Individual deflections and overall clearance with reference to the engine speed transient. As expected, the rotor initially responds quickly because of the centrifugal forces induced by the engine speed transient. The shroud thermal growth is slower, but it eventually exceeds that of the rotor. According to this formulation, the blades contribute the least to the clearance change. Like the rotor, they grow initially quickly because of the rapid increase in centrifugal and thermal stresses, but their growth is significantly less than that of the rotor. This should be expected considering the relatively short length of the blades in comparison to the rotor radius. The predicted clearance generally follows the expected trend for a takeoff transient. Engine experience indicates an initial reduction in clearance of about 0.76 to 1.27 mm (30 to 50 mils), and the recovery due to the thermal growth of the shroud is expected to be 0.25 to 0.51 mm (10 to 20 mils). Exact ranges for a given engine would depend on the parameters and transient conditions specific to that engine. This model predicts a reduction of approximately 0.51 mm (20 mils) and a recovery of about 0.13 mm (5 mils), which roughly models the trends.
Last updated: September 3, 2004 11:01 AM
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