Subcooled boiling heat transfer to R 12 in an annular vertical channel

A study of subcooled boiling heat transfer under forced convection to the fluorocarbon R12 in a concentric annular vertical tube is reported. The experiments are carried out with temporal linear increasing heat flux density until onset of nucleate boiling occurs. With further increase in heat flux density a gradual transition towards fully developed boiling is observed. Then the heat flux density is reduced until the boiling curve approaches the curve for single-phase heat transfer again. The experimental data show an interesting phenomenon. The heat flux density to achieve the boiling initiation is significantly higher than that point, where boiling is terminated, during decreasing the heat flux. The wall superheat at the position of incipient boiling is relatively high and a sharp drop in wall temperature takes place by the boiling initiation. This very pronounced boiling hysteresis depends mainly on the very low surface tension, which increases the wetting abilities of the refrigerant Freon 12. Therefore the larger nucleating sites on the heated surface are flooded prior to boiling, leaving only very small sites as potential nucleating sources. This demands a higher surface heat flux to activate the residual sites. Finally, the obtained boiling curves and heat transfer coefficients are presented as a function of different mass flux densities (500-3000 kg/m/sup 2/*s), critical pressure ratio (0.242-0,605), heat flux densities (1-20 W/cm/sup 2/) and liquid subcoolings of 10-50K