In this study, experimental investigation and CFD analysis were performed in order to find out the effect of twisted tape on CHF enhancement.
CHF experiments were performed in the smooth tube and the twisted tape inserted tube. Inside diameter and heated length of the smooth tube is 10.4 mm and 1000 mm, respectively. The test section was vertically installed in the test loop and uniformly heated by electricity. The values of the twisted tape ratio y (the length for 180° twist of inside diameter) are approximately 5.77, 7.21, 9.62. The width and thickness of the twisted tape are 9.0 mm and 0.3 mm, respectively. The working fluid is R-134a, which entered into the test section upward with an inlet subcooling of 30-50 kJ/kg. The test pressure is ranged from 12 to 25 bar. The mass flux is ranged from 1000 to 5000 $kg/m^2s$. The experimental results of the smooth tube and the twisted tape inserted tube were compared with each other. The CHF in the twisted tape inserted tube was enhanced to 10-80 % comparing with CHF in the smooth tube. The pressure drop in the twisted tape inserted tube was also increased about 2 to 7 times than that of the smooth tube.
CFD analysis was performed at the same geometry and flow condition of the CHF test to investigate the detailed flow characteristics in the twisted tape inserted tube. From the CFD calculation, swirl flow was generated by the twisted tape and increases the tangential and axial velocity. Tangential velocity is the highest at the end of the twisted tape and the lowest between the twisted tape and the tube wall. Thus, the cases of different gap size between twisted tape and tube wall were also calculated. As the width between the twisted tape and tube wall becomes narrower, the swirl effect of the twisted tape on CHF enhancement becomes stronger without imposing additional pressure drop.