Influence of the Radial Clearance on Outlet Velocity Losses in the Unshrouded Turbine Cascade

Abstract

Small-size turbines of the aviation gas-turbine plants differ by their high output losses and a high sensitivity to the value of radial clearance over the rotor blades. An increase in this clearance, for example, in-operation results in the reduced power efficiency factor due to a change in the losses in the nozzle, in the turbine wheel and output losses. Test data of the cascade of an average reliability with the rotor blade height of 20.73 mm were given for the three clearances, in particular 0.22; 0.6 and 1.0 m. An increase in the losses in the upper zone was recorded for rotor and nozzle blades with an increase in clearance. It was shown that due to a change in the distribution behavior along the radius of absolute velocity С 2 output losses can either be increased or decreased or remain unchanged at a change in the clearance depending on the averaged angle of flow output from the cascade in the absolute motion α 2. To improve the methods of one-dimensional computation of the cascade using the integral characteristics of its lattices the method of definition of output losses was suggested taking into consideration radial nonuniformity of the velocity С 2 at a high clearance value. It is assumed that the flow velocity С 2 is constant along the radius at a minimum clearance value of 0.22 mm. A degree of the radial nonuniformity of velocity С 2 was taken into consideration using the cascade test data in different modes and at different clearances. The obtained test data on the influence of clearance on absolute losses with the output velocity are naturally not multi-purpose, because even for the cascade investigation, the kinetic energy of flow behind it depends not only on the clearance value and the averaged output angle but also on some other auxiliary factors.