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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St, New York, N.Y The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed In its publications. Discussion is printed only If the paper is published in an ASME Journal. Papers are available from ASME for 15 months after the meeting. Printed in U.S.A. Copyright 1994 by ASME 94 GT 309 DEVELOPMENT OF CERAMIC ROTOR BLADE FOR POWER GENERATING GAS TURBINE Tetsuo Teramae and Yutaka Furuse Engineering Research Center Tokyo Electric Power Company, Inc. Tokyo, Japan Katsuo Wade Hitachi Works Hitachi, Limited lbaraki-ken, Japan Takashi Machida Mechanical Engineering Research Laboratory Hitachi, Limited lbaraki-ken, Japan ,L ABSTRACT To cope with the increasing demand of electric power, many research and development programs have been performed in the field of electric power industry. Among them, the application of highly thermal resistive ceramics to hot parts of the gas turbines is one of the most promising ways to raise the thermal efficiency of the gas turbine, and several projects have been executed in the U.S.A., Europe and Japan. Tokyo Electric Power Co., Inc. (TEPCO) also has been conducting a research project to apply ceramic components to hot pans of a 20MW class gas turbine with a turbine inlet temperature of 1300C. In this project. TEPCO and Hitachi have been conducting the cooperative research work to develop a first stage ceramic rotor blade. After several design modifications, it was decided to select ceramic blades attached directly to a metal rotor disc, and to insert metal pads between the dovetail of the ceramic blade and metal disc to convey the centrifugal force produced by the blade smoothly to the metal disc. The strength of this ceramic blade has been verified by a series of experiments such as tensile tests, room temperature spin tests, thermal loading tests, and high temperature spin tests using a high temperature gas turbine development unit (HTDU). In addition, the reliability of the ceramic blade under design and test conditions has been analyzed by a computer program GFICES (Gas turbine - Fine Ceramics Evaluation System) which was developed on the basis of statistical strength theory using two parameter Weibull probability distribution. These experiments and analyses demonstrate the integrity of the developed ceramic rotor blade. INTRODUCTION In modern thermal power stations, combined cycle power plants have been constructed and operated, since they have high thermal efficiency and excellent load adjustment capability. The thermal efficiency of the combined cycle power plants have been raised mainly by increasing the turbine inlet temperature (TIT) of the gas turbine. The increase of TIT itself has been attained by the introduction of superalloys along with advanced turbine cooling technologies. The increase in TIT, however, causes problems associated with the increase in NOx emission. Lean-premixed combustion is one of the most promising NOx abatement systems. In this system, a large portion of compressed air is utilized for premixing with fuel, and this leads to lack of cooling air. It seems, therefore, that the increase in TIT of gas turbines will reach its limit in near future as far as air cooled metal components are used. Ceramics, especially the silicon based nonoxide ceramics such as silicon carbide (SiC) and silicon nitride (Si 3N,), have very attractive combinations of physical and mechanical properties at high temperatures. The application of these ceramics enables gas turbines to use increased TITs with a small amount of cooling air of hot parts, so that it becomes possible to raise the thermal efficiency of the gas turbine. This will also bring about considerable fuel savings, increase of output power, and low NOx emission. These advantages have caused a strong drive towards the application of ceramic components to gas turbines. Since 1984, TEPCO has been conducting a research and development project for the application of ceramics to hot parts of a power generating gas turbine in cooperation with Toshiba Corporation, Mitsubishi Heavy Industries, Ltd.. and Hitachi. Ltd., (HARA et al, 1987). The objective of this project is to verify the adaptability of ceramics for a three stage gas turbine which is designed to have an output of 20MW and a TIT of 1300C. Ceramics were considered for components which are exposed to hot combustion gas, such as the liner and transition piece of the combustor, the first and second stage nozzle vanes, and the first stage rotor blade. Among these components, the developments of ceramic combustor and nozzle vanes had been Presented at the International Gas Turbine and Aeroangine Congress and Exposition The Hague, Netherlands June 13-10, 1994 Downloaded From: on 01/24/2018 Terms of Use:

2 completed successfully and the details of.these researches were presented in other papers (MAEDA et al, 1991, and MIYAZAKI etal. 1991). TEPCO and Hitachi have been conducting a research program to develop a first stage ceramic rotor blade. Since rotor blades are required to have very high reliability for the integrity of gas turbine, the research has been carried out most carefully and several design modifications of ceramic blades have been made to reflect a variety of strength test results. In this paper, the authors show a series of design modifications of a ceramic rotor blade, and also present the results of component tests such as tensile tests, room temperature spin tests, static thermal loading tests and high temperature spin tests. In addition, the reliability of the developed ceramic rotor blade is evaluated relative to design and test conditions using a reliability analysis program, named GFICES, which was developed based on Weibull probabilistic strength theory in the course of this project. DESIGN OF GAS TURBINE AND THE FIRST STAGE ROTOR BLADE Based on the preliminary design conducted during the period, the heat balance of a model gas turbine at the design point was decided as shown in Fig. I. The output power of 20MW was selected as the target of the project because this class seemed to be suitable to apply developed techniques to 100MW or 200MW class power generating.gas turbine. and creep strength at elevated temperatures, they have disadvantages such as lower fracture toughness, lower thermal shock resistance and a wider scatter of fracture strength compared to metal materials. The design of ceramic components must be done carefully taking account of these disadvantages. As for the first stage rotor blade, we decided not to adopt an all ceramic rotor but a hybrid type rotor which consists of ceramic blades and a metal rotor disc. The fundamental subject of this hybrid type structure is how to mount ceramic blades to a metal rotor disc. This structure must satisfy the following requirements. (1) Prevention of high local stress concentration at the root of the ceramic blade. (2) Heat insulation between ceramic blades and metal disc. (3) Adequate damping characteristic against blade vibration. (4) Sealing mechanism to prevent the inverse flow of hot combustion gas. RESEARCH AND DEVELOPMENT OF HYBRID TYPE ROTOR BLADE STRUCTURES Among these requirements, the most important problem in developing a ceramic rotor blade is how to minimize the stress on the blade induced by the centrifugal forces of rotation. Moreover, if the ceramic blade is designed to be in contact with the metal, the temperature of the ceramic should be limited to below around 600C at the contact area to avoid chemical reaction between them. Taking these points into consideration, three types of hybrid structure as shown in Fig. 2 have been investigated. IS? Ceramic Blade Ceramic Blade Ceramic Compliant Layer Maud Shenk Metal Pad Metal Disc Metal Disc Type A Type Et TWO C Fig. 1 Heat balance at design point Fig. 2 Hybrid structures of ceramic rotor blade According to this preliminary design, the design condition at the inlet of the first stage rotor blade is given as the relative gas temperature of 1150C on the average and the gas pressure of 9.5 ata. The pitch circle diameter and the blade height are specified as 790mm and 58mm, respectively. Although ceramics show the superior characteristics of heat resistance, oxidation resistance, wear resistance, fracture strength Rotor blade structure with a ceramic compliant laver Firstly, the structure of type A in Fig. 2 was investigated. It consists of a ceramic blade, a metal shank and a ceramic compliant layer. SiC was initially selected as the candidate 2 Downloaded From: on 01/24/2018 Terms of Use:

3 material of blade because of its excellent properties in creep strength and oxidation resistance. The shank is mounted in the metal disc using the conventional fir-tree attachment. The gap between ceramic blade and metal shank is filled up with a ceramic compliant layer which is made up of ceramic fabric injected with ceramic adhesive and then sintered. Fig. 3 shows the photograph of this structure. Then, spin tests were performed at room temperature using the specimens which have simplified plate-type airfoil. Fig. 4 shows the test results. It was revealed that most of the type B specimens made of S1 3N4 and Sialon were able to endure the centrifugal force up to 120% of the rated rotational speed. And. some of the blades could survive 140% of the rated speed. 0 Sw,,..d j TYP0.13:_Auochment_with Metal...111mA Fasled I I 61, ,11. III Fig. 4 Results of room temperature spin tests Fig. 3 Type A structure of ceramic rotor blade on type B structure Although this structure has good properties on the points of thermal insulation and prevention of high local stress concentration on the contact surfaces, room temperature spin tests of SiC blades resulted in some fractures at the root of dovetail below the rated speed because of the lack of material strength. Furthermore, the long term reliability of the ceramic compliant layer was investigated by tensile tests. It was found that the ceramic compliant layer could endure the load corresponding to the rated speed under static loading but could not endure the same load when it was repeated cyclically. Rotor blade structure with metal pads Secondly, the type B structure shown in Fig. 2 was developed reflecting the weakness of type A structure, and loose attachment was adopted using two metal pads instead of the ceramic compliant layer. The contact surface between the metal shank and metal pads is formed into a cylindrical shape and the ceramic blades can slide on this contact surface and no additional stress due to local constraint occurs. In addition to SIC, Si 3N1 and Sialon were also selected as candidate materials for the ceramic blade. Tensile tests were performed both at room temperature and at 600C. The specimens were designed to have root configurations on both ends in order to apply the tensile load. The test results at room temperature showed that the specimens of Si 3N4 and Sialon endured the static loading up to 140% and cyclic loading up to 120% of the rated speed. The tensile test at 600C showed slightly lower strength than at room temperature. Although the type B design showed higher strength compared to type A, spin tests revealed that the fracture of ceramic blades originated at the place of contact with the pads. Numerical analyses showed that non-uniform contact was caused by the opening deformation of the shank. This result indicated the necessity to increase the rigidity of shank in order to reduce the local stress concentration. However, this was very difficult since the size and configuration of shank is limited by the requirement of airfoil pitch. Rotor blade structure directly attached to metal disc The type C structure shown in Fig. 2 was evaluated. In this structure, the metal shank is eliminated and ceramic blades are directly mounted to a metal rotor disc. The attachment is also done by inserting metal pads. This design is able to reduce the local stress at the root by preventing the non-uniform contact. The configuration of the root was investigated by FEM stress analyses and experiments. These studies showed that a decrease in the angle between the contact surface and radial direction resulted in a decrease in peak stress at the root of the dovetail and an increase in contact stress. The angle of the contact surface was finally selected as 45 degree by considering the above analytical results and the balance between normal force and shearing force on the contact surface. Fig. 5 shows the detail of the type C structure. A small amount of cooling air is introduced between the ceramic blade and the metal disc in order to provide heat insulation and to prevent the inverse flow of hot gas. Moreover, gaps along the edges of the blade platform are automatically sealed by pins of 3 Downloaded From: on 01/24/2018 Terms of Use:

4 Table 1 Material properties of a candidate Si Temperature ( C) R.T Density [g/cml 3.5 Young's modulus [Gpal Poisson's ratio Coefficient of thermal expansion 11/ Cx Thermal conductivity [W/m Specific beat [Jig IC] RS 4-point bending strength fh4pal Weibull modulus 20 Ceramic Rotor Blade Ceramic Seal Pin Metal Pad Cooling Air Tensile tests were performed both at room temperature and at 600C to examine the strength of the dovetail. Fig. 6 shows the specimen configuration for the tensile tests. As shown in Fig. 7, all specimens exceeded the strength corresponding to 140% of the rated speed. Room temperature spin tests were performed to confirm the reliability of ceramic blades subject IO centrifugal force. The test results showed that most of the type C specimens could survive the centrifugal force up to 140% of rated speed as shown in Fig. 8. These tensile and room temperature spin tests proved that the type C structure has a safety factor of 2 relative to the rated centrifugal force in fast fracture mode. 0 r1 4 Fig. 5 Detail of type C structure of ceramic rotor blade cylindrical ceramic bar, which are pushed and held automatically at the proper positions by their own centrifugal forces. In order to locate blade and pads at their correct positions at static conditions, a plate-type spring is inserted between the bottom of the blade and the metal disc. Two kinds of Si 3N4 were finally selected as the candidate materials for the ceramic blade, since Si 3N4 has shown much progress in its strength and heat resistance at elevated temperatures. Table 1 shows the properties of one of the candidate Si 3N4 materials. 52 Fig. 6 Configuration of tensile test specimen 4 Downloaded From: on 01/24/2018 Terms of Use:

5 Zse Following these strength tests, the thermal stress analyses and thermal loading cascade tests were performed in order to verify the reliability of the ceramic blade subjected to thermal load. Figs. 9 and 10 show the variation of calculated maximum temperature and maximum principle stress of the ceramic blade on the trip condition. The maximum thermal stress of 360MPa is generated at about 5 seconds after the shutdown of fuel. This stress is well below the material strength. P 1200 CI a 1000 CI Trip Time (s) Ratio of fracture load to rated centrifugal force Fig. 9 Calculated variation of maximum temperature on trip condition Fig. 7 Weibull plots of tensile test results on Si 3N4 specimens of type B structure Ro ta t io na l Speed MI ' and Typo_CLDiroct Allochinent_to. Metal_Ditt gra Failed Ma x imu m p r inc ip a l s tre ss (M Pa ) t Trip 1 1 I I I j I 5 10 Time (s) 7 ID i.14.0 Si,N PO Fig. 8 Results of room temperatu e spin tests on type C structure Fig. 10 Calculated variation of maximum principal stress on trip condition 5 Downloaded From: on 01/24/2018 Terms of Use:

6 Table 2 Comparison of design condition and HTDU specification Design HTDU Average gas temperature at combustor exit 1,300 C 1,300 C Gas pressure at combustor exit 15 ata 3 ata Average gas temperature at rotor inlet 1,150 C 1,080 C Rated rotation speed 10,800 rpm 11,264 rpm Circumferential speed at PCD 413 m/s 413 m/s Thermal loading cascade tests were carried out under the gas condition of design temperature and design pressure in a static test rig. A cascade of five ceramic blade specimens with two side ceramic blades were set up into a metal base which has the same root configurations as the rotor disc. The tests were conducted two times. Firstly the steady state test followed by normal shutdown and secondary followed by the trip. After each test, the cascade was disassembled and the ceramic blades were examined carefully by visual appearance check and penetrant inspection. Fig. II shows the cascade of ceramic blades after the trip test. No cracks were found and the ceramic blades were confirmed to be sound against the severe thermal transient load caused by the trip. Since the strength reliability of type C structure were examined against centrifugal force and thermal transient stress, it was decided to conduct high temperature spin tests using the high temperature gas turbine development unit (HTDU), which is an one stage gas turbine. Table 2 shows the fundamental specifications of the HTDU test. Fig. 12 shows the setup of ceramic blades to the metal rotor disc. Preceding this test, room temperature spin tests were conducted as a proof test of ceramic blades up to 120% of design rated speed. All tested blades survived without any sign of damage detectable by penetrant inspections. Six ceramic blades were mounted to a metal rotor disc. To maintain the rotating balance, two sets of three blades were placed axi-symmetrically. Fig. 12 Setup of ceramic blades to metal rotor disc Fig. 11 Cascade of ceramic blades after the trip test The tests were performed four times totally as shown in Table 3. At first, a spin test by compressed air was performed up to the rated speed, and the vibration of the rotor shaft and ceramic blades were found to be very small and well below allowable limits. The other three tests were conducted using combustion gas up to the design rated speed. After these tests were completed, the rotor was disassembled and inspected. As to the 6 Downloaded From: on 01/24/2018 Terms of Use:

7 Table 3 The condition of high temperature spin tests by HTDU Test I Test 2 Rotational test up to 102% of rated speed by compressed air of 200 C Combustion test up to rated speed at 1100 C for a half hour and 1200 C for one and a half hours, and then normal shutdown Test 3 Combustion test up to rated speed at 1300 C for 2 hours, and then normal shutdown ' Test 4 Combustion test up to rated speed at 1300 C for 2 hours, and then fuel damp ceramic blades, there were no detectable cracks or damage. But, the plate-type spring showed some permanent deformation due to its own centrifugal force. So, it is necessary to make some design modification on this spring. RELIABILITY ANALYSES OF CERAMIC BLADE Although ceramics have very attractive combinations of physical and mechanical properties at high temperatures, their toughnesses are very low and their strengths show wider scatter compared to metal materials. These undesirable properties create the necessity of 'design by analysis" of ceramic components. As a part of TEPCO's project, therefore, a program GFICES was developed for the strength evaluation of structural ceramic components on the basis of statistical strength theory using two parameter Weibull probability distribution (TERAMAE et al, 1993). This program is coupled as a postprocessor to the general purpose structural analysis program MSC/NASTRAN. The reliability of the ceramic rotor blade was evaluated for the fast fracture mode under the assumption that ceramic blades contain random volume flaws. At first, stress analyses were conducted corresponding to design and experimental conditions, and then the reliability analyses were performed. Table 4 summarizes the safety factors which are defined as the ratio of the load corresponding to failure probability of 0.5 to the specified load. As shown in this table, the safety factor of normal design condition was raised almost twice by conducting 120% spin test at room temperature as a proof test. The design trip condition is the most severe for the ceramic rotor blade and the safety factor on design trip condition is The thermal loading test was proven somewhat effective in raising the reliability on the design trip condition. CONCLUSION In this paper, the authors reported on the research and development of the first stage ceramic rotor blade for a 20MW class industrial gas turbine with a TIT of 1300C. The concept of hybrid structures which consist of ceramic blades and a metal disc was adopted, and the strength of the ceramic rotor blade was evaluated analytically and experimentally. The main results of this research are as follows. (1) The hybrid structure with a ceramic compliant layer was proposed first. Although it has excellent performance in heat insulation, cyclic loading tests revealed that the strength of the ceramic compliant layer was not enough. (2) Secondly, the hybrid structure with metal pads was investigated. It showed the higher reliability against monotonic and cyclic centrifugal forces. However, the opening deformation of the shank caused non-uniform contact between the dovetail of ceramic blade and metal pads, and higher stress concentration was generated at the root section of the ceramic blade. (3) Therefore, it was decided finally to adopt the design concept in which ceramic blades are mounted directly to a metal disc. SOL was selected as the candidate material since its mechanical properties have. been improved extensively. Tensile tests and room temperature spin tests showed the excellent reliability of this structure, and a safety factor of 2 against centrifugal force was confirmed. Moreover, the thermal loading cascade tests revealed the soundness of ceramic rotor blades against a trip condition. (4) To verify the soundness of the ceramic rotor blades at high temperatures, hot spin tests were conducted using a high temperature gas turbine development unit. It was found that ceramic blades directly mounted to a metal rotor disc can endure the centrifugal force up to the rated design speed at elevated temperatures. (S) The reliability of ceramic blade was numerically examined using the reliability analysis program GFICES. The analyses showed that the spin test and the thermal loading test raise the reliability of ceramic blades to a great extent. In addition to the preceding developments of combustor and stator nozzle vanes, the development of the first stage ceramic rotor blade was successfully completed, and the initial target of the project has been accomplished. During this research and development program, however, the technologies related to gas turbines have shown much progress. and the 1300C class gas turbine has come to be manufactured using metal components for combined cycle power plants. Therefore, although the superiority of ceramic components to metal ones in performance and thermal efficiency of gas turbines is still valid, it seems suitable to change our target for the application of ceramic components to gas turbines with much higher TIT than 1300C. It seems that the material characteristics 7 Downloaded From: on 01/24/2018 Terms of Use:

8 Table 4 Numerical results on the safety factor of the ceramic rotor blade Condition Proof test condition Safety factor Room temperature spin test 120% of normal design speed Thermal loading test Steady state Trip condition High temperature spin test Steady state Trip condition 120% room temperature spin test Design condition Normal condition 2.21 Steady state of thermal loading test % room temperature spin test and steady state of high temperature spin test Trip condition Trip condition of thermal loading test 120% room temperature spin test and trip condition of high temperature spin test of ceramics have been improved year by year. So, it is now planned to conduct a research program evaluating the applicability of both monolithic ceramics and ceramic composite materials for gas turbines operating at much higher TITs than 1300C. REFERENCES Hara, Y., Hisa, S., Wada, K., and Tsuji, I., 1987, "Research and Development of Ceramic Components for Power Generating Gas Turbine," JSME Reports No , pp Maeda, F., Okabe, N., Tshuchiya, T., and Watanabe, N., 1991, "Development of Ceramic Combustor for Industrial Gas Turbines," Proceedings of the 1991 Yokohama International Gas Turbine Congress, Vol. 3, pp Miyazaki, S., Tsuji, 1., Mori, M., Hara, Y., Furuse, Y., and Hamada, S., 1991, "Development of Ceramic Vanes for an Industrial Gas Turbine," Proceedings of the 1991 Yokohama International Gas Turbine Congress, Vol. I, pp Teramae, T., Hamada, S., and Hamanaka, J., 1993, "Probabilistic Structural Analyses of Ceramic Gas Turbine Components," IUTAM Symposium on Probabilistic Structural Mechanics - Advances in Structural Reliability Methods, in printing. 8 Downloaded From: on 01/24/2018 Terms of Use: