Film cooling aerodynamic performance

Abstract

As a response to a socio-economic framework which demands lower fuel consumption and CO2 emissions, gas turbine manufacturers strive to attain higher thermal efficiency and specific work output in their engines. The enhancement of these two performance parameters is linked to higher turbine inlet temperatures (TIT), which explains the increasing trend in TIT accompanying aero engines industrial development.  Turbine cooling technology is one of the disciplines strongly contributing to this aim, enabling operational hot gas temperatures to be higher than the melting temperature of the material. This study deals with film cooling: an external type of turbine cooling. Coolant air, bled from the compressor, is injected into the turbine blades and vanes and discharged through small holes into the airfoil´s external boundary layer, creating a thin insulating layer that reduces convective heat transfer from the hot gas to the surface. However, this gain in thermal capability brings along an aerodynamic penalty.  The purpose of this work is to understand the aerodynamic performance of a NACA 0012 airfoil with four rows of holes on the pressure side, as a follow-up experimental study of a previous one using the same model but with suction side injection (Lanzillotta, et al., 2017). A configuration with angle of attack α=0º, freestream velocity V∞=15m/s and air as secondary flow is tested as a baseline to understand the effect of blowing ratio BR∈(0,2) on flow field characteristics. Then, other configurations are tested to analyse the effect of angle of attack, freestream velocity, single row injection and density ratio by using CO2 as secondary flow, to simulate the temperature ratio existing in real gas turbine applications. Pointwise pressure measurements at a location downstream of the airfoil (x= 1.25c) and planar and stereo PIV are used as flow measurement techniques. Wake velocity profile characteristics and aerodynamic losses are retrieved from pressure measurements. Results for the baseline configuration show how the wake velocity profile displaces towards the pressure side when blowing is introduced. For low blowing ratios, the low momentum of the coolant induces high mixing losses whereas for high blowing ratios, the energizing effect of the high momentum coolant outweighs the mixing losses. Maximum losses are found for BR=0.5 and they decrease for higher blowing ratios. For BR=1.4, a shift from wake to jet local behaviour is observed in the wake velocity profile. The high and low velocity regions in the 2D average velocity fields computed from planar PIV measurements show the same trend with blowing ratio and provide further information about the mixing shear layer at a location close to the cooling holes. Finally, 3D average velocity fields computed from stereo PIV display the evolution of the mixing shear layer and jet in crossflow in the streamwise direction.