Abstract: The basic concept of viscosity. When Liquid flows, the molecules
in the internal friction known as the viscosity of the liquid, the value of the
viscosity used to characterize the liquid properties which related to the
resistance factor. Viscosity is divided into dynamic viscosity, kinematic
viscosity and conditional viscosity.
The basic concept of viscosity
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| Zonwon IVS300-2 Ubbelohde Viscometer |
Liquid in the flowing, the friction between the
molecules known as the viscosity of the liquid. the value of the viscosity used
to characterize the liquid properties which related to the resistance factor.
Viscosity is divided into dynamic viscosity,
kinematic viscosity and conditional viscosity.
The flowing liquid is consist of many parallel moves layers parallel. Different
layers has different speed gradient (dv / dx), which is the basic
characteristics of the flow. Due to the presence of the velocity gradient, the
slower flow of the liquid layer blocks the flow of the faster liquid layer, so
the liquid produces movement resistance. In order to maintain a certain
velocity gradient, it is necessary to apply a contrast force to the liquid
layer to the opposite resistance.
This force applied on the unit liquid layer area
is called shear stress or shear force τ (N / m2).
Shear rate (D) D = d v / d x (S-1) Shear stress and shear rate are two basic
parameters that characterize the rheological properties of the system. Two different planes but parallel fluid, have the
same area "A", at a distance "dx", and "V1" and
"V2" flow at different flow rates to the same direction, Newton
assumed to maintain this power at different flow rates proportional to the
fluid Relative velocity or velocity gradient, ie: τ = ηdv / dx = ηD (Newton
formula) where η is related to the properties of the material, we call
"viscosity".
Put two plates (each plate 1 ㎡)
in an area of 1 meters liquid,1 meter
distance of two plates, when added 1N shear stress, so that the relative
velocity between the two plates is 1m / s, the viscosity of this liquid is
1Pa.s
Newtonian fluid: a fluid conforming to the Newton
formula. Viscosity is only related to temperature, independent of shear rate,
and τ is proportional to D.
Non-Newtonian fluid: does not meet the Newton
formula τ / D = f (D), ηa that a certain (τ / D) under the viscosity, said the
apparent viscosity.
Also known as the viscosity coefficient, shear
viscosity or dynamic viscosity. A physical property of a fluid used to measure
the viscosity of a fluid. For Newtonian fluids, the Newtonian viscosity law can
be defined:
Where μ is the viscosity of the fluid; τyx shear
stress; the velocity component UX; x, y coordinate axes; dux / dy is the shear
strain rate. The ratio of the viscosity μ of the fluid to its density ρ is
called the kinematic viscosity, denoted by v.
Viscosity varies significantly
depending on the temperature, but usually varies slightly with the pressure.
Fluid viscosity decreases with increasing temperature, the viscosity of the gas
increases with increasing temperature. For solutions, μr represents relative
viscosity than the conventional solution viscosity and solvent viscosity μ of
μ,
Relative relationship between C concentration and viscosity can be expressed
as:
μr=1+【μ】C+K′【μ】C+…
[Μ] is the intrinsic viscosity of the solution,
K 'is the coefficient.【μ】, K' are independent of concentration.
The viscosity of the different fluids are
different. Under a pressure of 101.325 kPa, at 20 ℃ condition, air, water, and
a kinematic viscosity and dynamic viscosity of glycerol:
Air μ=17.9×10-6Pa·s， v=14.8×10-6 m2/s
Water μ=1.01×10-3Pa·s， v=1.01×10-6m2/s
Glycerol μ=1.499Pa·s， v=1.19×10-3m2/s
Due to the role of viscosity,
so that the movement of objects in the fluid by the frictional resistance and
pressure resistance, resulting in mechanical loss (see flow resistance).
Viscosity data for various
fluids are measured primarily by experiments. Commonly used viscometer capillary
type, drop ball type, cone plate type, drum type and so on. Sometimes to meter
a determine condition viscosity by using specific forms of viscometer. As
commonly used in the oil refining industry Engler viscosity (or viscosity Enge
La) as an indicator of petroleum products, which represents the ratio of time
with the same volume of oil 200cm 20 ℃ water outflow from the required Engler
viscometer at a temperature. The relationship between viscosity and kinetic
viscosity can be calculated according to empirical formula. Another example is
the Mooney viscosity of the rubber used in the industry to measure the average
molecular weight rubber and a plasticity index.
In the absence of viscosity test data, the
viscosity can be estimated by theoretical or empirical formula. For gases with
less pressure, the estimates are more accurate and poor for liquids. The
viscosity of a heterogeneous fluid, such as a low concentration suspension, can
be estimated using the Einstein formula:
μm is the viscosity of the suspension; μ is the
viscosity of the continuous phase liquid; φ is the volume fraction of the
dispersed phase in the suspension; and μd is the dispersed phase viscosity.
When the dispersed phase is solid particles, μd → ∞, when the dispersed phase
is bubble, μd → 0, μm = (1 + φ) μ.
Viscosity is a measure of fluid viscosity, which is a representation of the
fluid flow force on its internal friction phenomenon. The viscosity showed
large frictional force, the greater the molecular weight, the more the binding
of hydrocarbon, the greater this force. The viscosity of various lubricants
decisive, determining the quality of identification and use, and combustion
properties of the fuel oil and the like of various costs. At the same
temperature the distillate, alkanes as the main component of a low viscosity
petroleum products, and better viscosity-temperature, i.e. high viscosity
index, i.e. a small viscosity change with temperature changes the amplitude;
cycloalkanes containing (or Aromatic hydrocarbons) components of the oil
viscosity is higher, that is, poor viscosity; with colloids and aromatics more
oil viscosity, viscosity of the worst, that the lowest viscosity index.
Viscosity commonly used kinematic viscosity, in mm2 / s. Heavy fuel oil
viscosity, kinematic viscosity preheated to achieve 18 ~ 20mm2 / s (40 ℃),
favor a uniform injection nozzle.
Three ways to measure the viscosity: Dynamic
viscosity, kinematic viscosity and conditional viscosity.
(1) Dynamic
viscosity: ηt is the resistance of the two liquid layers separated by 1 cm and
its area is 1 cm / s relative to the moving speed of 1 cm / sec in grams per
cent centimeter. 1 g / cm · s = 1
poise General: The industrial kinematic viscosity unit is expressed in poise.
(2) Kinematic
viscosity: at the temperature t ℃, the kinematic viscosity is expressed by the
symbol γ, in the international unit system, the kinematic viscosity unit is Sri
Lanka, that is, square meters per square meter (m2 / s), the actual measurement
commonly used cue, Cst) indicates that the unit of cent seconds is square
milliseconds per second (ie 1cst = 1mm2 / s). Kinematic viscosity is widely
used in the determination of jet fuel oil, diesel, lubricants and other liquid
petroleum products dark petroleum products, after use of lubricants, crude oil
viscosity, kinematic viscosity measured by countercurrent method
(3) Conditional viscosity: refers to the use of different specific viscometer measured in the unit of viscosity, the commonly used conditions for the viscosity of the following three:
① known as Si Gele Engler viscosity (Engler)
viscosity. Certain sample volume, at a predetermined temperature: the lower
(e.g., 50 ℃, 80 ℃, 100 ℃), flow time count the time required for 200 ml of
distilled water and effluent samples at the same volume from the desired 20 ℃
Engler viscosity (Seconds) ratio. When the temperature t, viscosity of about
represented by the symbol Et, Engler viscosity condition of the unit.
② Saybolt
viscosity that Saybolt (sagbolt) viscosity. A certain amount of the sample, at
the specified temperature from the Cessler viscometer 200 ml out of the
required number of seconds to "seconds" units. Saybolt viscosity is
divided into Saybolt Universal and Saybolt viscosity of heavy oil (or Saybolt
Florence (Furol) viscosity) two kinds.
③ Raywell viscosity known as
Redwood viscosity. A certain amount of the sample, at the specified
temperature, from the Rayleigh meter out of 50 ml of the required number of
seconds to "seconds" as a unit. Raytheon viscosity is divided into
two types of Ray 1 (Rt) and Raytheon 2 (expressed in RAt).
Viscometry
above three conditions, commonly used in Europe and America, in addition to use
of Engler viscometer dark lubricating oil and residual oil, the other two are
rarely used in the viscometer. The three conditions of viscosity representation
and units are different, but the relationship between them can be converted
through the chart. At the same time the viscosity and kinematic viscosity can
also be converted, so easy and more flexible.
There are many ways of measuring the
viscosity, such as the drum method, a falling ball method, the vibration
damping method, a cup viscometer method, capillary method and the like. For the
smaller viscosity of the fluid, such as water, ethanol, carbon tetrachloride,
commonly used capillary viscometer measurement; and viscosity of the larger
fluid, such as castor oil, transformer oil, oil, glycerin and other transparent
(or translucent) Drop ball method; for the viscosity of 0.1 ~ 100Pa · s range
of liquid, can also be used to determine the drum method.
