Page 33 - Fluid mechanics, heat transfer, and mass transfer

FLUID MECHANICS BASICS
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example, in one brand of viscometers, the load and & Saybolt viscometers are of two types, namely,
speed decrease by pushing the level upward, and Saybolt Universal and Saybolt Furol, based on the
vice versa. The viscosity can be found by adjusting diameter of the orifice used. Universal orifice, with
the load until the rpm is 200. By examining the load smaller bore, is intended to measure viscosities of
applied and comparing tables found on ASTM lighter oils. Furol orifice is of larger bore for mea-
D-562, one can find the viscosity in Krebs units suring viscosities of high-viscosity oils such as fur-
(KU),uniqueonlytoStormer-typeviscometers.This nace oil. For the Saybolt viscometer, the amount of
method is used for paints applied by brush or roller. oil to be measured is 60 mL. Viscosity is expressed as
& Capillary glass tube viscometers. Saybolt Seconds Universal (SSU) or Saybolt Sec-
onds Furol (SSF).
➢ Vibrational viscometers operate by measuring the
damping of an oscillating electromechanical res- & Redwood viscometers are of two types, Redwood No.
onator immersed in a fluid whose viscosity is to be 1orRedwoodNo.2.Theformerhavinga smallerbore
determined. The resonator generally oscillates in orifice and the latter a larger bore orifice are intended
torsion or transversely (as a cantilever beam or a to measure lower and higher viscosity oil samples,
tuning fork). The higher the viscosity, the larger respectively. When the flow time exceeds 2000 s,
the damping imposed on the resonator. The damp- Redwood No. 2 will be more convenient to use.
ing of the resonator may be measured by one of the & Redwood seconds refer to the number of seconds
several methods: required for 50 mL of the oil to flow out of the device
- Measuring the power input necessary to keep at a predefined temperature.
the oscillator vibrating at a constant amplitude. & For Engler viscometers, the reading is the time, in
The higher the viscosity, the more the seconds, required for 200 mL of the oil to flow
power needed to maintain the amplitude of through the device at a predefined temperature.
oscillation. & Conversion charts are used to convert viscosities
- Measuring the decay time of the oscillation once from these instruments into kinematic viscosities.
the excitation is switched off. The higher the
. What type of viscometer is used for viscosity measure-
viscosity, the faster the signal decays.
ment for non-Newtonian liquids?
- Measuring the frequency of the resonator as a
& Rheoviscometers.
function of phase angle between excitation and
. Give selected approximate viscosity conversions.
response waveforms. The higher the viscosity,
& Table 1.2 gives viscosity conversions among kine-
the larger the frequency change for a given phase
matic, Saybolt, Redwood, and Engler viscosities.
change.
& Rheoviscometers are used to measure viscosities of
high-viscosity fluids such as molten polymers.
1.5 FLUID STATICS
. Name some petroleum testing viscometers and describe
their characteristics. . What are static and dynamic pressures?
& These are Saybolt, Redwood, and Engler viscometers, & Static pressure is the pressure exerted by a fluid at
used for petroleum testing using ASTM or IP stan- rest. Dynamic pressure is the pressure exerted by a
dards.(ASTM andIPstandardsarepublishedannually fluid in motion.
byAmericanSocietyfor TestingMaterialsandEnergy & Static pressure is uniform in all directions, so pres-
Institute of United Kingdom, respectively.) sure measurements are independent of direction in an
& They do not determine absolute viscosity of petro- immobile (static) fluid. Flow, however, applies ad-
leum products, tar or fluid products, but empirically ditional pressure on surfaces perpendicular to the
test fluidity of petroleum oils, by conforming to flow direction, while having little impact on surfaces
requirements of the test methods giving results that parallel to the flow direction. This directional com-
are reproducible. ponent of pressure in a moving (dynamic) fluid is
called dynamic pressure.
& These viscometers basically involve a tube of certain
dimensions to accommodate fixed volumes of test oil . “Values of high vacuum are generally expressed in
and precision orifices of specified bores, attached to terms of Torr.” What is Torr?
the tube at the bottom. The tube is immersed in a & TheunitofpressureinmmHgisoften called
constant temperature bath. Time, in seconds, taken Torr, particularly used in vacuum applications.
for flow of fixed quantity of oil into a calibrated glass 760 mmHg ¼ 760 Torr. 1 kPa ¼ 1Torr 0.13332.
bottle is measured. . What is a barometer? How does it work?