Abstract
This paper presents an experimental investigation on forced convective boiling of refrigerant HCFC123 in a mini-tube, and visualization of boiling phenomena at the onset points of nucleate boiling is conducted. It was indicated in the former report that heat transfer characteristics of forced convective boiling in mini-tubes, of which inner diameter is φ0.51mm and φ0.30mm, are different from those in conventional tubes. In order to clarify this difference of heat transfer characteristics, bubble formation and departure at the onset points of nucleate boiling were observed and investigated, using a glass tube on which ITO film is formed. The bubble diameter at departure becomes small with mass flux and becomes large with heat flux in a mini-tube. The bubble occupies almost the whole tube cross section and grows up into a long thin vapor plug as heat flux becomes larger, and thereafter, the flow pattern changes into an annular flow right after the onset points of nucleate boiling. Accordingly, forced convective evaporation can be dominant for low mass flux and high heat flux, which is so different from heat transfer characteristics in conventional tubes. From these results, there seems to be some potential that nucleate boiling influences formation of flow pattern in mini-tubes.
