A New “Bounce-Normal” Boundary in DPD Calculations for the Reduction of Density Fluctuations

Dissipative Particle Dynamics (DPD) is a mesoscopic fluid modeling method, which facilitates the simulation of the statics and dynamics of complex fluid systems at physically interesting length and time scales. Currently, there are various applications of DPD, such as colloidal suspensions, multi-phase flow, rheology of polymer chains, DNA macromolecular suspension, etc., which employ this technique for their numerical simulation. The DPD technique is capable of modeling macroscopic properties of the bulk flow very well, but difficulties arise if the flows are confined through wall-bounded regions, or when different boundaries simultaneously exist in the simulation domain. These boundaries cause negative effects on the macroscopic temperature, density and velocity profiles, as well as the shear stress and pressure distributions. In particular, the interaction of DPD particles with solid boundaries causes large density fluctuations at the near wall regions. This density distortion leads to pronounced fluctuations in the pressure and shear stress, which are not actually present. To overcome these serious deficiencies, we introduce a new method in this work, which uses a combination of randomly distributed wall particles and a novel reflection adaptation at the wall. This new methodology is simple to implement and incurs no additional computational cost. More importantly, it does not cause any distortion in the macroscopic properties. This novel reflection adaptation is a novel version of the bounce back reflection, which we shall term the bounce-normal reflection. The most important characteristic of this method is that it reduces density fluctuations near the boundaries without affecting the velocity and temperature profiles. This new method is easily applicable to any wall-bounded problem with stationary boundaries and it has a very good consistency with macroscopic features. The eventual objective of this numerical development work is to investigate suspension flow through micro/nano channels of fluidic NEMS/MEMS devices, with applications to DNA and protein separation. These micro/nano channel devices, consisting of many entropic traps, are designed and fabricated for the separation of proteins and long DNA molecules.

Characteristics of Heat Transfer and Flow on a Surface With Small-Scale Concave Spherical Pits

Small-scale concave spherical pits, which have a special effect on heat transfer enhancement and turbulent drag reduction, are investigated by numerical simulation in detail. Two kinds of small-scale concave pits structures are designed on surface of a plate, which are located in the bottom of a rectangle channel. The characteristics of heat transfer and flow in channel are investigated and compared with a same channel with plate bottom by means of LES. Flow structure and temperature distribution near the pits are analyzed. The numerical simulation results indicate that the concave spherical pits disturb the flow field and vortex is induced by the pits. The turbulent coherent structure is affected by the induced vortex. The numerical simulation indicates that small scale pit can generate the vortex in couple. The range of vortex is accord with the array of small scale pit. The small scale pit can enhance the intensity of vortex. As a result, the temperature field near the pit is changed with generation of the vortex. The heat transfer mechanism on plate with small scale concave spherical pit is summarized.

A Molecular Dynamics Study on Effects of Nanostructural Clearances at an Interface on Thermal Resistance

The classical molecular dynamics simulation was conducted in order to clarify the effects of structural clearances in nanometer scale on thermal resistance at a liquid-solid interface. A liquid molecular region confined between the solid walls, of which the interparticle potential was Lennard-Jones type, was employed as a calculation system. The solid walls consisted of three atomic layers where the temperature of the middle layer was controlled by the Langevin method. Heat flux in the system was calculated numerically by integrating the forces that acted on the temperature controlled atoms by the Langevin method. The temperature jump between the solid wall and the liquid molecular region was calculated numerically. The thermal resistance at a liquid-solid interface was calculated numerically with changing the surface structural clearances in nanometer scale. Temperature gradient and liquid density were also changed as calculation parameters. With changing the surface structural clearances from 0nm to 2.5nm the thermal resistance at the interface once decreased and became the minimum value when the structural clearances were between 0.6 to 1.0 nm. The thermal resistance between the solid and the liquid increased when the structural clearances were more than 1.0nm. With the increase of the liquid density the thermal resistance between the solid and the liquid substantially decreased regardless of the temperature gradient and the surface structures in nanometer scale.

Experimental Researches on Micro-Droplet Freezing on a Solid Surface Under Atmospheric Conditions: Part I

Atmospheric ice accretion on structures is a problem of fundamental importance to a number of industries. Examples of engineering problems caused by ice accretion involving aircraft, power transmission lines, telecommunication towers, electrical railway contact-wires, and other structures. Under atmospheric icing conditions two basic types of ice may form; rime or glaze. The supercooled micro-droplets in clouds are an important factor in icing. The objective of this study was to develop a new experimental method to investigate a single supercooled micro-droplet freezing process, in order to better understand the mechanism of rime or glaze ice accretion. The experimental device and principles are described in this paper. The experimental set has two small cold rooms, which is separated by a board with a central hole. A droplet with diameter of 15∼40 μm, temperature of 0∼−5°C was levitated in the cold air stream by electrostatic force. A CCD camera tracked its trace. The air temperature is from 0∼−10°C, the micro-droplet diameter is from 15∼40μm, and its temperature is from 0∼−5°C in the experimental study. This article focused on the experimental set and the experimental principles, and the next article will focus on the experimental data analysis.

Enhanced Natural Convective Heat Transfer of CNT-Ethylene Glycol-Water Suspensions

This paper presents an experimental study on the rheology and steady state natural convective heat transfer of suspensions of Carbon nanotubes (CNT) in the Water - Ethylene Glycol (WEG) base liquid, heated in a cylindrical cavity. Two series of experiment were performed in two orientations of the central axis of the cavity; vertical, and, horizontal. In vertical axis experiments, the heat was supplied from the bottom. The cylindrical cavity was made out of Aluminium, 10mm in height and 240mm in diameter. The heat input was 215W/m2. The CNTs used had an aspect ratio of ∼150. There were six suspensions investigated in either series of tests; CNT 0.1wt%, and EG 0, 10, 25, 50, 75 & 100wt%. It was found that the vertical axis orientation deteriorates heat transfer in all cases. However for horizontal orientation, there is a spectacular enhancement of up to 83% depending upon the EG concentration. The results also show that WEG-CNT suspension demonstrates non-Newtonian behaviour, which augments with increasing EG concentration.

Convective Heat Transfer Coefficient and Pressure Drop of Al2O3-Water Nanofluid in a Single-Phase Microchannel for Cooling of High Heat Output Devices

The paper describes an experimental procedure performed to obtain the convective heat transfer coefficient of Al2O3 nanofluid working as cooling fluid under turbulent regimen through arrays of aluminum microchannel heat sink having a diameter of 1.2 mm. Experimental Nusselt number correlation as a function of the volume fractions, Reynolds, Peclet and Prandtl numbers for a constant heat flux boundary condition is presented. The correlation for Nusselt number has a good agreement with experimental data and can be used to predict heat transfer coefficient for this specific nanofluid, water/Al2O3. Furthermore, the pressure drop is also analyzed considering the different nanoparticles concentration.

Thermal Behavior in a Multi-Layer Metal Thin-Film With Interfacial Contact Conductance by the Parabolic Two-Step Model

The effect of the contact conductance on micro-scale heat transport in a multi-layered metal thin-film subjected to ultra-fast pulse laser heating is investigated in this study. The interfacial contact conductance existing at the interface is included in the analysis. The parabolic two-step (PTS) model is applied to predict the electron and lattice temperature. The equations can be solved by Laplace transform and the Riemann-sum approximation. The results show that the contact conductance plays an important role in the micro-scale heat transport processes for a multi-layered metal thin-film. Hence, in order to eliminate the thermal failure of the surface layer, the thermal resistance existing at the interface should be as small as possible.

Measurements for the Moisture Permeations and Thermal Resistances of Cyclo Olefin Copolymer Substrates Deposited a Silicon Dioxide Film

Plastic substrates for organic light-emitting devices (OLED) are extremely sensitive to moisture and oxygen. A new amorphous engineering thermoplastic, nominated cyclic olefin copolymer (COC) has been used for this application, because of higher transparence, lower birefringence, lower dispersion and lower water absorption. However, COC plastic substrates can’t sustain plasma-based processing temperatures at 350°C. In this study, experiments of the moisture permeation rate testing and the thermal resistance experiments are conducted to explore the moisture diffusion barrier and thermal barrier characteristics of COC substrate deposited a SiO2 thin film on it. Silicon dioxide layer of thickness, 0.25μm, 0.5μm, and 1 μm, respectively, are fabricated by PECVD. For the permeation rate measurement, the Ca-test method is adopted. For the thermal resistance measurements, two methods of the thermocouple in vacuum environment and the IR thermography are adopted and measured results are compared. Different surface temperatures, 323.15K, 373.15K, 408.15K, and 473.15K, respectively, are applied upon the silicon dioxide film and temperature differences for varied thickness of silicon dioxide film are measured. Experimental results are presented to investigate the behaviors of moisture diffusion barrier and thermal barrier characteristics of the COC/SiO2 structure.

Growth of Micro Bubbles on Micro-Configured Metal-Graphite Composite Surfaces and Boiling Enhancement

A series of studies in nucleate boiling phenomena on metal-graphite composite surfaces has been investigated by Prof. Wen-Jei Yang and their associates. It has been discovered that the unique micro-configured construction of the composite surfaces plays a crucial role in the enhancement of boiling heat transfer. The present paper focuses on the formation and growth processes of micro bubbles and the micro/nano scale boiling behavior to reveal the mechanism of boiling heat transfer enhancement on the unique surfaces. The growth processes of the micro and macro bubbles are analyzed and formulated followed by an analysis of bubble departure. Based on these analyses, the enhancement mechanism of the pool boiling heat transfer on the composite surfaces is clearly revealed. The micro-configured composite surfaces provide more even distribution of a great number of stable boiling active sites through the graphite fibers. Consequently, the heat conduction through the layers is increased, which provides the power of phase change at the interfaces on bubble bottoms. Experimental results convincingly demonstrate the enhancement effects of the unique structure of metal-graphite composite surfaces on boiling heat transfer.

Nanofludic Analysis on Methanol Crossover of Direct Methanol Fuel Cells

Direct methanol fuel cells (DMFCs) are considered as a competitive power source candidate for portable electronic devices. Nafion® has been widely used for the electrolyte of DMFCs because of its good proton conductivity and high chemical and mechanical stability. However, the major problem that must be solved before commercialization is the high methanol crossover through the membrane. There are a number of studies on experiments about the methanol crossover rate through the membrane but only few theoretical investigations have been presented [1–3]. In this paper, an atomistic model [4] is presented to analyze the molecular structure of the electrolyte and dynamic properties of nanofluids at different methanol concentration. In the same time, the nano-scopic phenomenon of methanol crossover through the membrane is observed. The simulation system consists of the Nafion fragments, hydronium ions, water clusters and methanol molecules. Fig. 1 shows the simplified Nafion fragment in our simulation. Both intra- and inter-molecular interactions were involved in this study. Intermolecular interactions include the van der Waals and the electrostatic potentials. Intramolecular interactions consist of bond, angle and dihedral potentials. The force constants used above were determined from the DREIDING force field. The SPC/E model was employed for water molecules. The three-site OPLS potential model was utilized for the intermolecular potential in methanol. Each proton which migrates inside the electrolyte is assumed to combine with one water molecule to form the hydronium (H3O+). The force parameters for the hydronium were taken from Burykin et al [5]. The atomistic simulation was carried out on the software DLPOLY. First, a 500 ps NPT ensemble was performed to make the system reach a proper configuration. This step was followed by another 500 ps NVT simulation. All molecular simulations were performed at a temperature of 323K with three-dimensional periodic boundary conditions. The intermolecular interactions were truncated at 10 Å and the equations of motion were solved using the Verlet scheme with a time step of 1 fs. Fig. 2 shows the calculated density of the simulation system for different methanol concentrations at 323K. It can be seen that the density decreases with the methanol uptakes. The volume of the system increases as the methanol concentration increases, which means that the membrane swelling with methanol uptakes. The radial distribution functions (RDFs) of the ether-like oxygen (O2) toward water and methanol molecules for different methanol concentrations at 323K are shown in Fig. 3. From this figure, we find that methanol molecules can reside in the vicinity of the hydrophobic part of the side chain while water can not. Fig. 4 shows the RDFs between the oxygen atom of the sulfonic acid groups (O3) and solvents for different methanol concentrations at 323K. As shown in Fig. 4, both water and methanol have a tendency to cluster near the sulfonic acid groups, but water molecules prefer to associate with the sulfonic acid groups in comparison with methanol molecules. The mean square displacements (MSDs) of water and methanol molecules for different methanol concentrations at 323K are displayed in Fig. 5. It is shown that MSD curves have a linear tendency, which means both water and methanol molecules are diffusing in the system during the simulation. As the methanol concentration increases, the slope of MSD curve increases for methanol and decreases for water. This indicates higher methanol content constrains the mobility of water molecules but enhances the mobility of methanol molecules that cross the electrolyte. In summary, molecular simulations of the Nafion membrane swollen in different methanol concentrations (0, 11.23, 21.40, 46.92 wt%) at 323K have been carried out. Both methanol migration mechanism and hydronium diffusion phenomenon have been visualized by monitoring the trajectories of the specific species in the system. MSDs are used to evaluate the mobility and shows that the higher the methanol concentration, the greater the tendency of methanol crossover.