A CFD investigation of the effect of non-Newtonian behavior of Cu-water nanofluids on their heat transfer and flow friction characteristics

In the present study, a finite volume method is used to investigate heat transfer and flow friction behavior of non-Newtonian nanofluids. To study a practical application of the mentioned concept, a simple model of a parabolic trough solar receiver is simulated. The main objective of the present study is to investigate the effect of non-Newtonian behavior of the working fluid on the performance of a parabolic trough solar collector. The heat transfer fluid is assumed to be a non-Newtonian nanofluid, and the flow regime is considered to be laminar. The effect of Cu nanoparticle addition on heat transfer coefficient and flow friction of Newtonian, shear-thinning and shear-thickening nanofluids are studied. To comprehensively investigate the effect of buoyancy-driven secondary flows on the average Nusselt number and friction factor of the absorber tube, simulations are carried out for different Grashof numbers (Gr = 10(5), 10(6), 10(7)), Reynolds numbers (Re = 200, 500, 1000), Cu nanoparticle volume fractions (phi = 0, 0.01, 0.03) and non-Newtonian power-law indexes (n = 0.25, 0.75, 1, 1.25, 1.75). It is concluded that when shear-thickening nanofluids are utilized as the working fluid, nanoparticle addition makes no sensible changes in the average Nusselt number. Besides, for all values of non-Newtonian power-law index, variations in volume fraction values do not have any significant effect on the friction factor. Furthermore, it is shown that when the working fluid is shear-thinning, nanoparticle addition triggers to considerable increment in Nusselt number. At high Grashof and Reynolds numbers, the ratios of Nusselt number and friction factor of shear-thinning nanofluids (n = 0.25) to those of shear-thickening nanofluids are up to 3.57 and 0.08, respectively.