摘要:

AbstractThe flow in a single‐cavity extrusion die is considered by using 1‐D equations. Since the fluid flow in a well‐performing die does not deviate much from the case of perfect widthwise distribution, linearization of 1‐D governing equations is often justified. It is demonstrated how linearization can lead to significant simplifications in the analysis of die flow. A self‐consistent asymptotic expansion is identified to accomplish this linearization; the expansion can be generalized to obtain higher‐order corrections if necessary. In contrast to previous analyses, which have generally been numerical, analytic solutions are obtained to predict widthwise flow nonuniformities in a die having a specific cavity area variation and in which cavity inertial effects are important; these solutions are valid for power‐law and Newtonian fluids. A novel truncated power‐law approach to modeling the flow of shear‐thinning fluids in a die is proposed that combines, by using analytical criteria, the solutions for purely Newtonian or power‐law flow in the cavity and slot. The validity of this approach is demonstrated through comparison with numerical results of a 1‐D die model for a Carreau

ISSN:0001-1541    

DOI:10.1002/aic.690420602

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractHeat and mass transfer from a circular cylinder exposed to a convective environment with a surface reaction of arbitrary order is studied based on the stream function‐vorticity formulation. A hybrid numerical scheme combining the Fourier spectral method in the angular direction and the spectral element method in the radial direction is used to solve the conservation equations along with an influence matrix technique to resolve the vorticity boundary conditions. Results showing the temporal evolution of the flow, temperature and concentration fields are presented for cases with and without vortex shedding with the latter triggered by the cylinder's rotation. A parametric study is also performed to examine the influence of the Reynolds number, Grashof number, Prandtl number, Schmidt number, Damköhler number, the reaction order, the flow alignment, the heat of reaction, the rotational velocity and the flow pulsation on the effectiveness of the reaction surface. With the exception rotation, they all exhibit strong dependen

ISSN:0001-1541    

DOI:10.1002/aic.690420603

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractThis work concerns some selected 3‐D flow effects on residence time distribution (RTD) in a screw extruder. The Newtonian problem is emphasized in which the cross‐ and down‐channel flow components can be formally discoupled and treated as two parallel 2‐D problems: our stream‐function‐based analysis of the Newtonian cross‐channel flow was combined with the existing 2‐D solution for the down‐channel flow. The RTD was determined by tracing the streamlines generated from both velocity components for the industrially important cases of fully‐ and partially‐filled channels. The effect of the cross‐channel flow on the RTD is discussed in connection with the stagnation flows, the down‐channel pressure gradient, and the presence of a free surface in a partially‐filled channel. The cross‐channel pressure distribution is compared to the conventional 1‐D theory. The non‐Newtonian effects were assessed usin

ISSN:0001-1541    

DOI:10.1002/aic.690420604

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractAxial solid velocity, solid volume fraction, and solid shear viscosity were computed in the riser of a circulating fluidized‐bed reactor using a two‐phase 2‐D computational fluid dynamic model. The time‐averaged model predictions agree well with the experimental data of Miller and Gidaspow (1992). The model predicts a core‐annulus flow in the riser, similar to that found experimentally. The maximum velocity in the core agrees well with the measurements, but the downflow in the annulus is somewhat overpredicted. The solid volume fractions profiles agree well in both core and annulus, with discrepancy in the core at the level close to the inlet. The radial profile of solid shear viscosity computed by the turbulent kinetic energy model is ten times lower in the core than that found experimentally, but with a linear function of solid volume fraction in the measurement, the computed profile agrees well with ex

ISSN:0001-1541    

DOI:10.1002/aic.690420605

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractTransient drop‐size distributions of stirred dispersions undergoing breakage were experimentally measured at long stirring times. The results show that drops continue to break the entire duration of the experiment (at least up to 10 h) and force a reevaluation of the widely held concept of a maximum stable drop diameter, dmax. Transient distributions show the existence of self‐similarity, which is the same as that observed in transient distributions obtained at short times (up to ∼ 1 h, Sathyagal et al., 1995b) indicating that the nature of breakage does not change with time. The existence of similarity at long stirring times can be used to obtain good estimates of breakage rates of small drops by an inverse problem proc

ISSN:0001-1541    

DOI:10.1002/aic.690420606

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA numerical model developed simulates inlet noise‐driven wave dynamics on a falling film at relatively high Reynolds numbers. Two parameters, a normalized Reynolds number and a noise index, are sufficient to specify the wave statistics on most channels. Observed phenomena, like wave inception, downstream wave texture coarsening, initial deceleration and subsequent acceleration of wave speeds, are quantitatively reproduced and explained. Statistical analysis from our simulations suggests that beyond a critical Reynolds number this complex noise‐driven spatio‐temporal dynamics can be modeled by a deterministic interaction theory based on stable solitary waves that resemble onehump pulses and a statistical theory with a random‐phase description of noise. The “chaotic” wave dynamics at higher Reynolds number is hence due to both noise amplification/filtering and intrins

ISSN:0001-1541    

DOI:10.1002/aic.690420607

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractComplex hydrodynamic behavior of circulating fluidized beds makes their scale‐up very complicated. In particular, large‐scale lateral solids segregation causes a complex two‐phase flow pattern which influences significantly their performance. Lateral solids segregation has been attributed to direct collisional interactions between particles as well as to interaction between gas‐phase eddies and dispersed particles. However, these phenomena have not been investigated thoroughly.This article discusses an advanced 2‐D hydrodynamic model developed for circulating fluidized beds based on the two‐fluid concept. Because theory to model the interaction between gas‐phase eddies and dispersed particles is not available, turbulence was modeled on a macroscopic scale using a modified Prandtl mixing length model. To model the influence of direct particle‐particle collisions the kinetic theory for granular flow was applied based on the Chapman‐Enskog theory of dense gases. For model validation purposes, a cold flow circulating fluidized bed was employed in which sand was transported with air as fluidizing agent. The column is equipped with pressure transducers to measure the axial pressure profile and with a reflective optical fiber probe to measure the local solids concentration and axial solids velocity. Theoretically calculated solids concentration and axial solids velocity agree satisfactorily with experiment, especially when one realizes that the model contains no adjustable parameters. In general, however, the model slightly underpredicted the experimentally observed lateral solids segregation and yielded a more peaked velocity profile compared to its experim

ISSN:0001-1541    

DOI:10.1002/aic.690420608

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractThe ribbing instability hinders the roll coating process, as it produces wavy coating films. The technique studied here prevents ribbing, whereby a string is placed parallel to the rolls, in contact with the unstable meniscus. In this experiment, the fluid is a Newtonian oil. A free‐surface visualization technique allows measurement of the interface profile where the string interacts with the forming films. The ribbing can be eliminated under conditions at least 20 times its natural onset. The influence of the string position on the mass transfer is then examined. Measurements show that the streamwise position of the string controls the total flow rate of the process, while its transverse position accurately controls the relative thicknesses applied on each roll. The string proves to be both an efficient concept to eliminate ribbing, and a reliable way of fixing the coating film thickness, without resorting to traditional gap width and roller speeds selectio

ISSN:0001-1541    

DOI:10.1002/aic.690420609

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractCohesive (Group C) particles have been widely used in various industries. To handle and process such fine particles, a clear understanding of the flow behavior and interparticle force, is needed. To achieve that objective, a Laser Doppler Anemometer system was used to measure particle velocity, fluctuating velocity, and size and extent of agglomeration or cluster formation of particles in a dilute gas/fine oil shale particle flow system with particle density of 2,082 kg/m3, average particle volumetric concentration of 1.5%, and average particle mass flux of about 100 kg/m2·s in a controlled‐moisture environment. The flow behavior of the particles was also studied for a mixture of 99% shale particles and 1% antistatic agent (Larostat powder, a quaternary ammonium compound) to examine the role of electrostatic force in gas/cohesive particle flow behavior. The addition of Larostat powder significantly reduced the electrostatic force and, in turn, made Group C particles behave similar to Group A or in some cases to Group B particles. In addition, our experimental data showed that the Maxwellian distribution function is a reasonable assumption to describe the velocity probability density function of the shale particles with or without antistatic agen

ISSN:0001-1541    

DOI:10.1002/aic.690420610

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA method for generating theoretical breakage distribution functions for multiple particle breakage is presented. It starts with the joint probability function that accounts for all the child particles; it is then reduced to the marginal probability function commonly used in the breakage equation. This method is flexible enough to allow the user to choose the number of child particles and the functional form to be used. The method is demonstrated with both product and summation functions with a power‐law form. To facilitate the use of these theoretical functions for statistical analyses, a companion discretized breakage equation is developed. The new equation guarantees the conservation of mass and correct prediction of the total number of particles despite discretization. It is easy to use because it is a set of ordinary differential equations and applicable to both equal‐size and geometric‐size intervals. Simulation results show that different breakage distribution functions coupled with different breakage rates can produce almost indistinguishable particle‐size distributions, signifying the need for further work in th

ISSN:0001-1541    

DOI:10.1002/aic.690420611

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY