Chengzhi Hu

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.

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.

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

Numerical Investigation into the Cooling Process of Conventional Engine Oil and Nano-Oil Inside the Piston Gallery

A numerical simulation model has been developed for the transient flow and heat transfer problems of oil inside the piston gallery using a coupled VOF/Level Set method. Detailed cooling processes of Cu-oil nanofluids, with the nanoparticle size of 50 nm and the volume fractions of 1, 2 and 3 %, have been investigated. The oil fill ratio (OFR) and heat transfer coefficients (HTC) variations at different crank angles have been examined as well. The results have demonstrated that the nano-oil is able to improve the heat transfer capacity by a large margin. Compared with the conventional engine oil, the overall average heat transfer coefficients of the nano-oil, with the volume fractions of 1, 2 and 3 %, increase by 5.80, 14.51 and 28.11 % respectively.

On the Influence of Nanoparticle Shapes for Nanofluids Flow Behaviors by Molecular Dynamics Simulation

Understanding how the nanoparticles influence flow behavior of nanofluids is important for revealing mechanism of heat transfer enhancement by using nanofluids. The aim of this work was to study the microscopic change in base fluid and micro-motion of nanoparticles due to Brownian motion by molecular dynamics simulation. The present work established shearing flow simulation models considering different shapes of nanoparticles. Velocity distribution and number density distribution of fluid, and angular velocity components and translational velocity components of nanoparticles were statistically analyzed. The results of velocity distribution and number density distribution showed that adding nanoparticles reduces flow boundary layer and causes uneven distribution of mass; and the results for angular velocity components and translational velocity components of nanoparticles showed that nanoparticles rotate fast in the fluid, and vibrate irregularly. The present study suggests that adding nanoparticles causes microscopic change for base fluid including reducing thickness of flow boundary layer and uneven density distribution in fluid. In addition, the micro-motions of nanoparticles including rotation and vibration due to Brownian motion strengthen micro-flow effect and momentum transfer in nanofluids. Furthermore, by comparing motion behaviors of nanoparticles in different shapes the present work reveals that shapes of nanoparticles influence deeply flow behavior of nanofluids.Copyright