381598 Thermohydraulic Characteristics of a MICRO-Fin TUBE with Twisted Tape Inserts

Monday, November 17, 2014
Galleria Exhibit Hall (Hilton Atlanta)
Smith Eiamsa=Ard, Mahanakorn University of Technology, Bangkok, Thailand

The convective heat transfer coefficient and pressure drop characteristics of turbulent tube flow through a micro-fin tube with twisted tape in co/counter-arrangement have been studied experimentally. In the experiments, the twisted tapes with different twist ratios (y/W) were placed along a constant heat flux micro-fin tube in turbulent regime. Water as the tested fluid is passed in a Reynolds number range of 5000 to 26,000. Five twisted-tapes of different twist ratios, y/W = 1.5, 2.0, 3.0, 4.0 and 5.0, are introduced in each run.  In the current work, the twisted tape was used as vortex generators while the micro-fin was used as turbulators for enhancing the heat transfer coefficient by breakdown of thermal boundary layer and by mixing of fluid flow in tubes. Heat transfer enhancement is evaluated by comparing the present experimental results with the results of present plain tube (non-swirl flow) and also those obtained from standard correlations.  The experimental results reveal that at similar conditions, the Nusselt numbers in the system with a micro-fin tube and twisted tape are considerably higher than those in the system without tape.  Evidently, the tape with the smaller twist ratio gives higher heat transfer rate, friction factor as well as thermal performance factor than the one with larger twist ratio as a result of a larger contact surface area, and stronger turbulence intensity and thus better fluid mixing which leads to a thinner thermal boundary layer. The experimental result reveals that for using the micro-fin tube with twisted tape, the increases in the mean Nusselt number and friction factor are, respectively, up to 226% and 7.47 times of the plain tube and the maximum thermal performance factor is 1.93.  It is also obvious that the heat transfer rate and friction factor obtained from the micro-fin tube with twisted tape in counter-arrangement is higher than that from the micro-fin tube with twisted tape in co-arrangement. In addition, correlations for Nusselt number, friction factor and performance evaluation criteria to assess the real benefits in using micro-fin tube with twisted tape in co/counter-arrangement are reported.

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