Forced convection heat transfer and pressure drop in horizontal tubes with internal twisted fins

The results of experimental studies on heat transfer and pressure drop during forced convection of air and liquid ethylene glycol in pipes with inner fins are presented. Tubes with integrated fins were made by extrusion from an aluminum alloy; the fins were straight or twisted spirally along the tube and their tips almost reached the tube axis. Depending on the process parameters, the heat transfer coefficients of the finned tubes that were obtained with these liquids were higher by a factor of 3 to more than a factor of 8 compared to plain tubes. The pressure drop was correspondingly higher for finned than for smooth tubes. The improvement in heat transfer cannot be explained quantitatively simply by linear coupling with the enhancement in the surface area due to finning. In addition to this quantitative effect, there is also a qualitative effect that is caused by the shape of the fins. The real improvement in heat transfer appears to be determined by efficient secondary flows that form in the flow channels between adjacent fins and interact with the vortex core flow in the middle of the tube cross section. © 2020 by Begell House, Inc.