D. Banerjee V. K. Dhir Mechanical and Aerospace Engineering Department, University of California, Los Angeles, Los Angeles, CA 90095 Study of Subcooled Film Boiling on a Horizontal Disc: 1 Part 2—Experiments Experiments were performed to study subcooled film boiling of performance liquid PF- 5060 (made by 3-M Company) on a horizontal copper disc. The experiments were per- formed for two regimes of film boiling involving departing vapor bubbles (low subcool- ing) and nondeparting vapor bubbles (high subcooling). By employing high speed digital camera, data were obtained for temporal variation of bubble height, bubble shape and bubble growth rate over one cycle. Heat flux data were deduced from temperatures measured with thermocouples embedded in the solid. The results from the numerical model are compared with experimental data and are found to be in general agreement. Particle Tracking Velocimetry (PTV) experiments were performed for a configuration of non-departing vapor bubbles to study the flow field in the liquid phase. The PTV experi- ments point to the existence of natural convection flow in the liquid phase and is in qualitative agreement with the predictions available in the literature. DOI: 10.1115/1.1345890 Keywords: Boiling, Film, Heat Transfer, Phase Change, Two-Phase 1 Introduction In subcooled film boiling the bulk temperature of the liquid phase is lower than the saturation temperature. In such a situation the heat flux at the wall is partitioned at the interface between phase change and convective heat transfer into the subcooled liq- uid. This results in higher heat transfer compared to saturated film boiling. Dhir and Purohit 1 observed experimentally that film boiling heat transfer coefficients in subcooled film boiling on a sphere were 50–60 percent higher than those predicted by the laminar plane interface theory. The significant enhancement in liquid side heat transfer was attributed to the alteration of flow field in the liquid by interfacial waves. Experimental results of Vijaykumar and Dhir 2,3 show that degree of subcooling significantly affects the liquid side heat transfer in subcooled film boiling on a flat vertical plate. Nishio and Ohtake 4 observed that subcooled film boiling in the ‘‘small cylinder diameter’’ regime could be divided into two distinct regimes, ‘‘normal’’ and ‘‘singular.’’ In the ‘‘normal’’ re- gime of film boiling for R-113 as the test fluid and for heater wire diameter greater than 0.2 mm the boiling heat transfer coefficient was found to increase with degree of subcooling. In the ‘‘singu- lar’’ regime of film boiling for R-113 as the test fluid and heater wire diameter less than 0.2 mm, the heat transfer coefficients were found to decrease to a minimum with increasing subcooling. However, the authors did not provide any explanation for this peculiar behavior in the ‘‘singular’’ regime. Kikuchi et al. 5 found that in saturated film boiling of water on silver coated spherical and cylindrical probes the liquid-solid L-S contacts occurred with a much higher frequency compared to subcooled film boiling, in which the L-S contacts were almost nonexistent. This pointed to the stabilizing influence of subcool- ing on the vapor-liquid interface in subcooled film boiling. They also found that the L-S contact frequency was lower for a cylinder than for a sphere. Linear stability analysis performed by Busse and Schubert 6 and also discussed by Busse 7, for a system undergoing first order phase change by heating from below pointed to the exis- tence of different regimes of vapor-liquid interfacial instability. An application of this analysis was in a geothermal situation where, for instance, water can be stably stratified over steam. The results of the aforementioned analysis were experimentally veri- fied by Ahlers et al. 8. The experiments involved isotropic- nematic phase transition of a liquid crystal. They reported various conditions where conduction temperature profile was obtained in the heavier overlying nematic phase. Under certain conditions of subcooling and wall heat flux val- ues the vapor film is stably stratified under the liquid pool in subcooled film boiling under pool boiling conditions. Study of such a configuration is helpful in understanding the hydrodynamic aspects of the vapor-liquid interface and heat transfer into the subcooled liquid. Such a situation was reported by Ayazi and Dhir 9, for subcooled film boiling of water on a horizontal cylinder. The authors argued that such a configuration of stationary vapor- liquid interface can exist only when the vapor production rate at the film matches the condensation rate at the bubble interface. Based on the experimental results, the authors proposed a criterion for the onset of collapse of subcooled film boiling on a horizontal cylinder. Though there is a huge body of literature on subcooled film boiling—very few investigations have been performed to under- stand the hydrodynamics of film boiling on a horizontal flat plate. Part of the reason can be ascribed to the difficulty in gathering experimental data for evolution of the interface on a flat plate compared to say, on a horizontal cylinder and also in proper post processing of the experimental data. The difficulty arises because of obstruction of the view by bubbles departing in front and back of the focal plane. This study was performed to enhance the un- derstanding of subcooled film boiling on a horizontal flat plate under pool boiling conditions. The objectives of the present inves- tigation were to: 1 compare the hydrodynamic and wall heat transfer experimental data with numerical predictions for a con- figuration of departing bubbles at low subcoolings; and 2 study the occurrence of a stably stratified vapor layer under the liquid pool for subcooled film boiling on a horizontal disc at high sub- coolings and compare the results with the numerical predictions of Banerjee et al. 10. For this purpose, film boiling experiments were performed from low to high subcoolings. In the experiments, spacing of the bubble 1 This work received support from the National Science Foundation. Contributed by the Heat Transfer Division for publication in the JOURNAL OF HEAT TRANSFER. Manuscript received by the Heat Transfer Division January 20, 2000; revision received October 31, 2000. Associate Editor: V. Carey. Copyright © 2001 by ASME Journal of Heat Transfer APRIL 2001, Vol. 123 Õ 285 Downloaded 24 May 2010 to 165.91.74.118. 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