NPs naturally tend to sediment and agglomerate, which leads to having a non-uniform suspension. Suspending the nanoparticles (NPs) into the base fluid with the minimum sedimentation and agglomeration of the particles is a long-lasting challenge in employing NFs in practical applications. It is accepted by all researchers that the most crucial step towards conducting experimental investigations using NFs is the sample preparation step. Adding nano-sized particles to conventional coolants (i.e., water, oil, ethylene glycol, and so forth) leads to improving the thermal properties of the conventional coolants, which is highly desirable from the heat transfer point of view, although it causes some penalty in pumping power and pressure loss, which is not desirable from the energy management point of view. It is known that thermophysical properties of each fluid play an important role in heat transfer performance of the fluids: viscosity directly affected the pumping power and the pressure loss, thermal conductivity indicates the heat transfer effectiveness of the fluid, and specific heat capacity shows the capability of the fluid in storing and moving heat away. After this pioneering study by Choi and Eastman 1, many researchers investigated the stability and thermophysical properties 2, 3, 4, heat transfer 5, 6, 7, 8, 9, 10, 11, applications of artificial intelligence in predicting the thermophysical properties of nanofluids 12, 13, 14, 15, 16, and applications of various NFs 17, 18, 19, 20, 21. There is no denying the fact that introducing nanofluids (NFs), which are suspensions of nano-sized particles into the conventional working fluids, by Choi and Eastman 1 in 1995 has opened new doors to improve the heat transfer performance in different applications. As for pumping power, it is observed that the maximum increase in fanning friction factor ratio is less than 3%, which shows that adding MWCNTs to water does not impose a considerable penalty in the required energy for pumping power. As for dynamic viscosity, it is observed that increasing ultrasonication time to 60 min leads to decreasing the dynamic viscosity of the samples. The results revealed that prolonging the ultrasonication time to 60 min leads to improving the samples’ stability while prolonging ultrasonication time to higher than 60 min resulted in deteriorating the stability. The samples’ stability is investigated in different periods. Moreover, a probe-type ultrasonic device has been used, and different ultrasonication times have been applied. The two-step method has been employed to prepared the samples. Investigating the effects of ultrasonication time on samples’ stability, rheological properties, and pumping power of a water-based nanofluid containing MWCNTs nanoparticle is the main objective of the present study. The present study is the continuation of the authors’ previous research on the effects of ultrasonication on the thermophysical properties of Multi-Walled Carbon Nanotubes (MWCNTs)-water nanofluid. It is known that ultrasonication has a certain effect on thermophysical properties and heat transfer of nanofluids.
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