Tip clearance effects in a turbine rotor: Part II - Velocity field and flow physics

A comprehensive experimental investigation was under taken to explore the f low field in the tip clearance region of a turbine rotor to understand the physics of tip leakage flow. Specifically the paper looks at its origin, na ture, development interaction with the secondary flow, and its effects on p erformance. The experimental study was based on darn obtained using a rotat ing five-hole probe, Laser Doppler Velocimeter; high-response pressure prob es on the casing, and static pressure taps on the rotor blade surfaces. The fil st part of the paper deals with the pressure field and losses. Part II presents and interprets the vorticity, velocity, and turbulence fields at several axial locations. The data provided here indicates that the tip leak age vol tex originates in the last half chord. The leakage vortex is confin ed close to the suction surface col-ii er near the blade tip by the relativ e motion of the blade and the casing, and by the secondary flow ill the tip region. The rip leakage flow clings to the blade suction surface until mid chord then lifts off of the suction surface to form a vortex in the last 20 percent of the blade chord. The relative motion between blades and casing lends to the development of a scraping vortex that, along with the secondar y flow, reduces the propagation of the tip leakage flow into the mainflow. The rotational effects and coriolis forces modify the turbulence structure in the tip leakage flow and secondary; flow as compared to cascades.