Boiling in an unconstricted bed with 65% porosity formed by spherical nickel particles with 28.3 μm mean diameter was studied in two sets of experiments. In the first set, the granular bed had the shape of a cylindrical annulus and heat was added through the outer cylinder surface with a heat flux up to 1 kW m-2. Visual observation established in ascending order the evidence of a dry-out region at the bottom of the column, a capillary region in which liquid, as well as vapor, occurred in the voids of the bed, and a chimney region with essentially horizontal cracks and vertical channels and with this uppermost region in a heaving motion. Whether all of these regions existed depended on the total height of the bed. In the second set of experiments, performed in a pressure vessel containing an array of nine heated tubes with the granular medium outside the tubes, the establishment of a dry region below the wetted region (comprising the capillary and chimney regions) was determined by the measurement of the vertical temperature profile along the tube wall at a heat flux up to 50 kW m-2. The wetted region in the bed is independent of the pressure level and of the total height of the bed. The dry region forms when the total bed height is larger than the wetted region. The height of the capillary region changes inversely proportional to the square root of the heat flux whereas the chimney region is essentially independent of it.
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Acknowledgement-The topic of this study was suggestedb y the Electric Power Research Institute, which also provided financial support. Helpful dicussions with Dr S. Green and Mr D. Steininger of that organization are gratefully acknowledged. Partial support was also provided by the National Science Foundation, Heat Transfer Program.