THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St., Now York, N.Y. 10917 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed In its publications. Discussion is piinted only If the paper is pub- lished in an ASME Journal. Papers are available from ASME for 15 months after the meeting. Printod in U.S.A. 94.GT•359 INFLUENCE OF TIP CLEARANCE ON THE INTER BLADE AND EXIT FLOW FIELD OF A TURBINE ROTOR CASCADE M. Govardhan and N. Venkatrayulu Department of Mechanical Engineering Thermal Turbomachinery Laboratory Indian Institute of Technology Madras, India V. S. Vishnubhotla Department of Mechanical Engineering College of Engineering Osmania University Hyderabad, India 1111111111 1111111111111 ABSTRACT A detailed study of flow through the blade passage and down- stream of a linear turbine cascade was carried out for four cases of tip clearance including zero clearance. Apart from inlet tra- verse, a total of eight stations were chosen for inter-blade flow traversing between 5% and 95% of axial chord from leading edge. Downstream flow surveys were made at distances of 106% of axial chord from the blade leading edge. Pitchwise and span- wise traverses were conducted for each tip clearance at these stations using a small five hole probe. Provision was also made for the measurement of static pressure distribution on the suc- tion and pressure surfaces and also on the blade tip surface when clearance is present. At about 40% of axial chord from the leading edge, the presence of clearance vortex is identified inside the passage. The growth of the clearance vortex in size, its movement to- wards the suction surface and its increase in strength with the gap size were observed beyond 55% of axial chord till the trail- ing edge region. The rate of growth of the losses in the endwall region increased with clearance. Home shoe vortex was not observed for the highest clearance. The overall losses increase rapidly with clearance in the rear half of the blade. NO : Velocity : Axial Velocity C„ : Tangential Velocity eh : Blade Chord Cp : Static Pressure coefficient : Axial Chord EW : Endwall : Blade span L.E : Blade Leading Edge MS : Blade Midspan : Static Pressure Pp : Total Pressure PS : Pressure Surface : Blade Spacing SS : Suction Surface T. E : Trailing Edge : Axial Direction : Pitchwise Direction : Spanwise Direction alb : Inlet Blade Angle on : Outlet Blade Angle : Tip Gap Height Subscripts : Normal Component : Spanwise Component Superscripts : Pitchwise Mass or Momentum Averaged : Pitch and Spanwise Mass or Momentum Averaged INTRODUCTION Many turbomachine impellers are not shrouded and the leakage flow through the tip clearance of blade is an unavoid- able factor which deteriorates the performance. Denton and Cumpsty (1987) mentioned about two distinct and equally im- portant aspects to the tip clearance flows. First, there is a reduction in the blade force, therefore, the work done. This occurs because the leakage flow passes over the blade tip essen- tially without being turned. As a consequence of viscous effects in the tip clearance gap entropy is also produced. The second major aspect is the mixing of the flow that passes through the tip clearance gap with that which passes between the blades. The effect of tip leakage is found to be more severe in turbines as compared to compressors. Since the lift coefficient of a turbine blade is much larger than that of a compressor, the vortex core diameter is likely to be larger, thus influencing the flow over a large region near the tip. Additional losses are noticed when the endwalls of the cascade move with respect to the blades. A strong accumulation of losses at the blade end on the suction side of the blades was observed by many investigators. Bindon (1987, a, b, c) conducted experiments in a lin- ear cascade with tip clearance to extend the understanding of the flow physics. He measured both static pressure flow field and the boundary layers inside the tip gap and on the end wall. He aided his experiments with smoke flow visual- isation. Bindon (1989) from his later experiments concluded that, of the tip clearance flow losses irenerated unto the trailing. Presented at the International Gas Turbine and Aeroengine Congress and Exposition The Hague, Netherlands - June 13-16, 1994 Copyright © 1994 by ASME Downloaded from https://asmedigitalcollection.asme.org/GT/proceedings-pdf/GT1994/78835/V001T01A121/2404664/v001t01a121-94-gt-359.pdf by guest on 20 July 2020