Time-resolved Studies of High Density Ratio Film-cooling Flows

Open Access
Author:
Eberly, Molly Kate
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 16, 2012
Committee Members:
  • Karen Ann Thole, Thesis Advisor
Keywords:
  • Film-Cooling
  • High Density Ratio
  • Heat Transfer
  • Turbomachinery
Abstract:
Gas turbine engines are essential to the aircraft propulsion and power generation industries. Increasing the turbine inlet temperature is one way to gain efficiency in an engine, but it results in the need for cooling techniques such as film-cooling. Film-cooling is a method by which compressor bleed air is injected through the turbine surfaces to form a layer of coolant that protects components from the hot combustion gases. This study investigated the effects of high density ratio film-cooling injection for various film-cooling geometries in the heat transfer and flowfield regimes. A new facility was developed that combined the use of a heated freestream with cryogenic coolant to achieve high engine-realistic density ratios. Adiabatic effectiveness measurements, which quantify the performance of film-cooling, were conducted and general trends between momentum flux ratio, blowing ratio, density ratio, and velocity ratio were observed. Results showed that a high density ratio (DR = 1.6) consistently contributed to increased lateral spreading of the film-cooling jets relative to a low density ratio jet (DR = 1.2) at the same blowing ratio. High momentum flux ratio jets (I > 0.6) were found to contribute to jet detachment. Particle image velocimetry measurements were made to quantify the time-averaged and instantaneous flowfields. High levels of turbulence intensity were measured to be most prevalent in the jet shear layers and near the hole exit. At the hole exit, turbulence intensity scaled with momentum flux ratio. The shear layers, conversely, exhibited the lowest peak turbulence at M = 1 and strengthened as mass flux deviated from that value. Higher turbulence was generated at high density ratio than low density ratio when the jets were attached contributing to the increased lateral spreading. The time-resolved data showed shear layer vortices in the form of Kelvin-Helmholtz instabilities. The vortices had a consistent length scale and broke down into turbulence approximately four cooling diameters downstream from the injection.