Date of Award

January 2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Forrest E. Ames

Abstract

The stagnation region of a turbine vane is a critical area for assessing heat transfer. The heat loads at that region are influenced by many factors such as the turbulence intensity, length scales, interaction of the turbulent eddies, vortex stretching and rapid straining of the fluid streamlines. In such a situation, it becomes difficult for gas turbine designers to accurately predict the heat transfer rate at leading edge stagnation region.

The purpose of this study is to investigate the response of high intensity free stream turbulence (FST) near the stagnation region of two different diameter leading edge cylinders in order to better understand the physics and to expand the parameter range for vane designers. Since FST has significant impact on the heat transfer augmentation, this study will examine the influence of elevated turbulence in the highly accelerating flow near the stagnation region. In the presence of the stagnation region of a body, turbulence can be intensified due to the straining field that elongates turbulent eddies or be blocked due to the presence of the wall. This amplification of turbulence allows eddies to penetrate closer to stagnation region surfaces and enhance the heat transfer augmentation.

In this research, a comprehensive set of data including velocities, turbulent components, and turbulent spectral information were acquired for two different diameter (0.1016 m and 0.4064 m) cylinders. Data for local heat transfer was previously recorded

by a previous graduate student. Hot wire measurements were acquired at various locations along the upstream stagnation stream line for a range of cylinder diameter Reynolds numbers and turbulence intensities. Turbulence measurements and energy spectra were acquired using hot-wire technique. Mean velocity profiles along the stream line were compared with computational fluid dynamic (CFD).

All these experiments were performed in UND's large scale, low speed cascade wind tunnel facility. Results from the larger cylinder and smaller cylinders of 0.4064 m and 0.1016 m diameters accordingly indicated in the previous studies that increasing turbulence intensity augments heat transfer at the stagnation region and promotes transition to turbulent flow. However, it was also evident from the previous experiments that, on the small cylinder, augmentation levels were closer to the TRL model prediction than that on the larger cylinder. The smaller cylinder with aft body tends to exhibit more rapid straining of the turbulent eddies from the oncoming turbulence, which intensified turbulence near the stagnation region.

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