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An experimental investigation of dry patch formation and stability in thin liquid films McAdam, Donald

Abstract

An experimental study was conducted on the formation and stability of dry patches in a thin liquid film flowing down a heated plate. The occurrence of such a dry patch is of great practical significance since, in some cases, the loss of liquid cooling effect allows the temperature of the heated surface to exceed its melting point. This is particularly dangerous in a high heat flux, two-phase system such as the core of a boiling-water-cooled reactor. Previous theoretical analyses indicate that dry patch stability and possible rewetting depends on a balance between inertia forces at the upstream edge tending to rewet the dry patch and surface tension and thermocapillary forces which cause it to spread. Dynamic contact angle is particularly significant in these analyses. Direct experimental confirmation of these analyses was not previously possible because measurements of dynamic contact angle under film break up conditions have never before been made. Film break up was observed in a thin liquid film of carbon dioxide at bulk conditions from 2°C and 3.62 MPa (megapascals) to 18°C and 5.5 MPa by high-speed (2500 frames per second) photography. Reynolds numbers based on film thickness ranged from 185 to 1000. Dynamic contact angle was measured using a specially developed schlieren system and photodensitometer measurements of the recording film. The technique is based on the angle of refraction of light by the curved liquid surface upstream of the dry patch. The angle of refraction is related to the slope of the liquid surface and is transformed to an optical density field by a graded filter. Surface profile and film thickness were calculated knowing the slopes upstream of the dry patch. Contact angle and surface profile as a function of time were found by analysing successive frames of movie film. A quasi-stable dry patch is established by a balance of 'the forces acting' at the stagnation point. Contact angle increases as the stagnation point advances until the surface tension force balances the inertia force. The stagnation point then becomes stationary and begins to recede, due to the surface tension force. Contact angle decreases as the stagnation point recedes until surface tension force is again balanced by inertia force and the cycle is then repeated until the dry patch rewets. Rewetting occurs when large waves hit the dry patch. The present experiments indicate that one criterion for a stable dry patch is a balance of body forces by surface tension forces and this is characterized by the Bond number. This is approximately equal to one, based on measured film thickness. Measured results compare well with theoretical analyses in laminar films at Reynolds numbers less than 250. At higher Reynolds numbers theoretical results do not agree with measured results. Further models are required which consider the three-dimensional nature of the stagnation point, the dynamic effect of surface waves on contact angle and which give good predictions of minimum thickness in turbulent films

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