Qulan Zhou

Water infiltration behaviours in carbon nanotubes under quasi-static and dynamic loading conditions

The mechanisms of pressure-driven water infiltration into single walled carbon nanotubes are explored using molecular dynamics simulations. Both quasi-static and dynamic loading conditions are investigated, and the influence of tube size is examined. Under quasi-static loading, the water molecules flow into the tube via surface diffusion at a low pressure and when the external pressure reaches a critical value, the infiltrated water flux can sharply increase to a steady state. Upon dynamic loading, the nominal infiltration length per unit external work is employed to measure the comprehensive effect of the loading rate. It is found that such factor is larger (i.e. infiltration is easier) at a lower loading rate and a larger tube size, which is closely related with the interactions between water molecules and nanotube wall atoms.

Effect of Electric Field on Liquid Infiltration into Hydrophobic Nanopores

Understanding the variation of nanofluidic behavior in the presence of an external electric field is critical for controlling and designing nanofluidic devices. By studying the critical infiltration pressure of liquids into hydrophobic nanopores using molecular dynamics (MD) simulations and experiments, important insights can be gained on the variation of the effective liquid-solid interfacial tension with the magnitude and sign of electric field, as well as its coupling with the pore size and the solid and liquid species. It is found that the effective hydrophobicity reduces with the increase of electric intensity and/or pore size, and the behavior is asymmetric with respect to the direction of the electric field. The underlying molecular mechanisms are revealed via the study of the density profile, contact angle, and surface tension of confined liquid molecules.

Electrolyte solution transport in electropolar nanotubes

Electrolyte transport in nanochannels plays an important role in a number of emerging areas. Using non-equilibrium molecular dynamics (NEMD) simulations, the fundamental transport behavior of an electrolyte/water solution in a confined model nanoenvironment is systematically investigated by varying the nanochannel dimension, solid phase, electrolyte phase, ion concentration and transport rate. It is found that the shear resistance encountered by the nanofluid strongly depends on these material/system parameters; furthermore, several effects are coupled. The mechanisms of the nanofluidic transport characteristics are explained by considering the unique molecular/ion structure formed inside the nanochannel. The lower shear resistance observed in some of the systems studies could be beneficial for nanoconductors, while the higher shear resistance (or higher effective viscosity) observed in other systems might enhance the performance of energy dissipation devices.

Temperature dependence of fluid transport in nanopores

Understanding the temperature-dependent nanofluidic transport behavior is critical for developing thermomechanical nanodevices. By using non-equilibrium molecular dynamics simulations, the thermally responsive transport resistance of liquids in model carbon nanotubes is explored as a function of the nanopore size, the transport rate, and the liquid properties. Both the effective shear stress and the nominal viscosity decrease with the increase of temperature, and the temperature effect is coupled with other non-thermal factors. The molecular-level mechanisms are revealed through the study of the radial density profile and hydrogen bonding of confined liquid molecules. The findings are verified qualitatively with an experiment on nanoporous carbon.

A conceptual thermal actuation system driven by interface tension of nanofluids

In a system containing nanoporous materials and liquids, the well-known thermo-capillary effect can be amplified by the ultralarge specific surface area of the nanopores. With appropriate temperature change, the relative wetting–dewetting transition can cause the liquid to flow in or out of the nanopores, and part of the thermal energy is converted to significant mechanical output. A conceptual design of such a thermal actuation/energy conversion/storage system is investigated in this paper, whose working mechanism, i.e. the thermally dependent infiltration behaviors of liquids into nanopores, is analyzed using molecular dynamics simulations. The fundamental molecular characteristics, including the density profile, contact angle, and surface tension of the confined liquid molecules, are examined in considerable detail. The influences of pore size, solid phase and liquid species are elucidated, which couple with the thermal effect. The energy density, power density, and efficiency of the thermal actuation system are evaluated. An infiltration experiment on a zeolite/water system is performed to qualitatively validate these findings.

Infiltration behaviour of water in a carbon nanotube under external pressure

The wetting behaviour and associated pressure effect of water in single-walled carbon nanotubes (SWCNTs) are investigated through molecular dynamics (MD) simulations. It is found that water molecules can enter SWCNTs via surface diffusion, and the effective infiltration rate increases with pressure. The effect of pressure on infiltration rate is highly non-linear, exhibiting characteristics of both hydrophilic and hydrophobic surfaces. There exists a nominal infiltration pressure that is dependent on the SWCNT size, above which the water flux is significantly increased.

An electroactuation system based on nanofluids

We propose the conceptual design of an electrically controlled actuation system by adjusting the relative hydrophobicity of a nanoporous material/liquid mixture. When the variation in wettability is amplified by the large surface area, a considerable mechanical work is output. The energy density, power density, and efficiency are explored and their variations with pore size, solid phase, and liquid phase are explored. An infiltration experiment on a nanoporous silica system is performed to qualitatively validate these findings.

Effect of particle size in a limestone–hydrochloric acid reaction system

Experimental characterization of the wet flue gas desulfurization process is carried out using a model limestone-hydrochloric acid reaction system, with in-situ measurement of the dissolution rate and particle size distribution. The limestone source, initial particle size distribution, working temperature and pH value are varied in large ranges. The dissolution rate is found to be higher when the average particle size is smaller, the temperature is higher, or the pH is lower. An empirical equation is established to correlate the dissolution rate with the particle size and working conditions, which agrees well with measurements. The results may be useful for providing insights to improve the efficiency of the wet flue gas desulfurization process, as well as other solid particle-liquid solution reactions.

An ECT System Based on Improved RBF Network and Adaptive Wavelet Image Enhancement for Solid/Gas Two-phase Flow

Abstract Electrical capacitance tomography (ECT) is a non-invasive imaging technique that aims at visualizing the cross-sectional permittivity distribution and phase distribution of solid/gas two-phase flow based on the measured capacitance. To solve the nonlinear and ill-posed inverse problem: image reconstruction of ECT system, this paper proposed a new image reconstruction method based on improved radial basis function (RBF) neural network combined with adaptive wavelet image enhancement. Firstly, an improved RBF network was applied to establish the mapping model between the reconstruction image pixels and the capacitance values measured. Then, for better image quality, adaptive wavelet image enhancement technique was emphatically analyzed and studied, which belongs to a space-frequency analysis method and is suitable for image feature-enhanced. Through multi-level wavelet decomposition, edge points of the image produced from RBF network can be determined based on the neighborhood property of each sub-band; noise distribution in the space-frequency domain can be estimated based on statistical characteristics; after that a self-adaptive edge enhancement gain can be constructed. Finally, the image is reconstructed with adjusting wavelet coefficients. In this paper, a 12-electrode ECT system and a pneumatic conveying platform were built up to verify this image reconstruction algorithm. Experimental results demonstrated that adaptive wavelet image enhancement technique effectively implemented edge detection and image enhancement, and the improved RBF network and adaptive wavelet image enhancement hybrid algorithm greatly improved the quality of reconstructed image of solid/gas two-phase flow [pulverized coal (PC)/air].

Experimental study on thermal effect on infiltration mechanisms of glycerol into ZSM-5 zeolite under cyclic loadings

Understanding the fundamental infiltration mechanisms under thermal response is of crucial importance to design and develop nanoporous energy systems. In this work, a glycerol/ZSM-5 zeolite-based pressure-driven energy absorption system was built, while the temperature-dependent intrusion of glycerol molecules into lyophobic nanopores of ZSM-5 zeolite and the underlying mechanisms were experimentally studied. By changing the system temperature, the correlations of infiltration pressure with the infiltration and defiltration percentages of the liquid phase under thermal response were explored. It turns out that lifting the system temperature will reduce the critical infiltration pressure barriers and change the systems wettability. The equivalent surface tension and contact angle are calculated to elucidate the thermal dependence of the systems wettability. Elevating system temperature can also help enlarge the entry area of the nanochannels and trigger more glycerol molecules to flow out of the nanochannels, which means an increase of the infiltration and defiltration percentages. Weakened hydrogen bonding interaction, temperature sensitivity of glycerol viscosity, and the inherent gas phase in the nanoporous channels may contribute to the infiltration and outflow process at a higher temperature level. Cyclic loadings were applied under each working condition to test the recoverability of the built system. Results showed that the systems throughput shrank in the first three/four cycles and became stable afterwards. Lifting the system temperature could enhance both intrusion and extrusion processes, thus helping the system reach a faster throughput balance, which is beneficial in establishing a recoverable and reusable energy absorption/storage/conversion system.