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precision drilled cooling holes. Since a ceramic combustor wall operates at a higher temperature than a metal combustor wall, it helps to stabilize combustion process, especially at part-load conditions for lean premixed combustion, which is prone to combustion instability and flameout. Six ceramic materials were considered for the micro-turbine combustor, four monolithic ceramics; silicon carbide (SiC), silicon nitride (Si3N4), siliconized silicon carbide (SiSiC), alumina (Al2O3), and two ceramic matrix composites (CMCs)- Oxide/Oxide and SiC/SiC. Although CMC combustors have been successfully demonstrated on industrial gas turbine engines [5,6,7,8], their cost was found to be prohibitive for the cost sensitive micro-turbine. In addition, the small size and low stress expected in the micro-turbine combustor warranted monolithic ceramics as past engine experience has illustrated [9,10,11]. As a result, CMCs were not pursued for the ST5+ micro-tubine combustor. distributions are shown in Figure 3. The temperature is lower at the flange area, necessary for attachment, but higher at the main combustor can body. The temperature difference between metal and ceramic at attachment generates substantial thermal stress and insulation material was added to minimize heat flow from the ceramic can to the metal support, thereby raising temperature at the ceramic can attachment area while still maintaining a low temperature for the metal support. The four monolithic ceramics, they were ranked according to their strength, resistance to thermal shock, and cost of manufacturing. Al2O3 and Si-SiC were eliminated because of (a) 2081 1465 -2.0 17.8 low thermal shock resistance and low strength respectively. SiSiC was also found to be prone to creep at stress and temperature regime expected for the ceramic combustor can. In- situ toughened Si3N4 offers the highest strength and toughness, but it is more costly than SiC. Therefore SiC was selected as the primary candidate material for combustor can. The ST5+ has a single silo type combustor (see Figure 1) and the current metal combustor in the baseline configuration is impingement cooled. The metal can is supported at the fuel nozzle and is free to slide at its exit in and out of the combustion transition duct. The ceramic combustor adopted the same arrangement at exit, but special attention was paid to the attachment method at the fuel nozzle end in order to minimize thermal stress resulting from the thermal expansion difference between the metal support and the ceramic combustor. Figure 2: Two Ceramic Combustor Attachment Methods Two ceramic combustor cans were designed; both with a flange at the fuel nozzle end for attachment (see Figure 2). One design has a right angle flange, while the other design has a 45- degree flange. The ceramic cans are clamped down to a metal support through a spring that purports to absorb the thermal expansion mismatch between the can and the metallic support. Extensive thermal and stress analyses were performed to ascertain the temperature, thermal stress and probability of failure. The temperature and maximum principal stress Figure 3: (a) Temperature distribution (F) and (b) maximum principal stress (ksi) in Ceramic Combustor Can The final results for the two designs are summarized in Table 1, where three materials other than SiC are also included for comparison purpose. The ceramic can design with a 45 degree flange has a lower thermal stress and corresponding failure probability than the 90 degree flange design because of its lower constraint on the thermal expansion of the hotter main ceramic can body. Table 1: Maximum Temperature and Stress in Two Ceramic Combustor Designs In order to determine if there were any issues with acoustic vibration, baseline combustor configuration tests were performed. The measured frequency-dependent impedance was incorporated in a thermo-acoustic model, which predicted that at 100% power, system instability most likely exists near 480 Hz. Due to the high stiffness and low density, the first natural frequency of the ceramic can is 1605Hz; therefore well above the excitation frequency. (b) Material Max./Min. Max.Stress Temp (oF) (ksi) Honeywell AS800 Hexoloy SA SiC CoorsTek SCRB-210 Kyocera SN282 90 Degree Flange 2103/1330 14.6 2081/1465 17.8 2090/1391 16.6 2103/1330 11.3 Design 45 Degree Flange Max./Min. Max. Stress Temp (oF) (ksi) 2103/1320 7.9 2080/1457 7.8 2090/1392 8.1 2103/1320 6.1 107 Copyright © 2004 by ASMEPDF Image | ADVANCED MICROTURBINE SYSTEMS Final Report for Tasks 1 Through 4 and Task 6
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