Eiamsa-ard S.   Somravysin P.   Changcharoen W.   Promthaisong P.   Chuwattanakul V.   Pimsarn M.  

Flow-field and thermal behaviors of laminar flow through a round tube equipped with dual twisted tapes

Reporter: Eiamsa-ard S.

Twisted tape inserts extensively appear in heat exchanger systems for redeveloping the thermal boundary layer, inducing swirl flow and therefore enhancing the heat transfer perfor-mance. To date, twisted tapes can be used in compact heat exchangers because of its low cost, ease of maintenance and manufacture. Tubes with twisted tape insert have been also used in several thermal system applications; for example, heat recovery processes, air conditioning and refrigeration systems, and chemical reactors. Many researches on heat transfer enhancement for such kinds of configurations have been carried out for the past decades. Twisted tape was early introduced as heat transfer enhancement device in form of typical twisted tape. However, at some operating conditions, the typical twisted tape generated friction loss within the heat exchanger of unsatisfactory level, gave rise to higher pumping power, resulting in performance factor at the same pumping power below unity.

Flow/Thermal visualizations and heat transfer measurements had been employed to study the flow-field and thermohydraulic characteristics in a heat exchanger tube equipped with dual twisted tape (DTs).  Influences of the dual twisted tape (DTs) on the heat transfer, friction factor and thermal performance factor are presented along with those of a single twist-ed tape (ST).   The flow Reynolds number was studied in range between 1000 and 2000.  The DTs with three twist ratios y/w = 2.0, 2.5 and 3.0) were employed for comparative study with the ST under similar conditions. The article also presents the application of a mathematical model for numerical simulation of the dual swirling flows in a tube induced by DTs insertion.  The numerical simulation was performed in order to gain an understanding of physical behav-ior of the fluid flow (dual swirling flows), fluid temperature and local Nusselt number behaviors of a tube fitted with DTs in the laminar flow regime. The Navier-Stokes equation in common with the energy equation was solved using the SIMPLE technique with the RNG k- turbulence model.  The numerical results shown that, on the heat transfer rate, the DTs performed better than that of the plain tube and ST.  The results also show that heat transfer rate and friction increased with decreasing twist ratios (y/w).  At similar conditions, DTs (y/w = 2.0) offered high-er heat transfer rate, friction factor and thermal performance factor than DTs ones (y/w = 2.5 and 3.0) as they induced more consistent swirling flows and thus turbulence. This reveals that it is possible to gain promising tradeoff between enhanced heat transfer and increased friction by selecting the dual twisted tapes with proper geometries.

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