The object of this program was to conduct, at approximately atmospheric pressure, a visual and quantitative study of two-phase one-component flow in a horizontal tube evaporator under conditions of subcooled and bulk boiling over a range of uniform heat fluxes and test fluid flow rates.

A forced convection test loop including a horizontal twelve foot long test section comprised of five, .430" I.D., transparent electric resistance heating film coated glass tubes was constructed and operated over a range of inlet subcoolings (to 46F), heat fluxes (to 8000 BTU/hr-ft²) and flow rates (to 1392 lbm-hr) using "Freon-113" as the test fluid.

In addition to the measurement of all inlet conditions and the temperature, pressure, and heat flux distributions, high speed photographs were taken of each final type run in order to permit an accurate interpretation of the data.

The experimental results are (1) bubbles originating at the wall in the subcooled boiling region do not collapse but continue to grow as they move downstream; (2) the flow becomes annular soon after the bulk of the fluid becomes saturated; (3) the bulk of the fluid is superheated several degrees in the bulk boiling region; and (4) nucleation at discrete points does not contribute to the heat transfer once an annular flow is established.

Examination of the data in the light of existing correlations indicates that the actual void fraction must be included in any correlation for the subcooled and low quality boiling region where the quality is nearly zero for organic fluids and that a correlation including the effect of an actual mass quality will be the best for the annular flow region where the void fraction is nearly constant.

For the range of variables studied, location of the initial nucleate boiling point in the subcooled boiling region has a pronounced effect on the local heat transfer coefficient. However, the effect is due to a significant change in the fluid mechanics rather than a change in the nucleate boiling heat transfer contribution.

Units: I-P