Abo Bibliothek: Guest

ISSN Online: 2377-424X

International Heat Transfer Conference 12
August, 18-23, 2002, Grenoble, France

Film Boiling Heat Transfer from a Downward-Facing Hemisphere of Relatively Large Diameter

Get access (open in a dialog) DOI: 10.1615/IHTC12.4580
6 pages

Abstrakt

The strategy of in-vessel retention through external vessel cooling (IVR-EVC) was suggested to protect the lower head from being overheated due to relocated material from the core during a severe accident. The cavity flooding was selected because of relatively simpler installation than flooding within the thermal insulator. However, the cavity flooding tends to take much more time than flooding within the thermal insulator. The differential time between the two flooding strategies was estimated to be as large as forty minutes in a typical pressurized water reactor (PWR). It is thus questionable whether the reactor vessel could indeed be soaked prior to relocation of the molten core unless the core damage state is recognized early enough to allow for timely flooding of the lower head. Once the core material has been accumulated prior to flooding, the initial heat removal mechanism may most likely be transient, turbulent film boiling of water. The current understanding is mostly limited to steady-state, laminar film boiling on the sphere, however. Further the correlations were developed from the test sections much smaller than the reactor vessel. The laminar film boiling heat transfer coefficients will tend to underestimate the actual heat transfer from the lower head. In this study the film boiling heat transfer coefficients for a downward-facing hemispherical surface are measured from quenching tests. The test section is made of copper to maintain the Biot (Bi) number below 0.1. The results of this experiment are compared with predictions by the laminar film boiling correlations. It is observed that the higher thermal conductivity of copper results in the lower wall superheat and heat fluxat the minimum heat flux condition in the tests The maximum heat flux from the present transient experiments is found to be smaller than the critical heat fluxreported from previous steady-state experiments.