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Table 2: Fast Fracture Failure Probability of IBR under Four Loading Conditions 60000 58000 56000 54000 52000 50000 48000 46000 Rotor Maximum Principal Stress 0 5 10 15 20 25 30 35 Time (sec) CARES/LIFE Analysis of ST5+ Ceramic IBR IBR Location / Strength Combined Probability of Failure for IBR Average values were used for N Charac. Strength ksi (Weibull Modulus) 90.79 (14.73) 71.44 (14.14) 30.71 (8.93) - T154 strength and Weib 4.32E-05 3.78E-04 3.71E-01 3.71E-01 ull modulus Probability of Failure Cold Spin Hot Spin Transient Transient Shut-down Start-up 1.51E-04 1.46E-03 1.17E-01 1.18E-01 1.11E-04 1.19E-03 1.10E-01 1.11E-01 9.61E-04 Bore and Disk Volume / Longitudinal ground Disk Surface / Transverse ground Blade / As processed 2.00E-08 3.10E-07 9.61E-04 Figure 9: Emergency shutdown maximum principal stress as a function of time Ceramic IBR Life Prediction The NASA CARES/Life code [9] was used to predict the probability of failure for the final IBR design. NT154 silicon nitride from Saint Gobain Ceramics was selected as the material for IBR. For this material, as-processed material has a different strength and Weibull distribution than longitudinally or transversely ground material. Additionally, the strength and Weibull modulus is a function of temperature. Since the IBR will have all three types of surfaces as well as a temperature distribution during operation, the FEA model was partitioned into three sets to more accurately predict the probability of failure. Figure 10 shows the partitioning of the FEA model for ceramic life prediction. CERAMIC SHROUD d (Longitudinally Ground) Due to the low thermal expansion of silicon nitride materials, the ceramic IBR of ST5+ would grow thermally approximately 1/3 that of the metallic ST5 compressor turbine rotor. In addition, the low density and high stiffness of the ceramics makes the ST5+ ceramic IBR expand less than the ST5 metallic rotor under centrifugal forces. Combining these two factors together, a large gap could be expected between the ceramic IBR and the metal shroud. This could result in severe blade tip flow losses that can be translated to low turbine and engine efficiency. To assess the effect of such loss, a tip clearance analysis was conducted for three shrouds: metal shroud, SiC and Si3N4 shrouds. These analyses included checking pinch points, where clearance might go to zero or interference, during startup. Aero thermal heat transfer to the shroud, and its effect on shroud absolute temperature was also determined and added to the analysis. The analysis showed that if a metal shroud is used with the ceramic rotor, excessive leakage would ensue due to the large tip clearance of 0.0261in. However if a ceramic shroud is employed, the tip clearance is reduced to 0.0085in for SiC and 0.0062in for Si3N4. For small engines such as ST5+, a 0.001 in tip clearance equates 0.3% turbine efficiency. Therefore, use of a ceramic shroud may result in a 0.45% increase in turbine efficiency. Four ceramic design concepts for the turbine shroud were generated and down selected to two. Detailed thermal and stress analyses were performed to determine which of the two concepts would be better as the final design. Due to the high rotation speed of the rotor blades, heat transfer coefficients for the shroud were determined as a function of the engine axial length only. Sink temperature variation in the circumferential direction assumed the same profile as predicated by CFD at the exit of the scroll. Considering the averaging effect resulting from the rotor blade rotation, such a profile represents a worse case. Thermal distortion of the shroud support, i.e., the turbine support casing or the vane ring outer platform, strongly influences thermal stress in the shroud. Steady state thermal analysis showed that the OD platform of the vane grows radially by 0.0158 in under hot streak conditions and 0.014 in under average conditions. This means the maximum vane ring outer B B o or re ea an n d D Di is s k k V V o o l lu u m me e Blade (As processed) Disk Surface (Transverse Ground) Figure 10: IBR FEA model partitioned into three sets for CARES reliability analysis Table 2 contains a summary of the fast fracture reliability results. It is apparent from examining the table that the cold spin case has the highest probably of failure. The cold spin conditions would only be encountered during rotor proof testing. The high probability of failure is due to the large hoop stress in the rim. This stress is greatly reduced as the rotor heats up and develops a thermal gradient. The probability of failure of the blade and rim would be reduced to approximately 0.1% if the blade surfaces were machined. This would obviously have an adverse effect on overall IBR cost. In all cases the probability of failure is controlled by the blade and rim. 110 Copyright © 2004 by ASME Stress (psi)PDF Image | ADVANCED MICROTURBINE SYSTEMS Final Report for Tasks 1 Through 4 and Task 6
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