenon is consequence of the high temperature of the turbine, and to avoid it, materials with high melting temperature must be used, as well as creep test data needs to be consulted during the material selection. Some innovative solutions for the temperature problems of the turbine have appeared during the last years and are still being tested on micro-turbines. One of them are the Thermal Barrier Coatings (TBC) with a ceramic material. This type of coating should be selected so that is refractory enough to resist the high temperatures at the surface and has a low bulk thermal conductivity to minimize heat transfer to the metallic zone. Moreover, the thermal expansion of the selected coating should closely match the metallic one to minimize potential stresses. Nowadays, the most widely used ceramic coating material in turbines industry is Yttria stabilized zirconia (YSZ). Another consideration is that this coating must have a grain and pore structure to minimize thermal conduction to the metal-ceramic interface. The coating should have enough porosity, so it reduces the thermal conductivity while simultaneously adhering to the metal turbine bond-coat layer (Estrada, 2007). The current techniques used to TBC are Electron beam physical vapor deposition (EBPVD), Air plasma spray (APS), High velocity oxygen fuel (HVOF), Electrostatic spray-assisted vapour deposition (ESAVD) and Direct vapor deposition (DVD). 9 The last years SLM techniques are increasing sharply in the turbine manufacturing. Materials like Inconel or Hastelloy X are examples of high temperature and corrosion resistant nickel-based alloys which can be manufactured with this method. In most cases, these alloys contain chrome, iron, niobium and molybdenum and other alloy components, and they are often known as super alloys. Nickel-based alloys withstand higher temperatures than steels and they are also highly weldable. Their resistance to temperature is achieved through a mixture of dispersion hardening, precipitation hardening and solid solution strengthening. Nickel-based alloys exhibit good mechanical characteristic values such as high tensile strength and good endurance strength. Inconel can be used at temperatures of up to 700°C. Hastelloy X can even be used at temperatures of up to 1200°C. This makes these alloys ideally suited for aerospace technologies and for turbine production. Moreover, through post processing, such as hardening, heat treatment or hot isostatic pressing (HIP), the components’ properties can be adapted to meet specific requirements (SLM Solutions, 2017). Continued improvements in powder synthesis, processing, and densification have resulted in the development of a current generation of silicon nitride ceramics having controlled microstructures consisting of elongated grains. These so-called self-reinforced or in-situ toughened materials exhibit superior performance as reflected by increased strength, higher fracture toughness, and enhanced resistance to creep rupture (Soares, 2007). A silicon nitride rotor for a radial-flow micro gas turbine with a capacity of 30KWel was developed by Fraunhofer. The ceramic rotor exhibits long-term stability up to 1200ºC at maximum operating loads and can be mass-produced. Depending on chemical composition, sintering and post processing, specific properties can be improved. The post processing led to high strength as well as high oxidation resistance and fatigue strength up to 1200ºC. The final data can be seen in Table 1 (Stockmann et al., n.d.). 10 Table 1: Silicon nitride data. Operating temperature (ºC) 1200 Fracture toughness (MPa·m1/2) 6.8 Yield Strength (MPa) 1000 Fatigue strength at 1200ºC (MPa) 500 2.2.6 Turbine wheel manufacture The turbine is the part of the micro-turbine machine which gives the motion to the shaft, therefore it is the most important part of the device and needs a sophisticated method of manufacture, in order to accomplish the hard specifications required due to the strong conditions of stress and temperature. During the last years, advanced manufacture plays a key role in manufacture, and it influences positively to the fabrication of micro-turbines. 2.2.6.1 Conventional manufacture of micro-turbines: Investment casting Producing a turbine wheel with such specific demands of performance, is a technique developed over many years, since the micro-turbines appeared. The turbine wheel is usually manufacture with nickel-based superalloys, a high strength investment casting. These types of alloys are designed to withstand high temperatures and it is a trade-off in machinability. Therefore, an investment casting process has been developed during many years to achieve the best results, and minimize the machining on it (Ramamurthy and Thennavarajan, 2008). The conventional process of investment casting has the following steps, showed in Figure 4 (James, 2015): Figure 4: Investment casting steps. 3D model Wax Shell Casting 11 The first step for the investment casting is to manufacture the wax pattern for the process from a 3D model. The wax is used for the ease of melt out and the possibility to reuse it. Several wax patterns are assembled on a tree and typically the pattern is destroyed during the process, so a new one is required for each casting. Once the tree and the patterns are assembled, metal casting pattern is then dipped in a refractory slurry whose composition includes extremely finegrained silica, water and binders. A ceramic coating is built over the wax surface. The pattern is then repeatedly dipped into the slurry to increase the thickness of the ceramic coat. Once the ceramic coat has the toughness required, it is dried in air to harden it. Then the wax is melted at 90 – 175°C and it flows out the shell (The library of Manufacturing, n.d.). The ceramic shell is then heated to 550 - 1100°C to strengthen further the mould and eliminate any rest of wax, water or contaminants. The metal casting is poured while the mould is still hot allowing the liquid metal to flow easily through the mould cavity and shrinking together as they cool, which gives a better dimensional accuracy. After the casting is solidified, the ceramic shell is broken from the piece and all the parts are cut away from the tree (The library of Manufacturing, n.d.). When the casting process finish, grinding, milling and electrical discharge machining (EDM) processes are required (Figure 5). Figure 5: Conventional manufacture post-processing. To finish the micro-turbine manufacture, a final coating is carried on, increasing the material properties. 2.2.6.2 Advanced Manufacturing of micro-turbines The most widely additive manufacturing technique used for micro-turbine manufacturing is Selective Laser Melting (SLM). This type of manufacturing, as a difference of conventional casting, allows an initial fast testing and validation process before to start the final integrated development. Grinding Milling EDM Coating 12 2.2.6.2.1 Selective Laser Melting Concerning the advanced manufacturing techniques on micro-turbines manufacture, testing and validation with SLM process can be seen in Figure 6 (James, 2015). This process is completely different from conventional manufacturing, allowing efficiency and precision to the process. Moreover, this type of additive manufacturing allows the fabrication of nickel-based alloy micro-turbines wheels. The first stage starts with the design of the model and its simulation to approve the model before to start the real process with Selected Laser Melting (SLM). SLM is a rapid prototyping, 3D printing or additive manufacturing practise which uses a high-power density laser to melt and fuse metallic powder together. This process has a limiting factor, the high surface roughness (Ra ≥ 5μm). Is for this reason that a post-processing method to meet the requirements of surface roughness (Ra ≥ 0.8μm) is needed. Therefore, to achieve this small roughness, micro machining process is required (Such and Meiners, 2015). After the post-processing, during instrumentation stage, all the measurements and critical dimensions are required to be checked to see if the turbine meets the initial requirements. To finish the cycle, a test is carried out showing the quality of the product through an objective way. When the testing and validation cycle is completed, advanced manufacturing allows integrated development with iterative and fast cycles. The process above has many advantages, it allows parallel and integrated development process, radical development approaches, ambitious and short development goals, and short iterative cycles (James, 2015). Once the testing and verifying cycle and the real performance of the micro-turbine prototype is accepted, the process chain for SLM production starts (Figure 7). The raw material for this process are the powder (with diameter lower than 50μm), 3D design and simulation SLM Processing Post processing Instrumentation Testing Integrated development Figure 6: Additive manufacturing testing cycle. 13 then the machine is conditioned and the process designed. The control parameters for the machining are implemented on the SLM machine and the turbine processing starts (James, 2015). Figure 7: SLM process. The post-processing starts when the micro-turbine is ready after the SLM process, then a quality control detects if defects appeared during the additive manufacture process. If the SLM process pass the quality control, the microturbine receive a heat treatment, to improve the strength of the turbine material, but this process can however result in undesirable residual stresses, which may lead to cracking and distortion of the component treated (Fry, n.d.). Another quality control is carried out to check if the heat treatment is right and the turbine goes to the machining part to finish the turbine (Figure 8). Figure 8: Post-processing. At the final part of the micro-turbine machining the piece is submitted to a final quality control. Advanced manufacturing process reduces costs and improves the response time for customers during the micro-turbine design process or for small production quantities, as well as, provides a good opportunity for improved efficiency within the repair and refurbishment business. However, the underlying characteristics and properties of many metal powders (especially high-temperature super-alloys) are not yet well-understood and tested to the industry's exacting standards (Aller, 2016). It is expected that properties of laser built parts will be different from traditional manufacturing. One of the problems of this technique is the large residual stress found on the fabricated pieces because the rapid temperature cycles and steep temperature gradients occur in the scanned layers. This residual stresses in Powder supply SLM machine condition Process design Machine setup SLM processing Quality control Heat treatment Quality control Machining Finishing and final quality control 20 3 METHODOLOGY The methodology used for the calculation of the stresses consists with three main parts. At the beginning, a simulation of the flow on the turbine with CFX was done to know the temperature and pressure generated on the micro-turbine surface. Once these values were known it was possible to import them to a Thermalsteady analysis and know how the temperature was distributed in the turbine geometry. At the end, the pressure from CFX and the temperature from Thermalsteady were imported to a Structural-steady analysis in order to get the stress generated in the micro-turbine. The study was carried out with the material properties of Inconel 625 manufactured by casting and with Inconel 625 manufactured by SLM. All the procedure can be seen in Figure 16, which shows the Workbench interface of the project, and all the parts created to calculate the final stress distribution of the micro-turbine. Figure 16: ANSYS project interface. 3.1 Geometry The geometry analysed was a radial turbine provided by the company HIETA Technologies, an additive manufacturing company. This geometry has a 21 diameter of 6.2cm and 11 blades. The geometry can be seen in Figure 17, which is composed by the turbine wheel and the subjection shaft. Figure 17: Isometric (left) and frontal (right) view of the radial turbine analysed. This geometry allowed to carry out a study with a real geometry, which gives more realistic results and the possibility to see the difference between the stresses generated, depending on the two types of manufacturing. 3.2 Material properties The material used for the comparison between the two types of manufacturing was as stated above Inconel 625. The properties depending on the temperature, to make even more realistic the results could not be obtained during this brief period of the thesis, and this could be one of the next steps of the project. Therefore, to carry out the study, the curves of the Young Modulus depending on the temperature were approximated for both materials. This approximation allowed to achieve the results of this thesis, but they should be compared with the real values, and it requires experimentation. 3.2.1 Conventional manufacturing: Casting The mechanical properties used for Inconel 625 manufactured by casting were obtained from the web page MatWeb (Appendix B) and are shown in Table 2. 22 Table 2: Main properties of Inconel 625 manufactured by casting. Mechanical properties at 20°C Tensile Strength [MPa] 836.3 Yield Strength [MPa] 390 Modulus Elasticity [GPa] 208 Poisson Ratio 0.28 Shear Modulus [GPa] 81.4 The Young Modulus used was 208GPa at 20°C, corresponding to Inconel 625 (Annealed 1180°C + Aged 760°C). This value was extrapolated to make it dependant on the temperature, following the same slope as the natural Inconel 625 properties (Appendix C). The final curve used for the Young Modulus (E) versus the temperature is shown in Figure 18. Figure 18: Approximated curve of the Young Modulus of Inconel 625 manufactured by casting versus the Temperature. 0 50 100 150 200 250 0100 200 300 400 500 600 700 800 Ecasting [GPa] Temperature [°C] Young Modulus Inconel 625 (Casting) 23 As can be seen in the plot above, the Young Modulus decreases about 50GPa with an increment of the temperature of 800°C. For the thermal study, the thermal conductivity was required to perform the analysis. Knowing that the thermal conductivity is approximately the same for the Inconel 625 manufactured by casting and by SLM, the following thermal conductivity, showed in Figure 19, was taken for both type of materials and was extracted from the properties of Inconel 625 showed in Appendix C. Figure 19: Thermal conductivity of Inconel 625 versus the Temperature. 3.2.2 Additive manufacturing: Selective Laser Melting The mechanical properties used for this material were taken from the CRP Meccanica web page, a CNC machining company. The data sheet can be seen in Appendix A and the main properties are stated in Table 3. Table 3: Main properties of Inconel 625 manufactured by SLM. Mechanical properties at 20°C Tensile Strength in horizontal direction XY [MPa] 827 Tensile Strength in vertical direction Z [MPa] 827 Yield Strength in horizontal direction XY [MPa] 414 0 5 10 15 20 25 0100 200 300 400 500 600 700 800 900 Thermal conductivity [W/(m·K)] Temperature [°C] Thermal conductivity Inconel 625 24 Yield Strength in vertical direction Z [MPa] 414 Modulus Elasticity in horizontal direction XY [GPa] 170 Modulus Elasticity in horizontal direction Z [GPa] 160 Maximum operating temperature under load [°C] 650 The Young Modulus was also approximated with the same criteria as the casting manufacture and following the slope of the Young Modulus in Appendix C, due to the impossibility to find specific and accurate data, for example the σ-ε curve depending on the temperature. Following the section 2.2.7.2, the difference for this type of manufacturing is that the Inconel 625 manufactured with SLM (stress relieved) introduce anisotropy to the piece and the properties change depending on the direction as can be seen in Table 3 and in Figure 20. Figure 20 Young Modulus of Inconel 625 manufactured by SLM versus the Temperature. 3.3 CFX This Computational Fluid Dynamics (CFD) software tool delivers reliable and accurate solutions for turbomachinery applications. And it is recognized for its outstanding accuracy, robustness and speed with rotating machinery (ANSYS, 2017). The CFX analysis is the first part of the model, and gives the conditions of pressure and temperature under the turbine works. 100 110 120 130 140 150 160 170 180 0100 200 300 400 500 600 700 800 E [MPa] Temperature [°C] Young Modulus SLM (stress relieved) XY direction Z direction 25 The main problem of this analysis was that the real conditions of this geometry were not known. Therefore, it was necessary to carry out an accurate analysis in order to realise which were the approximate operating conditions for this turbine, and making the simulation more reliable. 3.3.1 Fluid domain The first step was to construct the fluid domain. This domain was designed with Solidworks, and after imported to ANSYS. In ANSYS was used the operation Boolean to subtract the turbine geometry from the fluid domain, and the result is shown in Figure 21. Figure 21: CFX fluid domain for the turbine wheel geometry (left) and sectional view of the turbine and the fluid domain (right). The inlet height of the fluid domain is 7.8mm and the total length of the geometry 50mm. 3.3.2 CFX Mesh This was the most important part of the CFX analysis to get reliable and accurate results and it required a special attention. The final model was analysed without any simplification, because the appearance of many difficulties to apply circular symmetry and after import the results to ANSYS Thermal and ANSYS Structural due to the geometry given. 26 To catch the wall effects was necessary to calculate the first boundary layer for the mesh over the blades. The first step was to calculate the velocity at the inlet: 𝑣𝑖𝑛𝑙𝑒𝑡 =𝐹𝑙𝑜𝑤 𝐴𝑟𝑒𝑎 =155.57𝑚 𝑠 Equation 3 The dynamic viscosity of the fluid at 4bar and 800ºC is 4.579x10-5 Pa·s-1, then: 𝑅𝑒=𝜌∙𝑣𝑖𝑛𝑙𝑒𝑡 ∙𝐷ℎ 𝜇=261,191.5 Equation 4 Where 𝜌 is the density of the air, 𝑣𝑖𝑛𝑙𝑒𝑡 the velocity at the inlet, 𝐷ℎ the hydraulic diameter and 𝜇 the dynamic viscosity. To calculate the skin friction coefficient, the Blasius equation was used: 𝐶𝑓 =0.079·𝑅𝑒−0.25 =3.495𝑥10−3 Equation 5 Then, it was possible to calculate the wall shear stress and shear velocity (also called friction velocity): 𝜏𝑤=0.5·𝐶𝑓·𝜌·𝑣𝑖𝑛𝑙𝑒𝑡2=52.44 𝑃𝑎 Equation 6 𝑢𝑇=(𝜏𝑤 𝜌)0.5 =6.503𝑚 𝑠 Equation 7 Therefore, taking a y+ = 30, the first layer thickness had to be: 𝛿𝑦=𝑦+·𝜇 𝜌·𝑢𝑇=30·4.579𝑥10−5 𝑃𝑎 𝑠 ⁄ 1.24𝑘𝑔 𝑚3 ⁄·6.503𝑚𝑠 ⁄=1.7𝑥104 𝑚 Equation 8 The final input parameters for the mesh generation were a Size function of Proximity and Curvature, Fine Relevance Centre, Medium Smoothing, Slow Transition and Fine Span angle centre. The minimum face and edge size was 0.015mm and the maximum 3mm. The growth rate was the common value of 1.2. To generate the right boundary layer - Figure 22 -, the inflation mode was activated on the hub and blades surfaces, with an inflation option of First boundary layer equal to 0.017mm as calculated above. 27 Figure 22: CFX fluid domain mesh for the turbine wheel geometry. The final mesh had 764,048 elements and 1,582,908 nodes. The mesh sensitivity explained below was carried out to generate the optimum mesh and check the robustness of the model. 3.3.2.1 Mesh sensitivity A total of four meshes were analysed to get the optimum number of elements. The parameter analysed for the mesh sensitivity was the isentropic efficiency with an operating conditions close to the final expected values. The difference between the biggest mesh with 1,480,211 elements got a difference between the mesh three with 764,048 elements, of 0.34%. This value was lower than 1%, so was assumed that the isentropic efficiency converged. Figure 23 shows the isentropic efficiency against the number of elements, and can be seen that after 764,048 elements, the curve became almost constant. 28 Figure 23: Mesh sensitivity for the CFX model. 3.3.3 Boundary conditions The fluid domain mesh was imported to CFX in order to study the operating conditions and knowing the temperature and pressure distributions on the turbine wheel. The analysis was with a steady state. The basic boundary conditions defined were at the inlet and the outlet of the turbine. As can be seen in Figure 24 the inlet domain was marked with inflow arrows and the outlet domain as outflow arrows. Once the inlet and outlet were defined, the angular velocity was introduced, as well as the direction of rotation. The value of the rotation speed as explained in section 2.2.3, varies from 80,000 to 120,000rpm for the micro-turbines, so a rotation speed sensitivity was done in section 3.3.4, between the velocities 90,000 and 110,000rpm. The Figure 24 also shows the direction of rotation for the radial turbine. 81.5 82.0 82.5 83.0 83.5 84.0 84.5 85.0 85.5 86.0 200,000 600,000 1,000,000 1,400,000 Isentropic efficiency [%] Mesh Elements CFX Mesh sensitivity 29 Figure 24: Inlet and Outlet domain and the direction of rotation of the turbine. The fluid assigned was Air Ideal Gas, as the effect of the fuel was considered negligible, because as explained in section 2.2.3, is less than 1% of the total flow. The reference pressure was set at 1bar, the heat transfer was defined as Total energy and the turbulence model assigned was the Shear stress transport, the common model used for this type of turbines. The inflow/outflow boundary conditions were studied and, as the rotation speed, it was seen how the variation of this parameters changed the final operating conditions. Therefore, following the section 2.2.3, the value for the pressure at the inlet were studied between 2 and 4bar, and the temperature between 550 and 650ºC. The outlet pressure was set constant at 1 bar. For the inflow direction condition, a value of 70 degrees respect the radial direction was assigned, as seen in Figure 25. 36 3.5.1 ANSYS Structural mesh The micro-turbine mesh designed for the structural analysis - Figure 33 - differs from the thermal analysis mesh. It required more accuracy at zones where the stress was higher to catch all the effects. The most important refinement zone was the joint between the blades and the hub. This is the zone with higher stresses on the turbine (excluding the shaft because this thesis focuses only on the turbine geometry without the shaft). Therefore, an element size of 0.8mm was specified at this zone. Figure 33: Micro-turbine mesh for the Structural analysis. On all the corners between the junction of the blade-hub and the bottom part of the turbine, a sphere of 0.5mm of radius and a sizing of 0.08mm was applied, and some changes were observed before and after it. Moreover, a sizing of 1mm and hard behaviour on the shaft was applied to avoid small elements. The Size Function used was Proximity and Curvature, a Medium Relevance Centre, a Medium Smoothing, a Medium Span Angle Centre and Fast transition. The minimum size for the faces and edges was 0.08mm and the maximum 1mm. 37 The total elements for the optimised mesh was 960,582 as can be seen in the mesh sensitivity below. 3.5.1.1 Mesh sensitivity Since the zone of interest is the joint between the blades and the hub, the maximum stress generated on the joint at both parts of the blade was plotted. Figure 34 shows the mesh sensitivity carried out at this zone. Figure 34: Mesh sensitivity of the Structural analysis. The mesh sensitivity showed that the key point for the mesh elements was 960,582. The stress value at this point of elements differed from the last value less than 1%, then it was considered that the mesh converged. 3.5.2 Boundary Conditions The boundary conditions applied for the Structural analysis were the pressures imported from CFX, the temperatures imported from the Thermal analysis, and the subjection of the turbine. The pressure imported is shown in Figure 35, which was divided by the hub zone and the blades zone. 35 45 55 65 75 85 100,000 350,000 600,000 850,000 1,100,000 1,350,000 Stress [MPa] Mesh elements P1 max P2 max 38 Figure 35: Imported pressure of the hub (left) and the blades (right). The temperature imported is shown in Figure 36. It shows the body temperature of the turbine, which influences the stresses generated depending on the temperature of the point. Figure 36: Imported temperature from Thermal analysis. And the last input needed to specify was the subjection point of the turbine. It is shown in Figure 37. The exact subjection does not influence the stresses on the blade studied, since the stresses on the shaft were not studied. 39 Figure 37: Subjection point of the turbine. 40 4 RESULTS AND ANALYSIS 4.1 CFX The operating conditions for this geometry were not known. Therefore, to observe how the operating conditions influence the final stress generated on the geometry, a sensitivity analysis was done for the inlet pressure, the inlet temperature and the rotation speed. The inlet pressure was studied between 2 and 4bar and the temperature at the inlet between 550 and 650°C (the maximum working temperature of Inconel 625 is approximately 650°C). Moreover, each of these ranges were simulated for the rotation speeds between 90,000 and 110,000rpm. The outlet pressure was fixed during all the analysis at 1bar and the inflow angle at 70° with respect to the radial direction. The results obtained for the Efficiency and Power with a rotation speed of 90,000rpm are shown in Figure 38 and Figure 39. Figure 38: Efficiency versus Inlet pressure for a rotation speed of 90,000 rpm. 82 84 86 88 90 92 94 1.5 2 2.5 3 3.5 4 4.5 Efficiency [%] Inlet pressure [bar] 550°C, 90000rpm 600°C, 90000rpm 650°C, 90000rpm 41 Figure 39: Power versus Inlet pressure for a rotation speed of 90,000rpm. It is clear that when the pressure is 2bar, the efficiency does not change so much with the temperature variation, and the values are between 91.5 - 92.5%. However, when the pressure increases, the value for the efficiency starts to differ depending on the temperature, between 83 - 86%. The power plot shows that the temperature does not have a considerable influence on the output power. However, the output power increases considerably when the inlet pressure arises (between 12 - 40kW). Moreover, with an increase of the temperature the output power slightly increases, but the efficiency decreases. The same behaviour is shown in Figure 40 and 41, where the rotation speed was fixed to 100,000 rpm. However, the efficiency and power is slightly higher for this rotation speed than the efficiency and power for 90,000rpm. 10 15 20 25 30 35 40 1.5 2 2.5 3 3.5 4 4.5 Power [KW] Inlet pressure [bar] 550°C, 90000rpm 600°C, 90000rpm 650°C, 90000rpm 42 Figure 40: Efficiency versus Inlet pressure for a rotation speed of 100,000 rpm. Figure 41: Power versus Inlet pressure for a rotation speed of 100,000rpm. For a rotation speed of 110,000rpm, the behaviour obtained is quite different, especially when the inlet pressure is 2bar. The plots are shown in Figure 42 and Figure 43. 87 88 89 90 91 92 93 94 95 1.5 2 2.5 3 3.5 4 4.5 Efficiency [%] Inlet pressure [bar] 550°C, 100000rpm 600°C, 100000rpm 650°C, 100000rpm 10 15 20 25 30 35 40 45 1.5 2 2.5 3 3.5 4 4.5 Power [KW] Inlet pressure [bar] 550°C, 100000rpm 600°C, 100000rpm 650°C, 100000rpm 43 Figure 42: Efficiency versus Inlet pressure for a rotation speed of 110,000rpm. Figure 43: Power versus Inlet pressure for a rotation speed of 110,000rpm. Figure 42 and Figure 43 show that the efficiency and output power when the inlet pressure is 2bar and the rotation speed 110,000rpm is lower than the efficiency and power obtained for 90,000 and 100,000rpm at the same pressure. This behaviour is due to the inflow angle which is not the right one. Compared with the flow when the inflow angle is the right one (Figure 44), this situation generates a vortex (Figure 45), thereby a decrease of the turbine efficiency. 87 88 89 90 91 92 93 94 95 1.5 2 2.5 3 3.5 4 4.5 Efficiency [%] Inlet pressure [bar] 550°C, 110000rpm 600°C, 110000rpm 650°C, 110000rpm 5 10 15 20 25 30 35 40 45 1.5 2 2.5 3 3.5 4 4.5 Power [KW] Inlet pressure [bar] 550°C, 110000rpm 600°C, 110000rpm 650°C, 110000rpm 44 Figure 44: Velocity flow without vortex generation under operating conditions of Pin = 3bar, Tin = 650°C and rotation speed 110,000rpm. Figure 45: Velocity flow with vortex generation under operating conditions of Pin = 2bar, Tin = 550°C and rotation speed 110,000rpm. 45 4.2 Thermal analysis Since this step is not relevant for the study and is only a transition between the input parameters at the initial part of the model and the final stresses calculated at the output, in this section is explained only the temperature distribution for a turbine under the operating conditions of Pin = 3bar, Tin = 650°C and a rotation speed of 110,000rpm, which generates an output power of 29.5kW and an efficiency of 92.4%. The behaviour against the temperature can be extrapolated to the other operating conditions studied in this thesis. The results show that the maximum temperature is located at the inlet point of the turbine, where the hot inlet flow insides. The temperature decreases through the blade from the inlet to the outlet. For this case, the flow temperature at the inlet part of the turbine has approximately 620°C as can be seen in the Figure 46, and this flow generates a temperature on the turbine geometry of 614°C at the inlet part (Figure 47). Following the same criteria, the flow temperature at the outlet is 470°C, thereby a temperature of 440°C at the outlet part of the turbine. Figure 46: Flow temperature distribution on the turbine surface under operating conditions of Pin = 3bar, Tin = 650°C and rotation speed 110,000rpm. 52 and for Pin = 4bar the casting value of 5.97MPa is also higher than the SLM value of 5.82MPa. Figure 54: Stress vs X under different operating conditions, 100,000rpm and Inconel 625 (Casting). Figure 55: Stress vs X under different operating conditions, 100,000rpm and Inconel 625 (SLM). As it is done at the previous section 4.3.1, Figure 56 shows how the difference between the values of stress between the casting manufacture and the SLM manufacture decrease with the increment of the pressure and the stress value. 0 1 2 3 4 5 6 7 0 5 10 15 20 25 30 Stress [MPa] X [mm] Casting 550°C, 2bar 550°C, 3bar 550°C, 4bar 600°C, 2bar 600°C, 3bar 600°C, 4bar 650°C, 2bar 650°C, 3bar 650°C, 4bar 0 1 2 3 4 5 6 7 0 5 10 15 20 25 30 Stress [MPa] X [mm] SLM 550°C, 2bar 550°C, 3bar 550°C, 4bar 600°C, 2bar 600°C, 3bar 600°C, 4bar 650°C, 2bar 650°C, 3bar 650°C, 4bar 53 Figure 56: Difference between the values for Casting and SLM versus the pressure with 100,000rpm. 4.3.3 Structural analysis results for a rotation speed of 110,000rpm The same explanation than sections 4.3.1 and 4.3.2 can be used for a rotation speed of 110,000rpm, the plots in Figures 57 and 58 shows the same behaviour against the increment of pressure and temperature. However, for this rotation speed, the pressure when the inlet pressure is 2bar and the inlet temperature 550°C has a different behaviour than the other rotation speeds. This is because of the higher pressure on the blade at the inlet induced by the vortex generated. The points with maximum stress at the middle zone of the edge for this rotation speed are X = 7.32mm (Casting 0.66MPa) and X = 9.85mm (SLM 0.72MPa) for Pin = 2bar. For Pin = 3bar, the values are X = 7.33mm (Casting 1.78MPa) and X= 9.85mm (SLM 1.93MPa), and for Pin = 4bar are X = 7.33mm (Casting 3.18MPa) and X = 9.9mm (SLM 3.32MPa). The minimum stress for Pin = 2bar is located at X = 24.3mm (Casting 0.35MPa) and X = 18.5mm (SLM 0.26MPa). When Pin = 3bar, the minimum stress points are X = 22.1mm (Casting 1.05MPa) and X = 21.2mm (SLM 0.89MPa) and for Pin = 4bar the values are X = 22.1mm (Casting 1.56MPa) and X = 21.1mm (SLM 1.38MPa). 0 5 10 15 20 25 30 1.5 2 2.5 3 3.5 4 4.5 Difference [%] Inlet pressure [bar] 100,000rpm σmax σmax (middle zone) σmin 54 The maximum stress when Tin = 650°C appears also at the end of the path, for Pin = 2bar the casting value is 1.06MPa and the value for SLM is 1.04MPa. For Pin = 3bar is 3.41MPa for casting and 3.34MPa for SLM, and for Pin = 4bar, 4.92MPa is the maximum stress value for casting and 4.85MPa for SLM. Figure 57: Stress vs X under different operating conditions, 110,000rpm and Inconel 625 (Casting). Figure 58: Stress vs X under different operating conditions, 110,000rpm and Inconel 625 (SLM). 0 1 2 3 4 5 6 0 5 10 15 20 25 30 Stress [MPa] X [mm] Casting 550°C, 2bar 550°C, 3bar 550°C, 4bar 600°C, 2bar 600°C, 3bar 600°C, 4bar 650°C, 2bar 650°C, 3bar 650°C, 4bar 0 1 2 3 4 5 6 0 5 10 15 20 25 30 Stress [MPa] X [mm] SLM 550°C, 2bar 550°C, 3bar 550°C, 4bar 600°C, 2bar 600°C, 3bar 600°C, 4bar 650°C, 2bar 650°C, 3bar 650°C, 4bar 55 In Figure 59 can be seen the differences between the values studied for casting and SLM follow the same trend than the values studied in sections 4.3.1 and 4.3.2, but for 110,000rpm, the difference for the stress values at higher pressure are lower than the values obtained at 90,000rpm and 100,000rpm. Figure 59: Difference between the values for Casting and SLM versus the pressure with 100,000rpm. 4.3.4 Comparison The studied path points for the σmax at the middle zone in Figure 60 shows how the SLM manufactured turbine has always higher stress at these points, and it increases with the pressure. The trend for the stress at this point is to decrease with the increment of rotation speed. 0 5 10 15 20 25 30 1.5 2 2.5 3 3.5 4 4.5 Difference [%] Inlet pressure [bar] 110,000rpm σmax σmax (middle zone) σmin 56 Figure 60: Stress versus rotation speed for σmax (middle zone). Figure 61 with σmin shows approximately the same behaviour, but with a significant difference, the SLM manufactured turbine has always the lower minimum stress. The same behaviour can be seen in Figure 62, where the maximum stress is plotted for the different pressures and rotation speeds. It also shows how the stress for the casting is higher than the SLM manufactured turbine. This behaviour is due to the proximity to the free surface and the anisotropy of the material. Moreover, another important thing to explain is that the stress becomes also equal for both types of manufactures when the pressure and the rotation speed increase. 0 0.5 1 1.5 2 2.5 3 3.5 4 85000 90000 95000 100000 105000 110000 115000 Stress [MPa] Rotation speed [rpm] σmax (middle zone) Casting, 2bar Casting, 3bar Casting, 4bar SLM, 2bar SLM, 3bar SLM, 4bar 57 Figure 61: Stress versus rotation speed for σmin. Figure 62: Stress versus rotation speed for σmax. To conclude the structural analysis, section 4.3, Figure 63 shows how the difference between the SLM manufacture and the casting manufacture decreases when the stress increases. Furthermore, when the rotation speed arises, the difference decreases together with the pressure. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 85000 90000 95000 100000 105000 110000 115000 Stress [MPa] Rotation speed [rpm] σmin Casting, 2bar Casting, 3bar Casting, 4bar SLM, 2bar SLM, 3bar SLM, 4bar 0 1 2 3 4 5 6 7 8 85000 90000 95000 100000 105000 110000 115000 Stress [MPa] Rotation speed [rpm] σmax Casting, 2bar Casting, 3bar Casting, 4bar SLM, 2bar SLM, 3bar SLM, 4bar 58 Figure 63: Difference between casting and SLM results versus Inlet pressure comparison. 0 5 10 15 20 25 30 35 1.5 2 2.5 3 3.5 4 4.5 Difference [%] Inlet pressure [bar] σmax, 90000rpm σmax, 100000rpm σmax, 110000rpm σmax (middle zone), 90000rpm σmax (middle zone), 100000rpm σmax (middle zone), 110000rpm σmin, 90000rpm σmin, 100000rpm σmin, 110000rpm 59 5 DISCUSSION Several points can be extracted from the results and analysis section. Firstly, concerning the overall performance of the turbine. When the inlet pressure is low the overall efficiency is less influenced by the temperature, an increment of the inlet pressure generates an increase of output power, if the temperature rises the output power is higher but the overall efficiency decreases. Moreover, when the rotation speed increases, the inflow angle may change to avoid the vortex generation, because the presence of a vortex decreases the efficiency and the output power. Secondly, with reference to the operating conditions influencing the stress distribution, the stress on the blade base increases when the inlet pressure of the turbine increases, however, it increases more from 2 to 3bar than from 3 to 4bar. The inlet temperature generates an increase of the stress, but it is only noticeable at the inlet part of the turbine, where the temperatures and stresses are higher. Furthermore, if a vortex appears on the turbine blade, the stress at the inlet zone becomes higher. Thirdly, regarding the study of the impact introduced on a micro-turbine due to SLM manufacture. It includes the impact of the SLM manufacture compared with the investment casting, the traditional manufacturing and widely used for microturbines manufacturing. The difference between both types of manufacturing declines with the increase of the inlet pressure and with the increase of the rotation speed as it was seen in Figure 63. The higher stress difference between both types of manufacturing is reached by σmin, followed by the point with maximum stress at the central zone of the path studied and σmax respectively. Furthermore, the stress at the point with maximum stress at the central path zone is higher for the SLM manufacture in contradistinction to the σmin and σmax points. This behaviour is due to the proximity of σmax to the free surface and the anisotropy of the material. The decrease of the σmax for SLM manufacture influences the σmin point, generating also a lower stress for this point than the casting manufacturing. Therefore, the SLM manufacture generates lower stress at the inlet part of the turbine, and higher stress at the outlet. 60 Therefore, several conclusions are extracted from the study. The joint between the blade and the hub is the critical area of the turbine for the two types of manufacturing and the turbine manufactured by SLM has lower stress at the inlet part than the turbine manufactured by casting. However, the stress generated at the middle of the critical area is higher for the turbine manufactured by casting. This shows the necessity to consider the type of manufacturing before to start the production, in order to strengthen the areas affected by the anisotropy (specially the middle of the critical area, where the stress fluctuation is higher). Moreover, the increase of the stress in this area and the necessity to reinforce it, can create design problems. As well as, the reliability of materials manufactured by selective laser melting is lower than casting and the final results depend a lot on the right study of the optimum conditions (velocity, laser power, working atmosphere, etc.) of the manufacturing process. If these conditions are not the optimum, the variability of the mechanical properties introduced, generates more variability at the middle of the critical area, giving the possibility to appear higher stresses where it was not expected and in consequence higher fatigue. 61 6 CONCLUSIONS AND FUTURE RESEARCH The overall aim of this thesis was to determine the impact that additive manufacture have on a micro-turbine wheel. This was assessed through a Workbench 17.1 model, including a CFX analysis, a Thermal analysis and a Structural analysis to calculate the stress generated on the turbine. To conclude, the results obtained from the Workbench model are: • The critical area is the joint between the blade and the hub for both types of manufacturing. • If the type of manufacturing is SLM, it needs to be determined before the micro-turbine design to avoid problems at the middle of the critical area and reinforce it. • The stress fluctuation at the critical zone is higher for SLM and can increase the stress fatigue. • The reliability of materials manufactured by SLM is lower allowing the possibility to future problems. However, the results of this thesis are obtained with a CFD and FEA model and using an approximation of the material properties for both manufacturing processes due to the inaccessibility to the real data. Therefore, a further study should be carried out, studying through an experimental method the real properties and stresses generated, in order to compare them with the results of the model designed in this thesis. Moreover, this thesis can be complemented by the study of varied materials and several types of additive manufacturing, and finding what is the best for the micro-turbines manufacturing. Another improvement can be the study of the rugosity introduced on the microturbine surface due to the additive manufacturing processes, what can influence the aerodynamics of the turbine. 68 Appendix C : Inconel 625 properties C.1 Mechanical properties C.2 Thermal properties 69 Appendix D Mesh sensitivity results D.1 CFX D.2 Thermal D.3 Structural 70 Appendix E : CFX results 71 Appendix F : Structural data results for 90,000rpm from ANSYS Structural (example) The table below has the values for the stress on the path at the concave part of the blade (front), what was studied at the results section, and the convex part of the blade (Back). 72 73 74 75 76 77