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have been mostly made as singlets [3-7]. In such a design, the platforms of one vane are not connected to those next to it and the thermal deformation of two neighboring vanes are decoupled, which leads to less constraint and lower thermal stress. However, there are also disadvantages associated with such a vane concept. Cost per vane tends to be higher for singlet vanes in comparison to a full vane ring, as more mating surfaces require extra post-processing machining. Other issues include sealing and attachment. As cost competitiveness is one key parameter for small gas turbine engines, a full ceramic vane ring was chosen as the first design concept. The vane ring is attached to the engine by a tab-and-slot arrangement. This allows the ceramic vane ring to grow radially relative to a metallic retaining plate without introducing thermal stresses. The tabs and slots need to be machined to a high dimensional tolerance so that an accurate position of the vane ring relative to turbine rotor can be maintained. The outer platform of the vane ring has been modified to reduce thermal stress (see Figure 5). The thickness of the platform has been reduced and the front and rear fins on the platform have been removed to decrease the thermal mass, which is beneficial in controlling transient thermal stresses. Compressive pressure loading is introduced by a metallic spring ring positioned between the turbine support casing and the outer platform. Insulating material is added between the ceramic vane ring and the surrounding metallic structure to thermally shield the metal parts and to lower the temperature gradient and stress in the vane. was calculated as 232, indicating that time dependent strength degradation is not an issue at high temperatures. Tanaka et al. had reported much higher n values (>10,000) from stress rupture tests at 1200 and 1400°C [8]. The material also has excellent creep resistance up to and well beyond the maximum vane temperature of ~1200°C. Silicon nitride under high temperature steam environment is subject to grain boundary degradation and recession [9]. An environmental barrier coating will be used to protect the ceramic vanes from accelerated oxidation. The thermal gradients and the resulting thermoelastic stresses in the vane cascade were estimated using the finite element code ABAQUS. The analysis was performed on the existing ST5 metal design, as if manufactured as a single silicon- nitride (SN282) part. Sequential thermal and stress analysis was performed for average-inlet conditions for the complete vane ring. The steady-state maximum and minimum temperatures in the vane were predicted as 1103 and 894°C and the resulting maximum principal stress was 178 MPa. The maximum stress occurred at the inner platform due to lower temperatures in that region. The stresses at the trailing edge were approximately 103 MPa. The mechanical constraints and attachment schemes were not included in the analysis. Detailed stress and life analyses were also performed on the new ST5+ design. Due to a non-uniform combustor exit temperature profile (pattern factor), the vane gas temperature field will be more severe than average-inlet conditions. The FEA was performed for both average-inlet and hot-streak conditions. Figures 6 and 7 show the temperature gradients and thermal stress distribution in the vane cascade for hot-streak conditions, respectively. As expected, the temperature gradients were more severe and the stresses (179 MPa) were higher in the hot-streak case as compared to the average-inlet case (107 MPa). The stresses in both cases, however, were lower in the ST5+ design than in the ST5 design due to less severe thermal gradients. Transient analyses for engine start-up and emergency shutdown condition are currently underway. Figure 6. Steady state temperature (oC) in the ST5+ vane for hot-streak conditions metal spring insulating ceramic vane Figure 5. Ceramic vane ring attachment The candidate ceramic material for the vane ring is silicon nitride (SN282) from Kyocera Industrial Ceramics Corp. (KICC). Vendor supplied temperature dependent physical and mechanical properties (strength and strength distribution) were used for finite element analysis and life predictions. The slow crack growth (SCG) susceptibility of this material was determined by dynamic fatigue testing at the Oak Ridge National Laboratory, TN (ORNL). The SCG parameter (n) at 1260°C 101 Copyright © 2002 by ASME 1205 1173 1140 1108 1076 1043 1011 979 948 915 883 851 818PDF Image | ADVANCED MICROTURBINE SYSTEMS Final Report for Tasks 1 Through 4 and Task 6
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