An Investigation of Mist/Air Film Cooling with Application to Gas Turbine Airfoils

Film cooling is a cooling technique widely used in high-performance gas turbines to protect the turbine airfoils from being damaged by hot flue gases. Film injection holes are placed in the body of the airfoil to allow coolant to pass from the internal cavity to the external surface. The injection of coolant gas results in a layer or "film" of coolant gas flowing along the external surface of the airfoil.;In this study, a new cooling scheme, mist/air film cooling is investigated through experiments. A small amount of tiny water droplets with an average diameter about 7 microm (mist) is injected into the cooling air to enhance the cooling performance. A wind tunnel system and test facilities are built. A Phase Doppler Particle Analyzer (PDPA) system is employed to measure droplet size, velocity, and turbulence. Infrared camera and thermocouples are both used for temperature measurements.;Mist film cooling performance is evaluated and compared against air-only film cooling in terms of adiabatic film cooling effectiveness and film coverage. Experimental results show that for the blowing ratio M = 0.6, net enhancement in adiabatic cooling effectiveness can reach 190% locally and 128% overall along the centerline. The general pattern of adiabatic cooling effectiveness distribution of the mist case is similar to that of the air-only case with the peak at about the same location.;The concept of Film Decay Length (FDL) is proposed to quantitatively evaluate how well the coolant film covers the blade surface. Application of mist in the M = 0.6 condition is apparently superior to the M = 1.0 and 1.4 cases due to the higher overall cooling enhancement, the much longer FDL, and the wider and longer film cooling coverage area.;Based on the droplet measurements made through PDPA, a profile describing how the air-mist coolant jet flow spreads and eventually blends into the hot main flow is proposed. A sketch based on the proposed profile is provided. This profile is found to be well supported by the measurement results of the turbulent Reynolds stresses. The location where a higher magnitude of turbulent Reynolds stresses exist, which indicates a higher strength of the turbulent mixing effect, is found to be close to the edge of the coolant film envelope. Also, the separation between the mist droplet layer and the coolant air film is identified through the measurements: large droplets penetrate through the air coolant film layer and travel further into the main flow.;Based on the proposed air-mist film profile, the heat transfer results are re-examined. It is found that the location of optimum cooling effect is coincident with the point where the air-mist coolant starts to bend back towards the surface. Thus, the data suggests that the "bending back" film pattern is critical in keeping the mist droplets close to the surface, which improves the cooling effectiveness for mist cooling.