Heat transfer augmentation in a rectangular channel by the use of porous sinusoidal screen inserts.

Abstract

Most regions in Southern Africa, South Africa in particular receives an abundancy of solar energy when compared to other regions across the globe. The most economical way to harness this energy is to design and implement economical heat exchangers with air as the working fluid. The performance of heat exchangers can be enhanced by implementing passive techniques such as coated or roughened surfaces, extended surfaces, displaced insert devices, flow swirl devices and coiled tubes among others. The addition of inserts in heat exchangers can increase the overall efficiency of the heat exchanger by increasing the convective heat transfer at the wall by generating local turbulence and mixing. The porous insert aims to the increase the rate of heat transfer by either increasing the convective heat transfer coefficient, the heat transfer area or both. The porous inserts act in such a way to repeatedly grow and destroy thin boundary layers on the surface providing a higher heat transfer coefficient. The porous inserts are lightweight, increase the structural integrity of the heat exchanger walls and can be retrofitted to the heat exchanger. By measuring the effects on the convection heat transfer rate, pressure drop, and turbulence caused by a wavy screen insert in a rectangular channel with air as the working fluid will help determine the thermal performance of the wavy screen insert. By independently investigating the effects of geometrical properties of the wavy screen insert, such as the porosity and periodicity of the insert, the size and shape of the insert can be determined to increase the thermal performance and reduce the size of the compact heat exchanger. This research aims to increase the thermal effectiveness of the heat exchanger channels while minimizing the increase in pressure penalty in the same channels. Increased thermal effectiveness directly affects the convection heat transfer, compact size, initial cost of the heat exchanger and how a small pressure penalty affects the operating cost of the heat exchanger. This research aims to investigate the effects of porosity and periodicity of the sinusoidal screen insert in a rectangular channel on the convection heat transfer rate and pressure drop across the channel to improve the thermal performance of the heat exchanger employed in solar panels, energy recovery systems, electronic chips, and machine components. To investigate the thermal performance of sinusoidal screen inserts in flat plate heat exchangers an experimental test facility is constructed which is capable of replicating the conditions of the heat exchanger. Six inserts are fabricated with varying wavelength and porosity. The height or amplitude of the sinusoidal waveform remains constant at 14 mm for all the inserts. The wavelength, λ is investigated at 12 mm, 16 mm and 20 mm. The effects of the porosity of the insert, ζ is investigated for at 48 % and 68 % porosity. The inserts are placed inside a 203 x 14 mm rectangular test section and wall temperature and pressure measurements along the test section are recorded over a range of 400 < Re < 35 000. The study shows that the use of sinusoidal inserts in flat plate heat exchangers can increase the thermal performance over the range of 400 ≤ Re ≤ 3000. Above Re = 3000 the increased pressure penalty decreases the performance of the heat exchanger.