STUDY ON TIME-SPATIAL CHARACTERISTICS OF HEAT TRANSFER BY VISUALIZATION OF INFRARED IMAGES AND DYE FLOW

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The effect on time-spatial heat transfer characteristics of large-scale vortices was investigated by means of observing both dye flow and infrared images. Longitudinal vortices were artificially generated by a single winglet vortex generator in a pipe. The patterns of iso-velocity, static pressure, friction factor, and heat transfer coefficients show good correspondences to phenomena of the main vortex spirally flowing downstream. The quantitative analogy between the heat transfer and the shear stress is confirmed except for some regions, where the effects of the down-wash or blowaway of the secondary flows is caused by the main vortex rotating on its own axis on the surface at the wall. When the blunt body placed at the center of the duct height, the side vortex is observed to occur at the wall in an interlocking motion with a Karman vortex. These periodical phenomena of growing side vortices play a role in the increase of heat transfer. At narrow clearances, Karman vortex was not shedding, the maximum heat transfer is near the separation point of flow after it has passed the clearance space. When the body was attached to the duct surface, the vortices in the shear layer flow in good order to downstream direction, then combine with each other in downstream region, and after that large vortex attaches the surface. Near the reattachment point, the temperature shows high and low values alternatively. Attaching vortices to the surface caused to decrease the temperature on there, and the low temperature region moves downstream according to the vortices. The reattachment point was kept a high temperature until next vortex attaches to there successively. The heat transfer was augmented by the reattachment and detachment of large-scale vortices.