Viscous Flow Transport Research Articles

Two-phase explicit CBS procedure for compressible viscous flow transport in porous materials

PurposeIn this work, a new two-phase version of the finite element-based Artificial Compressibility (AC) Characteristic-Based Split (CBS) algorithm is developed and applied for the first time to heat and mass transfer phenomena in porous media with associated phase change. The purpose of this study is to provide an alternative for the theoretical analysis and numerical simulation of multiphase transport phenomena in porous media. Traditionally, the more complex Separate Flow Model was used in which the vapour and liquid phases were considered as distinct fluids and mathematically described by the conservation laws for each phase separately, resulting in a large number of governing equations.Design/methodology/approachEven though the adopted mathematical model presents analogies with the conventional multicomponent mixture flow model, it is characterized by a considerable reduction in the number of the differential equations for the primary variables. The fixed-grid numerical formulation can be applied to the resolution of general problems that may simultaneously include a superheated vapour region, a two-phase zone and a sub-cooled liquid region in a single physical domain with irregular and moving phase interfaces in between. The local thermal non-equilibrium model is introduced to consider the heat exchange between fluid and solid within the porous matrix.FindingsThe numerical model is verified considering the transport phenomena in a homogenous and isotropic porous medium in which water is injected from one side and heated from the other side, where it leaves the computational domain in a superheated vapour state. Dominant forces are represented by capillary interactions and two-phase heat conduction. The obtained results have been compared with the numerical data available in the scientific literature.Social implicationsThe present algorithm provides a powerful routine tool for the numerical modelling of complex two-phase transport processes in porous media.Originality/valueFor the first time, the stabilized AC-CBS scheme is applied to the resolution of compressible viscous flow transport in porous materials with associated phase change. A properly stabilized matrix inversion-free procedure employs an adaptive local time step that allows acceleration of the solution process even in the presence of large source terms and low diffusion coefficients values (near the phase change point).

Identification of the compensation effect in the characteristic sintering time model for population balances

The sintering of agglomerates under high temperature determines the diameter and morphology of particles. Accurate sintering model is essential to the process simulation for the particle dynamics. A method of combining population balance modeling and inverse problem methodology was applied in sintering simulation process to investigate relationship between effective kinetic parameters in the characteristic sintering time model, i.e., two dynamic parameters (the pre-exponential factor A and the apparent activation energy E). The polydisperse primary particle (PP) model was introduced to consider the inhomogeneous structure in agglomerates. Two inverse problem methodologies, tabulation method and response surface method, were employed by fitting simulation results to experimental measurements. A contour map about the difference between simulation results and experimental measurements as a function of various parameter sets was obtained. Optimal values were obtained when the difference is small. A linear relationship between the two uncertain kinetic parameters was identified, which is similar to the kinetic compensation effect in the Arrhenius equation for reaction rate. The linear relationship holds true for the sintering of both TiO2 and SiO2 agglomerates at least, which are dominated by the surface diffusion mechanism and the viscous flow transport mechanism, individually.

A bioinspired pumping model for flow in a microtube with rhythmic wall contractions

Inspired by respiratory system in insects, in particular the rhythmic wall contractions found in insect's tracheal tubes, we propose a bioinspired pumping model that can work particularly well in the low Reynolds number flow regime. Incompressible, viscous flow transport in a fluid-filled axisymmetric, inelastic tube with rhythmic wall contractions is modeled using the lubrication theory. The wall contractions are prescribed via a tube profile with two indentation sites that can move with time lags with respect to each other. The analytical model is validated using the method of fundamental solutions based on the Stokeslets-meshfree computational method. The velocity field, pressure and time averaged net flow rate induced in a complete contraction cycle are calculated. Results demonstrate that an inelastic tube with at least two contracting spots can produce unidirectional flow and working as pumping mechanism. The physical insight of this pumping model can be explained as follows: when the first constriction starts to increase with time while second constriction is slight, the resulting flow is distributed almost equally up and downstream. Now, if second constriction becomes large, then the flow resistance on the downstream side is larger during the decreasing phase of the first constriction, so less flow is drawn back upstream. Therefore, a unidirectional flow with net flow transport is produced.

Solvent flux behavior and rejection characteristics of hydrophilic and hydrophobic mesoporous and microporous TiO 2 and ZrO 2 membranes

This paper describes solvent flux and rejection behavior of different hydrophilic TiO 2 membranes and a hydrophobic ZrO 2 membrane. Solvents and solutes used range from polar to non-polar. Additionally, the temperature effect on permeability and rejection of solutes through these membranes was examined. It was found that the solvent permeabilities through microporous hydrophilic membranes with MWCO 275 and 1400 Da do not obey the viscous flow transport mechanism, in contrast to the permeability through mesoporous TiO 2 membrane with MWCO 7000 Da. Decrease of the pore size increased deviation from the viscous flow mechanism. Obtained aqueous and polar solvent fluxes were high. This raises opportunities for industrial applications. Moreover, permeabilities of the non-polar solvents can be increased to acceptable levels by working at elevated temperatures through the mechanism of activated permeability. The hydrophobic membrane showed very high non-polar solvent fluxes and remarkably low water permeabilities. Here, no surface interactions can be considered, due to the lack of hydroxyl groups on a membrane surface. Thus, transport through this membrane is related with viscosity decrease at higher temperature and is closer to viscous behavior than for the hydrophilic membranes with similar pore sizes. Rejections of the solutes did not change with temperature and were high when the hydrophilic membranes were used. The non-polar solute bromothymol blue in toluene showed high rejections, unlike some other literature data. The hydrophobic membrane showed much higher fluxes, but rejections were lower than expected. The results of this study are representative for the ceramic nanofiltration membranes currently available on the market.

Evaluation of porous ceramic membranes as O2 distributors for the partial oxidation of alkanes in inert membrane reactors

Abstract This study aimed to investigate the effects of different gas transport regime on the O2 distribution properties of porous ceramic membranes in inert membrane reactors. Three porous ceramic membranes, with different porous structure (microporous MFI zeolite, mesoporous SiO2 and meso-macroporous AlPO4 membranes), have been synthesised and characterised. The membranes were evaluated as O2 distributors in an inert membrane reactor under typical operating conditions used for the partial oxidation of alkanes before placing the catalyst bed. The C3H8 oxidation was chosen as a test reaction. The performance of the three membranes as O2 distributors, whose permeance are comparable, were evaluated by their ability (1) to control the O2 partial pressure in the tube side and (2) to limit the back-diffusion of C3H8 to the shell side by application of a transmembrane pressure. The effects of temperature on the membrane performance were also studied. On the basis of the above mentioned experimental conditions and criteria, the AlPO4 membrane, reported here for the first time, is clearly the most efficient membrane as porous O2 distributor. This outstanding property arises from its porous structure which allows a viscous flow transport and a high barrier effect.