Molecular dynamics simulation of annular flow boiling in a nanochannel is numerically investigated. In this research, an annular flow model is developed to predict the superheated flow boiling heat transfer characteristics in a nanochannel. To characterize the forced annular boiling flow in a nanochannel, an external driving force $$ \overrightarrow {F}_{\text{ext}} $$
 ranging from 1 to 12 PN (PN = pico newton) is applied along the flow direction to inlet fluid particles during the simulation. Based on an annular flow model analysis, it is found that saturation condition and superheat degree have great influences on the liquid–vapor interface. Also, the results show that due to the relatively strong influence of the surface tension in small channels, the interface between the liquid film and the vapor core is fairly smooth, and the mean velocity along the stream-wise direction does not change anymore. Also, it is found that the heat flux values depend on the boundary conditions. Finally, the Green–Kubo formula is used to calculate the thermal conductivity of liquid Argon. The simulations predict thermal conductivity of liquid Argon quite well.

Molecular dynamics simulation of annular flow boiling with the modified Lennard-Jones potential function

Numerical simulation of Poiseuille flow of liquid Argon in a nanochannel using the non-equilibrium molecular dynamics simulation (NEMD) is performed. The nanochannel is a three-dimensional rectangular prism geometry where the concerned numbers of Argon atoms are 2,700, 2,550 and 2,400 at 102, 108 and 120 K. Poiseuille flow is simulated by embedding the fluid particles in a uniform force field. An external driving force, ranging from 1 to 11 PN (Pico Newton), is applied along the flow direction to inlet fluid particles during the simulation. To obtain a more uniform temperature distribution across the channel, local thermostating near the wall are used. Also, the effect of other mixing rules (Lorenthz–Berthelot and Waldman–Kugler rules) on the interface structure are examined by comparing the density profiles near the liquid/solid interfaces for wall temperatures 108 and 133 K for an external force of 7 PN. Using Kong and Waldman–Kugler rules, the molecules near the solid walls were more randomly distributed compared to Lorenthz–Berthelot rule. These mean that the attraction between solid–fluid atoms was weakened by using Kong rule and Waldman–Kugler rule rather than the Lorenthz–Berthelot rule. Also, results show that the mean axial velocity has symmetrical distribution near the channel centerline and an increase in external driving force can increase maximum and average velocity values of fluid. Furthermore, the slip length and slip velocity are functions of the driving forces and they show an arising trend with an increase in inlet driving force and no slip boundary condition is satisfied at very low external force (<1 PN).

Nanoscale Poiseuille flow and effects of modified Lennard–Jones potential function

Boundary conditions at the solid-liquid surface over the multiscale channel size from nanometer to micron

The molecular dynamics (MD)–continuum hybrid simulation method has been developed in two aspects in the present work: (1) The energy equation has been combined into the coupling method in order to obtain the hybrid temperature profile and (2) the coupling method has been improved by the local linearization to obtain a smoother parametric profile. The developed method is primarily validated by analytical solutions and full MD results. Then, it is employed to study the scale effect on the flow and thermal boundaries in micro-/nano-channel flow. The hybrid velocity and temperature profiles are obtained with the channel height (H) ranging from 60σ to 2014σ and the solid–liquid coupling (β) ranging from 0.1 to 50. Scale effect has shown strong influence on the boundaries. Obvious slip characteristics can be found in the profiles, i.e. velocity slip and temperature jump, when H is small and β is large. However, the results also show that the profiles can be well predicted to converge to the macroscale non-slip/non-jump analytical solutions when H is large enough, where the effect of β can be omitted and the slip characteristics disappear. Correlations of relative slip length, relative temperature jump and pressure gradient with H are fitted from the simulation results. Copyright © 2009 John Wiley & Sons, Ltd.

Scale effect on flow and thermal boundaries in micro-/nano-channel flow using molecular dynamics–continuum hybrid simulation method

In this work, nonequilibrium molecular dynamics (MD) simulations were performed to investigate uniform liquid argon flow past a carbon nanotube. In the simulation, nanotubes were modeled as rigid cylinders of carbon atoms. Both argon-argon and argon-carbon interactions were calculated based on Lennard-Jones potential. Simulated drag coefficients were compared with (i) published empirical equation which was based on experiments conducted with macroscale cylinders and (ii) finite element (FE) analyses based on Navier-Stokes equation for flow past a circular cylinder using the same dimensionless parameters used in MD simulations. Results show that classical continuum mechanics cannot be used to calculate drag on a nanotube. In slow flows, the drag coefficients on a single-walled nanotube calculated from MD simulations were larger than those from the empirical equation or FE analysis. The difference increased as the flow velocity decreased. For higher velocity flows, slippage on the surface of the nanotube was identified which resulted in lower drag coefficient from MD simulation. This explains why the drag coefficient from MD dropped faster than those from the empirical equation or FE simulation as the flow velocity increased. It was also found that the drag forces are almost equal for single- and double-walled nanotubes with the same outer diameter, implying that inner tubes do not interact with fluid molecules.

Drag on a nanotube in uniform liquid argon flow

The molecular dynamics simulation of micro‐Poiseuille flow for liquid argon in nanoscale was performed in non‐dimensional unit system with the control parameters of channel size, coupling parameters between solid wall and liquid particles, and the gravity force. The molecular forces are considered not only among the liquid molecules, but also between the solid wall and liquid molecules. The simulation shows that a larger gravity force produces a larger shear rate and a higher velocity distribution. In terms of the gravity force, there are three domain regions each with distinct flow behaviors: free molecule oscillation, coupling and gravity force domain regions. Stronger fluid/wall interactions can sustain a larger coupling region, in which the flow is controlled by the balance of the intermolecular force and the gravity force. Strong surface interaction leads to small slip lengths and the slip lengths are increased slightly with increasing the shear rate. Weak surface interaction results in higher slip lengths and the slip lengths are dramatically decreased with increasing the shear rate. The viscosities are nearly kept constant (Newton flow behavior) if the non‐dimensional shear rate is below 2.0. At higher non‐dimensional shear rate larger than 2.0, the viscosities have a sharp increase with increasing the shear rate, and the non‐Newton flow appears.

X.D. Xu

Papers

Molecular dynamics simulation of micro‐Poiseuille flow for liquid argon in nanoscale