Axisymmetric multiphase lattice Boltzmann method

A lattice Boltzmann method for axisymmetric multiphase flows is presented and validated. The method is capable of accurately modeling flows with variable density. We develop the classic Shan-Chen multiphase model [ Phys. Rev. E 47 1815 (1993)] for axisymmetric flows. The model can be used to efficiently simulate single and multiphase flows. The convergence to the axisymmetric Navier-Stokes equations is demonstrated analytically by means of a Chapmann-Enskog expansion and numerically through several test cases. In particular, the model is benchmarked for its accuracy in reproducing the dynamics of the oscillations of an axially symmetric droplet and on the capillary breakup of a viscous liquid thread. Very good quantitative agreement between the numerical solutions and the analytical results is observed

Transport and distribution of gaseous impurities in UHP gas delivery systems and process equipment.

Removal of gaseous contaminants from process environment is essential for improving the yield and lowering the cost during the integrated device processing. Due to major advances in purification and delivery of high purity gases, the bulk and the process gases are no longer major sources of impurities. The burden is now shifted to the process tools which in most cases are the primary sources of impurities. This study focuses on the characterization of various sources of impurities in a manufacturing unit. An experimental methodology using different metrology tools is developed to extract the fundamental parameters of impurity transport modes. A simulation is also designed to predict the extent of contamination in a process tool, with an emphasis on ascertaining impurity distribution in a vertical diffusion furnace. The major sources of impurity in a manufacturing unit are permeation through polymeric seals, adsorption/desorption from the various surfaces and back-diffusion against the convection. The extent of leakage by permeation through polymeric materials depends primarily on permeation coefficient (product of solubility and diffusivity) of the impurity in the material and the concentration gradient across the surface. Rate of adsorption/desorption has strong dependence on type of impurity and the outgassing surface. In general, the adsorption process is not very activated. However, desorption is highly activated process and activation energy varies with the surface concentration. Back-diffusion involves both bulk and surface diffusion and is higher for lower pressures, lower flow rates, and for small nonadsorbing molecules. Back-diffusion is particularly significant in the gas boundary layer where the convective flow is small. The overall model for a typical purge process in the vertical reactor consists of modules representing individual impurity introduction and transport steps. Results show a stagnant gaseous volume in the wafer spacing, causing the pronounced concentration gradient of impurity over the wafer surface. The purge schedule of the wafers and furnace is significantly affected by gasket leakage, permeation through polymers and quartz lining as well as the feed gas impurities. A parametric study has also been done to determine the effects of wafer size, reactor size and purge flow rates on impurity distribution

Transient analysis of two-phase flow during blow down of a pipeline carrying flashing liquids

In this study, the controlled or uncontrolled release of a vapor-liquid mixture during the blowdown of a pipeline carrying a flashing liquid has been numerically simulated. The liquid may be pure such as propane or a mixture of two or more components, such as liquefied petroleum gas (LPG). With the help of a mathematical model, transient pressure and temperature profiles, quality of a 2-phase vapor-liquid mixture and the amount of the liquid remaining in the pipe during a typical blowdown are predicted. The theoretical analysis based on the assumption of a homogeneous two-phase flow (no slip condition between phases) allows for external heat transfer from the ambient to the liquid stored in the pipe. The simulation results show that the sudden depressurization of the liquid stored under supersaturated conditions may result in rapid cooling of the liquid possibly to the temperature below the ductile brittle transition temperature (DBTT) of the material of construction of the pipe. This may be hazardous as the pipe may rupture at weak locations along the length. The simulation results also show significant effects of ambient temperature, use of a low vapor pressure liquid as an additive, and pipewall roughness on total blowdown time of a long pipe. The present study is important from the point-of-view of maintenance and operation of pipelines carrying flashing liquids, especially in chemical industries and offshore oil and gas exploration operations. The model simulation results have been validated, wherever possible, with those published in literature

Preparation and characterization of ACF for the adsorption of BTX and SO<SUB>2</SUB>

Activated carbon fibers (ACFs) based on two precursors, namely viscose rayon and phenolic resin, were prepared by carbonization followed by physical activation with steam or CO<SUB>2</SUB>. The prepared ACF samples were examined for their suitability of adsorbing common atmospheric gaseous pollutants, SO<SUB>2</SUB> and benzene, toluene, and m-xylene (BTX), under non-equilibrium conditions in a fixed bed tubular reactor. The textural and surface properties of the ACF samples were correlated with the adsorption characteristic of ACF for BTX and SO<SUB>2</SUB>. The adsorption of BTX was found to increase with increase in either the BET area or micropore volume. In contrast, the surface groups containing electronegative oxygen atoms were found to have adverse impact on BTX adsorption. The adsorption of SO<SUB>2</SUB> was found to decrease with increase in the O-contents. The FTIR spectra and the elemental analysis revealed that the extent of oxygen groups (-OH, -COOH, -C=O, etc.) was larger in the steam-activated samples than in the CO<SUB>2</SUB>-activated samples, regardless of the type of the precursor. The results obtained here are germane to a quantitative tailoring of activation routes for optimal performance of ACFs in pollution control

Transient analysis of a gas manifold system

In the present work, a numerical study has been carried out to predict transient gas flow and pressure behaviour in a gas manifold system. The start-up and shutdown of the system, varying demands at the consumer ends, malfunctioning of compressors and valves are a few examples of common causes of transience in a gas delivery system. In particular, the sensitivity of oscillations in pressure and mass flux to variation in pipe dimensions, supply pressure and gas flow rate are ascertained under the aforementioned conditions of transience. The present results show that large pipe dimensions, high gas flow rate and high upstream pressure in the branch in which the disturbance is introduced, all cause greater amplitude in mass flux and pressure oscillations in the neighboring branches. The duration of oscillations is also found to be longer. The present study has practical importance in designing as well as in operating, a gas delivery system

Momentum and heat transfer from an asymmetrically confined circular cylinder in a plane channel

Unsteady momentum and heat transfer from an asymmetrically confined circular cylinder in a plane channel is numerically investigated using FLUENT for the ranges of Reynolds numbers as 10 ≤ Re ≤ 500, of the blockage ratio as 0.1 ≤ β ≤ 0.4, and of the gap ratio as 0.125 ≤ γ ≤1 for a constant value of the Prandtl number of 0.744. The transition of the flow from steady to unsteady (characterized by critical Re) is determined as a function of γ and β. The effect of β on the mean drag (C̅D) and lift (C̅L) coefficients, Strouhal number (St), and Nusselt number (Nuw) is studied. Critical Re was found to increase with decreasing γ for all values of β. C̅D and St were found to increase with decreasing values of γ for fixed β and Re. The effect of decrease in β on Nuw was found to be negligible for all blockage ratios investigated

Wall effects in flow past a circular cylinder in a plane channel: a numerical study

This paper describes a numerical study on the steady flow of an incompressible Newtonian fluid past a circular cylinder confined in a plane rectangular channel. Using FLUENT (version 6), two-dimensional steady state computations were carried out for an uniform inlet velocity and for different values of the Reynolds numbers in the range between 0.1 and 200 and blockage ratios (ratio of the channel width to the cylinder diameter) in the range between 1.54 and 20. The flow parameters such as drag coefficient, length of the recirculation zone, and the angle of separation are presented as functions of the Reynolds number and blockage ratio. The total drag coefficient (CD) was found to decrease with an increase in the blockage ratio (λ) for a fixed value of the Reynolds number (Re) and to decrease with increasing Reynolds number for a fixed value of λ. Similarly, for a fixed value of λ, both the angle of separation and the length of the recirculation zone increase with the increasing Reynolds number