Boiling heat transfer from a horizontal roll-worked tube enclosed by a concentric outer tube with two horizontal slots under reduced pressure conditions

An experimental investigation was carried out for the boiling heat transfer concerning a solar-powered absorption-type chiller. Two kinds of heat-transfer tubes (roll-worked surface and smooth surface) were tested in bulk liquid and in an annular narrow space formed by enclosing the heat-transfer tube within an isolated concentric outer tube with two horizontal slots under reduced pressure conditions, and the saturated boiling heat transfer characteristics were investigated experimentally. The experimental results indicated that the roll-worked tubes in bulk liquid had larger boiling heat transfer promotion than the smooth surface tube. The boiling heat transfer coefficients of the smooth tube were considerably enhanced when the space size was so selected as to take an optimum value. However, there was no definitive optimum space size and heat transfer enhancement for the roll-worked surface in the narrow space. Therefore, the compound enhancement effect of boiling heat transfer caused by narrow spaces and the surface treatment was not clearly observed in this experiment. Finally, the critical heat flux of boiling heat transfer in the narrow space was predicted by a modified correlation.