In this work, the main physical characteristics of plane water sheets flowing under gravity and surrounded by free air are experimentally investigated. By varying the mean flow rate through a 0.8 mm × 100 mm nozzle, water sheets are produced over a range of Reynolds and Weber numbers between 210 to 1240 and 0.37 to 13.52, respectively. First, the sensitivity of the sheets shapes to a mass flow rate variation is evidenced. For a sufficiently high flow rate, a liquid sheet forms with two rims bordering it. These rims join after a certain length L c resulting in a triangle-like shape of the sheet. This characteristic length is compared with a theoretical prediction given by a model valid for Re 1. Furthermore, some capillary waves forming a striped pattern are present at the sheet interface near the rims and are propagating towards the central axis as the sheet falls. These waves are interpreted as the consequence of the displacement of a high curvature gradient zone at the rim-sheet interface as suggested by their stationary shape. The critical mass flow rate at which the sheet destabilization is systematically observed is Q c = 0.056 kg.s −1 . It corresponds to a Weber number We 2.7, a value in line with the theoretical one We th = O(1) which usually indicates a sufficient condition to maintain a stable sheet. Such ruptures are characterized by the appearance of expanding hole(s), predominantly in the lower half of the sheet. The experimentally determined mean expansion velocity proved to be within ±20% of that provided by the well-known Culick expression. As expected, when considering mass flow rates below the above mentioned critical one, an intermittent regime of rupture is obtained characterized by the presence of sheets, threadlines, jets or drops.
During the quenching process, the liquid bath is usually agitated to homogenize the temperature and to enhance convective heat transfer. The purpose of this paper is to characterize on the one hand the agitation of a water bath due to the movement of a three-blade turbine and on the other the cooling of an Inconel 718 part being quenched in a stirred water bath. Velocity measurements were taken by PIV with and without the metallic part. We found that the velocity field became purely axial when we were far enough away from the turbine. Moreover, a high turbulent mixing level was shown for this type of jet. Velocity measurements were carried out for two agitation intensities. The axial velocity amplitude as well as the turbulent kinetic energy decreased dramatically as the rotational speed of the propeller decreased from 410 to 100 rpm. This caused the thermal behavior of the part to differ during quenching. Indeed, we found that the part cooled faster under stronger agitation. During the film boiling and transition phases, no appreciable effect of agitation could be observed. However, from the middle of the nucleate boiling phase, the part-bath heat transfer coefficient was found to decrease much less rapidly with the surface temperature if agitation was strong, than if it was weak or if the bath was completely calm. In such a case of strong agitation, both nucleate boiling and convection concomitantly ensure part cooling.
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