Orientation and related buoyancy effects in low-velocity flow boiling.

This work is an extension of experimental results reported previously, which might provide design guidance for approximating certain aspects of the flow boiling process in microgravity but taking place in Earth gravity. In that research the buoyancy effects on the bubble dynamics were minimized by the imposition of a liquid velocity parallel to a flat heater surface in the inverted horizontal position, or nearly horizontal (within +/-5 degrees ), thus holding the heated liquid and vapor formed close to the heater surface. For the fluid used, liquid velocities in the range U= 5-10 cm/s were judged to be critical for changes in the behavior of the flow boiling process. Using the hydraulic diameter of the rectangular duct used, with the heater surface embedded in one side, this velocity range gives rise to flow Reynolds numbers on the order of 4400-8800. It was subsequently judged to be of interest to extend the range of orientation of the flat heater surface relative to gravity to the full circular range of 0-360 degrees, in increments of 45 degrees, and the results of this work are presented here. A solid massive copper heater with a gold-plated boiling heat transfer surface 19 x 38 mm in size, previously used for critical heat flux measurements with boiling, provided a near-uniform surface temperature. Only steady measurements of heat flux and surface temperature were possible with the copper heater. R-113 was the fluid used; the velocity was varied over the interval of 4-28 cm/s; bulk liquid subcooling was varied over 5-11 degrees C; and heat flux varied over 0-10 w/cm(2).