Performance Evaluation of Twisted Tape Inserts inside an Evaporator on Flow Boiling Heat Transfer Enhancement and their Effect on Pressure Drop

In the present study, experimental investigations on flow boiling heat transfer enhancement (using twisted tape inserts inside an evaporator) and on its simultaneous effect on pressure drop were carried out. The main goal of this research was to obtain the maximum heat transfer enhancement with the least increase in pressure drop. The experimental set-up used was a well instrumented vapor compression refrigeration cycle. This set-up consisted of a test-evaporator as well. In addition, there were a pre-evaporator and an after-evaporator to achieve the required vapor quality. Refrigerant R-134a was electrically heated by the coil around it. The experimental investigation was carried out for a plain tube and for four twisted tapes inserted tubes with different twist ratios of 6, 9, 12, and 15. The results showed that the twisted tapes, in the best condition (twist ratio of 6), enhanced the heat transfer coefficient up to 68%, and in the worst condition (again twist ratio of 6), increased the pressure drop up to 180% above the plain tube values on a nominal area basis. Also, it was observed that among the tapes with different twist ratios, the tape with twist ratio of 12 in low mass velocities and the tape with twist ratio of 9 in high mass velocities, had the best performances.