EXPERIMENTAL RESEARCH ON THE ENHANCEMENT OF BOILING HEAT-TRANSFER OF LIQUID-HELIUM IN NARROW CHANNEL

In the nucleate pool where boiling is produced from a heated surface i mmersed in liquid, bubble growth comes mainly from the evaporation of a liquid microlayer between the bubble and the heated surface beneath it. If boiling is confined in a vertical narrow channel with a small g ap, the bubble size will be restricted between surfaces. A growing vap our bubble from the surface will soon touch the neighbouring surface, be forced to take a flat shape and leave its nucleation site earlier t han in a pool boiling, resulting in a higher boiling heat transfer rat e. On the other hand, rising vapour bubbles will collide with those bu bbles still attached to the surface and bring them away before they ca n leave the surface by themselves. This contributes further to enhance ment of boiling heat transfer. In our experiment it was observed that boiling in a narrow channel reduces the temperature difference between liquid helium and heated surface. Boiling heat transfer rate (or heat flux) in a narrow channel is also much higher than in a pool boiling. A new method of enhancing nucleate boiling heat transfer is presented in this paper. We believe the method will be useful in designing supe rconducting a.c. magnets that are bath cooled by liquid helium. (C) 19 97 Elsevier Science Limited.