摘要:

Nonuniform velocity and concentration distributions have been measured for suspensions of small, negatively buoyant particles flowing in horizontal tubes. All measurements were performed during statistically steady flow conditions with average fluid velocities up to 25 cm/s. The mean particle concentration φ̄sranged between 0.0≤φ̄s≤0.39. Time‐averaged measurements were recorded noninvasively by using a three‐dimensional, flow‐compensating, nuclear magnetic resonance technique. This method spatially resolves both the fluid velocity and particle concentration distributions from a single imaging experiment and permits detailed analysis of the flow‐induced structure of nondilute suspensions.

ISSN:0148-6055    

DOI:10.1122/1.550156

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

The application of nuclear magnetic resonance (NMR) flow imaging to the study of Poiseuille flows of single‐phase fluids and solid/liquid suspensions is demonstrated and investigated. Two fluids were studied: a mixture of a low‐molecular‐weight polyether oil and water which is Newtonian and an aqueous solution of a high‐molecular‐weight polymer which is non‐Newtonian. The velocity profiles of these fluids in Poiseuille flow measured by NMR are in excellent agreement with those predicted from viscometric data using a power law model. The suspensions investigated consisted of density‐matched compositions of solid poly(methyl methacrylate) beads of various concentrations up to 52% in a Newtonian, polyether‐based liquid. At particle loadings above 40% these suspensions exhibited rheological behavior akin to what has been reportedly observed for some solid‐rocket motor propellant compositions. NMR‐derived flow velocities of these suspensions at high particle loadings show clear deviations from the Newtonian parabolic behavior and some unusual intensity features which are not yet fully understood. The capabilities and limitations of NMR flow imaging as well as considerations for data interpretation in the special case of highly filled suspensions is discussed.

ISSN:0148-6055    

DOI:10.1122/1.550253

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Nuclear magnetic resonance (NMR) imaging was used to observe the evolution of radial concentration and velocity profiles of initially well‐mixed concentrated suspensions of spheres in viscous Newtonian liquids undergoing flow between rotating concentric cylinders (wide‐gap, annular Couette flow). In Couette flow, particles migrate from the high shear‐rate region near the inner rotating cylinder to the low shear‐rate region at the outer wall. The particle concentration near the outer wall approaches maximum packing for randomly distributed spheres at steady state, and velocity profiles reveal that the suspension is almost stagnant in these regions. For unimodal suspensions of spheres, the shear‐induced migration of large particles results in concentric two‐dimensional, circular sheets of particles arranged in hexagonal close‐packed arrangements extending inward from the outer wall. This paper examines the functional dependence of particle migration in concentrated suspensions undergoing shear flow in a wide‐gap Couette. The primary experimental parameters were strain, shear rate, viscosity of the suspending liquid, particle diameter, and degree of polydispersivity of the particle phase. The particle migration is irreversible, even under creeping flow conditions, and is a function of total strain. Particle migration rate increases with the mean particle diameter raised to 2.7±0.3 for sieved samples and 2.5±0.3 for unsieved samples. The particle migration does not depend on the strain rate nor on the suspending liquid viscosity. The migration rate was found to depend only weakly on the polydispersivity of the particulate phase.

ISSN:0148-6055    

DOI:10.1122/1.550157

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

A numerical method for the solution of low Reynolds number flow (Stokes flow) past particles of arbitrary shape is described. The method uses a predictor–corrector approach, with a rough initial calculation using the Dabros method followed by an application of the completed double layer boundary integral equation method (CDL‐BIEM). The method is especially well suited for particles with very rough and uneven surfaces. The mobility and stresslet of a sphere with a spiked surface are computed, to illustrate the utility of the new method. For spheres with small protrusions, our results show a small change in the stresslet and particle mobility. For protrusions on the order of the sphere radius, there is a significant increase in the stresslet, leading to Einstein viscosity coefficients on the order of 5, compared to 5/2 for the perfect sphere. We also examine algorithms and the underlying theory behind conjugate gradient and conjugate residual methods that appear to work well with the larger CDL‐BIEM problems. For simpler geometries, the discretized equations may be readily handled by a number of equation solvers, including direct methods such as LU factorization. For large (50 000 by 50 000 and greater) dense linear systems associated with multiparticle problems in complex geometries, the iterative methods show great promise, especially on high‐performance parallel computer architectures.

ISSN:0148-6055    

DOI:10.1122/1.550158

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Falling ball rheometry of suspensions of spheres and rods is simulated with a boundary element method. Fully three‐dimensional simulations are performed to obtain static, configuration‐specific solutions of the particle motions as well as to track the particles in time. The relative viscosities of suspensions of spheres (both unimodal and bimodal in size distribution) with solid volume fractions up to 0.05 are obtained. The effect of the spheres on the pressure drop from the top to the bottom of the cylinder is also investigated. Up to 40 interacting particles are modeled. Suspensions of rods of aspect ratio 5 and 10 (volume fraction of solids equal to 0.01) are also modeled. Results agree with available laboratory measurements. Ensemble averages of configuration‐specific solutions give good approximations of the relative viscosity of suspensions, even though flow‐induced structure is not accounted for. Each configuration‐specific solution requires orders of magnitude less computation time than the dynamic analyses. Further computation time can be saved without loss of accuracy by modeling only those particles within roughly 10 ball radii (about one cylinder radius) of the falling ball.

ISSN:0148-6055    

DOI:10.1122/1.550159

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

The following is a description of a new computer‐simulation technique for the quasi‐static particle mechanics of granular media and its application to the dilatant simple shear of periodic 2D arrays of nearly rigid frictional disks. By means of supercomputer implementation, we have investigated the effects of (Coulomb) particle friction and polydispersity on Reynolds dilatancy and plasticity of a granular mass subject to a constant isotropic confining pressure. The simulation employs deforming periodic‐cell boundary conditions, with subcells arranged to reduce the contact‐search effort, together with a quasi‐static motion algorithm which converts an assumed global deformation to a local forcing of particle motion. Nearly identical shearing dilatancys=dεV/dγ is obtained for samples of 56, 132, and 306 polydisperse disks under identical conditions. Our computed dilatancys=0.81 for the monodisperse assemblage obtained in this study is larger than the Reynolds‐type prediction of 0.5 for isotropic assemblages but is in good agreement with the theoretical value 0.87 which we obtain by a proper accounting for crystalline anisotropy. Our value of 0.35 for the polydisperse assemblage is in fair agreement with Kishino’s (1983) experimental value 0.4, Thornton and Barnes’ (1986) simulation value 0.3, and Bardet and Proubet’s (1990a, 1990b) simulation value of 0.37. In accordance with the prediction of Reynolds (1885) and with the results from previous experiments and simulations, it is found that particle–particle friction has no significant effect on dilatancy. We also present results on the shear and normal stresses, as well as the particle‐contact topology. The rheological behavior is found to be roughly in accord with the Reynolds–Rowe stress dilatancy picture of plastic yield and generally exhibits noncoaxiality of stRess and strain‐rate tensors. Both mono‐ and polydisperse disk asseMblages eventualLy te.d to devorm at constant rolume (area) at roughly the same solid fraction φ≊0.65 and at the geometric bond percolation threshold for particle contacts.

ISSN:0148-6055    

DOI:10.1122/1.550160

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

Barium ferrite particles were stabilized and suspended to 3, 5, 7, and 9 vol % in mineral oil. The particles were roughly hexagonal platelets, 100 nm wide and 17 nm thick. When sheared at rates less than 10 s−1in Couette rheometers of several gap sizes and surface treatment these suspensions gave erratic torque readings. Cycles in the torque corresponded to cycles of alternating stick and slip in the flow, as observed through a glass outer cylinder. Optical and transmission electron micrographs of a similar sample in which the matrix was polymerized indicated flocculated structures even at very low particle volume concentrations. At shear rates greater than 10 s−1the torque signal was steady and the flow became homogeneous in appearance. Similar behavior was observed in other flocculated suspensions, namely those made from both the magnetic and nonmagnetic forms of acicular (needle‐like) shaped ferric oxide particles.

ISSN:0148-6055    

DOI:10.1122/1.550161

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

During processing, solid rocket propellants are particle suspensions which exhibit complex rheological properties. We discuss measurements of the properties of two typical solid propellants, HTPB and high energy. Despite their complexity in terms of particle shape and size distribution, these suspensions are shown to exhibit properties similar to those found in model systems. Of particular importance is the development of microstructure during shearing, which affects the ultimate performance of the rocket engine.

ISSN:0148-6055    

DOI:10.1122/1.550162

出版商:The Society of Rheology    

年代:1991

数据来源: AIP

摘要:

During reactive urethane foaming a delicate balance involving bubble growth, phase separation, and network polymerization is established under adiabatic conditions. Little progress has been reported in the measurement of rheological changes for this relatively fast (3–5 min) and highly exothermic process. A flooded parallel plate fixture was designed and adapted to follow modulus build‐up in a Rheometrics System Four rheometer. The new geometry is able to accommodate the 20–40 fold volume change observed during foaming as well as to minimize interference by the skin on the foam surface. The influence of edge effects and fixture compliance was studied by testing different cup radii and gap heights with strain and frequency sweeps on a low viscosity polydimethylsiloxane and a shaving cream foam. The evolution of storage (G’) and loss (G‘) moduli during foaming was measured on a typical slabstock foam formulation at 2.0 and 4.2 g water per 100.0 g polyol. Four regions were identified during the foaming process: 1.Bubblenucleation, within the initial 30 s of reaction; 2.Liquidfoam, a network of interacting bubbles leads to a solid‐like plateau inG’; 3.Phaseseparationwhich leads to rapid modulus rise and cell opening; and 4.Foamedelastomer, where the final (foam) modulus is reached.

ISSN:0148-6055    

DOI:10.1122/1.550163

出版商:The Society of Rheology    

年代:1991

数据来源: AIP