Onset and cessation of nucleate boiling.

A model for nucleate boiling inception and cessation is presented which provides a rational unified explanation of the following experimentally observed phenomena: (1) the wall superheat at the onset of nucleate boiling is dependent upon the working fluid and past thermal history of the fluid-solid surface. (2) for most fluids other than water, a large difference exists between the wall superheat at the onset and at the cessation of nucleate boiling. (3) fluid flow has a negligible effect on the incipient boiling wall superheat for most fluids. Water is a notable exception. In this study fluids R-11 and water are used to illustrate the behaviour of well and poor wetting liquids respectively. The analysis allowed the following conclusions to be drawn: (1) only re-entrant cavities can retain vapour bubble embryos during a non-boiling period. As subcooling of a site fluid increases, cavities with the largest embryos are quenched first. Dormant site radii are of the order of 1 $\mu$m. (2) at incipient boiling conditions, stable bubble embryos of well wetting substances can only exist attached to the mouth of a re-entrant (sub) cavity. (3) under certain conditions stable bubble embryos of a poor wetting substance can also attach at the mouth of a conical cavity. Since incipient boiling is a function only of the wall superheat surrounding the bubble embryo, it can be affected by fluid flow only if the bubble protrudes into a non isothermal boundary layer or is physically deformed by the flow or the cavity pressure is affected by the flow. This is unlikely to occur with well wetting liquids and is very likely to occur for water. The common version of the integrated Clapeyron equation predicted liquid superheats which were very sensitive to the temperature at which the properties were evaluated. A comparison of these liquid superheats with those obtained using tabulated data showed errors of as much as 400%, for the small radius of curvature of the bubble embryos associated with dormant nucleation sites. Source: Dissertation Abstracts International, Volume: 56-01, Section: B, page: 0476. Adviser: T. W. McDonald. Thesis (Ph.D.)--University of Windsor (Canada), 1993