Experiments on onset of nucleate boiling (ONB) have been performed for the natural circulation flow in a one-sided heated narrow rectangular channel. With fast camera putting into use, the visualization study was conducted to capture the location of ONB. The experiments were performed at pressure of 0.2 MPa and 0.3 MPa, with subcooling temperature ranging from 40 to 60 K, and heating flux ranging from 70 to 220 kW/m2. Differing from conventional channel, the superheats of wall surface in the narrow rectangular channel needed for ONB is larger, and can be predicted with Thom’s correlation in developed flow region. Moreover, the superheat needed for ONB will decrease for the existence of entrance effect. The visualization study indicated that the traditional determination of ONB will bring in more error. In this study, valuable data and image have been provided for the error analysis of ONB.

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