The paper is dealing with a research carried out at the Institute of Thermal-Fluid-Dynamics to investigate the rewetting of a hot surface. In a previous work the rewetting of the hot surface by spray cooling has been analyzed. After the droplet impingement, the liquid film falls along the surface, and rewetting by falling film takes place. The experiment was characterized by a 1-dimensional liquid spray, i.e., droplets having a uniform, constant diameter, impinging on the heated surface. The cooling rate of the hot surface has been detected as a function of wall temperature, droplet diameter and velocity, and point of impact of the spray. The working principle of the spray is based on the varicose rupture of a liquid jet: imposing a periodic (symmetrical) perturbation of appropriate amplitude and frequency on the jet surface, the flow is "constrained" to break soon after leaving the nozzle, eventually obtaining constant diameter drops, depending on the nozzle diameter and liquid velocity. In this paper the previous results with spray cooling are compared with experimental runs in which the spray injection is substituted with a falling film all along the test section. The rewetting velocity has been calculated from the response of the thermocouples placed on the heated wall and from a digital image system based on the video image registered during the runs.
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