An Investigation of Mist/Air Film Cooling with Application to Gas Turbine Airfoils

<p>Film cooling is a cooling technique widely used in high-performance gas turbines</p> <p>to protect turbine airfoils from being damaged by hot flue gases. Film injection holes are</p> <p>placed in the body of the airfoil to allow coolant to pass from the internal cavity to the</p> <p>external surface. The ejection of coolant gas results in a layer or “film” of coolant gas</p> <p>flowing along the external surface of the airfoil.</p> <p>In this study, a new cooling scheme, mist/air film cooling is proposed and</p> <p>investigated through experiments. Small amount of tiny water droplets with an average</p> <p>diameter about 7 μm (mist) is injected into the cooling air to enhance the cooling</p> <p>performance. A wind tunnel system and test facilities were build. A Phase Doppler</p> <p>Particle Analyzer (PDPA) system is employed to measure droplet size, velocity and</p> <p>turbulence. Infrared camera and thermocouples are both used for temperature</p> <p>measurements.</p> <p>Mist film cooling performance is evaluated and compared against air-only film</p> <p>cooling in terms of adiabatic film cooling effectiveness and film coverage. Experimental</p> <p>results show that for blowing ratio M=0.6, net enhancement in adiabatic cooling</p> <p>effectiveness can reach 190% locally and 128% overall along the centerline. The general</p> <p>pattern of adiabatic cooling effectiveness distribution of the mist case is similar to that of</p> <p>the air-only case with the peak at about the same location.</p> <p>The concept of Film Decay Length (FDL) is proposed to quantitatively evaluate</p> <p>how well the coolant film covers the blade surface. Application of mist in the M=0.6</p> <p>condition is apparently superior to the M=1.0 and 1.4 cases due to the higher overall</p> <p>cooling enhancement, the much longer FDL, and wider and longer film cooling coverage</p> <p>area.</p> <p>Based on droplet measurements through PDPA, a profile describing how the airmist</p> <p>coolant jet flow spreads and eventually blends into the hot main flow is proposed. A</p> <p>sketch based on the proposed profile is provided. This profile is found to be well</p> <p>supported by the measurement results of Turbulent Reynolds Stress. The location where</p> <p>a higher magnitude of Turbulent Reynolds Stress exists, which indicates higher strength</p> <p>of turbulent mixing effect, is found to be in the close neighborhood of the edge of the</p> <p>coolant film envelope. Also the separation between the mist droplets layer and the</p> <p>coolant air film is identified through the measurements. In other words, large droplets</p> <p>penetrate through the air coolant film layer and travel further over into the main flow.</p> <p>Based on the proposed air-mist film profile, the heat transfer results are reexamined.</p> <p>It is found that the location of optimum cooling effect is coincident with the</p> <p>starting point where the air-mist coolant starts to bend towards the surface. Thus the data</p> <p>suggests that the “bending back” film pattern is critical in keeping the mist droplets close</p> <p>to the surface which improves the cooling effectiveness for mist cooling.</p>

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation-Restricted

Discipline thesis:degree_discipline

Mechanical Engineering

Year

2012

Author dc:creator

• zhao, lei

Contributors dc:contributor

• Dr. Ting Wang

• gas turbine
• turbine airfoil
• heat transfer
• film cooling
• experimental study
• mist cooling
• Engineering
• Heat Transfer, Combustion
• Mechanical Engineering

Repository record dc:identifier

https://scholarworks.uno.edu/td/1499

OAI identifier oai:identifier

oai:scholarworks.uno.edu:td-2494