The outflow area of a vertical annulus with closed bottoms was varied to investigate its effect on pool boiling heat transfer. For the test, a smooth stainless steel tube of 19.1mm diameter and the water at atmospheric pressure were used. The ratio of areas of the outflow and the gap space was varied between 0.04 and 1.13. As the area ratio decreased a clear change in heat transfer was observed at high heat fluxes. It was found that heat transfer on the tube surface was strongly dependent on the intensity of the liquid agitation and the evaporative mechanism under the coalesced bubbles. As the ratio was lower than 0.1 the generation of critical heat flux was observed even at a low heat flux less than 100kW/m²².

pool boiling, annulus, closed bottoms, outflow area.