An Experimental Study of Mist/Air Film Cooling with Fan-shaped Holes on an Extended Flat Plate - Part 2: Two-Phase Flow Measurements and Droplet Dynamics

摘要

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 prescribed for both cylindrical and fan-shaped holes. The mist film layer consists of a typical coolant film layer and a wider droplet layer. The droplet layer is identified by a wedge-shaped enclosure prescribed by the data rate (droplet number per second) distribution. The apex of the enclosure, depicting by the maximum data rate, roughly indicating the core region of the coolant film. The upper boundary of the film layer, characterized by active mixing with the main flow, is found to be close to relatively high values of local Reynolds shear stresses. Thanks to higher inertia possessed by larger droplets (＞20 μm in diameter) at the injection hole, the larger droplets tend to shoot across the coolant layer, resulting in a wider droplet layer than the cooling film layer. With the prescribed coolant film and droplet layer profiles, the heat transfer results on the wall presented in Part 1 are re-examined. The 3-D droplet measurements show that the droplets injected from the fan-shaped holes tend to spread wider in lateral direction than cylinder holes and accumulate at the location where the neighboring coolant film layers meet. This flow and droplet behavior explains the higher cooling performance as well as mist-enhancement occurs between the fan-shaped cooling holes, rather than along the hole's centerline as demonstrated in the case using the cylindrical holes.