Subcooled boiling bubble condensation experiments were conducted in a vertical-upward annular channel by using water as the testing fluid at atmosphere pressure. The test runs comprised of bulk liquid temperatures, velocities and wall heat fluxes ranging from 75.0 degrees Celsius to 98.0 degrees Celsius, 0.25 m/s to 1.0 m/s and 150 kW/m2 to 200 kW/m2 respectively. A particle/droplet image analysis system was employed to capture the flow channel at four locations downstream of heated section for a total of 13 test conditions. The bubble Sauter-mean diameter was obtained in the range of 0.1 mm to 0.9 mm. It is also found that bubble sizes increase with the increase of liquid temperature or the decrease of liquid velocity. The condensation Nusselt number was calculated to be in the range of 10-4 to 10-1, which is much smaller than the typical range of 100 to102. This might due to the existence of non-condensable gas in the bubble
