摘要:

AbstractLagrangian statistics are explored as a means of describing the transport of solids in a turbulently flowing liquid. The essential feature of the approach is to represent the concentration field as resulting from a distribution of sources of particles. It is argued that this provides a better framework to understand the physics than the Eulerian analysis currently being used. Fully developed concentration fields are calculated, using the assumptions of homogeneous turbulence and plug flow. It is found that the configuration of solids and the suspended load depend primarily on the ratio of the settling velocity to the friction velocity, which is a measure of the relative importance of turbulence and of settling in depositing particles. The analysis emphasizes the need for a better understanding of the mechanism and the rate of entrainment of particles into a turbulently flowing liquid.

ISSN:0001-1541    

DOI:10.1002/aic.690391002

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe velocity contour profiles within Dean vortices produced by flowing water in a solid‐walled curved tube are quantitatively measured. Magnetic resonance flow imaging using flow encoding with spin warp imaging is used in two and three dimensions to obtain these velocities. These experimental measurements are then compared with simulations obtained from solutions of the Navier‐Stokes and continuity equations for three axial flow rates. Numerical solutions were obtained with a finite volume method for curved tube Poiseuille f

ISSN:0001-1541    

DOI:10.1002/aic.690391003

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA mixing length model describes the recently observed (Bilger et al., 1991) effect of chemical reactions on turbulent diffusivities. From this viewpoint, the coupling between microscale reactions and macroscale mixing occurs because the reaction rate is a quasitransferable property which causes more reaction in fluctuating fluid lumps leaving regions of high rate than low rate. The net effect, which raises or lowers reactant diffusivities, depends on a characteristic macromixing time and the rate at which the mean reaction rate changes with mean concentration:\documentclass{article}\pagestyle{empty}\begin{document}$$ \frac{{{\rm \varepsilon}_A}}{{\rm \varepsilon}} = 1 - \tau _M \frac{{d\bar r_A}}{{d\bar C_A}} $$\end{document}The model shows correct qualitative behavior and appears to agree quantitatively with much of the existing data. It adds an apparent flux, ετM(drA/dy), to the convective diffusion equations which can enhance or hinder the normal turbulent fluxes, but is likely to have little effect on the mean concentration fiel

ISSN:0001-1541    

DOI:10.1002/aic.690391004

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractAn original technique for simultaneous measurements of concentration and velocity in a turbulent liquid flow with and without a chemical reaction is developed by combining a two‐component He‐Ne laser Doppler velocimeter with a Ar+laserinduced fluorescence technique. The mass flux and eddy diffusivity of mass are estimated from the simultaneous measurements of concentration and velocity in a liquid shear‐free mixing layer downstream of a turbulence grid. The results show that the present measuring technique enables us to simultaneously measure velocity and concentration with high resolutions and that the approximation of the mean gradient diffusion causes about 10% error in estimating the mass flux in a reacting liquid

ISSN:0001-1541    

DOI:10.1002/aic.690391005

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA dispersionless, adsorption equilibrium, plug‐flow model of a countercurrent moving‐bed chromatographic separator operating at steady state is investigated. The flux of the constituents of a binary mixture is examined via concentration‐phase plane plots, which reveal a complex behavior depending on a parameter ó, the ratio of solid‐ and fluid‐phase flow of the mobile phase. When the model is modified by incorporating finite mass‐transfer rates, axial concentration profiles exhibit behavior that is characteristic of experimental observations, rather than the predictions of the ideal model. Numerical simulations with the finite mass‐transfer model reveal an optimum opera

ISSN:0001-1541    

DOI:10.1002/aic.690391006

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractThe heterogeneous macropore, surface and micropore diffusion model recently proposed by Hu et al. (1993) for single‐component systems is exiended to multicomponent systems in the description of sorption kinetics of gaseous adsorbates in bidispersed (macropore and micropore diffusions), heterogeneous microporous particles. This model describes sorption of gaseous adsorbates with allowance for the energy distribution of adsorption site for both equilibrium and diffusion of the adsorbed species. A uniform distribution is used to describe this energy distribution, and the cumulative energy distribution function is assumed the same for all species in the analysis of multicomponent mixtures. Extensive experimental data of ethane and propane in Ajax activated carbon are collected to study the effects of energy distribution on the binary adsorption, desorption, and displacement dynamic

ISSN:0001-1541    

DOI:10.1002/aic.690391007

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractMeasurements of temperatures (secondary outputs) and flows (inputs) are used to estimate product compositions (outputs) in a distillation column. The problem is characterized by strong collinearity (correlation) between temperature measurements and an ill‐conditioned model from inputs to outputs. In a linear study, three estimator methods, the Kalman‐Bucy filter, Brosilow's inferential estimator, and principal component regression (PCR), are tested for performance with μ‐analysis. One can achieve remarkably good control performance with the static PCR estimator, which is almost as good as the dynamic Kalman filter. The quality of the estimate for these two estimators is improved by additional temperature measurements, although the improvement is only minor for more than about five measurements. On the other hand, the performance of the Brosilow inferential estimator may not improve by adding measurements due to sensitivity to modeling errors. For all estimators, the use of flow (input) measurements does not improve the estimator performance and does in fact damage the performance if a static estimator i

ISSN:0001-1541    

DOI:10.1002/aic.690391008

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA method of nonhlinear static and dynamic process modeling via recurrent neural networks (RNNs) is studied. An RNN model is a set of coupled nonlinear ordinary differential equations in continuous time domain with nonlinear dynamic node characteristics as well as both feedforward and feedback connections. For such networks, each physical input to a system corresponds to exactly one input to the network. The system's dynamics are captured by the internal structure of the network. The structure of RNN models may be more natural and attractive than that of feedforward neural network models, but computation time for training is longer. Our simulation results show that RNNs can learn both steady‐state relationships and process dynamics of continuous and batch, single‐input/single‐output and multiinput/multioutput systems in a simple and direct manner. Training of RNNs shows only small degradation in the presence of noise in the training data. Thus, RNNs constitute a feasible alternative to layered feedforward back propagation neural networks in steady‐state and dynamic process modeling and model‐base

ISSN:0001-1541    

DOI:10.1002/aic.690391009

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractParametric sensitivity of a two‐dimensional pulverized‐fuel (PF) combustion model is studied extensively for the effect of parametric uncertainty on model predictions. Results show that error in coal devolatilization/oxidation parameters has the dominant effect on predicted burnout, NOxformation, local gas temperature, and coal‐gas mixture fraction. Uncertainty in the turbulent particle dispersion parameters appears to have a secondary effect, while error in the particle‐gas radiation parameters has little impact on model predictions. Regions of the computational domain exhibiting sensitivity to specific parameters are identified. Specific parameter sensitivity implies the relative importance of various mechanisms in the overall process. Turbulent particle dispersion seems to be important early in the reactor with kinetic processes dominating at and following the predicted ignition point. Radiation appears to be of minor importance. These results indicate the need for a better method of predicting the overall volatiles yield and further understanding of the devolatilization/oxidation mechanism and its role in the overall PF combustion process. The study provides fundamental direction for future comprehensive model development and focuses on experimental work to better quantify critical input par

ISSN:0001-1541    

DOI:10.1002/aic.690391010

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY

摘要:

AbstractA pseudohomogeneous model is used to analyze the steady state of a sharp reaction front in a porous medium. The reaction rate is described by the flame sheet approximation, and an asymptotic matching analysis is used to determine the temperature, conversion and position of the reaction front. Both adiabatic and nonadiabatic operations are considered, and the effect of radiation is included in the adiabatic case. The results show that there exists a maximum molar flux before blowout occurs. The critical molar flux is decreased by higher activation energies, but it is increased by higher rate constants and flame temperatures. Radiation further stabilizes the flame against blowout. Expressions are provided for the flame temperature, reactant conversion and the conditions when blowout occur. It is also shown that under certain conditions the front has two steady‐state position

ISSN:0001-1541    

DOI:10.1002/aic.690391011

出版商:American Institute of Chemical Engineers    

年代:1993

数据来源: WILEY