Yousheng Xu

Fractal analysis of effective thermal conductivity for three-phase (unsaturated) porous media

A fractal analysis of effective thermal conductivity for unsaturated fractal porous media is presented based on the thermal-electrical analogy and statistical self-similarity of porous media. Here, we derive a dimensionless expression of effective thermal conductivity without any empirical constant. The effects of the parameters of fractal porous media on the dimensionless effective thermal conductivity are discussed. From this study, it is shown that, when the thermal conductivity of solid phase and wet phase are greater than that of the gas phase (viz., ks∕kg>1, kw∕kg>1), the dimensionless effective thermal conductivity of unsaturated fractal porous media decreases with decreasing degree of saturation (Sw) and increasing fractal dimension for pore area (Df), fractal dimension for tortuosity (Dt), and porosity (ϕ); when the thermal conductivities of solid phase and wet phase are lower than that of the gas phase (viz., ks∕kg<1, kw∕kg<1), the trends were just opposite. Our model was validated by comparing ...

Controllable transport of water through nanochannel by rachet-like mechanism

By using molecular dynamics simulation, we have investigated systematically the feasibility of continuous unidirectional water flux across a deformed single-walled carbon nanotube (SWNT) driven by an oscillating charge outside without osmotic pressure or hydrostatic drop. Simulation results indicate that the flux is dependent sensitively on the oscillating frequency of the charge, the distance of the charge from the SWNT, and the asymmetry of the water-SWNT system. A resonance-like phenomenon is found that the water flux is enhanced significantly when the period of the oscillation is close to twice the average hopping time of water molecules inside the SWNT. These findings are helpful in developing a novel design of efficient functional nanofluidic devices.

A vibration-charge-induced unidirectional transport of water molecules in confined nanochannels

We propose a novel nanoscale design for unidirectional transport of water molecules through a single-walled carbon nanotube (SWCNT). This is achieved by using a vibration charge and a composite SWCNT with asymmetrical surface energy. With the proposed system, we demonstrated, using molecular dynamics simulations, that a continuous unidirectional water flow can be driven by a vibration charge without osmotic pressure or a drop in hydrostatic pressure. It is shown that the net flux of continuous unidirectional water flow can be controlled by adjusting the parameters of periodic vibration charge, temperature, and the degree of heterogeneity in surface energy. The remarkable net flux was the combined effect of the kinetic energy provided by the vibration charge, and the water density gradient resulted from the heterogeneous surface energy of the SWCNT. The present nanoscale design can efficiently convert the energy of vibration charges to the transport of water molecules. It may find applications in liquid circulation without a pressure gradient, lab-on-a-chip technology, desalination of sea water, filtration of polluted water, etc.

Optimizing the design of nanostructures for improved thermal conduction within confined spaces

Maintaining constant temperature is of particular importance to the normal operation of electronic devices. Aiming at the question, this paper proposes an optimum design of nanostructures made of high thermal conductive nanomaterials to provide outstanding heat dissipation from the confined interior (possibly nanosized) to the micro-spaces of electronic devices. The design incorporates a carbon nanocone for conducting heat from the interior to the exterior of a miniature electronic device, with the optimum diameter, D0, of the nanocone satisfying the relationship: D02 (x) ∝ x1/2 where x is the position along the length direction of the carbon nanocone. Branched structure made of single-walled carbon nanotubes (CNTs) are shown to be particularly suitable for the purpose. It was found that the total thermal resistance of a branched structure reaches a minimum when the diameter ratio, β* satisfies the relationship: β* = γ-0.25bN-1/k*, where γ is ratio of length, b = 0.3 to approximately 0.4 on the single-walled CNTs, b = 0.6 to approximately 0.8 on the multiwalled CNTs, k* = 2 and N is the bifurcation number (N = 2, 3, 4 ...). The findings of this research provide a blueprint in designing miniaturized electronic devices with outstanding heat dissipation.PACS numbers: 44.10.+i, 44.05.+e, 66.70.-f, 61.48.De

PREDICTION OF EFFECTIVE THERMAL CONDUCTIVITY OF POROUS MEDIA WITH FRACTAL-MONTE CARLO SIMULATIONS

On the basis of the fractal scaling laws of pore distribution in natural porous media, a probability model is developed for thermal conductivity in porous media by combining fractal theory and Monte Carlo technique. The current numerical model, which was validated by comparison with the existing experimental data, shows that the thermal conductivity of porous media is a function of the thermal conductivities of volume fraction, pore area fractal dimension, tortuosity fractal dimension and random number. The effect of microstructure parameters on the effective thermal conductivity of porous media is studied. The proposed fractal Monte Carlo simulation technique has advantages compared with conventional numerical methods and may have the potential in analyzing other transport properties of porous media.

Migration-collision scheme of Lattice Boltzmann method for heat conduction problems involving solidification

The phase-change problem is solved by the migration-collision scheme of lattice Boltzmann method. After formula derivation, we find that this method can give a rigorous numerical value for the phase-change temperature, which is of crucial importance. One-dimensional solidification in half-space and two-dimensional solidification in a corner are simulated. The phase change temperature and the liquid-solid interface are both obtained, and the results conform to the analytical solution.

Propagation of liquid surface waves over finite graphene structured arrays of cylinders

Abstract Based on the multiple scattering method, this paper investigates a benchmark problem of the propagation of liquid surface waves over finite graphene (or honeycomb) structured arrays of cylinders. Comparing the graphene structured array with the square structured and with triangle structured arrays, it finds that the finite graphene structure can produce more complete band gaps than the other finite structures, and the finite graphene structure has less localized ability than the other finite structures.

Optimizing Permeability in Fractal Tree-like Branched Networks

The overall permeability of composites with self-similar fractal tree-like networks is studied. Under the constraint of total volume, we derived a dimensionless expression of effective permeability and discussed in detail the relationship between the dimensionless effective permeability and the geometrical parameters of the tree-like network including diameter ratio, length ratio, branching number and fractal dimension. From the study, it was shown that, the dimensionless effective permeability of the tree-like network decreases with the increase of bifurcation number (N), branching length ratio ( ^ ) , branching levels (m) or fractal dimensions of channel length (D) when other parameters are kept constant. It was also found that, the dimensionless effective permeability of the tree-like networks reaches maximum when the diameter ratio Ρ satisfies ß , where ^ = 3 , Ν is the bifurcation number N=2, 3, 4, This optimal diameter ratio for maximum effective permeability of the fractal tree-like networks obeys Murrys law, but does not obey the WBE model of plants.

A new cross-scaling method to deal with the porous flow problem

The finite volume method (FVM) and the lattice Boltzmann method (LBM) are coupled with each other to construct a new cross-scaling method to deal with the porous flow problem. To check the effectiveness of our developed cross-scaling LBM—FVM, the above mentioned problem is also solved by the well known LBM—LBM. Based on the data checking of the published data and the results of LBM—FVM and LBM—LBM, good agreement is observed.

Simulation of flow through porous anode in MFC at higher power density

Microbial fuel cell (MFC) is a novel environmental friendly energy device which has received great attention due to its technology for producing electricity directly from organic or inorganic matter by using bacteria as catalyst. To date, many experiments have been carried out to achieve the maximum power output with advective flow through porous anode to the cathode in the MFC. However, the precise mechanical mechanism of flow through anode and the quantified relationship between electrode spacing and MFC performance are not yet clearly understood. It has been found experimentally that the power output can be increased apparently at certain electrode spacing configuration. Based on these available experimental data, this paper investigates the effect of spacing between electrodes, the Darcy number of porous anode and the Reynolds number on the power production performance of MFC by using lattice Boltzmann method. The numerical simulation results present that the distance between electrodes significantly influences the flow velocity and residence time of the organic matter attached to the anode in the MFC. Moreover, it is found that the Darcy number of porous anode and the Reynolds number can regulate the output efficiency of MFC. These results perform better understanding of the complex phenomena of MFC and will be helpful to optimize MFC design.