*4.2. Size and Shape of the Nanoparticles*

*4.2. Size and Shape of the Nanoparticles*  The morphology of the nanoparticles has a considerable impact on the thermal conductivity enhancement. For instance, in the experimental work developed by [94], nanofluids were prepared by dispersing titanium oxidenanoparticles in rodshapes measuring10 nm diameter and 40 nm length, and in spherical shapes measuring15 nm diameter in deionized water.The results revealed that the particle size and shape have effects on the thermal conductivity improvement. At 5 vol. %, with the titanium oxiderodsand spheres, the enhancement was found to be around 33% and 30%, respectively, compared with that of the base fluid itself. Moreover, for instance, in the case of ethylene-glycol-based nanofluids [95], the nanoparticle size effect on the thermal conductivity of the nanofluids was also not conclusive as some experimental works reported that the nanofluids with larger-sized nanoparticles exhibitedgreater enhancements in the thermal conductivity than the ones having smaller nanoparticles,whereas others found that the smaller the nanoparticles, the greater the improvement in the thermal conductivity. Hence, the size and shape of the added nanoparticles can also impact on the HTC and CHF. This is related to the fact that different parameters, including post-sintering pore shape, permeability, surface roughness, and thermal conductivity and diffusivity of the deposited layer are influenced by the morphology of the nanoparticles. Moreover, the authors Peng et al. [96] performed an empirical study on the impact of the size of copper nanoparticles on the nucleate pool-boiling heat transfer of a R113/oil thermal fluid. The researchers reported that the maximum increase of 23.8% was achieved for the HTC by reducing the size of the nanoparticles from 80 to 20 nm. Moreover, the smaller copper nanoparticles led to an The morphology of the nanoparticles has a considerable impact on the thermal conductivity enhancement. For instance, in the experimental work developed by [94], nanofluids were prepared by dispersing titanium oxidenanoparticles in rodshapes measuring 10 nm diameter and 40 nm length, and in spherical shapes measuring15 nm diameter in deionized water.The results revealed that the particle size and shape have effects on the thermal conductivity improvement. At 5 vol. %, with the titanium oxiderodsand spheres, the enhancement was found to be around 33% and 30%, respectively, compared with that of the base fluid itself. Moreover, for instance, in the case of ethylene-glycol-based nanofluids [95], the nanoparticle size effect on the thermal conductivity of the nanofluids was also not conclusive as some experimental works reported that the nanofluids with larger-sized nanoparticles exhibitedgreater enhancements in the thermal conductivity than the ones having smaller nanoparticles, whereas others found that the smaller the nanoparticles, the greater the improvement in the thermal conductivity. Hence, the size and shape of the added nanoparticles can also impact on the HTC and CHF. This is related to the fact that different parameters, including post-sintering pore shape, permeability, surface roughness, and thermal conductivity and diffusivity of the deposited layer are influenced by the morphology of the nanoparticles. Moreover, the authors Peng et al. [96] performed an empirical study on the impact of the size of copper nanoparticles on the nucleate pool-boiling heat transfer of a R113/oil thermal fluid. The researchers reported that the maximum increase of 23.8% was achieved for the HTC by reducing the size of the nanoparticles from 80 to 20 nm. Moreover, the smaller copper nanoparticles led to an increased pool-boiling HTC. The impact of the dimensions of the nanoparticles on the boiling behavior was also studied by Hu et al. [97] for a silica nanofluid. The authors observed an increasing tendency of

increased pool-boiling HTC. The impact of the dimensions of the nanoparticles on the boiling behavior was also studied by Hu et al. [97] for a silica nanofluid. The authors

contributes to the amelioration of the pool-boiling CHF, and as the size of nanoparticles increases, better boiling heat transfer performance is achieved for the nanofluids. Additionally, the researchers Minakov et al. [36] established that the CHF using nanofluids depends on the nanoparticle size and that the CHF increased with increasing nanoparticle size. This fact can be explained by assuming that the particle deposition on the heating surface plays the key role in the CHF enhancement. The larger the size of deposited particles, the larger the scale of the final roughness on the surface, which

the HTC by decreasing the nanoparticle size from 120 to 84 nm. Furthermore, it can be stated that the nanoparticle size strongly contributes to the amelioration of the pool-boiling CHF, and as the size of nanoparticles increases, better boiling heat transfer performance is achieved for the nanofluids. Additionally, the researchers Minakov et al. [36] established that the CHF using nanofluids depends on the nanoparticle size and that the CHF increased with increasing nanoparticle size. This fact can be explained by assuming that the particle deposition on the heating surface plays the key role in the CHF enhancement. The larger the size of deposited particles, the larger the scale of the final roughness on the surface, which promotes the formation of a deposit thickness enough for enhanced boiling. Shogl et al. [6] showed that the heating surface characteristics depended on the nanoparticles size and surface roughness. Hence, larger nanoparticle sizes improved the boiling performance of the nanofluid. Thus, the carbon nanotubes with water-based nanofluids enhanced the performance of the system and can be considered as the best heat removal method among the examined carbon nanotubes, alumina, and zinc oxide nanofluids, because both surface characteristics and boiling performance were improved with the carbon nanotubes.
