Does two identical twisted tape inserts each with opposite twist result in a different thermal hydraulic performance?
Detailed single phase heat transfer and pressure drop tests were conducted in a tube with two types of twisted tapes as shown in Figure 1. A twist ratio (L/D) is defined as the ratio of twist pitch at 180˚ to the tape width. Each stainless-steel tape had exactly similar physical parameters except that each had an opposite twist that resulted in a clockwise and anti-clockwise flow swirl, i.e., Left Turn (LT) and Right Turn (RT), respectively. An L/D of 4 and 5.5 of each type were tested.
Figure 2 shows Nusselt number (a heat transfer scale) versus Reynolds number for a fixed Prandtl number 14 (a fluid properties scale) for both types of twisted tapes with both L/D ratios. Heat transfer enhancement of 31% was observed with the RT tape and 33% with for the L/D=5.5. Similarly for the L/D = 4, RT and LT tapes showed 36 % and 38 % heat transfer enhancement, respectively.
As expected, the pressure drop with twisted tapes was higher than plain tube as shown in Figure 3. The least amount of pressure drop was 41% – 63% for RT tape at L/D = 5.5. A pressure drop of 42% – 66% was observed for LT tape L/D = 5.5 and a pressure drop of 47% – 68% was observed for RT tape L/D=4. The highest pressure drop of 56% – 70% was observed with LT tape L/D=4.
The goal of this study was to observe if flow swirl direction had any effect on heat transfer and pressure drop for similar operating conditions and if any difference were observed in the performance could that be attributed to the Coriolis effect due to the earth’s rotation. The study showed that the LT twist tape resulted in 2-3% better thermal performance. The enhancement is trivial enough that one could ignore it and consider it as a measurement error. However, on the other hand one could argue that the tests were almost identical using the same measurement instruments. This is a topic that could further be explored with various twist ratios, Reynolds and Prandtl number ranges and larger diameter where secondary swirls and even tertiary swirls could affect the thermal-hydraulic performance.
For further information read the following paper:
Ahmad Abbas, Zahid Ayub, Adnan Ayub, Wei Li and Shehryar Khan, “Comparing Single-Phase Thermal Hydraulic Performance of a Right and Left Turn Twisted Tape Insert in a Tube”, Journal of the Brazilian Society of Mechanical Sciences and Engineering, Vol. 43, 123 (2021). https://doi.org/10.1007/s40430-021-02828-6.
Figure 1. Left Turn and Right Turn twisted tape, L/D = 5.5 and L/D = 4
Figure 2. Nusselt number versus Reynolds number at Prandtl number 14
Figure 3. Pressure drop versus Reynolds number at Prandtl number 14