Masters Theses

Date of Award

8-2001

Degree Type

Thesis

Degree Name

Master of Science

Major

Aerospace Engineering

Major Professor

Ahmad D. Vakili

Abstract

The leakage flow in a stationary stepped labyrinth seal is investigated by means of flow visualization, pressure field measurements and Particle Image Velocimetry (PIV). The basis of the investigation is a generic stepped labyrinth seal currently used by the Tennessee Valley Authority (TVA) in their steam turbine generators. Geometric and flow parameters were varied in order to examine their influence on leakage through the seal. Following a brief theoretical development that details the physical mechanisms that cause flow energy loss and therefore leakage reduction in a single orifice, the discussion is extended to cover tire current theory describing labyrinth seals. Flow visualization results are presented for the baseline configuration, which was tested as a ten times scale water tunnel model at three different axial step locations. The observations made during these tests confirm the basic mechanisms of energy loss in labyrinth seals including turbulence induced viscous losses, chamber vortex generation, flow stagnation and increased flow streamline curvature. A 5 times scale 2-D airflow measurement stepped labyrinth seal model was constructed and tested over a range of seal pressure ratios from 1:1 to 10:1. Tested model configurations included the baseline stepped labyrinth seal and six additional variants of this basic design that were obtained by varying step height and knife angle. Results show that with relatively minor changes in geometry based on the physics of the flow through the seal, leakage reductions of up to 17% can be achieved. Finally, PIV measurements were carried out on the 2-D airflow seal models including both the five times scale baseline seal model and the improved design seal five times scale model which incorporates an increased step height and inclined long knives. Results show that the reduced leakage occurs due to an increased amount of flow stagnation and streamline curvature within the improved seal.

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