Determination of Cooling Parameters for a High Speed, True Scale, Metallic Turbine Vane Ring

Abstract

Film cooling technology has been around for many decades and many significant advances in cooling effectiveness have been made at many different facilities using several different methods. A large proportion of film cooling research is successfully carried out using simplified scaled-up models in wind tunnels coupled with novel measurement techniques. These tests have been very effective in assessing basic film cooling parameters for many cooling hole geometries, patterns, and blowing ratios. In real engines, however, film cooling designs are ultimately subjected to highly unsteady 3-D secondary flows and rotational effects. Few film cooling experiments have quantified these effects on real, true scale turbine hardware in a rotating test environment. The Turbine Research Facility (TRF) at the Air Force Research Laboratory has been acquiring uncooled heat transfer measurements on full scale metallic airfoils both with and without rotation for several years. The addition of cooling flow to this type of facility has provided new capability, and new challenges. The primary two issues being that the film temperature is unknown and that the airfoil is no longer semi-infinite. This makes it more difficult to extract the adiabatic effectiveness and the heat transfer coefficient from the measurements of surface temperature and surface heat transfer since conventional methods used in most other experiments are not valid in this case. In contrast another cooling parameter, the overall effectiveness, is readily obtained from measurements of surface temperature, internal coolant temperature, and mainstream temperature. The overall effectiveness is a normalized measure of metal surface temperatures expected for actual operating conditions. It is the goal of this paper to evaluate how measurements, obtained from a transient blowdown facility like the TRF, can be used to quantify the expected performance of a film cooled turbine airfoil. Additionally, it is imperative to properly correlate these experimental results to the true engine conditions. The data required for this analysis has been collected using an array of surface mounted thermocouples and thin film gauges in a series of experiments where freestream temperatures and coolant temperatures and mass flow rates were varied. The airfoil used in this investigation was a thin walled metallic airfoil with a showerhead cooling scheme and several rows of normal holes on both the pressure and suction sides of the airfoil. The flow is typical of that seen in a modern high pressure turbine — that is an inlet Mach number of about 0.1 accelerating toward sonic at the throat with a high inlet freestream turbulence level of about 20%.