Single Phase Convective Heat Transfer Enhancement In Turbulent Flow With Twisted Tape Inserts

Numerical simulations and experiments are carried out in this paper to research about characteristics of heat transfer,frictional resistance and comprehensive performance of turbulent water flow(3,000<Re<10,000)in a circular tube of the heat exchanger with twisted tape inserts.A new type of twisted tape which is modified on the base of the typical twisted tape(TT tape)is proposed to enhance the effects of twisted tapes on heat transfer.The new type of twisted tape is twisted with alternation of clockwise and counterclockwise periodically(ACCT tape)with a cycle of 180°and semicircular cuts are uniformly distributed on both sides of the tape(ACCT tape with gap).Effects of twisted tapes with different twist ratios(p/w=3,4,5)and different geometrical structure(TT tape,ACCT tape,ACCT tape with gap)are researched in numerical simulations.The results show that the twisted tape of twist ratio p/w=3 performed best on heat transfer among typical twisted tapes with three different twist ratios.Nusselt number with the twisted tape of twist ratio p/w=3 are 1.39-1.83 times to Nusselt number in the plain tube while that with the other two twist ratios are respectively 1.16-1.40,1.09-1.34 times to Nusselt number in the plain tube.The twisted tape of lower twist ratio leads in more frictional resistance.However,the twisted tape of twist ratio p/w=3 achieves the largest values of PEC,which is the reason that modifications of twisted tapes on geometrical structure are conducted with the twist ratio p/w=3.The ACCT tape performed better on heat transfer enhancement than the typical twisted tape among twisted tapes with different geometrical structure in numerical simulations and the ACCT tape with gap improve the frictional resistance caused by inserting the ACCT tape at the sacrifice of lessening a little effects on heating transfer.The results of numerical simulations indicate that the ACCT tape with gap shows best performance on heat transfer enhancement.Experiments mainly researched effects on heat transfer enhancement with different types of twisted tapes(TT tape,the ACCT tape with gap)and twisted tapes of different lengths(L=2400mm,1800 mm,1200mm,600mm)inserted in the inner tube of the dual-pipe heat exchanger.At first,values of Nu and f calculated with experimental data were compared with values calculated with data in numerical simulations and values calculated with empirical formulas to verify reliability of the experimental data.The experimental results showed that the twisted tape of full length(L=2400mm)achieved the best effects on heat transfer and led in the most frictional resistance.The full-length ACCT tape with gap showed superiority on heat transfer enhancement when compared with the full-length typical twisted tape.Nusselt number with the full-length ACCT tape with gap are 1.66-2.42 times to Nusselt number in the plain tube while that with the full-length typical twisted tape are 1.35-2.11 times to Nusselt number in the plain tube.Frictional resistance caused by the full-length ACCT tape with gap are obviously more than the full-length typical twisted tape.As a result,the full-length ACCT tape with gap achieved the best effects on heat transfer enhancement and the largest value of PEC reached 1.42.At the end of the paper,a multipoint linear fitting correction algorithm was proposed based on Nu(Nusselt number)and f(friction factor)calculated with the experimental data and experimental correlations were established.The errors of predicted values obtained by the correlations with experimental calculated values are within ±8%.