An experimental investigation was made of forced convection film
boiling of subcooled water around a sphere at atmospheric pressure.
The water was sufficiently cool that the vapor condensed before leaving
the film with the result that no vapor bubbles left the film. The experimental
runs were made using inductively heated spheres at temperatures
above 740°C. and using inlet water temperatures between
15°C. and 27°C. The spheres used had diameters of 1/2 inch, 9/16
inch, and 3/8 inch and were supported by the liquid flow. Reynolds
numbers between 60 and 700 were used.
Analysis of the collected non-condensables indicated that oxygen
and nitrogen dissolved in the water accumulated within the vapor film
and that hetrogeneous chemical reactions occurred at the sphere
surface. An iron-steam reaction resulted in more than 20% by volume
hydrogen in the film at wall temperatures above 900°C. At temperatures
near 1100°C. more than 80% by volume of the film was composed
of hydrogen. It was found that gold plating of the sphere could eliminate
this reaction.
Material and energy balances were used to derive equations which
may be used to predict the overall average heat transfer coefficients
for subcooled film boiling around a sphere. These equations include
the effect of dissolved gases in the water. Equations also were derived
which may be used to predict the composition of the film for
cases in which an equilibrium exists between the dissolved gases and
the gases in the film.
The derived equations were compared to the experimental results.
It was found that a correlation existed between the Nusselt number
for heat transfer from the vapor-liquid interface into the liquid and
the Reynolds number, liquid Prandtl number product. In addition,
it was found that the percentage of dissolved oxygen removed during
the film boiling could be predicted to within 10%.