Experimental result on rewetting of hot surfaces by droplet impingement

Abstract

The paper is dealing with a research carried out at the Institute of Thermal-Fluid Dynamics of ENEA (Casaccia Center) to investigate the rewetting of a hot surface cooled with droplet impingement. After the droplet impingement, the liquid film falls along the surface. The reference situation is the external surface of a tank which, after a fire, is subjected to flames or high temperature anyway, and is cooled with water or an extinguishing liquid. The experiment is characterized by a one-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 on the heated surface. The working principle of such a spray is based on the varicose rupture of a liquid jet (Rayleigh–Weber instability): imposing a periodic 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, for a given liquid, on the nozzle diameter and liquid velocity. Dimensions of droplets used in the present work range from about 0.3 to 1.5 mm. A digital image system allows to know, for each nozzle, droplet diameter and aspect ratio. Each nozzle disk is manufactured in order to have the same cross flow area independent of the number of nozzles in the disk. 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. A comparison of the two methods is provided. Two different locations of the jet impingement on the heated surface are tested. The spray is impinging outside the heated length, just upstream the copper clamp in one series of tests, while in a second series of tests the spray impinges directly on the first wall thermocouple, 25 mm below the beginning of the heated length. Prediction of the rewetting velocity obtained using available correlations has shown a good agreement with experimental data.