The outflow area has been varied to investigate its effect on pool boiling heat transfer in a vertical annular space. For the test, a smooth stainless steel tube of 19.1 mm diameter and the water at atmospheric pressure have been used. The ratio of areas of the outflow and the gap space has been changed from 0.04 to 1.13. As the area ratio decreases a clear change in heat transfer is observed. Throughout the area ratios the heat transfer coefficient is increased or decreased depending on the main mechanisms of heat transfer. The major cause for the tendency is attributed to the formation of a lumped bubble around the upper regions of the annulus and the reduced flow rate. (C) 2012 Elsevier Ltd. All rights reserved.