Presented at the International Gas Turbine and Aeroengine Congress and Exposition Houston, Texas - June 5-8, 1995

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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St, New York, N.Y SThe 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. Authorization to photocopy material for internal or personal use under circumstance not failing within the fair use provisions of the Copyright Act is granted by ASME to libraries and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of $0.30 per page is paid directly to the CCC, 27 Congress Street, Salem MA Requests for special permission or bulk reproduction should be addressed to the ASME Technical Publishing Department. Copyright O 1995 by ASME Al Rights Reserved Printed in U.S.A. g5-gt-20q RESEARCH AND DEVELOPMENT OF 300kW CLASS CERAMIC GAS TURBINE (DEVELOPMENT OF THE STATIC CERAMIC COMPONENTS FOR CGT303) Mitsuharu Murota, Issei Ohhashi, Yoshiyuki Ito and Sadao Arakawa CGT Project Team Technical Research Division Yanmar Diesel Engine Company, Limited Kyoto, Japan ABSTRACT As the result of setting the low pressure ratio at 4.5, sizes of the static ceramic components forming the gas passage in CGT303 have been increased, and establishing reliability of these components was thought to be the most important task. So, the heatcycle tests were conducted, in advance of the engine operation, and improvements have been made on their material and constructions. After conducting 600 times of the heat-cycle tests, so far, up to the gas temperature of 1200 C, we have succeeded in the engine operation at the turbine inlet temperature of 1200 C. Examples of the problems encountered in the test and of the solutions therefore are introduced in this paper. 1. INTRODUCTION Development of 300kW class ceramic gas turbine is under way from 1988 as part of the "Moonlight Project" undertaken by Agency of Industrial Science and Technology in Ministry of International Trade and Industry (AIST/MITI), which has later been renamed as the "New Sunshine Program". The target performance is to achieve thermal efficiency of 42% or better at the turbine inlet temperature (TIT) 1350 C. Two means are available for improving the thermal efficiency on the small type turbines above that obtainable with diesel engines; the one is to raise the turbine inlet temperature and the other is to recover the exhaust heat. For the former objective, ceramic components of excellent hightemperature strength are used to eliminate the need for cooling, while for the latter, energy possessed by the exhaust gas, which has formerly been discharged to the atmosphere, is recovered as the temperature to the inlet of the combustor by way of the heat exchanger. The CGT303 being developed by YANMAR DIESEL Co., Ltd., NIIGATA ENGINEERING Co., Ltd., KYOCERA CORPORATION, NGK SPARK PLUG Co., Ltd., and NKK CORPORATION is intended for use as a power unit for mobile power generation. To fill the requirement of compact design for the engines intended for such mobile power generation application, a rotary regenerator has been adopted for the heat exchanger. This rotary regenerative heat exchanger, while comes in the compact design, enables to achieve the high thermal effectiveness reaching around 90%. Our study on the pressure ratio that could attain the maximum thermal efficiency by making full use of these features, estimated that setting it at 4.5 would be optimum. As the result of setting the low pressure ratio at 4.5, sizes of the engine components parts have been increased. As it was thought to be disadvantageous in the use of ceramics of the lower fracture toughness, securing enough strength as the engine components was set our primary concern. In this context, conducting the heatcycle tests on those ceramic components has been put in our schedule from the outset of our.development work. In this paper, centering around the results of the heat-cycle test, some problems encountered in connection with such ceramic components and some Presented at the International Gas Turbine and Aeroengine Congress and Exposition Houston, Texas - June 5-8, 1995

2 Fig. 1 Cross Section of CGT303 cases in which we have successfully solved such problems are introduced. It is hoped that these data can serve as the reference in applying such ceramics of which much is expected as the suitable material for high-temperature applications in future. These are part of the achievements made in the research and development effort funded by the New Energy and Industrial Technology Development Organization (NEDO). 2. CERAMIC COMPONENTS COMPRISING OF THE CGT303 Cross section of the engine is shown in Fig. 1, and the components thereof are shown in Photo 1. Material of these components and the processes used for fabricating them are shown in Table 1. Material of the three components, gas generator turbine (GGT) nozzle, shroud and back shroud, which had been 13-sialon, has later been changed to silicon nitride (Si3N4). Material of all the other components is Si 3N HEAT CYCLE TEST METHOD In view of the potential problems unpredictable on the theoretical study, the test conditions were made as close as practical to those to be encountered in actual engine operation; the same components as used in the engine itself were used for the ceramic static components and the engine casing, as shown in Photo 2. The same system as adopted for the actual engine was used for supporting those ceramic components. Application of the ceramic components was executed

3 Combustor GGT Nozzle (19pcs.) Exhaust Diffuser Shroud Backshroud PT Nozzle (29pcs.) Scroll Intermediate Duct A Intermediate Duct B Photo 1 Ceramic Components in two stages; in the first stage upstream components including the shroud were replaced with the ceramic components. After solving the problems encountered with the upstream side, the ceramic versions were adopted, as the second stage, for the components used in the downstream from the intermediate duct. Fig. 2 shows the configuration of the system used for the testing. To allow a simulation of heating air with the heat exchanger, a preheating burner is set up at the upstream of the air supply side. In the initial engine operation, however, installation of the heat exchanger was not in schedule, so no such preheating burner was used in any of the tests reported here. It means that the outside surface of those ceramic components opposite the gas passage came into contact with the air fed from the blower, kept at around 200 C (at the compressor outlet temperature in case of the actual engine). Combustion gas temperature was varied in the range from 600 C to 1200 C. In this way, the temperature field was set at nearly equivalent conditions as to be encountered in the actual operation. Based on the assumption that the equivalent heat flux could be obtained as in the actual operation just by keeping the same Nusselt number,

4 i Table 1 Materials of the Ceramic Components and Forming Processes Components Materials Forming Processes Combustor Si3N4 Drain casting Scroll A Si3N4 CIP Scroll B Si3N4 Drain casting Scroll C Si3N4 Slip casting GGT Nozzle Shroud Backshroud (3-sialon Si3N4 (3-sialon Si3 N4 l3-salon Si 3 N4 Low pressure injection molding + HIP Slip casting Low pressure injection molding + HIP CIP Low pressure injection molding + HIP CIP Intermediate Duct A Si3N4 CIP Intermediate Duct B Si3N4 CIP PT Nozzle Si3N4 Injection molding Exhaust Diffuser Si3N4 Slip casting + CIP Main combustor fuel flow control the air flow rate in the test system was set at around 900m 31hr, whereat nearly equal gas speed would be obtained. The temperature schedule was set with the operation data obtained on the metal gas turbine tested in advance taken as the reference. Start-up time: about 30 seconds Gas temperature in assumed idling period: 750 C Temperature rise at the rate of 150 C/min. after start-up Held at the specified temperature for about 10 to 15 minutes. Temperature fell at the rate of 150 C/min. Came to a shutdown after having been held at 750 C for two minutes. Forced cooling started after having been put in a natural cooling for two minutes. Fig. 3 shows the temperature schedule at the gas temperature of 1200 C. 4. TEST RESULTS About 600 rounds in total of the heat-cycle tests were executed in the range of gas temperature from 600 C to 1200 C. During the period some of the components were broken necessitating geometry change or material change. On the other hand, no breakage occurred on such large-size components as Primary combustor fuel flow control I Fuel tank Gas tempera ture controller Gas temperature (equivalent to TIT) I i Air valve controller L------^ Preheating air temperature (Primary combustion gas temperature) Preheating air generator (Primary combustor) Fuel flow control valve Fuel flow control valve Gear pump Gear pump Feed pump Filter Main body of the ceramic static components heat-cycle test system Motoroperated air valve Exhaust gas Motor-operated air valve / Air flow meter Exhaust gas Blower I i - (high temperature) Discharge _^ L Exhaust gas Cyclone cooling tank Motor-operated air valve V 1 Fig. 2 System Diagram of the Heat-Cycle test System Cooling water 4

5 Photo 2 External View of the Main Unit of the Heat-cycle Test System the combustor or scroll, on which their reliability had been our concern. Results of study made on the main ceramic static components, till they were ready for use in the actual engine are described hereunder. (1) Combustor It is planned that with priority given to assuring the dependable ignitability and flame stability, the conventional diffusion type combustor is to be used in the engine in the initial stage, which is to be replaced with a premixed lean-burn combustor intended for low NOx emission, when such will be developed. Accordingly, description here deals with the single-can type diffusion combustor. Fig. 4 shows the construction of the combustor and the supporting system adopted therefore. The combustor is pressed downwardly by means of the spring provided at the upper part thereof and the tapered portion at the bottom of the combustor is supported at the four points on the tapered portion. Ceramic fiber is put between the combustor and the supports. To avoid an excessive thermal stress, the combustor liner is divided into three sections, with each of the matching faces being designed to have a spherical surface, so that misalignment, if any, can be absorbed. The combustor liner will be subject to severe conditions due to the the high-temperature flame inside. In the case when the engine is stopped suddenly in particular, creation of a high tensile stress was predicted at around the dilution port because of the inflow of cold air through the dilution port. In consideration thereof, preliminary study was made on the combustor liner on the potential thermal stress to which it would be subjected and on the expected life thereof, in addition to the review on the optimal shape and division lines. Transient analysis was made assuming the case where the engine was stopped suddenly during the rated operation (at TIT= C), and all the gas temperatures was lowered to the room temperature momentarily. As a result, it was anticipated, as shown in Fig. 5, that the maximum tensile stress of 107MPa would be created along the inner edge of the dilution port in 30 seconds after the emergency stop. Using the above mentioned results and the material characteristic data obtained with the test pieces, reliability evaluation on the combustor was made by means of the CARES program. The CARES is the program developed by NASA Lewis Laboratory for calculating reliability for fast fracture, which allows evaluation including such factors as the density of cracks under the multi-axial stress, graduation in the principal stress direction and the dependence on the shearing sensibility. As a result of the separate evaluation made on the volume fracture and the surface fracture, with the calculation on the total fracture probability, 2.46 x 10-9 was obtained for the fracture probability on the combustor, as shown in Fig. 6, providing that the ceramic combustor has sufficiently high degree of reliability. To endose this prediction, no case of breakage was observed with the item throughout the heat-cycle tests conducted in about 600 cycles. Another problem with the combustor is falling off of the adhesive used for fixing the dilution port. Aiming at reducing the thermal stress, an annular type collar is fit in the combustor liner, on which potential falling off of the adhesive could cause FOD (Foreign Object Damage). In view of the proven results of free of breakage without adopting the collar the dilution port will be made directly in the combustor liner in future.

6 a) a) E) a) CI a) cc x W a) S.- / - V Gas temperature inside the scroll Gas temperature inside the exhaust chamber 1, 1 a) S... Q. E a) 0 to E 0 Air -temperature at the periphery of the scroll Time (min.) Fig. 3 Temperature Schedule for the Heat-cycle Test ur points) Dilution Fig. 4 Construction of Combustor and Supporting System 6

7 CO CO I- CO N E E X C Time elapsed at the time of emergency stop (sec.) Fig. 5 Result of Thermal Stress Analysis on the Combustor i !2 a Fig. 6 Result of Reliability Analysis on the Combustor 7

8 A B C Fig. 7 Scroll Split into Three Sections (2) Scroll The scroll is a large-size component of over 400mm in diameter yet of the thin wall (5mm). It was necessary that the component is to be split into segments along the lines that can reduce the thermal stress while facilitating for fabrication. Through the studies made to locate the best split lines, it has finally been decided to have the structure divided into three sections as shown in Fig. 7. Although the heavy deformation was noted with the sintered product in the initial stage, it has been improved through the great effort made by the ceramics manufacturer to give the scroll of the high dimensional accuracy as shown in Photo 3. In its original design, the tapered fit-up system had been adopted for the scroll mating face, intending the concentric alignment automatically of the scroll, GGT nozzle, shroud and the like components. Upon actual measurement of the movement of the scroll, however, it was found that the scroll B could be tilted during the cooling process as shown in Fig. 8. (In this case no GGT nozzle was fit.) It was ascertained upon disassembling that the high rigidity at the part of scroll B where the combustor was fit into the scroll B caused ununiform deformation, which caused the scroll A to turn about this local fit-up part. As it became clear that 1.2 ombustio n Gas Temperature D I AL.0 _4 DIAL 06 ^ r 0IAL 1 DIAL -0.2 r ' f DIAL C) DIAL-1 cz 200 C) ci E C) I DIAL Fig. 8 Movement of Scroll B Time sec. 8

9 Photo 3 Assembled Scroll this would not only fail to hold them concentrically as originally expected, but also detrimental to the GGT nozzle, the butt-joint part has been modified to the spigot joint. We had been concerned about the results of the heat-cycle test as the scroll C was hit with the hightemperature gas from the combustor, but none of the case of breakage was found as in the case of the combustor. (3) Backshroud The backshroud of the shape as shown in Fig. 9 is one of important components, with the scroll, shroud, GGT nozzle, etc. being supported with the four pins on its back face. Its original shape as shown in Fig. 9, left, with the grooves provided at the four places on its perimeter to accommodate the locating pins, which, however, caused cracks originating from the bottoms of the grooves in the molding process. When fit in the engine as well, it was estimated that high stress would be created at the groove bottoms (Fig. 10), so those grooves have been changed to the pin holes as shown in Fig. 9, right. As a result, it has become possible to reduce its outer diameter to solve the problems coming from the fabrication and stress point of view. The three components, backshroud, shroud and GGT nozzle, which are to be exposed to the high temperature and the high-velocity gas flow, had been (Original design) Fig. 9 Backshroud (Modified design) 9

10 3 Y $ 8I Pa Iu II IHMPa Fig. 10 Result of Thermal Stress Analysis on the Backshroud planned with (3-sialon of the low toughness but the high oxidation resistance. It became evident that 13-sialon have such drawbacks as the heavy dependence of its strength upon the surface roughness, and the resultant high thermal stress caused by its low thermal conductivity. So, it was judged that the use of (3-sialon would be risky for these three components which are exposed to the collision damage due to combustion residue or the like foreign matters. The material has been changed to Si 3N4 later and cleared the heat-cycle tests conducted at up to 1200 C. (4) Intermediate duct A On the heat-cycle test conducted at the gas temperature of 1000 C, the intermediate duct A was broken with the hole for locating pin as the starting point as shown in Photo 4. It was supposed a high thermal stress was created around the pin hole, added with the pin load accompanying the locating. To cope with this, the duct part was separated from the flange part, as shown in Fig. 11, to reduce the thermal stress, while adopting the metal (IN601, with the thermal barrier coating applied on the inner periphery) to prepare for unpredictable mechanical stress. Taking this opportunity, it has been tried to measure temperature of individual ceramic components. Because of the poor reliability proven with the attempts using thermo-paint or thermocouple sheet, the same method as used on the metal components was tried, burying a sheathed thermocouple into a small hole drilled in the ceramic components (applied with the ultrasonic processing). Photo 5 shows the thermocouple buried in this way, and Fig. 12 shows the example of measurement taken on the intermediate duct A. Using the data obtained by the actual measurement done in this way, the predictions on the thermal stress and deformation were reviewed, which have lead to the improvement. (5) Intermediate duct B As shown in Fig. 13, the intermediate duct B is supported with the main support (metal) and positioning in the axial direction is done with the two sets of the ceramic pins. The main support is made to slide against the engine case in the axial direction, so that it will press the ceramic component with springs provided at the flange of the main support toward the front side of the engine at a compressive force of about 800kgf. The thermal expansion of the ceramic components, etc. is absorbed by the sliding action of the main support. ' In the heat-cycle test conducted at the gas temperature of 1200 C, the locating pin for the intermediate duct B was broken as shown in Photo 6 10

11 Photo 4 Broken Intermediate Duct (Original design) Support plate Thermal barrier coating. Intermediate UT Duct A (Modified design) Fig. 11 Intermediate Duct A and the portion around the pin hole was damaged. Cause of this damage was surmised that the main support was stuck against the engine case, causing the locating pin to be subject directly to the thermal expansion of the ceramic components. As the countermeasure, an insulator (comprising of the ceramic fiber) was inserted between the intermediate duct B and the main support to prevent the radiant heat, while fitting the solid lubricant to the peripheral portion of the main support to help the smooth sliding motion between the engine case. Based on these experience, construction of the main support part was reviewed for the modification as shown in Fig. 14. In this modification, the function of pushing the upstream ceramic components including the intermediate duct A toward the front side of the engine was separated, and the intermediate duct B and the variable mechanism for the power turbine nozzle being secured to the engine case by way of the main support A. 5. CONCLUSION Through the number of improvements, we have finally succeeded in establishing the geometries of the ceramic components and the construction of the metal members supporting them that can clear the heat-cycle 11

12 r r ". r wa 7K Photo 5 Intermediate Duct with embedded thermocouples 1400 Thermocouple 1200 Combustion gas U 400 C) C Cooling air G 200 Q E I- 0 Intermediate Duct A Fig. 12 Example of the Result of Fig. 13 Detail of the Area around the Main Temperature Measurement Support according to the Original Design 12

13 1, 7 Photo 6 Broken Intermediate Duct B tests at 1200 C without breaking. Also, with these ceramic components assembled in the engine, we have succeeded in running it at the turbine inlet temperature of 1200 C for about 30 minutes. As described above, a promising outlook has been obtained for the use of the large-size ceramic components as the engine components. It is realized that ceramics are now reaching the stage where they could be used as engine components, as we observe them being free of breakage even when a number of holes were drilled for temperature measurement, or propagation of cracks terminating within those ceramic components. Our frank feeling that has reached through the cases of disassembly and reassembly is that they should have better toughness. In this sense, we hope there should be the attempts of material development in pursuit of improved toughness, rather than higher strength to be achieved on such static ceramic components. Fig. 14 Detail of the Area around the Main Support according to the New Design 6. ACKNOWLEDGEMENT In closing, we wish to express our deep appreciation of the great cooperation given to us by all the staff committed to the project. 13