Studies on Heat Transfer Enhancement by Longitudinal Vortices Being Produced with Vortex Generator Insertion a Pipe

The longitudinal vortices were artificially generated by a single winglet vortex generator in a pipe. The longitudinal vortex enhances the heat transfer compared to other vortices produced by protrusions, because this longitudinal vortex does not decay until relatively further downstream. The purpose of this study is to know the informations about the flow motion of longitudinal vortex and the influence of vortices on heat transfer enhancement. The flow pattern was visualized by means of both fluorescein and rhodamine B, as a tracer of the water flow in a water table. The main vortex was spirally moved along the circumference and the behavior of other vortices was observed. Streamwise and circumferential heat transfer coefficients on the wall, wall static pressures, and velocity distribution in the overall cross section were also measured for air flow in the range of Reynolds number from 18 800 to 62 400. The distributions of streamwise heat transfer coefficients had a periodic pattern, and the peaks in the distribution moved circumferentially due to the spiral motion of the main vortex. Relation of the iso-velocity distribution and the wall static pressure to the heat transfer characteristics was shown. Moreover, the thermal performance was calculated under the condition of fixed pumping power, and it was found that the performance ratio is almost larger than unity