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2.1.2.2 Ceramic Shroud Due to the low thermal expansion of silicon nitride materials, the ceramic IBR the of ST5+ was expected to 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 expand less than the ST5 rotor due to centrifugal forces. Combining these two factors together, a large gap was expected between the ceramic IBR and the metal shroud. This gap would lead to severe blade tip flow losses and consequent 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. Aerothermal heat transfer to the shroud, and its effect on shroud absolute temperature was also determined and added to the analysis. The findings are summarized in Table 2.1.2 below. Table 2.1.2 Design changes and resulting clearance ST5 ST5+ Ceramic rotor ST5 ST5 SiC Si3N4 0.0078” 0.0261” 0.0085” 0.0062” It was evident from these data that if a metal shroud was used with the ceramic rotor excessive leakage would ensue due to the large tip clearance of 0.0261”. However, use of a ceramic shroud reduced the tip clearance to 0.0085” for SiC and 0.0062” for Si3N4. Only Si3N4 provided any real benefit over the baseline ST5 metal engine in terms of tip clearance control. For small engines such as ST5+, a 1 mil tip clearance equated to ~0.3% turbine efficiency. Therefore, use of a Si3N4 shroud was predicted to result in a 0.45% increase in turbine efficiency. Design concepts for ceramic shrouds were developed and one of the concepts is shown in Figure 2.1.10. This design included a slot arrangement to locate the shroud with respect to the vane outer platform. A total of four ceramic design concepts were developed for the turbine shroud, which were subsequently 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 worst case. Rotor Type Shroud Type Ave. Running Clearance 30PDF Image | ADVANCED MICROTURBINE SYSTEMS Final Report for Tasks 1 Through 4 and Task 6
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