Longitudinal vortex generator applied to heat transfer enhancement of a flat plate solar water heater

Abstract

The population growth and industrial development increase the demand for energy and the solar energy conversion is an important alternative to complement the world energy matrix. Flat plate solar collectors are devices that convert the solar energy into thermal energy and store the thermal energy in a working fluid, which could reduce the required electricity to heat water. However, an increase in the heat transfer in this type of equipment remains a great challenge. In the current research, two types of longitudinal vortex generators are placed into circular tube of a solar collector in order to enhance the heat transfer between tube walls and working fluid. The numerical modeling is performed by ANSYS 18.0 software and the flow pattern and heat transfer characteristics are analyzed in details for two types of vortex generators (delta-winglet and rectangular-winglet), Reynolds number of 300, 600 and 900, and angle of attack of 15 degrees, 30 degrees and 45 degrees. Ten vortex generators were uniformly arranged on a flat-plate inside the tube. The results show that the delta-winglet and rectangular-winglet longitudinal vortex generators are in fact a passive technique capable to enhance the heat transfer in a solar water heater. The highest heat transfer is obtained for attack of the angle of 45 degrees for both vortex generators, which is more pronounced for the rectangular-winglet vortex generator. However, the best ratio between the heat transfer and the pressure drop penalty is achieved for the delta-winglet vortex generator for angle of attack of 30 degrees. The secondary flow is successfully generated by the inserts, although the corner vortex is observed only by rectangular-winglet vortex generator. Finally, the first vortex generator only impacts on the friction factor and can be removed with no significant losses on the heat transfer.