Numerical investigation on the heat transfer enhancement by the combination of wavy tape, dimples and nanofluids in a PTC receiver.

Abstract

Parabolic trough collector (PTC) is the most developed concentrating solar technology. It represents a viable way to substitute fossil fuels in the production of heat process, however higher thermo-hydraulic performance is needed to be more competitive. This study presents a numerical investigation of dimples, wavy tape and nanofluids (Al2O3, TiO2 and Al2O3-TiO2 dispersed in water at 4% concentration) in combination, in a PTC receiver. Fluent was used to solve the fluid dynamics and heat transfer characteristics inside the PTC receiver with the different heat transfer enhancement techniques for Reynolds numbers ranging from 1.48x104 to 1.77x105. The study showed that dimples with Al2O3/water nanofluid lead to a higher thermo-hydraulic performance evaluated with the Thermal Performance Index with values as high as 1.78. It was also proven that the highest thermal enhancement is obtained when the three heat transfer augmentation techniques are used in combination (wavy tape, dimples, and Al2O3/water nanofluid) with a heat transfer coefficient enhancement of 3.12 times that of a plain PTC receiver with no thermal enhancement. Nonetheless, the combination of techniques also come with a high cost of pressure drop increase from 8.52 to 12.59 compared to the plain PTC receiver. The combination of all the techniques proved more useful at low Reynolds numbers because the flow is not as turbulent. As Reynolds number increases, the thermal increase is not proportional to the mean pressure drop increase, then leading poor performances at high Reynolds numbers. On the other hand, wavy tape with nanofluids proved to have better thermal performances at high Reynolds numbers. The use of nanofluids always leads to the higher thermal performance values. Regarding the different nanofluids, the difference among them is non-significant compared to each other in terms of mean pressure drop, however in terms of heat transfer coefficient improvement, there is about 1.34% difference between the highest thermal performance nanofluids (Al2O3, and Al2O3-TiO2) and TiO2/water nanofluid.