Takaharu Tsuruta

Condensation/evaporation coefficient and velocity distributions at liquid–vapor interface

Abstract Molecular dynamics simulations have been conducted for a system of argon molecules to study the effects of translational motion of monatomic molecules on condensation and evaporation coefficients. The results show that both coefficients depend on a surface-normal component of the translational energy. The vapor molecules with smaller energy can be easily reflected by the surface molecules, and the surface molecules with smaller energy can not evaporate. Also, velocity distributions of evaporated and reflected molecules are presented in the form of modified Maxwellian utilizing the condensation/evaporation coefficient expressed as a function of the normal component of translational energy.

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 ...

Unified theoretical prediction of fully developed nucleate boiling and critical heat flux based on a dynamic microlayer model

Abstract A new dynamic microlayer model has been proposed to predict theoretically the heat flux in fully developed nucleate boiling regions including critical heat flux (CHF). In this model, the heat transfer with boiling is mainly attributed to the evaporation of the microlayers which are periodically formed while the individual bubbles are forming. Since the initial microlayer thickness becomes thinner with the increase of wall superheat, both the local evaporation and the partial dryout speed of the microlayer increase. As a result, the time-averaged heat flux during the period of individual bubble has a maximum point, the CHF, at the predicted continuous boiling curve.

Experimental study of nucleate boiling heat transfer enhancement in a confined space

Abstract Saturated nucleate boiling heat transfer and critical heat flux (CHF) were experimentally studied in a confined space which consisted of two horizontal surfaces with a lower heated surface and an upper mesh screen. The nucleate boiling heat transfer was much enhanced in such a confined space, because the mesh screen kept primary vapor bubbles forming coalescence bubble within the confined space in lower heat flux region and allowed the vapor bubble to easily escape from the confined space in higher heat flux region. Observation by a high-speed camera showed that the behaviors of vapor mushroom over the mesh screen were not significantly different from that in the unconfined pool boiling at higher heat flux. The peripheral conditions of the confined space had no obvious effects on the heat transfer characteristics. The experimental CHF data showed a good agreement with the theoretical predictions based on the microlayer model proposed by some of the present authors.

Study on Shrinkage Deformation of Food in Microwave–Vacuum Drying

Drying shrinkage is an important problem in the food industry. Focusing on microwave–vacuum drying, we study the mechanism of deformation due to shrinkage of the food structure. A relationship between the strain and the water content is introduced for a finite element analysis. The temperature and water distributions are obtained by a finite difference method with the use of a variable permeability and diffusion coefficient depending on the water content. Comparisons with experimental data on radishes, carrots, and tofu indicate that the present model can express the deformation as well as the water content inside the materials.

Internal Resistance to Water Mobility in Seafood during Warm Air Drying and Microwave-Vacuum Drying

We present a novel microwave-vacuum drying technique as an effective method for seafood. Microwave is irradiated just for the latent heat supply to keep the material at room temperature. We made several experiments using the scallop and examined the internal resistance for water transport based on the porous media model. The experimental and theoretical study indicates that the microwave-vacuum drying keeps the channel for water transportation and results in the high permeability at the wide range of moisture content. The drying time was significantly shortened as compared with the warm-air drying and the room-temperature drying results in good quality of dried seafood.

Theoretical studies on transient pool boiling based on microlayer model

Abstract An analytical model for transient pool boiling heat transfer was developed in this study. The boiling curves of the transient boiling were obtained based on the microlayer model proposed by the authors and the mechanism of transition from the non-boiling regime to film boiling, i.e., direct transition was theoretically examined. Since the nucleate boiling heat flux is mainly due to the evaporation of the microlayer and its initial thickness decreases rapidly with increasing superheat, the duration of nucleate boiling is markedly decreased as the incipient boiling superheat is increased. It is found that the direct transition is closely connected to the rapid dryout of the microlayer which occupies almost the whole surface at high wall superheat.

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