To obtain effects of major geometric parameters on subcooled heat transfer in vertical annuli three gap sizes (7.05, 18.15, and 28.2 mm) and two bottom conditions (open or closed) have been investigated experimentally in water at atmospheric pressure. Up to 50 degrees C of liquid subcooling has been tested and 429 data points were obtained at both single phase and boiling regions. The increase in pool subcooling results in much change in heat transfer coefficients. The governing mechanisms are suggested as single-phase natural convection and liquid agitation for the annuli with open bottoms while liquid agitation and bubble coalescence are the major factors at the bottom-closed annuli. Four empirical correlations for the heat transfer coefficient have been suggested in terms of the gap size, the degree of subcooling, and the heat flux. The correlations predict the heat transfer data of single phase and of boiling within +/- 5% and +/- 20%, respectively. (c) 2006 Elsevier Ltd. All rights reserved.