3. Thermal properties of nanofluid

At first, the researchers only investigated the effect of particle volume concentration on thermal conductivity enhancement. Three possible approaches have been pursuing the study of

For producing high quality of nanofluids, some special conditions are prerequisites, e.g., stable suspension, permanent suspension, no particle agglomeration, and no chemical change of the fluids. Nanofluid preparation is a critical task with the use of nanoparticles for improving the thermal conductivity of base fluid. Fundamentally, there are two methods for producing

The two-step method is widely used in producing nanofluid synthesis considering the limitations of nanoparticle supplies commercially by some companies. In this means, nanoparticles are first created and then dispersed in the base fluids. Ultrasonic vibration is used to reduce the agglomeration of particles intensively. Making nanofluids utilizing the two-step processes is challenging because individual particles tend to quickly agglomerate. This agglomeration is due to attractive van der Waals forces between nanoparticles, and the agglomeration is a critical issue in all nanopowder technology, including nanofluid technology, and a crucial step to success in achieving high-performance heat transfer nanofluids [9]. The methods to prevent the agglomeration of particles, usually using a surfactant that regulates acidity (pH) of the base fluids.

This method simultaneously generates and disperses the nanoparticles into the fluid base, while the first method deploys previously manufactured nanoparticles into the base fluid.

Ions and other reactions products are then dispersed in the base fluid together with the nanoparticles since they are almost impossible to separate from their surroundings. For nanofluids containing high thermal conductivity, a one-step method is preferred to prevent particle oxidation. The advantage of a one-step technique is that nanoparticle agglomeration is the minimum, while the disadvantage is that very little nanofluid is produced. The one-step method has produced nanofluids in small quantities for research purposes only, and it is challenging to produce nanofluids commercially by this method [9]. They will be difficult to do for two reasons: firstly, a process that requires a vacuum significantly and slows the production of nanofluid, thereby limiting the production rate, and, secondly, producing

While most nanofluid productions to date have used one of the above techniques, other techniques are available depending on the particular combination of nanoparticle material and fluids [11]. The early studies on nanofluids focused on the measurement of the thermal conductivity. Later, more experiment regarding the convective heat transfer of nanofluids has

nanofluid: experimental, empirical, and numerical.

nanofluids, i.e., (1) two-step method and (2) one-step method.

Both methods involve reduction reactions or ion exchange.

nanofluids by this methods is expensive [10].

been developed continuously.

2.1. Preparation methods of nanofluid

2.1.1. Two-step method

280 Microfluidics and Nanofluidics

2.1.2. One-step method

Thermophysical property, especially the thermal conductivity, is a vital issue in nanofluid heat transfer phenomena. Prediction of thermal conductivity has been a severe challenge until now because many parameters affect the thermal conductivity values. Temperature, type of the base fluid, nanoparticle material, shape, size, volumetric fraction, production, and mixing methods may significantly change the thermal conductivity. The literature research on thermal conductivity of nanofluids is a guide to understand how different parameters affect the value and what kind of thermal conductivity model is selected for the calculation heat transfer enhancement.

Secondly, the viscosity is also crucial in nanofluid heat transfer performance, as the usage of nanofluid viscosity also increases. Prediction of the viscosity of nanofluids is also a challenging topic because of its increase in the pumping power. The similar parameters that affect thermal conductivity affect viscosity value.
