Heat transfer enhancement in a tube with twisted tape inserts placed separately from the tube wall

Abstract

Flow friction and heat transfer behavior in a twisted tape swirl generator inserted tube are investigated experimentally. The twisted tapes are inserted separately from the tube wall. The effects of twist ratios (y/D=2, 2.5, 3, 3.5 and 4) and clearance ratios (c/D=0.0178 and 0.0357) are discussed in the range of Reynolds number from 5132 to 24,989, and the typical one (c/D=0) is also tested for comparison. Uniform heat flux is applied to the external surface of the tube wall. The air is selected as a working fluid. The obtained experimental results from the plain tube are validated by using well known equations given in literature. The using of twisted tapes supplies considerable increase on heat transfer and pressure drop when compared with those from the plain tube. The Nusselt number increases with the decrease of clearance ratio (c/D) and twist ratio (y/D), also increase of Reynolds number. For all investigated cases, heat transfer enhancement (ζ) tends to decrease with the increase of Reynolds number and to be nearly uniform for Reynolds number over 15,000 and y/D lower than 3.0. The highest heat transfer enhancement is achieved as 1.756 for c/D=0.0178 and y/D=2 at Reynolds number of 5183. Consequently, the experimental results present that the best operating regime of all investigated twisted tape swirl generator inserts is detected at low Reynolds number, leading to more compact heat exchanger. The empirical correlations based on the experimental results of the present study are also given for prediction the heat transfer (Nu), friction factor (f) and heat transfer enhancement (ζ).