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 immersed 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 gap, the bubble size will be restricted between surfaces. A growing vapour bubble from the surface will soon touch the neighbouring surface, be forced to take a flat shape and leave its nucleation site earlier than in a pool boiling, resulting in a higher boiling heat transfer rate. On the other hand, rising vapour bubbles will collide with those bubbles still attached to the surface and bring them away before they can leave the surface by themselves. This contributes further to enhancement 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 superconducting a.c. magnets that are bath cooled by liquid helium. (C) 1997 Elsevier Science Limited.