STUDY OF MITIGATION OF TURBINE BLADE TIP LEAKAGE FLOWS USING TIP LEAKAGE INTERRUPTER

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Author:
Andichamy, Veerandra Chakkarava
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 17, 2017
Committee Members:
  • Cengiz Camci, Thesis Advisor
Keywords:
  • Turbine blade tip
  • Tip leakage flows
  • leakage vortex
  • Leakage vortex mitigation
  • turbine blade tip design
  • blade tip modification
Abstract:
The gap between the turbine rotor blade tip and turbine casing surface is essential to prevent rubbing of these two components. Due to presence of this gap, the differential pressure across the blade tip drives fluid flow from pressure side to suction side of the blade tip. This fluid flow is called tip leakage flow and is an important sources of multidisciplinary problems. The aerodynamic losses due to tip leakage flows in a turbine blade accounts for about one third of the overall aerodynamic loss of a present day turbine stage. The tip leakage fluid in general do not go through a significant expansion and work generation process. The solid surfaces in the tip area encounter the fluid areas with the highest absolute total temperature existing in the rotor frame of reference. The tip leakage flows also increase the thermal stresses, induce further thermal oxidation on the blade tip surfaces and on the casing surface. In addition, they may cause significant structural problems and also induce unsteadiness in the turbine flow field. Hence understanding of these tip leakage flows and reducing them are of outmost importance for improving energy efficiency, lifetime and reliability of any future turbine design. The current study uses Tip Leakage Interrupters (TLI) to mitigate some of the adverse effects of the tip leakage flows and improve the efficiency of the turbine stage. The TLI introduced in this study has the ability to alter the tip region flows on the airfoil surfaces. They operate by inducing controlled vortical structures originating from strategically shaped/oriented multiple and sub-miniature vortex generators. The TLIs in this investigation were attached on the suction side of the rotating blade tip sections in the Axial Flow Turbine Research Facility (AFTRF). The AFTRF is a high-pressure, single- stage cold-flow turbine with a 29 bladed rotor at the Department of Aerospace Engineering in the Penn State University. Three different experimental studies were completed on the TLI in which three different parameters such as the mounting location of TLI on the airfoil tip region, the number of TLIs mounted on the blade and the specific orientation of TLI were varied. The influence of the TLI on the leakage flow system was experimentally observed and interpreted using turbine exit total pressure maps obtained with very high spatial resolution. The time accurate and phase-locked total pressure data from the downstream of the modified turbine blade was collected by a high-time-response total pressure probe using a (100 KHz) dynamic pressure sensor. High resolution total pressure maps in a just downstream location of the AFTRF rotor provided detailed scans of the turbine exit field for further interpretations of flow field physics resulting from tip region flows and TLI induced flow systems. The current span-wise measurement resolution is about one percent of the blade span of 123 mm. The system generates 6000 equally spaced measurement points along one revolution of the rotor at a selected span-wise position. A phase-locked measurement system fills the 6000 bins during each revolution of the turbine rotor using a mil-spec optical shaft encoder with high encoding resolution. Each measurement bin occupies a 0.06 degrees in the circumferential direction. The system can collect about 206 data point along one blade pitch in one of the 29 rotor passages. The results obtained from these experiments was used to draw conclusion about the effectiveness of TLIs in reducing the tip leakage flows in the turbine tip region flow field. The ultimate goal of the TLI installations is to increase the total-to-total efficiency of axial flow turbines via the suppression or mitigation of individual tip vortices from each rotor blade.