There has been a surge in efforts aimed at producing more efficient heat exchangers. Typically, a heat exchange equipment is used to transfer heat between two or more fluids. Therefore, augmenting heat transfer rate can improve the performance of such systems. Contemporary research approaches mainly explored cylinders and ribs to enhance heat convection due to their structural simplicity. Over the years, researchers have explored other means to enhance thermal performance i.e. the use of stationary turbulence generators and passive vibrating enhancers. To this end, a plethora of literature has been reported on such systems, where several research papers have established that a properly designed turbulence generator can substantially improve the thermal performance of the system of concern. However, most of these reports were developed in the internal channel or the fintube heat exchangers, where the vortex development and the resulting heat transfer performance might be influenced by the tube and the channel walls.
In general, employing an appropriate turbulence generator is a simple and effective means to boost convective heat transfer. As such, further development of heat exchangers remains pivotal in enhancing their performance. To this end, a team of researchers from the Turbulence and Energy Laboratory at University of Windsor in Canada: Yang Yang (PhD candidate) and Professor David S-K. Ting, in collaboration with Steve Ray at the Essex Energy Canada, developed state-of-the-art convection heat transfer enhancers by focusing on the simple flexible strip placed normal to the freestream. The series of this research work which is now published in International Journal of Thermal Sciences was partly motivated to improve the solar photovoltaic efficiency by passively cooling the solar panel via a simple device.
In their work, a flexible 12.7mm wide by 38.1mm tall rectangular strip positioned normal to the freestream was explored for its effectiveness in enhancing the convective cooling of a heated plate in a wind tunnel. The researchers had particular interest on the effect of flexibility of the strip. As such, strip thicknesses of 0.1, 0.2 and 0.25mm were investigated at a wind velocity of 10 m/s, a Reynolds number based on the strip width and freestream velocity of 8.5 × 103.
The authors reported that the resulting Nusselt number augmentation with respect to the unperturbed reference case could be explained in terms of the turbulent flow characteristics detailed via a triple-sensor hotwire. Additionally, the team recorded a higher velocity toward the heated plate behind the 0.1mm-thick strip, contributing to the most effective heat transfer performance; approximately 0.1 higher than that associated with the 0.25 mm-thick strip in terms of the normalized Nusselt number.
In a nutshell, when the sun is high and most abundant in energy, the PV panel fails to capitalize the harnessing because the conversion efficiency decreases with PV panel (cell) temperature. the prevailing studies dealing with internal flow heat transfer cannot be applied directly, due to significant confinement effect. This confinement effect is largely eliminated in their “open” or “external” flow heat transfer study. Also, this kind of unconfined studies are more fundamental in nature, providing clearer elementary explanation in the absence of confinement complication.
Their study demonstrated the application of a 38.1mm tall and 12.7mm wide aluminum strip to enhance the convective heat transfer from a heated plate in a wind tunnel at 10 m/s wind velocity. Overall, the turbulent length scale was observed to be independent of the chosen thickness, instead, it seemed to be related with the dimension of strips, i.e. the height and the width. In a statement to Advances in Engineering, Professor David S-K. Ting, the corresponding author said “Simplicity, when appropriately engineered, becomes efficacy”.
Yang Yang, David S-K. Ting, Steve Ray. Convective heat transfer enhancement downstream of a flexible strip normal to the freestream. International Journal of Thermal Sciences, volume 145 (2019) 106059.