A molecular dynamics simulation on the convective heat transfer in nanochannels

Thermal Conductivity of a Fluid-Filled Nanoporous Material: Underlying Molecular Mechanisms and the Rattle Effect

Nanoporous materials are central to the energy and environmental crisis, with key applications in adsorption, separation, and catalysis. While confinement and surface effects on fluids severely confined in their porosity are well documented, the thermal behavior of nanoporous solids subjected to fluid adsorption remains puzzling in many aspects. With striking phenomena such as the so-called rattle effect, through which fluid/solid collisions decrease the overall thermal conductivity, the solid thermal conductivity and, more generally, heat transfer and dispersion in these complex systems challenge classical approaches (e.g., mixing rules including effective medium approaches fail to capture such effects as shown here). In particular, a robust molecular framework to describe the crossover between the decrease in thermal conductivity through the rattle effect in very narrow pores and the increase in thermal conductivity when replacing vacuum with a fluid phase in larger pores is still missing. Here, using a prototypical model of fluid-filled nanoporous materials (a Lennard-Jones phase confined in an all-silica zeolite), we perform a molecular simulation study to shed light on the parameters that govern the rattle effect in nanoporous solids. First, by varying the fluid/fluid, fluid/solid, and solid/solid interaction strengths as well as the fluid number density and mass density, we unravel the ingredients that lead to the essential coupling between fluid adsorption and phonon transport. Second, despite this complex interplay, inspired by pioneering molecular approaches on the rattle effect, we show that all data obey a simple statistical physics model that relies on the change in the speed of sound due to the fluid adsorbed density and the decrease in phonon lifetime due to scattering by fluid molecules. This framework, which provides a simple formalism to rationalize the thermal behavior of this class of solid/fluid composites, points to a decrease in thermal conductivity upon fluid confinement (up to 30% in some cases). Such an effect paves the way for the design of novel applications involving fluids in interaction with nanoporous materials.

Studying the influence of surface roughness with different shapes and quantities on convective heat transfer of fluid within nanochannels using molecular dynamics simulations

CONTEXT

In the past decade, rapid advancements in microtechnology and nanotechnology have propelled modern science and technology into the nanoscale domain, where miniaturization and high integration have found extensive applications across various fields, including industry, biotechnology, and information technology. Mechanisms of nanofluid flow and heat transfer properties have received increasing attention. In the flow and convective heat transfer of fluids at the nanoscale, the shape and dimensions of the surfaces play a crucial role. So, the main purpose of our paper is to investigate the influence of surface roughness with different shapes and quantities on fluid flow and convective heat transfer. In this study, we have chosen argon atoms as the fluid and used copper atoms to simulate the nanochannel walls. In order to investigate the influence of the shape and quantity of roughness on the convective heat transfer of fluids within nanochannels, we computed and analyzed the velocity, temperature, and density distributions of fluids inside channels with triangular, hemispherical, and rectangular roughness. Through simulation results, we found that triangular, hemispherical, and rectangular surface roughness at the same height can result in differences in temperature and velocity of the fluid within nanochannels. With a nanochannel roughness number of 5, the temperature and velocity of the fluid at the middle position of the nano-channel for the triangular roughness increased by 6% and 25% compared to the rectangular roughness, and by 4% and 10% compared to the hemispherical roughness. The fluid temperature and velocity are highest in channels with triangular surface roughness and lowest in those with rectangular roughness. Furthermore, increasing the quantity of surface roughness decreases the temperature and velocity of the fluid within nanochannels. When the quantity of rectangular surface roughness is 5, the fluid temperature within the nanochannel decreases by 12%, and the velocity decreases by 38% compared to a roughness quantity of 1. We also found, through velocity contours, that the presence of roughness increases the local fluid velocity in the rough regions of nanochannels. Roughness also reduces the density fluctuations of the fluid near the walls within the nanochannel. Roughness significantly affects the heat transfer performance of the fluid during its flow, and this influence should not be overlooked.

METHODS

In this study, molecular dynamics theory was employed, and simulations were conducted using the open-source software LAMMPS to investigate the influence of different shapes and quantities of surface roughness on fluid flow within nanochannels. The model in this paper was constructed using the LAMMPS software, and the surface roughness shapes on the walls were implemented as rectangular, hemispherical, and triangular. The wall surfaces were composed of copper atoms, while the fluid consisted of argon atoms. The copper atoms were arranged in a face-centered cubic (FCC) lattice with a lattice constant of 3.615 Å. Similarly, the argon atoms were arranged in a face-centered cubic (FCC) lattice with a lattice constant of 5.62 Å. The interactions between copper atoms were modeled using the EAM (Embedded Atom Method) potential, while the interactions between argon atoms were described using the LJ (Lennard-Jones) potential. The LJ potential was also employed to represent interactions between argon and copper atoms.

Heat transfer of graphene foams and carbon nanotube forests under forced convection

The effective dissipation of heat from electronic devices is essential to enable their long-term operation and their further miniaturization. Graphene foams (GF) and carbon nanotube (CNT) forests are promising materials for thermal applications, including heat dissipation, due to their excellent thermal conduction and low thermal interface resistance. Here, we study the heat transfer characteristics of these two materials under forced convection. We applied controlled airflow to heated samples of GF and CNT forests while recording their temperature using infrared micro-thermography. Then, we analyzed the samples using finite-element simulations in conjunction with a genetic optimization algorithm, and we extracted their heat fluxes in both the horizontal and vertical directions. We found that boundary layers have a profound impact on the heat transfer characteristics of our samples, as they reduce the heat transfer in the horizontal direction. The heat transfer in the vertical direction, on the other hand, is dominated by the material conduction and is much higher than the horizontal heat transfer. Accordingly, we uncover the fundamental thermal behavior of GF and CNT forests, paving the way toward their successful integration into thermal applications, including cooling devices.

Flow Condensation Heat Transfer Characteristics of Nanochannels with Nanopillars: A Molecular Dynamics Study

Flow condensation in nanochannels is a high-efficiency method to deal with increasingly higher heat flux from micro/nanoelectronic devices. Here, we study the flow condensation heat transfer characteristics of nanochannels with different nanopillar cross-sectional areas and heights using molecular dynamics simulation. Results show that two phases containing vapor in the middle of the channel and liquid near walls can be distinguished by obvious interfaces when the fluid is at a stable state. The condensation performance can be promoted by adding nanopillars. With the increase in nanopillar cross-sectional areas or heights, the time that the fluid spends to reach stability will be put off, while the condensation performance enhances. Different from the small enhancement of nanopillar cross-sectional areas, the condensation heat transfer performance improves significantly at a higher nanopillar height, which increases the heat transfer rates by 11.6 and 35.8% when heights are 6a and 8a, respectively. The preeminent condensation heat transfer performance is ascribed to the fact that nanopillars with a higher height disturb the vapor-liquid interface and vapor region, which not only allows vapor atoms with strong Brownian motion to collide with nanopillar atoms directly but also increases deviations of vapor-liquid potential energy to facilitate condensation heat transfer in nanochannels.

Molecular dynamics study of convective heat transfer mechanism in a nano heat exchanger

With the rapid development of micro/nano electro-mechanical systems, the convective heat transfer at the micro/nanoscale has been widely studied for the thermal management of micro/nano devices. Here we investigate the convective heat transfer mechanism of a nano heat exchanger by the employment of molecular dynamics simulation with a modified thermal pump method. First, the temperature jump and velocity slip are observed at the wall–fluid interfaces of the nano heat exchanger. Moreover, the larger Kapitza resistance in the entrance region weakens the convective heat transfer. Second, the heat transfer performance of the nano heat exchanger can be improved by increasing the surface wettability of the solid walls owing to more fluid atoms being involved in heat transport at the walls when the wall–fluid interaction is enhanced. Meanwhile, the strong surface wettability results in the appearance of the quasi-solid fluid layers, which improves the heat transfer between walls and fluids. Finally, we point out that when the surface wettability of the nano heat exchanger is weak, the heat transfer of the hot fluid side is better than that of the cold fluid side, while the convective heat transfer performances of the cold and hot fluid sides are reversed when the surface wettability is strong. This is because of the feebler temperature jump of the hot fluid side when wall–fluid interaction is small and the greater velocity slip of the cold fluid side for walls with large wall–fluid interaction.

The convective heat transfer mechanism in a nano heat exchanger is investigated using molecular dynamics simulation.

Molecular dynamics simulation of nanofluidics

This review reports the progress on the recent development of molecular dynamics simulation of nanofluidics. Molecular dynamics simulations of nanofluidics in nanochannel structure, surface roughness of nanochannel, carbon nanotubes, electrically charged, thermal transport in nanochannels and gases in nanochannels are illustrated and discussed. This paper will provide an expedient and valuable reference to designers who intend to research molecular dynamics simulation of nanofluidic devices.