Noninvasive interrogation of local flow phenomena in twisted tape swirled flow via positron emission particle tracking (PEPT)

Twisted tape inserts are often used to enhance the performance of heat exchangers. Previous simulations suggest the existence of secondary vortices and modified axial velocity profiles in these swirled flows, with only limited experimental evidence for these predictions. In this work, positron emission particle tracking (PEPT) is used to interrogate turbulent twisted tape swirl flow in a pipe. PEPT is a noninvasive, radiotracer based flow mea-surement technique that does not require optical access, making it suitable for measurements in a number of engineering flow systems. The existence of the modified velocity profile and secondary flows is confirmed using PEPT, and flow is seen to still be developing 20 diameters downstream of the twisted channel entrance. Sec-ondary vortices are further confirmed by the presence of increased vorticity in these regions, and turbulent fluctuations are seen to increase in magnitude near these flow structures. These results are in line with pre-dictions and point to possible regions of reduced heat transfer and hot spots in twisted tape based heat ex-changers. Suggestions are made for future experiments to investigate heated flows, utilizing the ability of PEPT to image flows in opaque systems for validating simulation of flows in engineering equipment, such as nuclear heat transport equipment.

Automated summary

In this study, PEPT technology was used to investigate the vortex and velocity distribution in twisted tape flows, confirming their existence and revealing their development up to 20 diameters downstream of the entrance. The increased vorticity and turbulent fluctuations suggest regions of reduced heat transfer and hot spots.