Direct observation of dryout processes of thin liquid films and $Al_2O_3$ nano-fluid performance in boiling crisis

Abstract

The boiling crisis phenomenon is the most enormously studied and disputed topic in the area of boiling heat transfer. The great interest is due to practical motives since it is desirable to design a heat exchanger or boiler to operate at as high a heat flux as is possible with optimum heat transfer rates, but without the risk of physical burnout. This study consists of two parts of boiling crisis mechanism-related visualization study and CI-1F enhancement-related nano-fluid study. In Part I, firstly subcooled flow boiling phenomena have been investigated photographically for water in vertical, one-side heated, rectangular channels under atmospheric pressure. At sufficiently high heat fluxes, three characteristic layers were observed in the heated channel: (a) a superheated liquid layer with small bubbles attached on the heated wall, (b) a flowing bubble layer consisting of large coalesced bubbles over the superheated liquid layer, and (c) the liquid core over the flowing bubble layer. In addition, the existence of a liquid sublayer under coalesced bubbles was identified photographically. According to visualization, the CI-1F mechanism for the present experimental condition could be related to the formation of large vapor clots resulting from coalescences of bubbles and the evaporation of the superheated liquid layer beneath those clots. Secondly, a photographic study of subcooled flow boiling with R-134a has been performed under higher pressure. The visualization made possible a detailed observation of the characteristic near-wall region, consisting of vapor remnants, an interleaved liquid layer, and coalesced adjacent bubbles. In addition, it is shown that the near-wall bubble layer of nucleate boiling beneath vapor clots is extinguished and, afterwards, the heated surface is locally covered by large vapor films, at CHF. Thirdly, a visualization study of pool boiling with nano-fluids has been performed. In the boiling, a thin liquid film adhering to a heater s...