Particulate Fluids

An electrorheological fluid (dispersion type), as defined for this purpose, is a mixture of micron-sized, high dielectric constant particles carried in an insulating base oil. When an electric field is applied transverse to the direction of any motion of the fluid, it causes an interaction between the particles, the field and the dispersant, and this results in an increase in the resistance to the flow of the mixture. 

The mechanism(s) of a particulate fluid electroviscous effect is still not fully resolved and quantified. It is not strictly relevant to this work and is therefore not dealt with in detail. At this stage it can only be said that it is a very multi-parameter and multidisciplinary event and, secondly, it should be understood that there is little change in the viscosity p of the fluid as it is normally defined in its continuum context save for a derived effective or non Newtonian viscosity sense. The term electroviscous, which has often been used to describe the present class of fluids, is misleading in this case. Rather, the held imposes a yield stress type of property on the fluid which is similar to, but not the same as, that which is a feature of the ideal Bingham plastic. This can readily be seen by referring to Figs. 6.63 to 6.66 inclusive. It is alternatively possible to claim that either the plastic viscosity changes with shear rate or the electrode surface yield stress does. 

The shear mode of operation is the term generally given to the simple shearing of the fluid, as in a Couette rotational or parallel plate type of viscometer but with an electric field applied between the moving and the stationary electrodes of gap size h (Fig. 6.63). With zero voltage V = 0) applied, most ER fluids exhibit near-Newtonian properties. When an electric field E = V/h) is applied to the fluid, there is an increased resistance to its movement which must be overcome before motion can take place (see Fig. 6.64 which is an idealised representation). Conventional constant temperature O and speed lv Couette laboratory techniques can normally only encompass shear rates (7 = cuR/h) up to several hundred s although cooled purpose made industrial clutch-type devices of similar geometry may reach 6000 s.  

The separation of the true response time of the electroviscous, Winslow or electrorheological effect from unsteady pressure recordings of the step input type is a complex and tedious affair. Experimentation carried out in this domain is expensive and time-consuming, not least on account of the many variations of electrode separation and length, the number of variables and the different input frequencies involved. Again, because of these problems any data presented for appraisal should be treated with caution. It is necessary to ensure exactly what information has been put forward and, from what kind of test and how it was derived. Also, a model of the constitutive form 

A further advantage of flow-mode testing is that the shear-rate magnitudes that would be encoimtered in a practical hydraulic device, often in excess of 40 000s, can be achieved [99]. However, the definition of shear rate needs to be subjected to scrutiny it is often derived from the Newto-nian/Poiseuille formula, albeit when plug flow is present [100]. In a Couette viscometer care must be taken to avoid plug flow the plug formed by radial 

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C. D. Williamson and S. J. Allenson. A new nondamaging particulate fluid-loss additive. In Proceedings Volume, pages 147-158. SPE Oilfield Chem Int Symp (Houston, TX, 2/8-2/10), 1989. 

Several solids conveying models were developed by Campbell and his students at Clarkson University [19, 20]. These models will be referred to as either the Clarkson University models or the Campbell models. They proposed that the movement of the screw flight was pushing the polymer bed as the screw turns rather than the frictional force at the barrel moving the polymer pellets down the screw. For these models, they assumed that the solid bed behaved more like an elastic fluid rather than a solid and removed the torque balance constraint. Campbell and Dontula [20] reasoned that because the solid polymer pellets more closely resemble an elastic particulate fluid, no torque balance in the bed would be necessary. They further assumed that the force normal to the pushing flight was due to a combination of the force due to the pressure in the channel and a force proportional to the frictional force exerted at the barrel by the solid bed. The Campbell-Dontula model was first published as ... 

This study was supported by the Particulate Fluids Processing Centre, a special research centre of the Australian Research Council and the United States of America National Science Foundation international research fellowship program. 

Some particulate fluids are affected by both electrical and magnetic fields with a high degree of synergy arising. Dependant on the relative direction of the fields a range of characteristics can be produced. 

ZAAH acknowledges the SLAB Fellowship (The Ministry of Higher Education, Malaysia and Universiti Sains Malaysia). The authors also thank Melbourne Ventures Pty. Ltd., The University of Melbourne for funding (Growing Innovation Fund (GIF)), Dr Andrea O Connor (Particulate Fluids Processing Centre (PFPC), The University of Melbourne for use of facilities and Sabina Zahirovic (Melbourne Venture Pty. Ltd) for helpful discussion. 

The basic fluid-bed unit consists of a refractory-lined vessel, a perforated plate that supports a bed of granular material and distributes air, a section above the fluid bed referred to as freeboard, an air blower to move air through the unit, a cyclone to remove all but the smallest particulates and return them to the fluid bed, an air preheater for thermal economy, an auxiUary heater for start-up, and a system to move and distribute the feed in the bed. Air is distributed across the cross section of the bed by a distributor to fluidize the granular soflds. Over a proper range of airflow velocities, usually 0.8-3.0 m/s, the sohds become suspended in the air and move freely through the bed. 

Depth filters are usually preferred for the most common type of microfiltration system, illustrated schematically in Figure 28. In this process design, called "dead-end" or "in-line" filtration, the entire fluid flow is forced through the membrane under pressure. As particulates accumulate on the membrane surface or in its interior, the pressure required to maintain the required flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particulate loading of the feed solution. In-line microfiltration of solutions as a final polishing step prior to use is a typical apphcation (66,67). 

The likelihood that materials will produce local effects in the respiratory tract depends on their physical and chemical properties, solubiHty, reactivity with fluid-lining layers of the respiratory tract, reactivity with local tissue components, and (in the case of particulates) the site of deposition. Depending on the nature of the material, and the conditions of the exposure, the types of local response produced include acute inflammation and damage, chronic... 

The degree to which inhaled gases, vapors, and particulates are absorbed, and hence their potential to produce systemic toxicity, depends on their solubihty in tissue fluids, any metaboHsm by lung tissue, diffusion rates, and equiUbrium state. 

In the atomizing process, a stream of molten zinc is broken into tiny droplets by the force of a pressurized fluid impinging on the stream. The fluid can be any convenient material, although air is normally used. The atomized drops cool and soHdify rapidly in a coUection chamber. The powder is screened to specified sizes. Particulate zinc is also produced by other methods such as electrolytic deposition and spinning-cup techniques, but these are not of commercial importance. 

The gels precipitated as described above are not useful in ion-exchange systems because their fine size impedes fluid flow and allows particulate entrainment. Controlled larger-sized particles of zirconium phosphate are obtained by first producing the desired particle size zirconium hydrous oxide by sol—gel techniques or by controlled precipitation of zirconium basic sulfate. These active, very slightly soluble compounds are then slurried in phosphoric acid to produce zirconium bis (monohydrogen phosphate) and subsequently sodium zirconium hydrogen phosphate pentahydrate with the desired hydrauhc characteristics (213,214). 

J. G. Wilson and D. W. Miller, "Removal of Particulate Matter from Fluid-Bed Catalytic Cracking Unit Stack Gases," f AirPollut. Mssoc. 7, 682 (Oct. 1967). 

Spouted beds are used for coarse particles that do not fluidize well. A single, high velocity gas jet is introduced under the center of a static particulate bed. This jet entrains and conveys a stream of particles up through the bed into the vessel freeboard where the jet expands, loses velocity, and allows the particles to be disentrained. The particles fall back into the bed and gradually move downward with the peripheral mass until reentrained. Particle-gas mixing is less uniform than in a fluid bed. 

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