Two distinctly different modes of cooling high-temperature bodies in subcooled liquids

Abstract

The paper sequentially substantiates an existence of the two distinct modes of film boiling during quenching the high-temperature bodies in subcooled liquids. Based on the vast investigations of film boiling regularities during cooling high-temperature metallic spheres in different subcooled liquids, the authors of the present paper revealed that among pure liquids, only in subcooled water there are the two distinctly different cooling regimes. According to the approximate physical model developed earlier (2018) by the authors, incipience of the micro-bubble boiling arises, if the local liquid/solid contacts occur between the surface roughness protrusions and the liquid crests of the wavy liquid/vapor interface. At such a contact, near a boundary of the three phases, a huge local heat sink (qev) acts. This heat sink is proportional to (σhlg/ν) and its value explains, in particular, why the fast cooling regime occurs in subcooled water at definite subcoolings, but never in cryogens or fluorocarbons. The model quantitatively predicts influence of the properties of a coolant and a cooled body on the incipience of the fast cooling regime. However, some issues remain open. The present paper considers the results of the numerical studies of a metallic body cooling under effect of the local heat sink qev. These studies shed light on some of these issues. The numerical simulation determines realistic values of a heat sink, a period of its action, and a size of the cooled area at the plane surface, which provide the conditions for spreading the wetted area at the surface of a cooled body. According to the simulation, the cooling process predictably occurs more rapidly for metals with lower thermal effusivity and for low-conductive and rather thick coating at the high-conductive metal. The strong effect of very thin low-conductive coatings from Teflon and silica exceeded expectation. At qev= 50 MW/m2 even 10-μm coating of the porous silica leads to the surface temperature drops larger 500 K during 0.01 s for the both basic metals studied. The results have confirmed that the local heat sink of high intensity, particularly at the modelled roughness protrusions, can provide the conditions of the liquid/wall contacts and spreading of the wetted area at the surface.