bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fractal nature of viscous fingering in two‐dimensional pore level models
Authors: Ferer M. , Sams W. Neal , Geisbrecht R. A. , Smith Duane H. 
Published: June 2004

AbstractUse of saturation‐dependent relative mobilities leads to linear flow; however, experiment and theory show that, in the limit of very large viscosity ratio, the flow is not linear but fractal. Generally, fractional flows and relative mobilities depend on both saturation and time. Use of a standard pore‐level model of 2‐D flow in the limit of infinite capillary number shows that this flow is fractal for large viscosity ratios (M = 10,000) and the sauration and fractional flows agree with the results of our general arguments. For realistic viscosities (M = 3 → 300), our modeling of the unstable flow shows that, although the flows are initially fractal, they become linear on a time scale, τ increasing as τ = τ0M0.17. Once linear, the saturation front advances as x ≈ v0M0.068 t; the factor M0.068 acts as a 2–D Koval factor.
Analysis of diffusion in polymers using evanescent field spectroscopy
Authors: Fieldson Gregory T. , Barbari Timothy A. 
Published: June 2004

AbstractA rigorous analysis confirmed by experimental results is presented for attenuated total reflectance, Fourier‐transform infrared (FTIR‐ATR) spectroscopy applied to the study of small molecule diffusion in polymers. FTIR‐ATR Fickian diffusion models are derived for a constant surface concentration and for an adjacent mass‐transfer boundary layer. An FTIR‐ATR Case II transport model demonstrates the ability to discriminate between transport modes. New experimental results for the acetone‐polypropylene, methanol‐polystyrene and methanol‐poly(methyl methacrylate) systems are consistent with gravimetric sorption and nuclear magnetic resonance measurements. An important result is the detection of multiple hydroxyl peaks for methanol during diffusion, indicating different hydrogen bonding states and the calculation of separate diffusion coefficients for each state in polystyrene.
Effects of operating conditions on stability of gas‐phase polyethylene reactors
Authors: McAuley K. B. , Macdonald D. A. , McLellan P. J. 
Published: June 2004

AbstractFluidized‐bed polyethylene reactors are prone to unstable behavior and temperature oscillations (Choi and Ray, 1985b). Their work is extended to show the effects of ethylene feed system operation, reactor cooling system design, catalyst properties, and gas composition on reactor stability and dynamics. The analysis is performed using a well‐mixed model, because heat‐ and mass‐transfer resistances between multiple phases are small and are not required to account for the observed bifurcation phenomena. The addition of a gas recycle and heat exchanger system to the model significantly affects dynamic performance, including the formation of limit cycles. The size and dynamics of the heat exchanger, however, have little effect on the overall stability. In contrast, automation of the ethylene feed system to replace the monomer in the reactor as it is consumed leads to substantially different dynamic behavior than if the ethylene feed is maintained at a constant rate. Catalyst properties (multiple sites, activation energy, and deactivation) significantly affect dynamics and stability, whereas comonomer and other gases affect them only mildly. The results confirm that without proper temperature control, gas‐phase polyethylene reactors are prone to instability, limit cycles, and excursions towards unacceptable high‐temperature steady states.
Binding of an antifreeze polypeptide to an ice/water interface via computer simulation
Authors: McDonald Shawn M. , White Angela , Clancy Paulette , Brady John W. 
Published: June 2004

AbstractThe interaction between a winter flounder antifreeze polypeptide and an ice/water interface was studied using Molecular Dynamics computer simulation techniques to study the mechanism of action of this class of antifreeze molecules. Simple Point Charge models were used for the water molecules, and a molecular mechanics program (CHARMM) was used to construct the model for the polypeptide. A (2021) face was exposed on the ice surface, as this is believed to be the experimentally favored ice face for peptide binding. The polypeptide binds strongly to the ice surface even though it was placed with its four polar threonine (Thr) groups pointing away from the ice surface. This tested the previously advanced hypothesis that adsorption occurs primarily between these groups and the ice due to a matching of the spacing between oxygen atoms in the ice lattice and the polar Thr residues. As well as contacts with other polar groups on the peptide, the binding to the ice produces a good steric fit of the peptide with the corrugated ice interface. The presence of the peptide did not induce any melting of the ice at 200 K.
Flow through curved rectangular channels of large aspect ratio
Authors: Targett Matthew J. , Retallick William B. , Churchill Stuart W. 
Published: June 2004

AbstractNumerical solutions were obtained for the field of velocity in angular forced flow through the annulus between two concentric cylinders of large and infinite aspect ratio with a gap‐to‐inner radius ratio of 0.05 using a finite‐element representation and the FIDAP code. For an infinite aspect ratio, velocity vectors reveal a purely angular motion below a critical Dean number of 37.31 and a secondary motion in the form of pairs of counterrotating vortices above that value. The wavelength of these vortices and the friction factor are correlated in terms of the Dean number. For large but finite aspect ratios a weak secondary motion around the periphery is found to occur below the critical Dean number, while for greater values the vortex at each end of the channel is greatly extended. The computed patterns of flow are in good agreement with prior experimental visualizations as well as with those carried out as part of this investigation. The computed characteristics are also in good agreement with prior theoretical results for limiting cases. The adaptation of the results for flow through an Archimedean spiral is described.
Rapid estimation of cylinder erosion rates in abrasive dust‐laden streams
Authors: Rosner Daniel E. , Tandon Pushkar , Labowsky Michael J. 
Published: June 2004

AbstractErosion yield data for particular combinations of target and projectile materials (via laboratory experiments carried out with a narrow‐size distribution over the important range of impact velocities and incidence angles), together with recently developed rational correlations for inertial impaction of suspended particles on a cylinder in high Reynolds number crossflow are used to provide a tractable framework for predicting the erosion rates of, say, heat exchanger tubes immersed in particle‐laden streams of combustion products.“Universal” results are cast in terms of the following accessible parameters: sensitivity of erosion yield to projectile incident velocity and angle, ratio of mean particle size to the threshold size required for impaction on the cylindrical target, spread of the mainstream particle size distribution (here log‐normal), and the characteristic “slip” Reynolds number for the critical size abrasive particle in the mainstream. Applications of the results are illustrated, and several generalizations are discussed.
Modeling of a monolithic catalyst with reciprocating flow
Authors: Wallace K. , Viljoen H. J. 
Published: June 2004

AbstractSeveral methods have been proposed to improve the conversion of catalytic oxidation reactions. Gases with low enthalpy contents or gases that contain small amounts of combustible compounds can be converted much more efficiently in a reactor with reciprocating flow. The solid phase exhibits an inertia behavior insofar as it does not reach equilibrium with the gas temperature at the reactor ends. The solid‐phase temperature at the center of the bed is considerably higher than in the unidirectional case. Results for the oxidation of CO at different inlet gas temperatures and switching periods are presented. An optimum period is found at which maximum conversion occurs. An estimate of the maximum temperature in the reactor is derived.
Noncatalytic gas‐solid reactions and mechanical stress generation
Authors: Rode Hilmar , Orlicki Dariusz , Hlavacek Vladimir 
Published: June 2004

AbstractThe reaction rate in gas‐solid systems can be affected by mechanical stresses that arise as the reaction proceeds. Stresses develop due to differences between precursor and product molar volumes and thermal expansion coefficients. Experimental evidence on the interaction of reaction rate and mechanical stress for the Ti/N2 and Ti/O2 systems is provided. A detailed and consistent mathematical model is developed for the reaction taking place at the constrained precursor/product interface. An elastic formulation for the stresses is adopted, and stress generation due to mismatches in linear thermal expansion coefficients and equivalent volume (Pilling‐Bedworth ratio) for the precursor and product are considered. The effect of surface energy, which becomes significant for particle sizes below 1 μm, is also included in the model. Both experimentally and theoretically, conditions exist where the mechanical stresses exceed the strength of the material, leading to mechanical breakdown of the product layer, thus causing a discontinuity in the observed reaction rate. The entire processing history, including the reaction, temperature, and pressure profiles, plays an important role in determining the overall reaction kinetics of the powder.
Surface interactions in a shear field
Authors: Dietsche Laura J. , Denn Morton M. , Bell Alexis T. 
Published: June 2004

AbstractThe dynamics of chain exchange between flowing bulk melt and the channel wall were studied for oligomeric polyolefins using attenuated total reflectance Fourier transform infrared spectroscopy (ATR/FTIR). The dynamics are dominated by a surprisingly slow first‐order process which depends on flow rate and materials of construction of the channel wall. The picture which seems to emerge is an entropically‐driven flow‐rate‐ (or stress‐) dependent adsorption equilibrium which retards the exchange between the surface and bulk.
Simulation of hydrosol deposition in granular media
Authors: Choo Chang‐Upp , Tien Chi 
Published: June 2004

AbstractAn algorithm is developed for simulating particle deposition from liquid suspensions flowing through granular media, specifically the prediction of the extent of deposition and the change of media permeability on a local basis. The formulation is based on the premise that granular media may be considered as an assembly of collectors represented by a dual‐configuration—either spherical or capillaric — to be applied for deposition and permeability reduction considerations. Comparisons with experiments indicate that the simulation algorithm can indeed be used as a predictive tool for estimating the transport and distribution of particulate matter in granular media.
Cake formation in 2‐D cross‐flow filtration
Authors: Lu Wel‐Ming , Hwang Kuo‐Jen 
Published: June 2004

AbstractThe mechanism of cake formation is studied in 2–D cross‐flow filtrations. By analyzing the forces exerted on a depositing particle, the value of the critical friction angle, βc, which leads the particle to be stuck on the contact position, is determined. Not only the probability of particle deposition but also the size distribution of deposited particles can be calculated from the theoretical value of βc. Numerical programs are constructed to simulate the packing structure and size distribution of particles in a formed cake and the path of cake compression. The local cake properties in the cake, such as local porosity and local specific filtration resistance, are also estimated by applying the design program and the data of filtration volume vs. time. The predicted values of average cake porosity and average specific filtration resistance agree very well with the experimental data.
Thermodynamic modeling of concentrated aqueous electrolyte and nonelectrolyte solutions
Authors: Kolder Alexander , De Pablo Juan J. 
Published: June 2004

AbstractA new approach is presented for calculation of activity coefficients in aqueous electrolyte solutions. This approach requires only standard thermodynamic properties for the pure components and for the solutes at infinite dilution. No adjustable mixture parameters are necessary. This new procedure has been applied to calculate: (1) the solubility of salts and the activity of water in binary aqueous solutions; (2) the temperature dependence of the solubility of salts; (3) the azeotropic composition of a system containing a volatile electrolyte, (H2HNO3); (4) the solubility of carbon dioxide in water at elevated pressures and at different temperatures.
Modeling of heat generation in ammonia‐treated solid rocket propellant
Authors: Raun Richard L. , Isom K. Bruce 
Published: June 2004

AbstractOne approach to demilitarize solid rocket propellants is treatment with ammonia. Ammonia extracts the oxidizers ammonium perchlorate and HMX, yielding a solid residue that is more suitable for incineration and less sensitive to impact and other modes of accidental initiation. Ammonia treatment of nitroglycerin‐containing propellants is complicated by an exothermic reaction between ammonia an nitroglycerin. If not removed, the heat generated by this reaction can cause propellant ignition. To help design safe treatment processes, a model for the ammonia‐propellant reaction was developed, which integrates transient energy and species conservation equations to simulate ammonia diffusion, heat generation, and heat flow in a propellant and in the solid residue resulting from ammonia treatment. It was calibrated using residue thickness and thermocouple data for one propellant. The calibrated model was used to predict conditions leading to ignition of thin propellant strips. The results agree well with experimental observations.
Potentialities and limitations of mixing simulations
Authors: Soulvaiotis A. , Jana S. C. , Ottino J. M. 
Published: June 2004

AbstractAs numerical simulations in mixing become pervasive, an analysis of errors becomes crucial. Purposely discretized examples with exact analytical solutions provide a reference point from which to judge the soundness of numerical solutions. Three types of errors are identified and examined: discretization, time integration, and round‐off, with emphasis on the first two. Theoretical derivations and numerical examples for 2‐D, steady (regular) and time‐periodic (chaotic) flows indicate that errors, in general, behave as material lines. In regular flows, their magnitude increases, on the average, with at most t2, while in chaotic flows it increases exponentially. Errors tend to align with the direction of the streamlines in regular flows and with manifolds in chaotic flows. As a result, even though exact and calculated trajectories diverge exponentially fast in chaotic flows, overall mixing patterns are reproduced, at least qualitatively, even when the velocity field is calculated using the main features of a line as it is deformed by the flow, although the error in its length may be more than 100%. It is concluded that accurate quantitative information, such as the location of periodic points or the length of a deformed line, can be obtained from numerical simulations. However, robust application of standard numerical analysis tools, such as mesh refinement, is necessary, which, in turn, can lead to nearly prohibitive computational costs.
Steady‐state drop‐size distributions in high holup fraction dispersion systems
Authors: Chatzi E. G. , Kiparssides C. 
Published: June 2004

AbstractMacroscopic phenomena in suspension polymerization reactors are extremely complex, and breakage and coalescene of polymerizing monomer droplets are not well understood, especially for high dispersed‐phase volume fractions. Depending on the agitation, concentration and type of surface‐active agent, the droplet size can exhibit a U experiementally and theoretically as the balance between breakage and coalescence rates of monomer drops. Both processes are related to the drop surface energy, which is proportional to the interfacial tension and its variation with time. In this study, the most comprehensive models describing breakage and coalescene processes in a dispersion system were incorporated into a generalized numerical algorithm to predict the steady‐state drop‐size distributinos in a high holdup (50%) liquid‐liquid distributions in high holdup dispersion systems, experiments were carried out with a model system of 50% n‐butyl chloride in water in the presence of a surface‐active agent, poly(vinyl alcohol), at different concentrations and agitation rates. The theoretical model can predict reasonably well the drop‐size distribution for all experimental investigation elucidates the relationships between the changing structure of PVA molecules at the monomer/water interface and their effects on breakage and coalescence frequencies at different agitation times and rates.
New concept of gas purification by electron attachment
Authors: Tamon Hajime , Mizota Hirotoshi , Sano Noriaki , Schulze Stefan , Okazaki Morio 
Published: June 2004

AbstractA new concept of gas purification by electron attachment is proposed. Low‐energy electrons generated in a corona‐discharge reactor are captured by electronegative impurities, producing negative ions. The ions drift to the anode in the electric field and are removed at the anode of the reactor. Two types of reactors were used to remove the negative ions: a deposition‐type reactor, which deposits negative ions at the anode surface; a sweep‐out‐type reactor, which sweeps out enriched electronegative impurities through the porous anode. Removals of dilute sulfur compounds, oxygen and iodine from nitrogen were conducted to verify the concept of gas purification. Simulation models were used to estimate removal efficiencies of these compounds, by taking into account electron attachment, and experiemental constants of the models were determined. The removal efficiency correlated by the models agreed well with the experiemental one.
Transient discharge of solid particles from upper outlet of vertical bubble columns
Authors: Hidaka Nobuyuki , Kakoi Kazuhiro , Matsumoto Toshitatsu , Morooka Shigeharu 
Published: June 2004

AbstractSieved glass beads of 230, 280 and 470 μm dia. were steadily suspended by upward concurrent gas and liquid flows in vertical bubble columns of 7 cm dia. and 4.85 m height or 15 cm dia. and 2.7 m height. Then the liquid velocity was suddenly increased to a higher value, which caused an unsteady‐state discharge of solid particles from the top of the column. The transient discharge rate was determined by collecting solid particles in the effluent, and transient axial distributions of gas and solid holdups along the column length were measured by sectioning the column with horizontal shutter plates just after stopping gas and liquid flows. A 1‐D sedimentation‐dispersion model was applied to describe the unsteady‐state discharge rate of solid particles as well as the change in gas and solid holdups in the columns. The model developed successfully expressed the transient behavior of solid particles over entire columns consisting of dense and dilute regions.
A theory for fast‐igniting catalytic converters
Authors: Leighton David T. , Chang Hsueh‐Chia 
Published: June 2004

AbstractUsing asymptotic expansion and numerical analysis, we demonstrate how the step‐response ignition time of an automobile catalytic converter depends on the ratio of the reaction rate to the interphase heat‐transfer rate, as measured by a key Damköhler parameter χ and the degree of monolith subcooling ŋ.In the region of low reactionrate at smallχ, the normalized ignition time tig scaled bythe homogeneous ignition time from the inlet gas temperature is (tigt) = 1 + 2 χ1/2∣ln( χ1/2/2 η)∣1/2, and the ignition takes place at a thermal front deep in themonolith. At large χ when the reaction rate is high, ignition occursat the leading edge of the monolith with (tig/t) = 2.50 + χ(ln η − 0.34).The delay in ignition time with increasing χ is due to a Taylor‐Aris dispersion mechanism induced by interphase heat transfer. Although the small‐ χ ignition mechanism is faster, its downstream ignition location leads to a very slow upstream propagation of the thermal front that follows ignition. An optimal converter system that ignites in 13 s, 25% of the current value in a standard step‐response test, is then designed by placing a small igniter, which ignites by the small‐ χ mechanism, upstream to preheat the current converter which then ignites by the large‐ χ mechanism. The length of the igniter is kept small by bypassing 2/3 of the exhaust since, from our theory, t is independent of the gas velocity.
Limiting cases and approximate solutions for fixed‐bed reactors with periodic flow reversal
Authors: Nieken Ulrich , Kolios Grigorios , Elgenberger Gerhart 
Published: June 2004

AbstractFixed‐bed reactors with periodic flow reversal are used to carry out reactions with low exothermicity in an autothermal way. Contrary to conventional autothermal reactors, the preheating of the feed gas is accomplished by regenerative heat exchange between the gas and fixed bed. The feedback of heat and direct coupling of heat exchange and chemical reaction give rise to an unusual reactor behavior. This behavior can be understood through consideration of the limiting cases of very long and very short cycle periods. Both cases can be calculated easily with the methods presented. Especially for the case of rapid flow reversal, simple equations can be derived that allow for a good physical understanding of the process and for a preliminary reactor design. In this case, it is also possible to compute the unstable solution, which is a lower bound of the temperature profile necessary for reactor startup.
Mass transfer with chemical reaction in thin‐layer electrochemical reactors
Authors: Devaux Régine , Bergel Alain , Comtat Maurice 
Published: June 2004

AbstractThin‐layer electrochemical reactors are analytical reactors where mass transfers are generally assumed not to affect the overall reaction rate. The validity of this hypothesis was tested for systems involving a second‐order or an enzymatic homogeneous reaction that consumed the product generated by the electrochemical heterogeneous reaction. The experiments were performed with the oxidation of hexacyanoferrate II into hexacyanoferrate III, coupled with the consumption of hexacyanoferrate III by the reduced form of nicotinamide adenine dinucleotide, catalyzed or not by a diaphorase. The tools commonly used for gas ‐ liquid interfaces were adapted to the description of mass transfer in a closed thin film. The theoretical model agreed well with the experimental data and served as the basis for testing the thin‐layer hypothesis. The validity of the hypothesis was plotted as a function of the Hatta and Damköhler numbers. The influence of concentration and diffusion parameters are discussed with focus on the validity criterion.