Critical heat fluxes for curved and straight surfaces during subcooled flow boiling

An analytical and experimental Investigation of the critical heat flux on concave, convex and straight heated surfaces has been carried out for subcooled, flow boiling in Freon 113. A total of 274 experiments have been conducted at five mean velocities and four subcoolings. The analytical study and the experimental data indicate that for equal velocity and subcooling, concave surfaces sustain larger critical heat fluxes than straight surfaces which, in turn, sustain larger critical heat fluxes than convex surfaces. The analytical results are compared with the experimental results of this and other work. An idealized vapor bubble cycle has been defined and is used as the basis for a critical heat flux model. In the model, vapor bubbles are assumed to depart from the heated surface when they have grown to the edge of the hydrodynamic sublayer. The frequency of the idealized vapor bubble cycle has been derived and combined with expressions for the energy associated with each vapor bubble cycle and the number of active sites present to produce a critical heat flux expression. The resulting expression contains only one empirical constant. Expressions for the ratio of the critical heat flux between concave and convex surfaces, and between straight and convex surfaces have been derived. These expressions contain no empirical constants and predict that for the conditions of this study, the concave surface will sustain critical heat fluxes of between 1,5 and 2.0 times those for the convex surface, depending upon the radial acceleration and subcooling of the bulk fluid. The straight surface was predicted to sustain between 1.0 and 1.2 times the critical heat flux magnitudes accommodated by the convex surface. An experimental, closed loop boiling apparatus with Interchangeable curved and straight test sections was constructed for this study. Details of the apparatus design and operating procedures for the experimental program are given. Tabular summaries of the experimental data are presented for all surfaces, along with graphical comparison of the experimental and analytical results. (author)