During the Three-Mile Island Unit 2 (TMI-2) accident, the lower part of the reactor pressure vessel had been overheated and then rather rapidly cooled down, as it was later found out in a vessel investigation project. These findings triggered a great deal of investigations to determine the critical heat flux (CHF) in narrow channels. Experiments were conducted to determine the CHF on a long downward heated rectangular narrow channel by changing the orientation of a copper crevice (5×105 mm2) type heater assembly. The test heater was placed in a demineralized, saturated water pool at atmospheric pressure. This work aims also to investigate the general boiling phenomena and the triggering mechanism for the CHF in the narrow channel through visualization of the bubble behavior in the vicinity of CHF. The test parameters include the channel size of 5 mm and the surface orientation angles from the downward facing position (180°) to the vertical position (90°). It was found that the CHF decreases as the surface inclination angle increases and as the gap size decreases. It was also shown that there exists a transition angle at which the CHF changes with a rapid slope, and that the inclination angle affects the bubble layer and the bubble discharge from the narrow gap.

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