Date of Award

Spring 5-18-2012

Degree Type

Dissertation-Restricted

Degree Name

Ph.D.

Degree Program

Engineering and Applied Science

Department

Mechanical Engineering

Major Professor

Dr. Ting Wang

Abstract

Film cooling is a cooling technique widely used in high-performance gas turbines

to protect turbine airfoils from being damaged by hot flue gases. Film injection holes are

placed in the body of the airfoil to allow coolant to pass from the internal cavity to the

external surface. The ejection of coolant gas results in a layer or “film” of coolant gas

flowing along the external surface of the airfoil.

In this study, a new cooling scheme, mist/air film cooling is proposed and

investigated through experiments. Small amount of tiny water droplets with an average

diameter about 7 μm (mist) is injected into the cooling air to enhance the cooling

performance. A wind tunnel system and test facilities were build. A Phase Doppler

Particle Analyzer (PDPA) system is employed to measure droplet size, velocity and

turbulence. Infrared camera and thermocouples are both used for temperature

measurements.

Mist film cooling performance is evaluated and compared against air-only film

cooling in terms of adiabatic film cooling effectiveness and film coverage. Experimental

results show that for blowing ratio M=0.6, net enhancement in adiabatic cooling

effectiveness can reach 190% locally and 128% overall along the centerline. The general

pattern of adiabatic cooling effectiveness distribution of the mist case is similar to that of

the air-only case with the peak at about the same location.

The concept of Film Decay Length (FDL) is proposed to quantitatively evaluate

how well the coolant film covers the blade surface. Application of mist in the M=0.6

condition is apparently superior to the M=1.0 and 1.4 cases due to the higher overall

cooling enhancement, the much longer FDL, and wider and longer film cooling coverage

area.

Based on droplet measurements through PDPA, a profile describing how the airmist

coolant jet flow spreads and eventually blends into the hot main flow is proposed. A

sketch based on the proposed profile is provided. This profile is found to be well

supported by the measurement results of Turbulent Reynolds Stress. The location where

a higher magnitude of Turbulent Reynolds Stress exists, which indicates higher strength

of turbulent mixing effect, is found to be in the close neighborhood of the edge of the

coolant film envelope. Also the separation between the mist droplets layer and the

coolant air film is identified through the measurements. In other words, large droplets

penetrate through the air coolant film layer and travel further over into the main flow.

Based on the proposed air-mist film profile, the heat transfer results are reexamined.

It is found that the location of optimum cooling effect is coincident with the

starting point where the air-mist coolant starts to bend towards the surface. Thus the data

suggests that the “bending back” film pattern is critical in keeping the mist droplets close

to the surface which improves the cooling effectiveness for mist cooling.

Rights

The University of New Orleans and its agents retain the non-exclusive license to archive and make accessible this dissertation or thesis in whole or part in all forms of media, now or hereafter known. The author retains all other ownership rights to the copyright of the thesis or dissertation.

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