*1.1. Theoretical to Experimental Perspective on Nanofluids*

The thermal properties of nanoparticles are explained through nanofluid theory, which supports physics and chemistry-based predictive models. The thermal conductivity of the nanofluids has not been explained for numerous reasons. First, nanofluids behave differently from solid-to-fluid suspensions or typical solid-to-solid composites. Reducing nanoparticle size increases nanofluid thermal conductivity. Second, nanofluids and traditional solid-to-liquid suspensions differ in thermal conductivity, concentration of the particles, and size. Thirdly, nanofluids are a new, highly multidisciplinary field that spans engineering, material science, physics, chemistry and colloidal science. Nanofluids hence need expertise in each field, which shows the difficulty to build a nanofluid theory [17,25–27]. Therefore, predictions are poorer when the nanoparticles are suspended in a liquid because these interactions include electromagnetic or particle-to-lattice heat transfer in addition to the lattice vibrational heat transfer predicted by the liquid models. Thus, a theory for nanofluid thermal conductivity can be developed by taking into account two crucial components, static and dynamic mechanisms. Near field radiation in nanofluids seems like a promising theory [26,28].

The most recent advances in fabrication technology have opened up exciting new possibilities for actively processing materials at nano-scale sizes. Materials that are either nanostructured or nanophase are composed of nanometer-sized components. Because of this, particles of a size less than 100 nm have characteristics that are distinct from those of traditional solids. The remarkable qualities that are associated with nanophase materials are a direct result of the relatively high surface area/volume ratio that these materials possess. This ratio is made possible by the presence of a significant number of constituent atoms that are located at the grain boundaries. Nanophase materials have superior thermal, mechanical, optical, magnetic, and electrical capabilities compared to traditional materials that have coarse grain patterns. These qualities include magnetism and electrical conductivity. As a consequence of this, the exploration of nanophase materials in research and development has attracted a significant amount of attention from both material scientists and engineers [29]. Several kinds of nanoparticles that are employed in nanofluids can be constructed out of a wide variety of materials, including oxide ceramics (Al2O3, CuO), carbide ceramic, metals (Cu, Ag, Au), semiconductors, composite materials and alloyed nanoparticles are some examples of advanced materials. Whereas, in the development of nanofluids a wide variety of liquids, including water, ethylene glycol and oil, have been employed as base fluids [29,30].

## 1.1.1. Volume Fraction and the Particle Size

Many experiments on nanofluid thermal conductivity have been described in recent years. Table 1 summarizes published experimental studies on nanofluids at room temperature. So, nanofluids with thermal conductivity higher than their base fluids, even at low nanoparticle concentrations, grow well with nanoparticle volume fraction. Below are several nanofluid thermal conductivity investigations.

Eastman et al. [31] first reported nanofluids' increased effective thermal conductivity. Al2O3 and CuO nanoparticles dispersed in water increased thermal conductivity by 29% to 60% for 5% nanoparticle volume fraction which later findings found a moderate increase in thermal conductivity for Al2O3 and CuO nanoparticles in water and ethylene glycol. Li et al. [32] recently studied the thermal conductivity of CuO and Al2O3 nanoparticles boosted waters thermal conductivity by 52% and 22% at 6% fractional part of volume at a temperature of 34 ◦C. Choi et al. [33] investigated multi-walled carbon nanotubecontaining oil suspensions' thermal conductivity. Thermal conductivity doubled with 1% volumetric loading. Even at low volume fractions, nanotube addition increases conductivity non-linearly. Strong thermal field interactions between fibers might be to cause. TiO2 nanoparticles are used to evaluate the thermal conductivity in deionized water by Murshed et al. [34]. Their findings demonstrated, for the very first time, that there was no anomalous improvement in the thermal conductivity of nanofluids containing a very low volume proportion of particles. This conclusion is in direct opposition to Patel et al.'s [35] unusual finding which shows the incorrect Patel's hypothesis. According to a comparison of the studies that have been conducted, the increases in thermal conductivities of various types of nanofluids are distinct from one another. The size and composition of the nanoparticles, in addition to the base fluids, both have an effect on the thermal conductivity of nanofluids. Particle size is essential to optimize results and build a relation to volume fraction which causes nano-scale mechanism in the suspensions. According to theoretical evidence, with a reduction in particle size, the effective thermal conductivity of nanofluids improved [36,37].


**Table 1.** An overview of various nanofluids and thermal conductivity.
