In the present study, heat transfer and thermal efficiency of double-pipe heat exchangers with novel Double- Perforated Inclined Elliptic (DPIE) turbulators were experimentally investigated. The range of Reynolds num- ber was between 5000 and 18,000 under turbulent flow regime. The inclination angle of the elliptic inserts (α) , and the perforation diameters (d) varied from 15 ◦ to 25 ◦ , and 0.5 mm < d < 1.5 mm, respectively. The perforated vortex generators can significantly increase the flow perturbations and disrupt the thermal boundary layer to enhance the heat transfer without noticeable impact on the friction loss. The experiments revealed that the average Nusselt number was increased by 217.4% by using DPIE turbulators compared to the tube without vortex generators. The recirculations through the perforations of the elliptic turbulators, increase the fluid mixing between the walls and the core area. The maximum thermal efficiency parameter of 1.849 was obtained for DPIE vortex generators with d/b = 0.25 and α = 25 ∘ . The Friction factor increment of 14.0% was observed for the double perforated elliptic turbulators with d/b = 0.25 compared to the typical elliptic inserts without perfora- tions. It was found that the heat transfer is increased around 39.4% by using DPIE inserts with d/b = 0.25 compared to the typical louvered strips without perforations. The main benefits of the proposed novel turbulators are their much higher thermal efficiency compared to the previous turbulators at wide range of Reynolds numbers, and their simple installation together with low manufacturing costs.

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