Experiments were carried out to investigate the subcooled flow boiling heat transfer and visualize the associated bubble characteristics for refrigerant 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 heat flux, mass flux, liquid subcooling and saturation temperature of R-134a on the resulting nucleate boiling heat transfer and bubble characteristics were 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 undershoot at ONE is rather large for most cases. The boiling heat transfer was slightly 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 bubbles and the bubble generation frequency is higher. But the size of the bubbles departing from the heated surface was only slightly affected by the imposed heat flux. At high mass flux and subcooling the bubble generation was suppressed to a noticeable degree. Besides, the bubbles are much smaller at a higher subcooling. Finally, empirical correlations for the heat transfer coefficient and bubble departure diameter in the subcooled flow boiling of R-134a were proposed. (C) 2000 Elsevier Science Ltd. All rights reserved.