Thermofluidic characterization of additively manufactured butterfly-shaped inserts for heat transfer enhancement in tubular heat exchangers

Heat transfer enhancement in heat exchangers still represents a crucial target for industrial plants efficiency. Passive methods, such as turbulators, are usually preferred to active ones due to their intrinsic capability to operate on the fluid flow without any need for auxiliary power. Butterfly-shaped inserts stand as valuable solutions for tubular heat exchangers due to their high degree of induced turbulence. However, their potential for turbulent regimes has not been fully disclosed yet. In the present work, different geometries of swirled butterfly inserts, directly integrated into tubular samples via additive manufacturing, are investigated with the aim of providing a full characterization of their thermofluidic behavior in the range 4000 < Re < 14,000. Steady-state tests are carried out by circulating water through the test section, heated by Joule effect. Pressure drops are monitored via liquid column gauges; inlet/outlet water temperature is measured by means of thermocouples, while the outer wall temperature is acquired via infrared thermography. The local and global Nusselt numbers Nu and Nu‾ are estimated by analytically correcting the measured outer wall temperature distributions. The fitting loss coefficients K related to each butterfly insert geometry are additionally estimated by considering the introduced local pressure losses. Finally, the Nusselt number and friction factor deviations from the benchmark geometry (plain tube) are compared through performance evaluation plots, by also quantifying performance indexes η. The results highlighted that holes in the butterfly insert layout are highly beneficial in terms of augmented heat transfer and mitigated pressure drops. The provided pieces of data can be useful for the design of butterfly-shaped inserts operating in turbulent flows, as well as for the conceptualization of novel heat transfer augmentation approaches based on modular, 3D printed sections for tubular heat exchangers.