Thermal performance enhancement in tubes using helically twisted tape with alternate axis inserts

Abstract

This article presents an investigation on heat transfer enhancement in a round tube inserted with a helically twisted tape. The effects of a helically twisted tape with alternate axis (HTT-A) on heat transfer, friction factor, and thermal performance factor behaviours are reported for the turbulent regime. HTT-A geometries are tape pitch to tube diameter, P/D = 1.0, 1.5, and 2.0; alternate length to pitch length, l/P = 1.0, 1.5, and 2.0; twisted length to tape width, y/W = 3.0; and tape width to tube diameter, w/D = 0.2. The experiment has been performed by varying the volumetric air flow rate in order to adjust Reynolds number ranging from 6 000 to 20 000. The wall of the testing tube is uniformly heated as a constant heat flux while the tests are covered with thermal insulations to reduce heat loss to surroundings. Thermal performance is evaluated by comparing the present experimental results with the results of the modified HTT-A and also those obtained from previous study (conventional helically twisted tape, HTT). The thermal performance of tested tube with HTT-A is evaluated to obtain the degree of heat transfer enhancement and friction factor induced by HTT-A with respect to the plain tube under the same test conditions. Evenly, it is interesting to observe that the tube with HTT-A consistently possesses higher heat transfer and thermal performance factor than those with the HTT around 14.1% and 1.9%, respectively. The HTT-A with the smaller pitch ratio and adjacent twist length provides higher heat transfer rate and friction factor than the one with larger pitch ratio and alternate length as a result of a larger contact surface area, stronger swirl intensity and, thus, better fluid mixing near the tube wall. In the range determined, the tubes with the largest pitch ratio (P/D = 2.0) and smallest alternate length (l/P = 1.0) give the highest thermal performance factor at around 1.35. In addition, the empirical correlations of the Nusselt number, friction factor, and thermal performance factor are also described.

Genetic Algorithm Integrated Fuzzy AHP-VIKOR Approach for the Investigation of W-Cut Insert Heat Exchanger for Cooling of Dielectric Fluid Used in Ultra-High Voltage Transformer

“…e presence of typical twisted tape in the tube leads to increased pumping power, which results in thermal performance below unity, so that proper design of twisted tape is required to increase the heat transfer of the equipment with reduced pressure drop [1]. Many research studies have been carried out in past decades to bring down the friction loss by modifying peripheral of twisted tape with different geometry, which includes double V-ribbed twisted tapes [2], horizontal wing-cut TT [3], perforated V-cut and U-cut TT [4], tapered twisted tape [5], TT consisting of wire nails [6], loose-fit perforated twisted tapes [7], twisted tape with alternate axis [8], ribbed twisted tape inserts [9], finned twisted tape [10], cross hollow TT [11], peripherally cut dual TT [12], helix TT with V-cut [13], square and V-cut twisted tape [14], centre-cleared twisted tape [15], dual TT [16], rectangular-cut TT [17], circular tube with lanced ring insert [18], and perforated helical TT [19].…”