Turbulent thermal, fluid flow and thermodynamic characteristics in a plain tube fitted with overlapped multiple twisted tapes

Turbulent thermal-hydraulic characteristics in a plain tube by using overlapped multiple twisted tapes (MTTs) with counter large/small combinations (C-L/S-C) were experimentally investigated. Reynolds number (Re) ranged from 5800 to 19200, tape number changed from 3 to 5 and overlapped twisted ratios (P1:P2) were 0.74 to 2.97. By using air as working fluid, heat transfer tests were performed under the constant heat flux conditions and pressure drop experiments were conducted at the isothermal conditions. Nusselt number (Nu) and friction factor (f), overall thermal performance evaluation criterion (PEC), entropy generation and entransy dissipation in the MTTs were compared. The results show that Nu and f increase with increasing tape number and decreasing overlapped twisted ratio. The experimental results demonstrates that both increasing tape number and decreasing overlapped twisted ratio result in the reduction of entropy generation due to heat transfer and the rise of entropy generation due to friction resistance. It is found that the indicator of equivalent temperature difference, reflecting entransy dissipation and heat transfer ability, has the same sequence to that of the heat transfer behaviors. In the scope of this study, both the maximum Nu and the largest increased f are obtained in the 5MTTs with values respectively about 98.4% higher and 9.13 times than that in the plain tube while the highest PEC is found in the 3MTTs with P1:P2 = 0.74 with a value of being about 1.08. The least total entropy generation is gained by the 3MTTs with P1:P2 = 0.74 at Re = 17090 and the smallest entransy dissipation per unit energy is achieved by the 5MTTs under Re = 17294. Furthermore, thermal-hydraulic empirical correlations with deviations of ±5% are developed and comparisons with previous studies are also conducted.