Fluids, viscous

If we assume laminar flow (Re 1), the time necessary for the liquid surface to reach a specified position can be derived by dimensional analysis (p fluid density, D characteristic length, g gravity)  

In Fig. 3.7, the time t taken by the oil to flow to a specific position inside the test tube is shown as a function of the shear viscosity r) for silicone oils of different viscosities. The experiment therefore confirms the proportionality t rj. 

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Polymers have found widespread applications because of their mechanical behaviour. They combine the mechanical properties of elastic solids and viscous fluids. Therefore, they are regarded as viscoelastic materials. Viscoelastic... 

The design and operation of a flow visualization system for highly viscous fluids, such as silicon rubber, has been reported by Ghafouri and Freakley (1994). This system consists mainly of a rotating roll and fixed-blade assembly, as is shown in Figure 5.7, and can be used to generate and maintain, essentially. 

Chemical Stabilization. The chemistry of the system determines both the rate at which the polymer phase is formed and the rate at which it changes from a viscous fluid to a dimensionally stable cross-linked polymer phase. It also governs the rate at which the blowing agent is activated, whether it is due to temperature rise or to insolubilization in the Hquid phase. 

Eriedel-Crafts reaction of naphthalene or tetrahydronaphthalene derivatives with those of styrene or alkylbenzenes has been used in the preparation of high viscous fluids for traction drive (195). Similarly, Eriedel-Crafts reaction of tetraline and a-methylstyrene followed by catalytic hydrogenation provided l-(l-decalyl)-2-cyclohexyl propane, which is used as a highly heat resistant fluid (196). 

For laminar flow (Re < 2000), generally found only in circuits handling heavy oils or other viscous fluids, / = 16/Re. For turbulent flow, the friction factor is dependent on the relative roughness of the pipe and on the Reynolds number. An approximation of the Fanning friction factor for turbulent flow in smooth pipes, reasonably good up to Re = 150,000, is given by / = (0.079)/(4i e ). 

Disk Pumps. When pumping shear-sensitive or highly viscous fluids, it is desirable to reduce internal turbulence caused by the vanes. The disk pump design rehes on the centrifugal frictional effect of a vaneless disk. Whereas the efficiency of this pump is lower than that of similar centrifugal pumps having vanes, it is often the only solution to certain pumping appHcations. 

Flow in tubular reactors can be laminar, as with viscous fluids in small-diameter tubes, and greatly deviate from ideal plug-flow behavior, or turbulent, as with gases, and consequently closer to the ideal (Fig. 2). Turbulent flow generally is preferred to laminar flow, because mixing and heat transfer... 

Figure 16 (145). For an elastic material (Fig. 16a), the resulting strain is instantaneous and constant until the stress is removed, at which time the material recovers and the strain immediately drops back to 2ero. In the case of the viscous fluid (Fig. 16b), the strain increases linearly with time. When the load is removed, the strain does not recover but remains constant. Deformation is permanent. The response of the viscoelastic material (Fig. 16c) draws from both kinds of behavior. An initial instantaneous (elastic) strain is followed by a time-dependent strain. When the stress is removed, the initial strain recovery is elastic, but full recovery is delayed to longer times by the viscous component.

The eugenol portion of the formula may consist of eugenol alone or may be compounded into a viscous fluid by the addition of various additives. The addition of rosin polymerized rosin hydrogenated rosin plasticizing oils, eg, oHve oil accelerators or retarders medicinal additives and other modifiers serve to extend the useflilness of the basic system. 

Reversible HydrocoUoids (Agar). The agar-based impression materials are thermally reversible, aqueous gels (230,231), that become viscous fluids in boiling water and set to an elastic gel when cooled below 35°C. The popularity of agar-based impression materials has diminished with the introduction of elastic impression materials such as alginate-based, polysulfide, silicone, and polyether impression materials, but agar [9002-18-0] materials are still used in substantial quantities. 

Fluids without any sohdlike elastic behavior do not undergo any reverse deformation when shear stress is removed, and are called purely viscous fluids. The shear stress depends only on the rate of deformation, and not on the extent of derormation (strain). Those which exhibit both viscous and elastic properties are called viscoelastic fluids. 

Purely viscous fluids are further classified into time-independent and time-dependent fluids. For time-independent fluids, the shear stress depends only on the instantaneous shear rate. The shear stress for time-dependent fluids depends on the past history of the rate of deformation, as a result of structure or orientation buildup or breakdown during deformation. 

For steady-state laminar flow of any time-independent viscous fluid, at average velocity V in a pipe of diameter D, the Rabinowitsch-Mooney relations give a general relationship for the shear rate at the pipe wall. 

Laminar Flow With highly viscous fluids the linear velocity along a streamline varies with the radial position. Laminar flow is charac teristic of some polymeric systems. Figure 23-21 shows how the conversion is poorer in laminar flow than with uniform flow over the... 

Viscous fluids Sterilized surface required Polished surface required Future expansion required Space restrictions Barrier coolant services Slurry applications... 

For heavy and viscous fluids, velocities should be reduced to about / values shown. 

Plate and Frame Composed of metal-formed thin plates separated by gaskets. Compact, easy to clean. Viscous fluids, corrosive fluids slurries. High heat transfer. Not well suited for boiling or condensing limit 350-500°F by gaskets. Used for Liquid-Liquid only not gas-gas. 0.8-1.5... 

In lagoons where there is liquid on the surface and a highly viscous fluid below the liquid surface, separating the water from the waste and sludge may be appropriate. The expienses for incineration of the removed sludge could be greatly reduced by removal of the water. 

K = Correction factor for viscous fluids, determined from Figures 15 and 16. A trial-and-error selection of orifice size is required in the determination of this factor. 

It IS the dynamic viscosity of the gas/fluid that determines its ability for free flow. Very viscous fluids require a large energy input to overcome the fric tional forces. 

The viscosities of non-haloaluminate ionic liquids are also affected by the identity of the organic cation. For ionic liquids with the same anion, the trend is that larger allcyl substituents on the imidazolium cation give rise to more viscous fluids. For instance, the non-haloaluminate ionic liquids composed of substituted imidazolium cations and the bis-trifyl imide anion exhibit increasing viscosity from [EMIM], [EEIM], [EMM(5)IM], [BEIM], [BMIM], [PMMIM], to [EMMIM] (Table 3.2-1). Were the size of the cations the sole criteria, the [BEIM] and [BMIM] cations from this series would appear to be transposed and the [EMMIM] would be expected much earlier in the series. Given the limited data set, potential problems with impurities, and experimental differences between laboratories, we are unable to propose an explanation for the observed disparities. 

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