An Experimental Study of Mist/Air Film Cooling On a Flat Plate with Application to Gas Turbine Airfoils- Part 2: Two-Phase Flow Measurements and Droplet Dynamics

A Phase Doppler Particle Analyzer (PDPA) system is employed to measure the two-phase mist flow behavior including flow velocity field, droplet size distribution, droplet dynamics, and turbulence characteristics. Based on the droplet measurements made through PDPA, a projected profile describing how the air-mist coolant jet flow spreads and eventually blends into the hot main flow is proposed. This proposed profile is found to be well supported by the measurement results of the turbulent Reynolds stresses. The coolant film envelope is identified with shear layers characterized by higher magnitudes of turbulent Reynolds stresses. In addition, the separation between the mist droplet layer and the coolant air film is identified through the droplet measurements—large droplets penetrate through the air coolant film layer and travel further into the main flow. With the proposed air-mist film profile, the heat transfer results on the wall presented in Part 1 are re-examined and more in-depth physics is revealed. It is found that the location of optimum cooling effectiveness is coincided with the point where the air-mist coolant stream 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.