The aim of this paper is to study the effect of flow conditions on nanoparticle fraction suspending in base fluid, using water-based TiO2 nanofluids under laminar flow conditions as an example. With the same initial concentration of nanoparticle (about 0.5% mass fraction), significant deterioration of stability is observed and the deterioration depends on the flow conditions (Reynolds number 500–2000, circulating time). The nanoparticle fraction in nanofluids flowing through bend tube with multi elbows is obviously lower than straight tube with fewer elbows, especially for that at higher Reynolds number. After 8 h, compared with the initial concentration (about 0.5% mass fraction), the relative concentration of nanofluids is 71.5% and 68.5% at Re = 500, for straight tube and bend tube, respectively. Whereas, that is 96.9% and 89.3% at Re = 2000 for straight tube and bend tube, respectively. Sample of nanoparticles in base fluid flowing through bend tube obviously appears the agglomeration (shown in TEM images), larger particle size (measured by laser particle size analyzer) compared with that flowing through straight tube. Moreover, the precipitation of particles can be apparently observed at the out curvature of elbows. These results imply that higher nanoparticle fraction can be maintained for nanofluids at higher Reynolds number and fewer elbows. It maybe helpful for better understanding heat transfer behavior of nanofluids.

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