Bottom up approach toward prediction of effective thermophysical properties of carbon-based nanofluids

Carbon-based nanofluids, mainly suspensions of carbon nanotubes or graphene sheets in water, are typically characterized by superior thermal and optical properties. However, their multiscale nature is slowing down the investigation of optimal geometrical, chemical, and physical nanoscale parameters for enhancing the thermal conductivity while limiting the viscosity increase at the same time. In this work, a bottom up approach is developed to systematically explore the thermophysical properties of carbon-based nanofluids with different characteristics. Prandtl number is suggested as the most adequate parameter for evaluating the best compromise between thermal conductivity and viscosity increases. By comparing the Prandtl number of nanofluids with different characteristics, promising overall performances (that is, nanofluid/base fluid Prandtl number ratios equal to 0.7) are observed for semidilute (volume fraction  ⩽ 0.004) aqueous suspensions of carbon nanoparticles with extreme aspect ratios (larger than 100 for nanotubes, smaller than 0.01 for nanoplatelets) and limited defects concentrations (<5%). The bottom up approach discussed in this work may ease a more systematic exploration of carbon-based nanofluids for thermal applications, especially solar ones

Thermal transmittance in graphene based networks for polymer matrix composites

Graphene nanoribbons (GNRs) can be added as fillers in polymer matrix composites for enhancing their thermo-mechanical properties. In the present study, we focus on the effect of chemical and geometrical characteristics of GNRs on the thermal conduction properties of composite materials. Configurations consisting of single and triple GNRs are here considered as representative building blocks of larger filler networks. In particular, GNRs with different length, relative orientation and number of cross-linkers are investigated. Based on results obtained by Reverse Non-equilibrium Molecular Dynamics simulations, we report correlations relating thermal conductivity and thermal boundary resistance of GNRs with their geometrical and chemical characteristics. These effects in turn affect the overall thermal transmittance of graphene based networks. In the broader context of effective medium theory, such results could be beneficial to predict the thermal transport properties of devices made of polymer matrix composites, which currently find application in energy, automotive, aerospace, electronics, sporting goods, and infrastructure industries

Heat Transfer at the Interface of Graphene Nanoribbons with Different Relative Orientations and Gaps

Because of their high thermal conductivity, graphene nanoribbons (GNRs) can be employed as fillers to enhance the thermal transfer properties of composite materials, such as polymer-based ones. However, when the filler loading is higher than the geometric percolation threshold, the interfacial thermal resistance between adjacent GNRs may significantly limit the overall thermal transfer through a network of fillers. In this article, reverse non-equilibrium molecular dynamics is used to investigate the impact of the relative orientation (i.e., horizontal and vertical overlap, interplanar spacing and angular displacement) of couples of GNRs on their interfacial thermal resistance. Based on the simulation results, we propose an empirical correlation between the thermal resistance at the interface of adjacent GNRs and their main geometrical parameters, namely the normalized projected overlap and average interplanar spacing. The reported correlation can be beneficial for speeding up bottom-up approaches to the multiscale analysis of the thermal properties of composite materials, particularly when thermally conductive fillers create percolating pathways

Thermal transport phenomena in nanoparticle suspensions

Nanoparticle suspensions in liquids have received great attention, as they may offer an approach to enhance thermophysical properties of base fluids. A good variety of applications in engineering and biomedicine has been investigated with the aim of exploiting the above potential. The multiscale nature of nanosuspensions raises several issues in defining a comprehensive modelling framework, incorporating relevant molecular details and much larger scale phenomena, such as particle aggregation and their dynamics. The objectives of the present topical review is to report and discuss the main heat and mass transport phenomena ruling their macroscopic behaviour, arising from molecular details. Relevant experimental results are included and properly put in the context of recent observations and theoretical studies, which solved long-standing debates about thermophysical properties enhancement. Major transport phenomena are discussed and in-depth analysis is carried out for highlighting the role of geometrical (nanoparticle shape, size, aggregation, concentration), chemical (pH, surfactants, functionalization) and physical parameters (temperature, density). We finally overview several computational techniques available at different scales with the aim of drawing the attention on the need for truly multiscale predictive models. This may help the development of next-generation nanoparticle suspensions and their rational use in thermal applications

A review on the heat and mass transfer phenomena in nanofluid coolants with special focus on automotive applications

Engineered suspensions of nanosized particles (nanofluids) are characterized by superior thermal properties. Due to the increasing need for ultrahigh performance cooling in many industries, nanofluids have been widely investigated as next-generation coolants. However, the multiscale nature of nanofluids implies nontrivial relations between their design characteristics and the resulting thermo-physical properties, which are far from being fully understood. This pronounced sensitivity is the main reason for some contradictory results among both experimental evidence and theoretical considerations presented in the literature. In this Review, the role of fundamental heat and mass transfer mechanisms governing thermo-physical properties of nanofluids is assessed, from both experimental and theoretical point of view. Starting from the characteristic nanoscale transport phenomena occurring at the particle-fluid interface, a comprehensive review of the influence of geometrical (particle shape, size and volume concentration), physical (temperature) and chemical (particle material, pH and surfactant concentration in the base fluid) parameters on the nanofluid properties was carried out. Particular focus was devoted to highlight the advantages of using nanofluids as coolants for automotive heat exchangers, and a number of design guidelines was suggested for balancing thermal conductivity and viscosity enhancement in nanofluids. This Review may contribute to a more rational design of the thermo-physical properties of particle suspensions, therefore easing the translation of nanofluid technology from small-scale research laboratories to large-scale industrial applications

Thermal transmittance in graphene based networks for polymer matrix composites

Graphene nanoribbons (GNRs) can be added as fillers in polymer matrix composites for enhancing their thermo-mechanical properties. In the present study, we focus on the effect of chemical and geometrical characteristics of GNRs on the thermal conduction properties of composite materials. Configurations consisting of single and triple GNRs are here considered as representative building blocks of larger filler networks. In particular, GNRs with different length, relative orientation and number of cross-linkers are investigated. Based on results obtained by Reverse Non-equilibrium Molecular Dynamics simulations, we report correlations relating thermal conductivity and thermal boundary resistance of GNRs with their geometrical and chemical characteristics. These effects in turn affect the overall thermal transmittance of graphene based networks. In the broader context of effective medium theory, such results could be beneficial to predict the thermal transport properties of devices made of polymer matrix composites, which currently find application in energy, automotive, aerospace, electronics, sporting goods, and infrastructure industries

A review on the heat and mass transfer phenomena in nanofluid coolants with special focus on automotive applications

Engineered suspensions of nanosized particles (nanofluids) are characterized by superior thermal properties. Due to the increasing need for ultrahigh performance cooling in many industries, nanofluids have been widely investigated as next-generation coolants. However, the multiscale nature of nanofluids implies nontrivial relations between their design characteristics and the resulting thermo-physical properties, which are far from being fully understood. This pronounced sensitivity is the main reason for some contradictory results among both experimental evidence and theoretical considerations presented in the literature. In this Review, the role of fundamental heat and mass transfer mechanisms governing thermo-physical properties of nanofluids is assessed, from both experimental and theoretical point of view. Starting from the characteristic nanoscale transport phenomena occurring at the particle-fluid interface, a comprehensive review of the influence of geometrical (particle shape, size and volume concentration), physical (temperature) and chemical (particle material, pH and surfactant concentration in the base fluid) parameters on the nanofluid properties was carried out. Particular focus was devoted to highlight the advantages of using nanofluids as coolants for automotive heat exchangers, and a number of design guidelines was suggested for balancing thermal conductivity and viscosity enhancement in nanofluids. This Review may contribute to a more rational design of the thermo-physical properties of particle suspensions, therefore easing the translation of nanofluid technology from small-scale research laboratories to large-scale industrial application

Thermal transmittance of carbon nanotube networks: Guidelines for novel thermal storage systems and polymeric material of thermal interest

Among other applications, the study of thermal properties of large networks of carbon nanoparticles may have a critical impact in loss-free, more compact and efficient thermal storage systems, as well as thermally conducting polymeric materials for innovative low-cost heat exchangers. In this respect, here, we both review and numerically investigate the impact that nanotechnology (and in particular carbon-based nanostructures) may have in the near future. In particular, we focus on the role played by some geometrical and chemical parameters on the overall thermal transmittance of large complex networks made up of carbon nanotubes (CNTs), that can be potentially added as fillers to (thermally) low-conductive materials for enhancing the transport properties. Several configurations consisting of sole and pairs of single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs), characterized by different dimensions and number of C-O-C interlinks, are considered. Based on the results found in the literature and using focused simulations using standard approaches in Non-Equilibrium Molecular Dynamics (NEMD), we highlight the dependence on the particle diameter, length, overlap and functionalizations of both thermal conductivity and boundary resistance across CNTs, which are indeed the relevant quantities for obtaining composite materials with desired unusual thermal properties. We observe that CNTs with short overlap length and a few interlinks already show a remarkable enhancement in the overall transmittance, whereas further increase in the number of C-O-C connection only carries marginal benefits. We believe that much understanding has been gained so far in this field thanks to the work of chemists and material scientists, thus it is time to draw the attention of engineers active in the energy sector and thermal scientists on such findings. Our effort, therefore, is to gather in this article some guidelines towards innovative thermal systems that may be manufactured and employed in the near future for addressing a great challenge of our society: Storage and use of thermal energy

Overall thermal transmittance in carbon nanotube networks for thermal storage systems and composite materials

The study of thermal properties of large networks of carbon nanoparticles may have an important impact in loss-free, compact sorption-based thermal storage systems, as well as thermally conducting polymeric materials for innovative low-cost heat exchangers. Here, we both review and computationally investigate on the role that nanotechnology (and in particular carbon-based nanostructures) may have in the near future in thermal sciences. In particular, we focus on the role played by some geometrical and chemical parameters on the overall thermal transmittance of large complex networks made up of carbon nanotubes (CNTs), that can be potentially utilized as fillers for enhancing the transport properties of energy in (thermally) low-conductive materials. Several configurations of single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs), characterized by different dimensions and number of C-O-C interlinks, are considered. Based on the results reported in the published literature and using focused simulations by standard approaches in Non-Equilibrium Molecular Dynamics (NEMD), we aim at highlighting the dependence on the particle geometry, overlap and functionalizations of the boundary resistance across CNTs, which is known to be the relevant quantity affecting thermal properties of composite materials. We find that CNTs with short overlap length and only a few C-O-C interlinks already show a significant enhancement in the overall transmittance, whereas further increase in the number of connection generates marginal benefits. We believe that much understanding has been gained so far in this field thanks to the work of chemists and material scientists. Hence, it is time to draw the attention of engineers active in the energy sector and thermal scientists on such findings. Our effort, therefore, is to collect in this study a few guidelines that can be useful for the design of innovative thermal systems to be manufactured and employed in the near future for addressing some of the challenges in thermal energy storage (e.g. enhancing the heat rate during charging/discharging processes)

Effect of interfacial thermal resistance and nanolayer on estimates of effective thermal conductivity of nanofluids

Colloidal suspensions of nanoparticles (nanofluids) are materials of interest for thermal engineering, because their heat transfer properties are typically enhanced as compared to the base fluid one. Effective medium theory provides popular models for estimating the overall thermal conductivity of nanofluids based on their composition. In this article, the accuracy of models based on the Bruggeman approximation is assessed. The sensitivity of these models to nanoscale interfacial phenomena, such as interfacial thermal resistance (Kapitza resistance) and fluid ordering around nanoparticles (nanolayer), is considered for a case study consisting of alumina nanoparticles suspended in water. While no significant differences are noticed for various thermal conductivity profiles in the nanolayer, a good agreement with experiments is observed with Kapitza resistance ≈10−9 m2K/W and sub-nanometer nanolayer thickness. These results confirm the classical nature of thermal conduction in nanofluids and highlight that future studies should rather focus on a better quantification of Kapitza resistance at nanoparticle-fluid interfaces, in order to allow bottom up estimates of their effective thermal conductivity. Keywords: Nanolayer, Kapitza resistance, Nanofluid, Effective medium approximation, Thermal conductivit