Abstract

Thermal and Flow Field Investigations of a Micro-Tangential-Jet Film Cooling Scheme on Gas Turbine Components
Othman Hassan, Ph.D.
Concordia University, 2013
Gas turbines play a major role in modern aerospace and in industrial power generation nowadays. Advanced gas turbines are designed to operate at increasingly higher inlet turbine gas temperature to increase their efficiency and specific power output. In order to enable this increase in the operating temperature, high-temperature resistant materials, Thermal-Barrier Coatings (TBCs), and advanced cooling techniques, are employed. Internal cooling, impingement cooling, and film cooling, are the typical cooling techniques that are being used nowadays for gas turbine engines cooling. For the past five decades, significant efforts have been implemented in the area of film cooling to design and investigate the performance of numerous cooling schemes at various operating conditions and geometries. However, the achieved effectiveness to date, especially over actual airfoil geometries, is still relatively low. Further efforts are essential to propose novel designs that are capable of providing the required cooling loads.
The present study investigates the thermal performance and flow characteristics downstream a new film cooling scheme over a gas turbine vane and a flat plate. The state-of-the-art transient Thermochromic Liquid Crystal (TLC) technique has been employed for film cooling measurements, while the Particle Image Velocimetry (PIV) technique has been employed for flow field investigations. Validation of all measurement techniques were conducted and good agreement with literature works has been achieved. The Micro-Tangential-Jet (MTJ) scheme is a discrete-holes shaped cooling scheme with micro sized exit height that supplies the jet parallel to the surface. The MTJ scheme consists of two main parts, a circular supply micro-tube, and a shaped exit parallel to the vane surface. The shaped exit of the scheme starts with a circular cross section. Lateral expansion angles are then applied in both directions and a relatively constant height is maintained throughout the scheme yielding a squared exit. Due to the micro thickness of the jet, a deep penetration inside the main stream is achievable, while maintaining a tangential injection direction to the surface, thereby avoiding jet lift off.
The film cooling performance of one row of MTJ scheme on the vane pressure side and another row on the suction side is investigated at different blowing ratios using the transient TLC technique. Comparisons with the film cooling performance of previously proposed shaped schemes are carried out to highlight the advantages and disadvantages of the new design. Mach number distributions over the airfoil surface are determined with and without the MTJ scheme to investigate the effect of the added material on the airfoil characteristics. A comprehensive analysis based on the current findings, previous efforts in the literature, and the flow field investigations using the PIV technique downstream the MTJ scheme is presented. Overall, the new design showed superior film cooling performance, compared to the best achieved results in literature. The effectiveness distribution downstream the MTJ scheme was characterized with superior lateral spreading over both pressure and suction surfaces. The measurements showed similarity in the characteristics of the 2-D film downstream the MTJ scheme and the one that accompanies the injection from continuous slot schemes. Moreover, the investigations showed that the presence of the MTJ scheme over the vane pressure or suction sides did not result in significant HTC augmentation, especially at blowing ratios less than unity. The MTJ scheme could be the first of a new generation of film cooling schemes over airfoil geometries.