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Dissipative particle dynamics (DPD) have been widely used for the simulations of dynamics of both simple and complex fluids at nano/micro scales. In these simulations, periodic boundaries are usually employed in the main flow direction and the characterization of the flow and heat transfer is based on fully developed conditions. In the real nano/micro-fluidic devices, however, there are entrances and exits and the flow and temperature fields are not the same at different positions, making the periodic boundary conditions ill-suited due to problems with conservation of energy and momentum. This is the motivation of the present study to generate the non-periodic boundary condition having an entrance and an exit in the the DPD system and study the heat transfer characteristics in the entrance region. In this study, the entrance and exit regions are modelled for simulations of the flow in a parallel-plate channel based on the available methodology originally introduced for molecular dynamics. In this methodology, a body force acts on the DPD particles at the entrance region of the solution domain to generate the entrance region. This is region is so-called pump region. Also, a region to initiate the DPDe temperature was located followed by the pump region. Forced convection heat transfer of water flowing through a parallel-plate channel with constant wall temperature was simulated using this method. The simulations were implemented for different body forces in the pump region. The results were evaluated in terms of velocity, temperature and number density distributions in the channel and showed the effects of the compressibility of the DPD fluid and random movement (or Brownian motion). In addition, the Reynolds and Nusselt numbers were calculated to investigate their effects on the heat transfer characteristics at the entrance region.

In this paper, energy conserving Dissipative Particle Dynamics (DPDe) is used to study liquid characteristics when the walls are kept at a melting temperature. The formulation of the phase change problem is based on the latent heat model available in the literature. It is incorporated into the DPDe method to simulate a one-dimensional solid-liquid moving boundary problem. The solution domain is considered to be a two-dimensional Cartesian box where DPDe particles are randomly distributed. Periodic boundary conditions are applied in the flow direction and solid DPDe particles are placed as additional layers on the top and bottom of the domain. The DPDe result was compared with the available analytical solution and the effects of the DPDe parameters and thermal characteristics are discussed.

The Mach number and pressure on the outlet plane of a straight micro-tube were investigated numerically for both laminar and turbulent flow cases. The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The LB1 turbulence model was used for the turbulent flow case. The compressible momentum and energy equations with the assumption of the ideal gas were solved. The computational domain is extended to the downstream region from the micro-tube outlet. The back pressure was given to the outside of the downstream region. The computations were performed for a tube whose diameter ranges from 50 to 500 μm. The average Mach number on the outlet plane of the fully under-expanded flow depends on the tube diameter and ranges from 1.16 to 1.25. The flow characteristics of the under-expanded gas flow in a straight micro-tube were revealed.

The heat conduction in a two-dimensional square plate at nano/micro scales is investigated using numerical and semi-analytical methodologies which are energy conserving dissipative particle dynamics (DPDe) and Boltzmann transport equation (BTE) with the gray relaxation time approximation. The solution domain was considered to be a two-dimensional square plate surrounded by walls having constant temperatures. The numerical and semi-analytical results for different Knudsen numbers ranging from 0.1 to 10 are presented in form of temperature distributions in the solution domain. These results are compared with each other and the applicability of the DPDe model to two-dimensional structures is discussed.

Diffusive-ballistic heat transport in a two-dimensional square plate was simulated using energy conserving dissipative particle dynamics (DPDe). The solution domain was considered to be a two-dimensional square plate surrounded by walls at constant temperatures, where DPD particles are uniformly distributed. The effects of phonon mean free path was incorporated by its relation to the cutoff radius of energy interaction. This cutoff radius was obtained based on Knudsen number (Kn) using the existing phonon-boundary scattering models. The simulations for 0.1 < Kn < 10 were conducted with different modifications of the cutoff radius. The results are presented in form of temperature distributions in the solution domain and the effect of Knudsen number is discussed.

The liquid flow in sub-micron channels is simulated using multi-body dissipative particle dynamics (MDPD) to study the effect of the surface tension between liquid and wall surface on the flow in sub-micon scale. The solution domain is considered to be two-dimensional, where DPD particles are randomly distributed. Periodic boundary condition is employed in the flow direction and the solid walls are created by distributing DPD particles in the additional layers on the top and bottom of the domain. The different surface tensions between liquid and wall surface are obtained by changing the interaction parameters between the liquid and wall DPD particles. The ratio of Capillary number (Ca) to Reynolds number (Re) is used to relate the DPD units to the physical units. The results are shown in the form of slip length and the effect of the surface tension on the liquid flow in sub-micron channels is discussed.

This paper focuses on under-expanded gas flow at a straight mini-tube exit. Pitot total pressure of gas flow (jet) in downstream region from a straight mini-tube exit was measured to give data for validation of numerical results. A mini-tube of 495μm in diameter & 56.3 mm in length and a pitot tube of 100 μm in outer diameter were used. The pitot total pressure was measured every 0.1 mm interval in the flow and radial directions. The measurement was done for the mass flow rates of 9.71×10−5 kg/s and 1.46×10−4 kg/s. The data were accumulated for validation of the numerical result to reveal the characteristics of the under-expanded gas flow at the exit of a mini-tube. Comparisons were conducted for sample computations and a slight discrepancy can be seen between numerical and experimentally measured pitot total pressures.Copyright © 2013 by ASME

Diffusive-ballistic heat transport in thin films was simulated using energy conserving dissipative particle dynamics (DPDe). The solution domain was considered to be two-dimensional with DPD particles distributed uniformly under constant temperature boundary conditions at the top and bottom walls and periodic boundaries at the side walls. The effects of phonon mean free path were incorporated by its relation to the cutoff radius of energy interaction. This cutoff radius was based on the Knudsen number using the existing phonon-boundary scattering models. The simulations for 0.1 < Kn < 10 were obtained with the different modifications of the cutoff radius. The results were presented in form of a nondimensional temperature profile across the thin film and were compared with the semi-analytical solution of the equation of phonon radiative transport (EPRT). When the phonon-boundary scattering is not considered, the DPDe simulation results have more discrepancies compared with the EPRT solution as Kn increases, indicating that the phonon-boundary scattering plays an important role when the heat transport across the film becomes more ballistic. The results demonstrate that the DPDe can simulate the diffusive-ballistic heat transport for a broad range of Kn, but phonon-boundary scattering should be considered for the accurate simulation of the ballistic heat transport. (C) 2013 Elsevier Ltd. All rights reserved. (Less)

The effect of nanoparticle volume fraction on hydrodynamic and thermally fully developed forced convection heat transfer in parallel-plate channels was numerically studied under constant wall temperature boundary condition using energy conserving dissipative particle dynamics (DPDe). The solution domain was considered to be two-dimensional with periodic boundary condition in the flow direction and no-slip boundary conditions at top and bottom walls of the channel. The results were shown via fully developed velocity and temperature distributions across the channel cross-section. Nusselt numbers are calculated from the gradient of temperature at the wall and compared with the analytical solutions. The effect of nanoparticle volume fraction on Nusselt number was discussed.Copyright © 2012 by ASME