Effect of Internal Twisted Tape on Critical Heat Flux of Subcooled Flow Boiling under Nonuniform Heating Condition.

Abstract

Critical heat flux (CHF) under circumferentially nonuniform heating conditions was experimentally investigated using direct current heating of a stainless steel tube. CHF in a smooth tube without internal twisted tape under nonuniform heating conditions was slightly higher than that under uniform heating conditions because of the lower average qualities at the tube outlet under nonuniform heating conditions than those under uniform heating conditions. CHF in tubes with internal twisted tape under nonuniform heating conditions was much higher than that under uniform heating conditions, though the average qualities at the tube outlet were the same for both cases. The higher CHF under the nonuniform heating condition was explained by the alternating development and disruption of the bubble boundary layer. The circumferential location of burnout initiation was observed by video camera, and it was confirmed that CHF started at the point where the bubble boundary layer developed the most.