Page 31 - Fluid mechanics, heat transfer, and mass transfer

FLUID MECHANICS BASICS
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TABLE 1.1 Rheological Characteristics of Non-Newtonian Fluids
Effect of Increased Time-
Fluid Type Shear Rate Dependent Examples
Pseudoplastic Thinning No Polymer solutions, starch suspensions, paints, greases, emulsions, biofluids,
detergent slurries, and so on
Thixotropic Thinning Yes Clay suspensions used in drilling muds, honey, ketchup, gelatin solutions, shaving
cream, and some food materials
Dilatant Thickening No Cornstarch pastes, slurries of clay and TiO 2 , wet beach sand, and sand-filled
emulsions
Rheopectic Thickening Yes Bentonite clay suspensions, gypsum suspensions, and some polyester solutions

. “Apparent viscosity of a pseudoplastic fluid flowing in a of finite diameter. These include Oswald glass vis-
pipe decreases as the flow rate increases.” True/False? cometer, capillary viscometer, Redwood No. 1 and
& False. No. 2 viscometers, and Saybolt Universal and Furol
viscometers.
. Summarize rheological characteristics of non-
Newtonian fluids. & Rising bubble or falling ball viscometers.
➢ Kits of bubble viscometers are available for
& Table 1.1 summarizes rheological characteristics of
different viscosity ranges in which a bubble of
non-Newtonian fluids.
a standard size is enclosed in sealed glass tubes
having different, but known viscosities. The liquid
under test is enclosed in a tube of same geometry
1.4 VISCOSITY MEASUREMENT
and the time of bubble rise in vertical position in
. How are viscometers classified? the tube with liquid under test is compared with
& Viscometers are broadly divided into two categories, bubble rise in the sealed tubes with liquids of
namely, rotational and tube type. known viscosities. Viscosity of the test liquid is
& Figure 1.2 gives subgroups of viscometers under the taken as the viscosity in a particular sealed tube
above classification. having the same rate of bubble rise, that is, time
taken for the bubble to rise the same distance in
➢ Glass capillary viscometers (e.g., Cannon–Fenske
both the tubes.
viscometers) are suitable for Newtonian fluids be-
cause the shear rate varies during discharge. ➢ In a falling ball viscometer, time taken for the ball
to travel vertically downward for a specified dis-
➢ Cone and plate viscometers are limited to moder-
tance in the liquid in a tube will giveviscosity from
ate shear rates.
the equation for Stokes’ law.
➢ Pipe and mixer viscometers can handle much
& Rotational viscometers (Figure 1.3) use the principle
larger particles than cone and plate or parallel
that the force required to turn an object in a fluid
plate instruments.
depends on the viscosity of the liquid. Absolute
➢ High-pressure capillaries operate at high shear
viscosity can be directly obtained from a rotational
rates.
viscometer that measures the force needed to rotate a
. What are the various types of viscometers and describe
spindle in the fluid.
their principles of operation?
& Brookfield viscometer belongs to this class and de-
& Flow viscometers work on the principle of time taken
termines the required force for rotating a disk or bob
for a fixed volume of liquid flowing through an orifice
in a fluid at a known speed.

Classification of viscometers. Rotating disk and parallel plate viscometers.
FIGURE 1.2 FIGURE 1.3