Swirling flow reorganization and heat transfer enhancement in a vortex tube with a diameter-transformed vortex generator

Abstract

The twisted-tape based vortex tube is recognized as a promising reactor for the highly energy-intensive steam cracking process. This study develops a short-length twisted tape, hereinafter termed the diameter-transformed vortex generator (DTVG), to reorganize decaying swirling flow through the coupling of bulk rotating flow and longitudinal vortices. Computational fluid dynamics with the RNG k–ε turbulence model was used to investigate macroscopic transport behavior, vortex structure evolution, sectional thermal–hydraulic performance and entropy generation. Based on vortex evolution, the DTVG vortex tube can be divided into induction, twin vortex and single vortex decay sections. This sectional swirling flow reorganization transforms the induction section into a vortex regulation region rather than a strong mixing region, while shifting the main heat transfer benefit to the twin vortex decay section. There, robust helical longitudinal vortex pairs promote wall-to-core transport, and near-wall shear renews the thermal boundary layer, yielding the highest Nusselt number and performance evaluation criterion values of 306.753 and 1.163, respectively. Entropy generation analysis further provides thermodynamic evidence for this functional transformation: compared with hollow twisted tape and conventional twisted tape, DTVG shows reductions of 21.7% and 21.3% in global entropy generation, with frictional entropy generation decreasing by 42.3% and 43.4%, indicating suppressed premature dissipation of mechanical energy during vortex initiation.