•Hydrodynamic and thermal slip is influential microscopic mechanisms in convection.•Microscopic mechanisms are correlated, but can also be controlled separately.•Influence and strength of microscopic ...mechanisms depend on atomistic behaviors.•Ratio of thermal slip to system size is the key factor in high-heat-flux convection.•Correlations between convection and relative thermal slip are suggested.
Convection heat transfer is assessed for laminarly flowing liquid water through graphene nanochannels via molecular dynamics (MD) simulations. The use of MD simulations allows for direct assessment of the minute details and mechanisms influencing overall heat transfer behaviors within our study; despite the presence of unrealistic axial conduction from temperature resetting and periodic boundary conditions within MD, hydrodynamically and thermally fully-developed water flow conditions are observed. It is indicated that the physics of convective heat transfer deviate from traditional macroscale theory as the no-slip boundary condition is violated with dimensional sizes descending towards the nanoscale; investigation into hydrodynamic slip and thermal slip, termed microscopic mechanisms, is performed for their influence on nanoscale convective outcomes. The parameters of graphene-water interaction strength, channel height, water velocity, and wall temperature are manipulated to evaluate resultant convection behaviors while comparing the effects of differing magnitudes of microscopic mechanisms imposed under various test conditions. This study finds microscopic interfacial mechanisms to significantly augment momentum and thermal behaviors and thus the conduct of convective heat transfer. Hydrodynamic and thermal slip are strongly correlated in all test case scenarios with the exception of velocity manipulation; the influence of thermal slip is found to dominate over that of hydrodynamic slip as surface advection is insignificant in high heat flux environments. Convective performance correlation is suggested as the ratio of thermal slip length to system size.
Thermal rectification in defect-engineered graphene with asymmetric hole arrangements is assessed via molecular dynamics simulations. Asymmetry in two different configurations (triangular and ...rectangular hole arrangements) is controlled by manipulating geometrical parameters, such as hole size; effects of geometry on the resultant rectification are investigated. Filtering of phonon propagation directions by geometrical confinement, and asymmetric relaxation distance induce a difference in heat transfer depending on transport direction, or thermal rectification. Increase in porosity, which results in additional confinement and larger difference in relaxation, produces more significant thermal rectification. While a rectangular arrangement of holes results in 70% of the maximum thermal rectification, up to 78% of rectification was achieved using a triangular arrangement within 47.5 nm of graphene, which can be attributed to more effective phonon-hole boundary scattering with a triangular arrangement. This study suggests a feasible approach to create thermal rectification and enables its fine control, contributing to the development of phononic devices and enhancement of thermal system design for electronics.
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Redirection of energy carrier propagation by geometric confinement is studied through the analysis of in-plane and cross-plane thermal transport within various graphene nanomesh (GNM) configurations ...using molecular dynamics (MD) simulations. As the transport channel width decreases with an increase in porosity, the effect of redirection increases; thus, the in-plane thermal conductivity of large-porosity GNM is more dependent on hole arrangement. Since higher porosities weaken the GNM structure due to a larger population of broken bonds, carbon atoms within the graphene structures are more easily influenced by interactions with the substrate silicon (Si) block. Subsequently, increase in porosity leads to the decrease of interfacial thermal resistance. At higher porosities, lower interfacial resistance and in-plane thermal conductivity cause diversions (and redirections) in heat flow from the GNM to the underlying Si substrate. Our study suggests that this method of heat flow redirection can be applied as an effective means to control and manage heat transfer within numerous applications; extension to the improved conductivity calculation accuracy can also be achieved through the inclusion of this diversion analysis.
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