## 4 Heat Transfer Mechanisms and Influencing Factors  
The heat transfer performance of the noble metals nanofluids can be affected by various factors and mechanisms. The most highlighted in the temperature are the effects of the solid-liquid interface Layer and interactions between the Molecules of the metallic Nanoparticles and the Molecules of base fluid. Some of these effects that influence the heat transfer of the noble metals nanofluids are the aggregation morphology of the incorporated nanoparticles and the match degree of the phonon density of state. The aggregation morphology influence was already numerically addressed by the authors [PERSON] et al. [14], [PERSON] et al. [15], and [PERSON] et al. [16; 17]. The aggregation was simulated in Molecular Dynamics by a diffusion-limit-cluster-aggregation model, where the influence of the size of the nanoparticle, concentration, and number in a cluster on the aggregation structure were evaluated. The researchers concluded that at a fixed volumetric concentration and size of the included nanoparticles, the thermal conductivity of the metallic nanofluids increased linearly with decreasing fractal dimension of the aggregation. The authors interpreted the fact based on the more direct heat transfer channels between the nanoparticles and on the larger interface between the nanoparticles and the base fluid produced by a lower fractal dimension. Also, the results showed that the nanoparticle aggregation can dramatically increase the thermal conductivity of the nanofluids, and the aggregation can be more influent for smaller nanoparticles and lower concentrations. The main mechanism is that the aggregation raises the phonon mean free path in the solid phase, which further results in the thermal conductivity enhancement of the nanoparticles and nanofluids. Additionally, the authors [PERSON] et al. [18] examined if nanoparticle additions with higher thermal conductivity are necessary for greater thermal conductivity of nanofluids and studied gold-water, silver-water, copper-water, and iron-water nanofluids by the dynamic molecular method. The results showed that the gold-water nanofluid exhibited the highest thermal conductivity and thicker nanolayer because of the stronger interaction forces between the gold and water molecules. Also, the authors proposed an index designated by match degree of phonon density of state and argued that the phonon match between the molecules of the metal and water molecules was the critical underlying mechanism for the thermal conductivity enhancement of the metal-water nanofluids. Moreover, the researchers [PERSON] et al. [19] proposed to add Janus nanoparticles into a fluid commonly named as Janus nanofluids, to further increase the thermal conductivity of the nanofluids. Throughmolecular dynamics simulations, the authors reported that the thermal conductivity can be appreciably improved by the incorporation of Janus nanoparticles into the base fluids in contrast with the traditional nanofluids. Based on the determination of the molecular radial distribution function around the added nanoparticles, and the diffusion coefficients of the base fluid and the Janus nanoparticles, the thermal conductivity improvement in the Janus nanoparticles containing nanofluids was attributed to the increased Brownian motion of the Janus nanoparticles that augments the likelihood of more inter-molecular collisions and enhances the heat transfer capability of the medium. Answering to the question of why can hybrid nanofluid improve thermal conductivity more, the authors [PERSON] et al. [20] evaluated the microscopic mechanism responsible for the enhancement in the thermal conductivity of hybrid nanofluids.