Turbulent thermal-hydraulic characteristics in a spiral corrugated waste heat recovery heat exchanger with perforated multiple twisted tapes

Waste heat recovery technology can undoubtedly improve energy saving and reduce carbon emissions, as one of the key equipment, heat exchangers always determine the waste heat recovery efficiency. In this study, to improve the heat transfer potentiality of conventional plain tube heat exchangers, a synergistic thermal enhancement technology of a spiral corrugated tube (SCT) with multiple twisted tapes (TTs) was devised by taking liquid-gas heat exchanging as a typical waste heat recovery scenario. The impacts of the TTs numbers, transverse space ratios (S/D), and longitudinal perforation length ratios (L/D) on tubular turbulent heat transfer -fluid flow characteristics were numerically investigated, consisting of flow fields, temperature contours, local Nusselt number (Nu) distributions, vortex structures, and turbulent kinetic energy contours, as well as average Nu, friction factor (f), and overall thermal performance evaluation index (ri). The findings proved that the fitting of multiple TTs into the SCT greatly homogenized the flow fields and temperature distributions, and increased the Nu and f, in which both of them also raised with the increase of the number of TTs. The ri dropped and subsequently augmented as the quantity of TTs increased. Furthermore, perforations on the surface of the quadruple TTs could attenuate the f with a relative less reduction in the Nu, which caused higher ri values than the ones without perforations. Notably, as demonstrated by the vortex structures, this could be ascribed to the fact that the perforations reduced excessive longitudinal vortex interactions in the mainstream regime without reducing the impingement to the thermal boundary layer too much. Both the Nu and f generally declined with rising S/D and L/D. Overall, by means of simple mounting perforations on the surface of the quadruple TTs, the maximum ri could be further enhanced up to around 7.9% and a maximum ri of 1.23 was obtained by the SCT with S/D = 0.152 and L/D = 0.348.