Ken Darcovich

Turbulent transport in membrane modules by CFD simulation in two dimensions

Abstract A numerical hydrodynamic simulation of the flow field inside a membrane module has been formulated and implemented. The membrane surface is treated as a porous wall. This simulation is modular in design and can support many optional features such as turbulent effects, pressure-related boundary conditions, and flexible module geometry. The diffusion-convection equation was incorporated in a straightforward manner to model a membrane separation showing the solute concentration field under realistic prevailing hydrodynamic conditions. The numerical results for one-component hydrodynamic simulations match existing but more cumbersome analytical models quite well.

CFD-assisted thin channel membrane characterization module design

Abstract This project involved the design of a thin channel cross-flow module for the characterization of flat ceramic membranes. A primary objective of this work was to ensure that the flow characteristics over the permeating area were uniform. To house these membranes, a thin channel module with a long rectangular base was envisioned. The module feed is supplied by a multi-inlet tube-type plenum meant to provide a uniform flow distribution through pressure equilibration attained in its volume. The design criteria for the module were minimization of both the flow non-uniformity and the pressure drop across the permeating area which was a central rectangular portion of a larger slab-style cell. The flow non-uniformity was taken as the normalized standard deviation of the velocity field above the permeating area. The pressure drops considered were those across the inlet plenum and across the permeating area normalized with respect to the outlet pressure. The computational fluid dynamics (CFD) scheme which calculated the above module characteristics was a k - e based turbulent transport model which used the finite difference method. Design variables considered were: the plenum diameter, module width, height and length, and the diameters, distribution and number of the inlets on the plenum. The distribution of the inlet diameters was determined by two variables: either a linear or parabolic profile of variable slope and model coefficient. The operating variables were the cross-flow velocity and the plenum inlet pressure. A two-level factorial design was used to screen the design variables. A refined three-level factorial design was used with a reduced set of design variables to optimize the module and study the response surface. The final module design parameters were chosen such that the design criteria of flow uniformity and low pressure drop were met under a preset range of operating conditions. The local gradients of the response surface were used to verify that the design criteria were not overly sensitive to the selected module design parameters.

Particle size distribution effects in an FEM model of sintering porous ceramics

A numerical simulation is presented on the sintering of porous alumina structures prepared by a controlled sedimentation technique. By forming this functionally gradient material with a very broad powder size distribution, the samples were able to remain flat through sintering. This experimental result is reflected in the present simulation results, which incorporated particle size distribution effects. In general, sintering functionally gradient ceramics can often introduce defects. Despite these common problems, the asymmetric structures considered in this paper featured a vertical functionality of continuously overlapping broad powder size distributions in the structure. This arrangement served to homogenize sintering rates. Modelling presented in connection with this shows that such structures can be readily sintered without warpage or cracking. To demonstrate these effects, a finite element method numerical simulation was developed to model the sintering characteristics of porous asymmetric ceramic structures by incorporating the powder particle size distribution into the model as a field variable. This work presents novel advances in the sintering model such that the contributions to the desired product properties attributable to particle size distribution effects can be demonstrated. These additions to the model produce numerical results which properly match observed structural profiles of physical samples.

An experimental and numerical study of particle size distribution effects on the sintering of porous ceramics

A single-step processing method has been established to prepare asymmetric porous alumina microstructures by a controlled sedimentation technique. Fine powder from an aqueous suspension is consolidated over a casting slab. Metastable surface chemical control of the suspension properties was able to induce a highly porous flat disc structure with a continuously increasing mean pore size from top to bottom. Formation of this gradient structure was facilitated by using a powder with a very broad particle size distribution. These structures can be used as either ultrafiltration media or as substrates for inorganic membrane making. Sintering can readily introduce defects into functionally gradient ceramics. Despite these problems, the asymmetric structures considered in this paper can be readily sintered without warpage or cracking. In this regard, a finite element method numerical simulation had been developed to model the sintering characteristics of functionally gradient ceramic structures. The key for being able to predict a non-warped structure was the incorporation into the model of the powder particle size distribution as a field variable. Across the vertical section of the structure, the distributions were broad and overlapping, all with a significant fines tail. These characteristics accelerate and homogenize local sintering rates, such that the net result is a non-warped fused structure. This paper presents recent advances with the simulation, where sample geometry, porosity and particle size distribution evolutions were traced alongside measurements made on physical specimens. In general the model corresponded well with the experimental observations. The correct accounting of observed trends lends confidence to the underlying sintering mechanisms incorporated into the model.

Trace amount formaldehyde gas detection for indoor air quality monitoring

Formaldehyde is not only a carcinogenic chemical, but also causes sick building syndrome. Very small amounts of formaldehyde, such as those emitted from building materials and furniture, pose great concerns for human health. A Health Canada guideline, proposed in 2005, set the maximum formaldehyde concentration for long term exposure (8-hours averaged) as 40 ppb (50 μg/m3). This is a low concentration that commercially available formaldehyde sensors have great difficulty to detect both accurately and continuously. In this paper, we report a formaldehyde gas detection system which is capable of pre-concentrating formaldehyde gas using absorbent, and subsequently thermally desorbing the concentrated gas for detection by the electrochemical sensor. Initial results show that the system is able to detect formaldehyde gas at the ppb level, thus making it feasible to detect trace amount of formaldehyde in indoor environments.

Propagation of electrical disturbances to automotive batteries in vehicle-to-grid context

This paper investigates the effects of disturbances originating in the electric grid as well as residential appliance inrush currents on the integrity of battery packs in electric vehicles that are connected to the grid or a residence for the purpose of V2G or V2H service. Simulation results show that the effect on battery capacity loss was negligible. The large size of an automotive battery pack allows it to easily withstand the levels of current caused by typical grid based disturbances and appliance inrush currents. Thus, power grid disturbances as they exist, need not be considered a reason to refrain from employing an electric vehicle for V2G or V2H service.