*3.5. Friction Factor of Nanofluids*

The nanofluids were evaluated for pressure loss and friction factor when flowing in a square heat exchanger at different Re-numbers. Figure 8a,b showed the measured pressure DW.

*3.5. Friction Factor of Nanofluids*

loss and friction factor for all samples versus the Re. The highest-pressure loss and friction factor increases were 20.8–23.7% and 3.57–3.85%, at a weight percentage of 0.1 wt. % and a velocity of 0.93 m/s, respectively. sure loss and friction factor for all samples versus the Re. The highest-pressure loss and friction factor increases were 20.8–23.7% and 3.57–3.85%, at a weight percentage of 0.1 wt. % and a velocity of 0.93 m/s, respectively.

The nanofluids were evaluated for pressure loss and friction factor when flowing in a square heat exchanger at different Re-numbers. Figure 8a,b showed the measured pres-

tube and bulk fluid contained in the test-section. The maximum rise in Nuavg was noted as follows: PEG@GNPs = 54%, PEG@TGr = 43%, SiO<sup>2</sup> = 28%, and Al2O<sup>3</sup> = 26% associated with

*Nanomaterials* **2022**, *12*, x FOR PEER REVIEW 14 of 21

**Figure 8.** Hydrodynamic properties of different nanofluid types against Reynolds number; (**a**) Friction factor, (**b**) Pressure drop. **Figure 8.** Hydrodynamic properties of different nanofluid types against Reynolds number; (**a**) Friction factor, (**b**) Pressure drop.

Brownian motion significantly affects the momentum transfer between solid nanoparticles and base fluid molecules at a low range of Re-numbers. The friction factor of samples increases slightly due to the Brownian motion [48]. However, this is an inactive mechanism in the high Re range. Mainly, the velocity of the nanofluids can be considered the most significant factor for the development of friction factor at a high Re-number range. The considerable differences between the observed friction factors of functionalized carbon nanostructures, metallic oxides, and distilled water at multiple Re numbers are due to the minor improvement in the viscosities of distilled water and their nanofluids. The variations in the friction factor are based on the nanofluid-related viscous drag. Typically, the density of nanoparticles is a crucial factor in enhancing the nano-coolant friction factor. The combination of dynamic and kinematic viscosities dramatically affects the pressure drop of different nanofluids. The excessive pumping capacity increases with increased dynamic viscosity. Brownian motion significantly affects the momentum transfer between solid nanoparticles and base fluid molecules at a low range of Re-numbers. The friction factor of samples increases slightly due to the Brownian motion [48]. However, this is an inactive mechanism in the high Re range. Mainly, the velocity of the nanofluids can be considered the most significant factor for the development of friction factor at a high Re-number range. The considerable differences between the observed friction factors of functionalized carbon nanostructures, metallic oxides, and distilled water at multiple Re numbers are due to the minor improvement in the viscosities of distilled water and their nanofluids. The variations in the friction factor are based on the nanofluid-related viscous drag. Typically, the density of nanoparticles is a crucial factor in enhancing the nano-coolant friction factor. The combination of dynamic and kinematic viscosities dramatically affects the pressure drop of different nanofluids. The excessive pumping capacity increases with increased dynamic viscosity.
