Boiling heat transfer experiments were preformed for narrow-horizontal-channels at 0.1 MPa. Test fluid was ethanol. The height and the width of the test channels were in a range of 1.0 mm ∼ 4.0 mm and 1.0 mm ∼ 3.0 mm, respectively. The channel length was 30 mm and 60 mm. The boiling surface was at the center of the bottom wall of the flow channel. Both ends of the channel were opened to wide space filled with liquid. Pool boiling experiments for the small heat transfer surfaces of the diameter of 1.0 mm ∼ 20.0 mm were also performed. Following conclusions were obtained. As the diameter of the boiling surface became small in pool boiling, boiling heat transfer was enhanced and the critical heat flux increased. The relation between the boiling heat flux and the wall superheat in the narrow rectangular channel was the same as that of the pool boiling for the same heat transfer surface size. As the channel height became narrow, the critical heat flux became low. In the case of the narrow-rectangular channel, large-coalescent bubbles occupied the top part of the flow channel. When the large bubble reached one outlet of the channel, almost all coalescent bubbles in the channel left from the outlet. After the selective departure of bubble from a specific outlet had established, the small reverse-inclination of the flow channel had no effect to alter the bubble departure outlet. It was suggested that the considerable steady-circulation flow had established. The CHF condition was not given only by the mass balance that the liquid supply to the channel was equal to the vapor generation rate. It was suggested that other effects such as the circulation flow should be included into the consideration for the CHF condition.

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