Heat transfer and fluid flow characteristics of heat exchanger tube with multiple twisted tapes and solid rings inserts

• Influence of circular ring with twisted tape on heat transfer is presented.
• Highest η of the order of 1.61 over smooth tube heat exchanger was obtained.
• Correlations for the Nu, f and η of the tubes with combined devices were developed.

Heat transfer and fluid flow characteristic of a roughened tube is investigated for constant heat flux. The inserts were employed for the swirl flow generation in the test section through the tube. Pitch ratios (l/D) = 1 and 2 of the solid ring tubular (SRT) and twist ratios (y/W) = 2, 3 and 4 of the twisted tapes (TT) are introduced into the core flow. The Reynolds number covered in the range of 6300–22500 for co-twist arrangements for the turbulent flow of air in the tested heat exchanger tube. The study reveals that the Nusselt number, friction factor and thermal performance factor of the integrated SRT and TT are in the range of 107–293, 0.93–0.99 and 1.46–1.61, respectively. The highest thermal performance factor in the order of 1.61 over smooth tube heat exchanger was obtained for the integrated device consisting of the SRT with l/D = 1.0 and y/W = 2. The quadruple co-twisted tape insert provides the maximum Nusselt number, friction factor, thermal performance factor. The correlations for the Nusselt number, friction factor and thermal performance factor of the tubes with combined devices were also developed in terms of flow and geometrical parameters.