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A comparative study on the enhancement mechanism by various twisted tapes (TTs) in association with the local flow pattern is investigated in detail via computational fluid dynamics (CFD). The effect of gap between TT and tube is further examined based on prior experimental validation. Result shows that the Nusselt number and friction factor will be increased by 12.6% and 18.1%, respectively, and the comprehensive thermal performance (CTP) will be improved from 1.05–1.07 to 1.12–1.15 when the gap is reduced from 1.0 mm to zero. The obtuse V-cut design in the experimental study shows superior performance, and the related enhancement mechanisms by windward, isosceles, and leeward V-cut configurations are numerically investigated. By comparing the local streamline at V-cut regions and the velocity distribution on the cross section, the windward V-cut might notably introduce fluid into the cut region and toward the tube wall subsequently, thereby resulting in effective heat transfer augmentation near the tube wall. Compared to the isosceles V-cut TT, the CTP of windward V-cut TT can be augmented from 1.15–1.18 to 1.16–1.21. However, the leeward V-cut TT will impair the CTP due to its inconsistent turbulence direction to the main helical flow. On the other hand, it is also indicated that fabricating three windward V-cuts at each helical pitch shows the highest improvement on CTP. Further increment of V-cut number may reversely reduce the CTP because of the rapid increase of flow resistance. Moreover, fabricating windward V-cuts on bilateral edges of the TT shows limited improvement.

The present study proposes a rationally based heat pump clothes dryer model which is capable of handling transient behaviors of both air-side and refrigerant side. The model can take into account the geometrical variation of the heat exchangers. Yet the proposed model is free from extra constraints that were normally imposed by previous studies. The simulation shows that there exist three stages in a typical drying process, including a preheat stage, followed by a constant evaporation stage, and finally a falling drying rate period. The calculation indicates that the variation of the clothes temperature and the system suction/discharge pressure show an appreciable rise in the first stage, remain nearly unchanged at the second stage, and increase again at the final stage of drying process. Also, the corresponding maximum specific moisture extraction rate (SMER) peaks at the end of the second stage while the corresponding coefficient of performance (COP) drops continuously. The simulation also indic...

This paper experimentally investigates the thermo-hydraulic performance of tubes with various twisted tapes (TTs) with the Reynolds number ranging from 4000 to 10,000 (at the turbulent transition region). The Nusselt number (Nu) and friction factor (f) of the tested plain tube shows good agreement with the Gnielienski and Filonenko equations. The average deviations are within 9.5% and 2.8%, respectively. For the successive TT tube, the existing correlation which includes the effect of gap between the TT insert and tube can well predict the present measurement, and the deviation upon Nu and f is within ±10%. Meanwhile, the successive TT with the twisted ratio of 4 can achieve the highest comprehensive thermal performance (CTP) by 1.08–1.10. Yet the Nu and f will simultaneously drop by 6.7% and 11.6% when raising the gap ratio from 0.043 to 0.135. The CTP is also pronouncedly reduced and can be negatively reversed than the plain tube at the turbulent transition region. On the other hand, it is validated that the U-cut and isosceles V-cut configurations might provide similar improvement on Nu and f compared to the successive TT. The obtuse V-cut configuration shows more pronounced improvement on Nu and f, and yields the highest CTP of 1.18–1.23 within the test range. The existing correlations applicable for the V-cut TTs are unable to predict the present measurements accurately.