A huge part of modern day power generation research and development strives to achieve higher thermal efficiencies and specific work outputs for both gas turbine Brayton and combined cycles. Advances in cooling technologies, both internal to turbine blades and external, provide the easiest way to accomplish this by raising the turbine inlet temperature far beyond the super-alloy's allowable temperature. Discrete film cooling injection, an external cooling technique, ensures a cool blanket of compressed air protects the blade surface from the harsh mainstream gas. To optimize the coverage and effectiveness of the film, a thorough understanding of the behavior and flow physics is necessary. The objective of the current study is to use hotwire anemometry as a tool to conduct 1D timeresolved turbulent measurements on the flow field of staggered multi-row film cooling arrays with cylindrical and diffuser shaped holes inclined at 20 degrees to the freestream. The study aims to investigate the flowfield to determine why the performance of diffuser shaped jets is enhanced even at comparatively high blowing ratios. In addition, blowing ratio effects and flowfield discrepancies at set downstream locations in the array centerline plane are also investigated. The experiments are conducted on an open-loop wind tunnel for blowing ratios in the range of 0.3 to 1.5 at a density ratio of 1. Boundary layer measurements were taken at 12 locations at the array centerline to obtain mean velocity, turbulence level, turbulence intensity, and integral length scales. Measurements were also taken at a location upstream of the array to characterize the incoming boundary layer and estimate the wall normal position of the probe in comparison with the logarithmic law of the wall. Mean effective velocity profiles were found to scale with blowing ratio for both geometries. A strong dependence of turbulence levels on velocity gradients between jets and the local fluid was also noticed. For cylindrical jets, attached cases displayed lower integral length scales in the near wall region compared with higher blowing ratio cases. This was found to be due to entrainment of mainstream fluid showing increased momentum transport below the jets. Diffuser cases at all blowing ratios tested do not show increased length scales near the wall demonstrating their enhanced surface coverage. Row-to-row discrepancies in mean velocity and turbulence level are only evident at extremely high blowing cases for cylindrical, but show significant deviations for diffuser cases at all blowing ratios. Unlike the cylindrical cases, jets from diffuser shaped holes, due to their extremely low injecting velocities, dragged the boundary layer with each row of blowing. Increased velocity gradients create a rise in peak turbulence levels at downstream locations. At high blowing ratios however, faster moving fluid, due to injection, at lower elevations acts as a shield for downstream jets allowing significantly further propagation downstream. Near the wall low magnitude integral length scales are noticed for diffuser jets indicating low momentum transport in this region. The results show good agreement with effectiveness measurements of a previous study at a higher density ratio. However, to accurately draw the comparison, effectiveness measurements should be conducted at a density ratio of 1. Recommendations were made to further the study of multi-row film cooled boundary layers. The scope includes a CFD component, other flowfield measurement techniques, and surface effectiveness studies using Nitrogen as the coolant for a much broader picture of this flowfield.
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Kapat, Jayanta S.
Master of Science in Mechanical Engineering (M.S.M.E.)
College of Engineering and Computer Science
Mechanical and Aerospace Engineering
Mechanical Engineering; Thermo-Fluids
Length of Campus-only Access
Masters Thesis (Campus-only Access)
Fernandes, Craig, "Statistical Analysis of Multi-Row Film Cooling Flowfields" (2017). Electronic Theses and Dissertations, 2004-2019. 5635.