| Summary: | The use of nanofluids as a heat transfer agent has spread and expanded as its thermal conductivity is higher than the base fluid, so its use has been increased in heat exchangers and PVT systems. In this study, water was used as a base fluid and nano-silicon carbide as an additive to improve the thermal conductivity of the resulted nanofluid and its thermophysical properties were studied using five surfactants. In this study, the stability time of nanofluid was the main variable investigated. The stability of nanofluid depends on the surfactant used to slow the suspended nanoparticles deposition in the emulsion. The deposition process significantly reduces the thermal conductivity of nanofluids and thus reduces heat transfer. The study results indicated that the time of nanoparticles suspended in emulsion depend mainly on the type of the surfactant used and the ultrasonication time. The used surfactants are: ammonium cetyl cetyl (CTAB), sodium dodecyl sulfate (SDS), tannic acid+ammonia solution, dodecylbenzenesulfonate (SDBS), and Sodium deoxycholate. Ammonium cetyl cetyl (CTAB) and tannic acid+ammonia solution gave the highest stability period (more than two months) with ultrasonication mixing time for six hours by adding 0.1 ml. The addition of larger amounts of surfactant (more than 0.75 ml) increased the stability time for all nanofluids used. The maximum stability time achieved was 88 days when cetyl cetyl (ammonium cTAB) was added by 0.5 ml. The density, viscosity, and thermal conductivity measurements showed that there was no significant change with the change of the surfactant used. The addition of larger amounts of surfactant (more than 0.5 ml) caused a very limited increase in the density and viscosity of nanofluids. It also caused a small reduction in thermal conductivity. Keywords: Nanofluid, Thermo-physical properties, Surfactant, Ultrasonication time, PVT
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