Subcooled flow boiling heat transfer of R-134a and bubble characteristics in a horizontal annular duct

Experiments were carried out to investigate the subcooled flow boiling heat transfer and visualize the associated bubble characteristics for refrigera nt R-134a flowing in a horizontal annular duct having inside diameter of 6. 35 mm and outside diameter of 16.66 mm. The effects of the imposed wall hea t flux, mass flux, liquid subcooling and saturation temperature of R-134a o n the resulting nucleate boiling heat transfer and bubble characteristics w ere examined in detail. In the experiment significant hysteresis was noted in the boiling curves during the onset of nucleate boiling (ONB) especially at low saturation temperature and high subcooling. The temperature undersh oot at ONE is rather large for most cases. The boiling heat transfer was sl ightly higher for a lower saturation temperature and was little affected by the mass flux. However, for a higher subcooling of the refrigerant better heat transfer results. Furthermore, the flow visualization revealed that at higher imposed wall heat flux the heated surface was covered with more bub bles and the bubble generation frequency is higher. But the size of the bub bles departing from the heated surface was only slightly affected by the im posed heat flux. At high mass flux and subcooling the bubble generation was suppressed to a noticeable degree. Besides, the bubbles are much smaller a t a higher subcooling. Finally, empirical correlations for the heat transfe r coefficient and bubble departure diameter in the subcooled flow boiling o f R-134a were proposed. (C) 2000 Elsevier Science Ltd. All rights reserved.