Minli Bai

The molecular dynamic simulation on impact and friction characters of nanofluids with many nanoparticles system

Impact and friction model of nanofluid for molecular dynamics simulation was built which consists of two Cu plates and Cu-Ar nanofluid. The Cu-Ar nanofluid model consisted of eight spherical copper nanoparticles with each particle diameter of 4 nm and argon atoms as base liquid. The Lennard-Jones potential function was adopted to deal with the interactions between atoms. Thus motion states and interaction of nanoparticles at different time through impact and friction process could be obtained and friction mechanism of nanofluids could be analyzed. In the friction process, nanoparticles showed motions of rotation and translation, but effected by the interactions of nanoparticles, the rotation of nanoparticles was trapped during the compression process. In this process, agglomeration of nanoparticles was very apparent, with the pressure increasing, the phenomenon became more prominent. The reunited nanoparticles would provide supporting efforts for the whole channel, and in the meantime reduced the contact between two friction surfaces, therefore, strengthened lubrication and decreased friction. In the condition of overlarge positive pressure, the nanoparticles would be crashed and formed particles on atomic level and strayed in base liquid.

Effect of Vibration on Forced Convection Heat Transfer for SiO2–Water Nanofluids

In the process of heat transfer, the fluid type and external parameters have a significant impact on heat transfer performance. For this reason, the physical properties, pressure differences, and heat transfer rates of SiO2–water nanofluids have been experimentally investigated in a straight circular pipe. Experimental results revealed a great difference in physical properties between SiO2–water nanofluids and purified water. The friction factor of low-volume-concentration nanofluids was slightly increased for laminar flow and tended to be almost independent of the Reynolds number for turbulent flow. The heat transfer coefficient can be enhanced either by adding nanoparticles to purified water or by imposing a transverse vibration on the heat transfer surface. Using these two methods at the same time (compound heat transfer enhancement), heat transfer performance is much better than that with either method alone. The largest increase of about 182% was observed under conditions of compound heat transfer enhancement.

Numerical investigation of the flow and heat behaviours of an impinging jet

The flow and temperature fields of a turbulent impinging jet are rather complex. In order to accurately describe the flow and heat-transfer process, two important factors that must be taken into account are the turbulence model and the wall function. Several turbulence models, including κ–ϵ turbulence models, κ–ω turbulence models, low-Re turbulence models, the κ–κl–ω turbulence model, the Transition SST turbulence model, the V2F turbulence model and the RSM turbulence model, are examined and compared to experimental data. Furthermore, for the near wall region, various wall functions are presented for comparison and they include the standard wall function, the scale wall function, the non-equilibrium wall function and the enhanced wall function. The distribution features of velocity, turbulent kinetic energy and Nusselt number are determined in order to provide a reliable reference for the multiphase impinging jet in the future.

On the Microscopic Flow Characteristics of Nanofluids by Molecular Dynamics Simulation on Couette Flow

Adding a small amount of nanoparticles to conventional fluids (nanofluids) has been proved to be an effective way for improving capability of heat transferring in base fluids. The change in micro structure of base fluids and micro motion of nanoparticles may be key factors for heat transfer enhancement of nanofluids. Therefore, it is essential to examine these mechanisms on microscopic level. The present work performed a Molecular Dynamics simulation on Couette flow of nanofluids and investigated the microscopic flow characteristics through visual observation and statistic analysis. It was found that the even-distributed liquid argon atoms near solid surfaces of nanoparticles could be seemed as a reform to base liquid and had contributed to heat transfer enhancement. In the process of Couette flow, nanoparticles moved quickly in the shear direction accompanying with motions of rotation and vibration in the other two directions. When the shearing velocity was increased, the motions of nanoparticles were strengthened significantly. The motions of nanoparticles could disturb the continuity of fluid and strengthen partial flowing around nanoparticles, and further enhanced heat transferring in nanofluids.

Numerical Study of Nanofluids Flow Characteristics Using LES–Lagrange Method and Molecular Dynamics Simulation

In order to reveal the mechanisms of heat transfer enhancement in nanofluids from the flow characteristics, this paper firstly used LES (Large eddy simulation)–Lagrange method to simulate the turbulent flow of nanofluids through a straight circular tube. It has been observed that nanoparticles would move up and down and sideways besides main flowing. The turbulent characteristics of nanofluids have been changed greatly in comparison with pure water: the turbulent intensity and Reynolds stress are enhanced obviously; there are more vortexes in the flow field. These flow characteristics of nanofluids can effectively strengthen the transport of momentum, mass and energy, which is the main reason for heat transfer enhancement in nanofluids. It is also found that nanofluids containing smaller diameter nanoparticles have higher turbulent intensity and flow activity. The flow characteristics of nanofluids are sensitive to the changes of smaller diameter nanoparticle size. While using different nanoparticle materials, the flow characteristics of nanofluids have a little change. At last, to verify the aforesaid views, the flow behaviors of nanofluids in the near wall region and main flow region have been simulated by molecular dynamics.© 2013 ASME

An investigation on the flow and heat transfer characteristics of nanofluids by nonequilibrium molecular dynamics simulations

ABSTRACT The flow and heat transfer characteristics of nanofluids are investigated by nonequilibrium molecular dynamics simulations. Both the effect of chaotic movements of nanoparticle (CMN) on flow properties and its resulting heat transfer enhancement are analyzed. Results show that compared with the base fluid, the effective thermal conductivity of nanofluids is increased, and the increase ratio in the shear flow field is much higher than that in the zero-shear flow field. Based on the models built in this paper, the contributions of increased thermal conductivity and CMN to the heat transfer enhancement of nanofluids are 49.8–68.6% and 31.4–50.2%, respectively.

Molecular dynamics simulation on the effect of nanoparticles on the heat transfer characteristics of pool boiling

ABSTRACT Molecular dynamics simulation was performed to investigate pool boiling of nanofluids on the metal wall. Nanoparticles were placed near the wall. Results showed that with the addition of nanoparticles the fluid temperature, net evaporation number and heat flux were increased, indicating that the boiling heat transfer was enhanced. In addition, the nanoparticles were able to move around the wall disorderly but did not move with the fluid. The effects of heated temperature and nanoparticle size on the boiling heat transfer were also investigated. By increasing heated temperature and nanoparticle size, the boiling heat transfer enhancement increased.

Molecular dynamics simulation on the effect of nanoparticle deposition and nondeposition on the nanofluid explosive boiling heat transfer

Abstract Molecular dynamics simulations were performed to investigate the effect of nanoparticle deposition and nondeposition on the explosive boiling heat transfer. Both particle state (deposition and nondeposition) and metal surface structure (smooth and rough) were considered to study the boiling behavior. Particularly for the rough surface, a special deposition case was simulated that the deposition nanoparticle was not filled with the pit. The results showed that the addition of nanoparticles enhanced the boiling behavior. The histories of argon temperature, net evaporation number, as well as heat flux demonstrated that deposition nanofluid boiling heat transfer enhancement behavior was the highest.

Characteristics of flow and heat transfer of nanofluids under laminar states

ABSTRACT The current article used four different models (single-phase, Mixture, Eulerian, and discrete phase model) to investigate the flow and heat transfer characteristics of nanofluids under a laminar state. We explored the Al2O3-water nanofluid inside a microscale trapezoidal channel and the CuO-oil inside a circular channel with a regular size. The velocity and temperature fields of nanofluids were discussed by comparing the differences among each model. It is revealed that the change of flow characteristics of nanofluid plays a more decisive role in its heat transfer enhancement besides the improvement of its physical properties.

Molecular Dynamics Simulation on the Effect of Micro-Motions of Nanoparticles in Heat Transfer Enhancement of Nanofluids

The flow and heat transfer characteristics of nanofluids in the near-wall region were studied by non-equilibrium molecular dynamics simulation. The nanofluid model consisted of one spherical copper nanoparticle and argon atoms as base liquid. The effective thermal conductivity (ETC) of nanofluids and base fluid in shear flow fields were obtained. The ETC was increased with the increasing of shear velocity for both base fluid and nanofluids. The heat transfer enhancement of nanofluids in the shear flow field (v≠0) is better than that in the zero-shear flow field (v=0). By analyzing the flow characteristics we proved that the micro-motions of nanoparticles were another mechanism responsible for the heat transfer enhancement of nanofluids in the flow field. Based on the model built in the paper, we found that the thermal properties accounted for 52%–65% heat transfer enhancement and the contribution of micro-motions is 35%–48%.Copyright