The effects of rarefaction on gas viscosity are investigated through the simulation of isothermal, low speed flow in a long straight channel using the Direct Simulation Monte Carlo (DSMC) method. Following convergence to the flow field inside the channel, the effective viscosity is calculated directly from its definition using shear stress calculations in each individual cell assuming that the gas flow is close to a local equilibrium state. Averaging over the cross-sectional area at different positions down the pressure gradient allows the determination of the gas viscosity as a function of the local Knudsen number (Kn) along the channel. Following an extensive investigation of this dependence over a wide range of Kn values, it was conveniently found that a Bosanquet-type of approximation describes very satisfactorily the Knudsen number dependence of the viscosity over the entire transition regime, i.e., from the slip-flow to the freemolecular flow limit. Such a simple functional dependence is expected to facilitate significantly phenomenological descriptions and numerical computations of rarefied flows that rely on the notion of an effective viscosity in the transition regime.
In the present work, the effect of wettability on CaCO 3 precipitation was approached by monitoring crystal formation in microchips of different wettability degree. Solutions of calcium chloride and sodium bicarbonate were mixed in hydrophilic and neutral-wet Y-junction microchips, and the precipitation of CaCO 3 crystallites was monitored. Sequential pictures showed the formation and growth of CaCO 3 crystals as a function of time, and the precipitates were identified by Raman spectroscopy. The obtained results indicated that in hydrophilic microchips, the increase of supersaturation ratio value resulted in higher growth rates and aggregate formation. The neutral-wet microchip surfaces were found to accelerate the precipitation of CaCO 3 compared to hydrophilic surfaces, and in the case of neutral-wet surfaces, crystallites were formed mainly close to the wall surfaces. In hydrophilic microchips, calcite was the main precipitate, while aragonite formation was favored in neutral-wet microchips.
Negative coefficients of thermal expansion have been reported for certain zeolite structures, including LTA, NaX, and DAY, with immediate consequences on the mechanical stability and performance of the supported membranes in gas separations. This unusual behavior is typically attributed to the systematic rotation of the framework tetrahedrals that operate as rigid units. A new interatomic potential is proposed in this work and used in lattice dynamics calculations for the computation of the thermal expansion coefficient as well as of the elastic and bulk moduli of zeolite faujasite with different aluminum contents. Comparison of the present simulations with literature measurements as well as with experimental data carried out in this work showed that the dealuminated faujasite (DAY) crystals contract upon heating over the entire temperature range examined, whereas aluminosilicate NaX contracts up to the ambient temperature and then expands with increasing temperature. Our analysis showed that the effect of temperature on the faujasite unit cell volume depends on the rigidity of the SiO4 and AlO4 tetrahedra as well as on their ability to rotate around their corner sharing oxygen atoms.
a b s t r a c tThe aim of the present paper is the development of an efficient numerical algorithm for the solution of magnetohydrodynamics flow problems for regular and irregular geometries subject to Dirichlet, Neumann and Robin boundary conditions. Toward this, the meshless point collocation method (MPCM) is used for MHD flow problems in channels with fully insulating or partially insulating and partially conducting walls, having rectangular, circular, elliptical or even arbitrary cross sections. MPC is a truly meshless and computationally efficient method. The maximum principle for the discrete harmonic operator in the meshfree point collocation method has been proven very recently, and the convergence proof for the numerical solution of the Poisson problem with Dirichlet boundary conditions have been attained also. Additionally, in the present work convergence is attained for Neumann and Robin boundary conditions, accordingly. The shape functions are constructed using the Moving Least Squares (MLS) approximation. The refinement procedure with meshless methods is obtained with an easily handled and fully automated manner. We present results for Hartmann number up to 10 5 . The numerical evidences of the proposed meshless method demonstrate the accuracy of the solutions after comparing with the exact solution and the conventional FEM and BEM, for the Dirichlet, Neumann and Robin boundary conditions of interior problems with simple or complex boundaries.
A new meshless numerical approach for studying heat conduction in particulate systems was developed that allows the efficient computation of the temperature distribution and the effective thermal conductivity in particle aggregates. The incorporation of the discretization-corrected particle strength exchange operator in meshless local Petrov–Galerkin calculations is suggested here, which was shown to perform better than previously tested trial functions, regarding the speed of convergence and accuracy. Moreover, an automated algorithm for node refinement was developed, which avoids the necessity for user intervention. This was quite important in the study of particle aggregates due to the appearance of multiple points of contact between particles. An alternative approach for interpolation is also presented, that increased the stability of the methods and reduced the computational cost. Test case models, commercial computational fluid dynamics software, and experimental data were used for validation. Heat transport in various aggregate morphologies was also studied using sophisticated aggregation models, in order to quantify the effect of aggregate fractal dimension on the nanofluid conductivity, targeting eventually the optimization of heat transfer applications. A trend of effective conductivity enhancement upon reduction of the fractal dimension of the aggregate was noted.
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