| An experimental study of forced convective subcooled boiling heat transfer was performed at heat fluxes that ranged beyond 10;Pressure drop, heat transfer, and CHF experiments were performed with metallic tubes having inside diameters ranging from 0.3 to 2.7 mm. Mass fluxes ranged from 5,000 to 40,000 kg/m;Over 200 CHF stable data points were obtained. CHF was shown to be an increasing function of both mass flux and subcooling, and an inverse function of diameter. CHF increased for length-to-diameter ratios less than 10, and decreased with increasing exit pressure. Unreasonably low CHF values were obtained for several series of tests; these premature failures are believed to be the result of thermal-hydraulic instabilities.;A "high flux" CHF data base containing over 800 data points was compiled. A new statistical correlation was developed, with an average deviation of 16 percent, that reproduced the parametric trends observed during the CHF experiments.;Visual flow regime observations were made beyond the exit of the heated test sections. These observations indicated that vapor bubbles generated at high heat fluxes are very small and appear as a nearly transparent "fog". Calculations were performed to gain additional insight into the the forces that affect bubble motion and the mechanics of boiling at high fluxes. These calculations showed that the flow regime consists of a thin rapidly condensing bubble boundary layer and an inner subcooled core.;Energy transport mechanisms were defined, and the most significant ones at high heat fluxes were identified. These include vaporization of near-wall superheated liquid, enhanced microconvection due to Marangoni forces, vapor-liquid interchange as bubbles depart from the heated surface, and latent heat transport by bubbles transported through the boundary layer. CHF occurs as a result of bubble crowding within the near wall bubble boundary layer that limits these heat transfer mechanisms. Bubble crowding results from limitations of heat transport from the outer region of the bubble boundary layer to the inner subcooled core, leading to an increase in bubble population and near-wall void fraction. |
