RP-219 -- Thermal Transport Improvement by Condensation Augmentation ✓ Most Recent

Passive heat transfer augmentation techniques were investigated with tube-side condensation of R-113 vapor. The basic experimental facility in the Heat Transfer Laboratory was extensively modified for this purpose. A smooth tube, the smooth tube with twisted-tape inserts of two different pitches, three internally finned tubes, two tubes with repeated-rib roughness, and a spirally fluted tube were tested.

The twisted-tape inserts increased overall average heat transfer coefficients approximately 30 percent above those of the smooth tube on a nominal area basis. The finned tube with the shortest and largest number of fins had the best performance, with heat transfer coefficients (nominal area basis) increased up to 120 percent. The repeated-rib tubes increased overall average heat transfer coefficients about 80 percent over the smooth tube values. Improvements in heat transfer coefficient for the spirally fluted tube were approximately 50 percent. The pressure drop in the smooth tube with the twisted tapes was 250 percent higher than that in the smooth tube, while the pressure drop increases in the other augmented tubes were modest.

The heat flux effect had an insignificant effect on the performance when the condensate flow was turbulent. The superheat effect was important only when the temperature difference was small.

Several of the available smooth tube correlations correlated reasonably well the smooth tube heat transfer and pressure drop results. Heat transfer and pressure drop correlations were developed for the three augmentation techniques by modifying the smooth tube correlations. These correlations provided reasonable descriptions of the present experimental results. The finned-tube correlation did not agree with the correlations reported previously; qualitative reasons for this behavior are given.

The relative performance of these augmented tubes was assessed using the experimental results for both heat transfer and pressure drop. The evaluation suggested that, at the same heat duty, the best performing finned tube or the repeated-rib tubes would permit a substantial reduction in condenser size without increasing pressure loss.

The repeated-rib tubes merit further study in view of the small amount of extra material required to form the ribs.