Heat Transfer Augmentation In the Post-Dryout Region with Inserted Enhancers

Abstract

Experimental studies have been conducted on the heat transfer augmentation and pressure drop characteristics in the post-dryout region of water/steam flow in circular heated tubes inserted with three types of enhancer: A core tube, a twisted tape, and a wire-wrapped tube. The experiments were performed using two once-through boiling water flow apparatuses of Joule heated and sodium-heated types. The experimental results of both tests with the inserted enhancers are evaluated, focussing on the heat transfer coefficient ratio and the pressure drop ratio compared to those without enhancers. Significant enhancement in heat transfer has been observed in the post-dryout region for each type of the inserts. It should be also noted that the degree of wall temperature fluctuation is remarkably reduced around the dryout point. The results indicate that the twisted tape is the best among the three types from the viewpoint of the combination of the heat transfer augmentation and pressure drop characteristics, and that the steam generator concept with enhancers inserted in the heating tubes has a highly cost-reducing potential for once-through type steam generators. It is estimated that the twisted tape insert could reduce the heat transfer surface area of the steam generator by 35% and the total weight by 30%