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CERAMIC VANE DESIGN AND ANALYSIS As stated in [4], there are two major reasons for selecting a vane ring rather than individual vanes: cost of manufacturing and reduction of sealing area. The primary loading on the ceramic vane ring is thermal in nature and transients during start-up and shutdowns present the worst cases. Therefore, the focus of the more recent work was to reduce the high stresses during transient conditions by design changes. The effects of platform thickness and attachment ring location on transient stresses were evaluated parametrically. The analysis also accounted for the presence of an environmental barrier coating (EBC) on the vane. A significant reduction in the transient stress from 59 ksi to ~26 ksi was achieved when the attachment ring was relocated towards the trailing edge and the outer platform cooling was modified. Detailed transient and steady state thermal analysis was performed on the aero-optimized airfoil designs. The steady state and transient thermal stresses in the vane ring were found to be acceptable for both average inlet and hot-streak conditions. The hot-streak and average inlet steady state stresses were found to be 25.3 and 13.9 ksi, respectively. The transient stress (in the hot streak zone) peaked to 25.9 ksi at the trailing edge fillet on the inner platform, 8 seconds into trip shut-down (Figure 4). Figure 4: Thermal stress during trip shutdown after 8 seconds Ceramic reliability analysis using NASA’s CARES/Life [12] predicted a probability of fast fracture failure of ~1 in 1,000,000 for individual vanes under steady state hot streak condition. Scaling the analysis to the entire vane ring predicted a failure probability of 1 in 100,000. The structural analysis was repeated to include the attachment tab (and the pressure loading on it) on the inner platform of the vane ring. The internal tab led to an increase in the stress to 34.5 ksi and the resulting probability of failure was predicted as 1.4 in 10,000 vane rings. No slow crack growth nor creep life was assessed so far. The final vane ring design was released for fabrication at Kyocera Industrial Ceramics Corp (KICC) in Vancouver, WA. Kyocera’s silicon nitride grade SN282 was selected for its high creep resistance at high temperature, which is a key issue for turbine vanes due to the presence of hot streaks from the combustor that exceed the average turbine inlet temperature by a considerable margin. Vane ring blanks were dry pressed and underwent a pre-sinter operation to prepare them for bisque machining (Figure 5a). A bisque machining approach was developed to enable machining of the complex vane ring geometry, which consisted of 15 vanes joined by integral inner and outer platforms. As a prelude to actual bisque machining of vane rings, a plastic model was machined using the CAM program to ensure that all surfaces could be accessed, and that there were no errors in CAM programming. A total of three bisque fired SN282 were pre-sinter machined followed by sintering. The yield on the components was outstanding, with all three being free of any shrinkage cracks, major distortion, or other defects. This yield is unusual in typical initial prototyping of large silicon nitride turbine components. Both throat width measurements and limited contact coordinate measurements indicated that the flow areas between each vane were within 4% of the original CAD model. One of the three vane rings had minor chip outs, which were deeper than the anticipated final diamond grinding envelope. The other two components were deemed acceptable for engine environment/gas generator testing. At the time of this writing, all three vane rings are undergoing diamond grinding of their attachment regions. All areas outside the actual gaspath will be diamond ground to ensure high dimensional tolerance with mating metallic components and seals. It is anticipated that one or two of the vane rings will undergo steady state and trip shutdown transient testing in UTRC’s ST5 gas generator rig during calendar year 2004. (a) (b) Figure 5: (a) Bisque machined vane ring; (b) Three vane rings after final processing CERAMIC INTEGRALLY BLADED ROTOR An integrally bladed rotor (IBR), rather than individual inserted blade design was chosen for cost and reduction of contact area between ceramic and metal interfaces. The latter represents a considerable saving in machining of contact surfaces and elimination of high contact stresses. The previous paper [4] focused on steady state thermal and stress analysis and reducing stress at the disc bore. This paper summarizes the efforts to reduce the stresses at disc rim under steady state conditions and stress during engine transients. 108 Copyright © 2004 by ASMEPDF Image | ADVANCED MICROTURBINE SYSTEMS Final Report for Tasks 1 Through 4 and Task 6
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