Gyoko Nagayama

A general expression for the condensation coefficient based on transition state theory and molecular dynamics simulation

A theoretical derivation of condensation coefficient based on transition state theory is presented in this paper by considering the three-dimensional movement of condensing molecules in the liquid–vapor interface region. The theoretical expression is a function of free volume ratio of liquid to vapor and activation energy for condensation. We have developed an evaluation of the activated state conditions in the interface region with the use of molecular dynamics (MD) simulations for argon and water. From the molecular scale consideration, it is found that a characteristic length ratio 3Vl/Vg has an important role in evaluating the condensation coefficient because the restricted translational motion is dominant in the condensation process compared with the rotational motion. Present theoretical values agree well with MD results in both monatomic and polyatomic polar molecules. Finally, we conclude that the condensation coefficient is an inherent physical property of a given pure liquid–vapor interface and ...

A Molecular Dynamics Approach to Interphase Mass Transfer Between Liquid and Vapor.

The study was conducted in order to understand a mechanism of interphase mass transfer between liquid and vapor. The molecular dynamics (MD) simulation is used to examine details of condensation and evaporation from the viewpoint of molecular kinetics. First, molecular boundary conditions for condensing, reflecting, and evaporating molecules are presented for an argon molecule as a function of the surface-normal component of translation energy. The velocity distributions can be expressed by the modified Maxwellian and making use of the condensation coefficient. The condensation coefficient of water is also examined for two kinds of intermolecular potential, the Carravetta-Clementi (C-C) model and the extended simple point charge (SPC/E) model, in order to consider the effect of the surface structure of the liquid on the condensation coefficient. The results indicate that the condensation coefficient of water is close to unity for both models and its dependence on the translation energy is small compared with argon. Finally, the condensation coefficient is studied based on the transition-state theory. An evaluation of the transition state is considered by applying the results of MD simulations for argon and water.

Molecular dynamics study on condensation/evaporation coefficients of chain molecules at liquid-vapor interface

The structure and thermodynamic properties of the liquid-vapor interface are of fundamental interest for numerous technological implications. For simple molecules, e.g., argon and water, the molecular condensation/evaporation behavior depends strongly on their translational motion and the system temperature. Existing molecular dynamics (MD) results are consistent with the theoretical predictions based on the assumption that the liquid and vapor states in the vicinity of the liquid-vapor interface are isotropic. Additionally, similar molecular condensation/evaporation characteristics have been found for long-chain molecules, e.g., dodecane. It is unclear, however, whether the isotropic assumption is valid and whether the molecular orientation or the chain length of the molecules affects the condensation/evaporation behavior at the liquid-vapor interface. In this study, MD simulations were performed to study the molecular condensation/evaporation behavior of the straight-chain alkanes, i.e., butane, octane, and dodecane, at the liquid-vapor interface, and the effects of the molecular orientation and chain length were investigated in equilibrium systems. The results showed that the condensation/evaporation behavior of chain molecules primarily depends on the molecular translational energy and the surface temperature and is independent of the molecular chain length. Furthermore, the orientation at the liquid-vapor interface was disordered when the surface temperature was sufficiently higher than the triple point and had no significant effect on the molecular condensation/evaporation behavior. The validity of the isotropic assumption was confirmed, and we conclude that the condensation/evaporation coefficients can be predicted by the liquid-to-vapor translational length ratio, even for chain molecules.

Molecular Dynamics Simulations of Interfacial Heat and Mass Transfer at Nanostructured Surface

The nanoscale heat and mass transport phenomena play important roles on the applications of nanotechnologies with great attention to its differences from the continuum mechanics. In this paper, the breakdown of the continuum assumption for nanoscale flows has been verified based on the molecular dynamics simulations and the heat transfer mechanism at the nanostructured solid-liquid interface in the nanochannels is studied from the microscopic point of view. Simple Lennard-Jones (LJ) fluids are simulated for thermal energy transfer in a nanochannel using nonequilibrium molecular dynamics techniques. Multi-layers of platinum atoms are utilized to simulate the solid walls with arranged nanostructures and argon atoms are employed as the LJ fluid. The results show that the interface structure (i.e. the solid-like structure formed by the adsorption layers of liquid molecules) between solid and liquid are affected by the nanostructures. It is found that the hydrodynamic resistance and thermal resistance dependents on the surface wettability and for the nanoscale heat and fluid flows, the interface resistance cannot be neglected but can be reduced by the nanostructures. For the hydrodynamic boundary condition at the solid-liquid interface, the no-slip boundary condition holds good at the super-hydrophilic surface with large hydrodynamic resistance. However, apparent slip is observed at the low hydrodynamic resistance surface when the driving force overcomes the interfacial resistance. For the thermal boundary condition, it is found that the thermal resistance at the interface depends on the interface wettability and the hydrophilic surface has lower thermal resistance than that of the hydrophobic surfaces. The interface thermal resistance decreases at the nanostructed surface and significant heat transfer enhancement has been achieved at the hydrophilic nanostructured surfaces. Although the surface with nanostrutures has larger surface area than the flat surface, the rate of heat flux increase caused by the nanostructures is remarkable.Copyright

Molecular Boundary Conditions and Accommodation Coefficient on A Nonequilibrium Liquid Surface

The non‐equilibrium molecular dynamics (NEMD) simulations have been carried out to obtain new evidence about inverted temperature profiles. We find that the inverted temperature profile occurs due to the excess energy of the reflecting molecules without contradiction to the second law. Therefore, a new definition of the accommodation coefficient for the reflecting molecule is proposed based on the energy of the reflecting molecule under the equilibrium condition. The accommodation coefficient decreases with increasing the mass flux in the vicinity of the liquid surface and this is the reason for the inverted temperature profile. Also, a direct simulation of Monte Carlo (DSMC) method has been performed with applying the molecular boundary condition developed in the non‐equilibrium molecular dynamics simulation. An inverted temperature profile is obtained because the energy of the reflecting molecule cannot reach accommodations to that of the equilibrium ones.

Scale effect of slip boundary condition at solid–liquid interface

Rapid advances in microelectromechanical systems have stimulated the development of compact devices, which require effective cooling technologies (e.g., microchannel cooling). However, the inconsistencies between experimental and classical theoretical predictions for the liquid flow in microchannel remain unclarified. Given the larger surface/volume ratio of microchannel, the surface effects increase as channel scale decreases. Here we show the scale effect of the boundary condition at the solid–liquid interface on single-phase convective heat transfer characteristics in microchannels. We demonstrate that the deviation from classical theory with a reduction in hydraulic diameters is due to the breakdown of the continuum solid–liquid boundary condition. The forced convective heat transfer characteristics of single-phase laminar flow in a parallel-plate microchannel are investigated. Using the theoretical Poiseuille and Nusselt numbers derived under the slip boundary condition at the solid–liquid interface, we estimate the slip length and thermal slip length at the interface.

Heat Transfer Enhancement at Nanostructured Surface in Parallel-plate Microchannel

In this paper, we examine the effect of the nanostructures on the parallel-plate microchannel flow experimentally as well as the nanochannel flow based on molecular dynamic studies. In the experiments, the microchannel surface is coated with nanostructures using plating and etching, and the sizes of nanostructures are rather different from the conventional surface roughness. The spacing of the parallel-plate microchannel is 300 µm, which is much smaller than its width and length, in order to reduce the effects of sidewalls as a well-studied geometry. It is found that the friction constant for the nanostructured surfaces of large contact angle (hydrophobility) is low compared with the surfaces of small contact angle (hydrophility). Also, the higher heat transfer coefficients have been obtained at the hydrophilic nanostructured surface with characteristic size of nanostructure of 200–700nm. In the molecular dynamics simulations of nanochannel flow, it is found that both the boundary condition at the solid-liquid interface depend on the surface wettability. The interface thermal resistance decreases at the nanostructured surface and significant heat transfer enhancement has been achieved at the nanostructured surfaces of hydrophility. Although the surface with nanostructures has larger surface area than the flat surface, the rate of heat flux increase caused by the nanostructures is still remarkable; however, it is unclear if the effect of the nanostructures could be detected in the microchannel experiments.

Microscopic Wetting at Microstructured Surface of Porous Silicon

Surface wettability is an important factor for micro/nanoscale thermal fluidic systems and it has attracted much interest for both fundamental research and practical applications. As one of the most attractive materials with controllable wettability, porous silicon is easy to be produced by the electrochemical etching. In this study, the effects of the microstructures of porous silicon on the wetting behavior of a pure water droplet were investigated experimentally. The solid-liquid contacting surface of the porous silicon substrate was prepared by varying both the geometrical microstructure and the chemical composition. The anodic etching was applied to the n type silicon substrate of orientation (100) and the geometrical microstructures of porous silicon were controlled by varying the fabrication conditions of the electrochemical etching. The pores of diameter ranging from 1–6 micrometers and the porosity up to 0.8 were obtained. Also, the surface chemical composition was controlled by coating the SiO2 layer or the CYTOP fluoropolymer layer directly on the porous silicon surface. The contact angle of the pure water droplet was measured at the prepared porous silicon surface in a room with constant temperature and humidity. The effects of the microstructures on the contact angle were discussed and the results were compared to both the classical theoretical models and a modified model based on the molecular dynamics simulations.Copyright

Topics on Boiling: From Fundamentals to Applications

This chapter deals with the various topics on boiling with regard to aspects of the fundamentals and applications to introduce the development of each author’s research in recent decades. The first four sections investigate the physics of boiling as phase change phenomena, including thermodynamic phase equilibrium state (Section 6.1), molecular dynamics of phase change (Section 6.2), computational analysis of boiling in micro-nano scale (Section 6.3), and transient boiling under rapid heating (Section 6.4). Section 6.5 deals with two-phase distribution measurement using neuron radiography. The following three sections then examine a specific boiling regime during highly subcooled boiling, called microbubble emission boiling (MEB). Each section treats the overall characteristics of MEB (Section 6.6), the occurrence conditions of MEB (Section 6.7), and vapor collapses in subcooled liquid related to MEB (Section 6.8). The next four sections are devoted to heat transfer augmentation with various techniques: thermal spray coating (Section 6.9), porous media (Section 6.10), patterned wettability refinement (Section 6.11), and self-rewetting fluid (Section 6.12). The last seven sections describe topics on applications of boiling. Sections 6.13 and 6.14 introduce boiling research in steel industries. Sections 6.15 and 6.16 explore vapor explosion. Boiling of refrigerant is discussed with heat pump systems in Section 6.17 and with automobile air conditioners in Section 6.18. Boiling related to emergency cooling core systems is considered in Section 6.19.

Effect of Micro-Structured Surface on Dropwise Condensation Heat Transfer

The micro/nano scale phase change phenomena become more and more important because the MEMS technology develops rapidly in the fields of electro- and bio-devices [1][2] and the MEMS enable us to control the surface wettability. In the dropwise condensation on the hydrophobic surface, the heat transfer coefficient is determined by the departing droplet size. In our previous paper, it was found that the droplets in radius around 7 μm made more significant contribution to the condensation heat transfer under the low-pressure conditions. That is, when the smaller droplets less than 7 μm cover the condensing surface, the higher condensing heat flux would be achieved than that of the ordinary dropwise condensation. However, it is still very difficult to keep the droplets to be continuous condensed within 7 μm at the surface.A challenging work has been conducted to fabricate a droplets exclusion structure on the condensing surface for the purpose of the enhancement of condensation heat transfer in our previous experiment [3]. By using the MEMS technology, we made the hybrid-condensing surface with hydrophobic and hydrophilic patterns in order to remove the grown droplets effectively. It was found that the hybrid-surface has a possibility to increase the condensation heat transfer coefficient but its drainage-ability of the condensate has the limitation due to the occurrence of the flooding over the surface structures. In order to reduce the flooding at the hydrophobic area, in this study, the new design of the condensing surface has been proposed and the condensation heat transfer coefficient is evaluated.© 2013 ASME