Thermohydraulic Performance of Heat Exchanger Tube Equipped with Single-, Double-, and Triple-Helical Twisted Tapes

Abstract

Heat transfer augmentation by dual-helical twisted tapes (D-HTTs) and triple-helical twisted tapes (T-HTTs) is presented in comparison with that by single-helical twisted tape (S-HTT). The effects of tape width ratio (w=D ¼ 0.1, 0.15, and 0.2) on heat transfer, friction factor, and thermal performance are also reported. The experiments were performed using air as a working fluid for Reynolds number between 6000 and 20,000. At similar conditions, the use of D-HTTs and T-HTTs leads to the increase of Nusselt number by respectively 15.6% to 17.6% and 19.5% to 23.4%, the increase of friction factor by respectively 83% to 206% and 143% to 335%, and the decrease of thermal performance factor by 3.9% to 20.3% and 8.3% to 26.2% compared to those obtained from the use of S-HTT=promoter. As the width ratio increases, both Nusselt number and friction factor are considerably increased. However, due to the poorer trade-off between the increases of Nusselt number and friction factor, lower thermal performance factor is consistently obtained at the conditions possessing higher Nusselt number. At similar conditions, S-HTTs consistently give a higher thermal performance factor than D-HTTs and T-HTTs; the maximum thermal performance factors given by S-HTTs with w=D of 0.1, 0.15, and 0.2 are 1.34, 1.31, and 1.29, respectively (at Reynolds number of 6000).

Heat transfer enhancement by helical screw tape coupled with rib turbulators

“…2) consisted of a test tube (calm and test sections), 3 hp high pressure blower, a set of T-type thermocouples, pressure taps, data acquisition system, personal computer, hot wire anemometer, inverter, U-tube or inclined manometer, power supply system and other support structures [22]. In the experiment, the pressure taps were located around 1260 mm (20D) upstream and 630 mm (10D) downstream of the test section.…”

“…In the experiments, air as the working fluid was tested under uniform heat flux condition [22]. Heat transfer rate gained by air is evaluated using the temperature difference across the test section as…”

“…The pressure drop (DP) across the test section length (L) was subjected to the calculation of friction factor via the following equation [22]:…”

“…Helical screw tapes (HSTs) have been used to introduce a swirl or turbulent near the tube wall which results in more efficient fluid mixing between near wall and core regions. The tapes were extensively modified for heat transfer enhancement such as regularly-spaced helical tape [11][12][13], helical screw-tape with/without core-rod [14,15], right-left helical screw tape [16][17][18], double helical tapes [19], triple helical screw tapes [20], co/counter helical screw tape [21] and double/triple helical twisted tapes [22].…”

“…Eiamsa-ard et al [21] reported that the helically twisted tapes inducing co-swirl flows (co-HTT) gave lower heat transfer coefficient and friction factor but higher thermal performance factor than the helically twisted tapes inducing counter-swirl flows (C-HTT). Recently, Eiamsa-ard et al [22] reported that although double helical twisted-tapes (D-HTTs) and triple helical twisted-tapes (T-HTTs) gave higher Nusselt number than the single helical twisted-tape (S-HTT), they caused significantly higher friction factor. This led to poorer overall result notified by the lower thermal performance factor.…”