摘要:

AbstractThe gas‐liquid slug flow pattern exists over a broad range of flow rate space at reduced gravity conditions and for that reason is expected to be widespread in many space applications. The objective of this work was to develop a model predicting the characteristics of slug flow at reduced gravity including the bubble length, shape, velocity and other properties of the slug flow. Observations derived from low‐gravity experiments suggested the use of a boundary layer‐type approach. Multiple solutions exist, and a criterion for selecting the solution which corresponds to experimental observation is indi

ISSN:0001-1541    

DOI:10.1002/aic.690390702

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractAn algebraic expression involving three universal constants is proposed for the eddy momentum diffusivity in fully developed turbulent pipe flow that adequately simulates its variation throughout the pipe cross‐section. The model permits analytical integration to provide a single universal correlation that, unlike other available expressions, smoothly and accurately describes the entire turbulent velocity distribution from the inner region in the immediate vicinity of the wall all the way to the core region near the pipe axis and compares well with the existing correlations and experimental data. Accurate values of the maximum velocity and other flow variables of engineering interest can also be calculated from the velocity profile expressio

ISSN:0001-1541    

DOI:10.1002/aic.690390703

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe propagation of surface waves is investigated theoretically and experimentally for the case of a single layer of viscous liquid flowing down an inclined plane, where the plane is oscillating in the flow direction. This work focuses on the linearized wavemaker problem, where the oscillations create waves which are small perturbations from the undisturbed flow. Downstream from the entrance region to the incline where the fluid is introduced, the undisturbed interface is parallel to the incline surface, and theory predicts that oscillations do not interact with waves that travel along the free surface. These waves grow as if there were no oscillation at all, and their propagation is governed by a dispersion relation between frequency, wavelength, and wave growth for single layer flow down a nonoscillating inclined plane. The entrance region to the incline is therefore responsible for exciting the various wave frequencies which are observed down the incline, as well as the initial amplitude of these waves. Experiments performed verify that waves propagate as predicted. Theory indicates that these conclusions are valid when the oscillations are perpendicular to the incline, as well as for the case of multiple stacked layers.

ISSN:0001-1541    

DOI:10.1002/aic.690390704

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractTwo‐phase flow in porous media is described based on the extended form of Darcy's law, which ignores momentum transfer at fluid‐fluid interfaces. Two forms of corrections to this simple description have been proposed in the literature: one on the relative permeability dependence on viscosity ratio; the other on the velocities assumed to be proportional to both phase pressure gradients and so introducing an additional saturation‐dependent cross coefficient.In this article, to identify the correct form of transport equations, a simple cubic network model of 30 × 30 × 30 bonds is used. The cross section of the bonds is that of a four‐cusp duct. The fluid interface in each duct is located by capillary equilibrium. The duct hydraulic conductances are then obtained as a function of viscosity ratio and phase volume fraction using a finite element calculation. These individual duct results are used in the network calculations for which a percolation algorithm is applied to simulate nonwetting phase displacing wetting phase, a process also known as initial drainage. Flow calculations show that both the nonwetting‐phase relative permeability and the cross coefficient are strong functions of saturation and viscosity ratio. Also, the off‐diagonal terms may contribute to a nonnegligible fraction of the flow. The proposed generalization of the Darcy equations is applicable to all problems involving multiphase flow in porous media. The current practices for relative permeability measurements and reservoir simulation may have to be reexamined in the context of the proposed transp

ISSN:0001-1541    

DOI:10.1002/aic.690390705

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractIn this work, the separation of carbon dioxide from methane by reactive liquid membranes consisting of the secondary amines, diethanolamine, and diisopropanolamine in polyethylene glycol with an average molecular weight of 400 was studied. A mathematical model was developed to describe the transport process. This model employs the zwitterion mechanism for the CO2amine reaction kinetics. Membrane flux experiments utilizing a flat plate device were carried out to verify the model predictions. It was found that large permeabilities and separations were achieved for low CO2feed concentrations. The results also show that DEA based solutions give better separation than DIPA based solutions.

ISSN:0001-1541    

DOI:10.1002/aic.690390706

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA simple strategy is developed for the design of model‐based fault‐monitoring systems. Several new features allow this approach suitable for practical applications. The core of the suggested monitoring system is a parallel parameter estimation method designed to reduce the chance of bias. Two important criteria, diagnostic observability and diagnostic resolution, are adopted to evaluate the performances of alternative designs. By following a systematic procedure, it is possible to determine the best sensor locations in a complex chemical process based only on the structural informations of the system model. After selecting the most appropriate measurement variables, diagnostic resolution can be enhanced further by exploiting additional insights gained from analysis of the physical meanings of the model parameters and by comparing the fault patterns in diagnosis. Several effective on‐line implementation techniques are also proposed to minimize computation for the estimation algorithm. The correctness and efficiency of the suggested design strategy are demonstrated by the simulation results in exa

ISSN:0001-1541    

DOI:10.1002/aic.690390707

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe direct oxidation of CH4to H2and CO in O2and in air at high temperatures over alumina foam monoliths coated with high loadings of Pt and Rh has been simulated using a 19‐elementary‐step model of adsorption, desorption and surface reaction steps with reaction parameters from the literature or from fits to previous experiments. The surface reaction model for Pt is in good agreement with previously reported low‐pressure(0.1 to 1 torr) reactor measurements of CH4oxidation rates at temperatures from 600 to 1,500 K and of OH radical desorption during CH4oxidation at 1,300 to 1,600 K over polycrystalline Pt foils. The model predictions for both catalysts are also consistent with product selectivities observed over monolithic catalysts in an atmospheric‐pressure laboratory‐scale reactor, and the differences between Pt and Rh can be explained by comparing individual reaction steps on these surfaces. Because of the good agreement between the model and both low‐and atmospheric‐pressure reactor simulations, a complete energy diagram for methane oxidation at low coverages is proposed. The model results show that under CH4rich conditions at high temperatures, H2and CO are primary products of the direct oxidation of methane via a pyroly

ISSN:0001-1541    

DOI:10.1002/aic.690390708

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractAn edge recession modeling method is proposed as a new approach to the prediction of carbon gasification kinetics. Using this method, the variations in reactive surface area (RSA) and specific reactivity (R) with conversion were simulated for several polynuclear aromatic molecules adopted as models of carbon crystallites. The effects of crystallite size and shape, edge site reactivity and vacancies on the changes in RSA and R with conversion were investigated. As a result, RSA and R were found to be essentially independent of crystallite shape, but to depend on crystallite size and the presence of vacancies. Good agreement was obtained between model predictions and experimental data for several kinds of model carbon crystallites. In the case of structurally disordered carbons (such as chars), simultaneous gasification of model crystallites of two different sizes gave better predictions than that of single‐size crystallites. The edge recession model is proposed as an alternative, or at least complementary, approach to the more conventional (and, arguably, less physically meaningful) pore structure development model

ISSN:0001-1541    

DOI:10.1002/aic.690390709

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractSolid‐state film ion exchange (Ag+‐Na+) in glass substrates has been investigated for the fabrication of passive, integrated, optical waveguides. A new ion exchange mechanism is proposed which couples the oxidation of silver metal to silver oxide and the diffusion of silver ions into the glass substrate. At high electric fields and/or high temperatures, the growth rate of a silver oxide film limits the formation of mobile silver ions and constrains the diffusion of silver into the glass. This new mechanism is able to predict the dopant distribution profile under those conditions and under conditions where a constant surface concentration is appropriate (infinitely thick oxide film). It also explains the characteristics of the current vs. time curve measured during the ion exchange process. The formation and growth of the silver oxide film were confirmed by X‐ray photoelectron spectroscopy and X‐ray diffraction a

ISSN:0001-1541    

DOI:10.1002/aic.690390710

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA model for a pilot‐scale chemical vapor infiltration (CVI) reactor in which multiple substrates are densified simultaneously by carbon deposition from a pyrolyzing hydrocarbon is presented. Kinetic expressions and parameters and transport properties for carbon deposition from propylene determined from experimental studies and a single substrate CVI model are incorporated into the pilot reactor model. The two‐dimensional transport equations for heat, mass and momentum transfer are solved for the entire reactor to simulate the effects of varied operating conditions on substrate densification throughout the reactor during the CVI process. Of the conditions simulated, the reactor temperature was found to have the most significant impact on both the uniformity of densification and the process time required. As the temperature increased, the uniformity of densification decreased and the time at which pore blockage at the outer surface occurred was shortened. Increasing the hydrocarbon feed concentration shortened the process time, but did not affect the final level of densification, while the feed rate only affected CVI at intermediate times. Comparison between experimental pilot reactor results and model predictions were satisfactory at short and long times, but agreement was less satisfactory at intermediate ti

ISSN:0001-1541    

DOI:10.1002/aic.690390711

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY