viscosity consistency

8/4/2019 Viscosity Consistency

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Every liquid has got certain

flow is due to the friction ca

In everyday terms (and for

water is "thin", having a lo

ut simply, the less viscous

!iscosity in a fluid is the

attraction in the liquid tha

applied to a fluid at rest, th

as shown in $ig. %elow.

The force F resisting motion

#here μ is the proportional

The applied force is therefo

fluid and inversely proporti

This equation can %e e&pres

.

Thus the shear stress %et

direction perpendicular to t

the velocity gradient

VISCOSITY

resistance to flow and is called its viscos

sed when one layer passes over the other

fluids only), viscosity is "thickness" or "in

er viscosity, while honey is "thick", havi

he fluid is, the greater its ease of moveme

resistance to its change of shape, whic

t resists any change due to flow or mot

molecular layers in the fluid tend to slid

in a fluid is given %y

ty factor called viscosity .

re proportional to the area (') and veloci

nal to the distance %etween the plates (y)

sed in terms of shear stress

een layers is proportional to the vel

he layers. This gives the relation %etwee

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ity. The resistance to

.

ternal friction". Thus,

ng a higher viscosity.

t (fluidity).

is due to molecular

ion. #hen a force is

on top of each other

ty gradient (u) in the

city gradient in the

the shear stress and



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Types of viscosity

There e&ist a num%er of forms of viscosity

• Newtonian fluids, such as water and most gases which have a constant viscosity.

• Shear thickening viscosity increases with the rate of shear.

• Shear thinning viscosity decreases with the rate of shear. hear thinning liquids are

very commonly, %ut misleadingly, descri%ed as thi&otropic.

• Thixotropic materials which %ecome less viscous over time when shaken, agitated, or

otherwise stressed.

• Rheopectic materials which %ecome more viscous over time when shaken, agitated, or

otherwise stressed.

• ' Bingham plastic is a material that %ehaves as a solid at low stresses %ut flows as a

viscous fluid at high stresses.• ' magnetorheological fli! is a type of "smart fluid" which, when su%*ected to a

magnetic field, greatly increases its apparent viscosity, to the point of %ecoming a

viscoelastic solid.

Viscosity coefficients

!iscosity coefficients can %e defined as follows

"ynamic viscosity, also a#solte viscosity, It is defined as the ratio of shear stress

(+) to the velocity gradient (dudy) produced %y it. Typical units is a-s, oise (). o / 0. a-s,o c / ma-s / 0.00 a-s.

o poise / 0. 12sm3

o #ater at 30 45 has a viscosity of .0030 c

$inematic viscosity is the dynamic viscosity divided %y the density (typical units

cm3s, tokes, t).

o #ater at 30 45 has a kinematic viscosity of a%out ct.

%li!ity& The reciprocal of viscosity is fluidity , usually sym%oli6ed %y φ / μ or

F / μ, depending on the convention used, measured in reciprocal poise (cm-s-g7),sometimes called the rhe.

SPECIFIC VISCOSITY '( S) is the ratio of a%solute viscosity of the fluid (8) to the

a%solute viscosity of a standard fluid (8st ) at the same temperature.

R*+,TIV* VISCOSITY '- R) is the ratio of a%solute viscosity of the fluid at a given

temperature (8t) to the a%solute viscosity (8st) of a standard fluid at 3045.



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VISCOSITY IN"*. 'IV) is an empirical num%er that indicates the effect of changes of

temperature on viscosity of a fluid. 9arge Iv means lower sensitivity to temperature.

Non/stan!ar! nits

The :eyn is a ;ritish unit of dynamic viscosity.

't one time the petroleum industry relied on measuring kinematic viscosity %y means of

the ay%olt viscometer, and e&pressing kinematic viscosity in units of Saybolt Universal

Seconds (<). =ther a%%reviations such as < (Saybolt Seconds Universal ) or <! (Saybolt

Universal Viscosity ) are sometimes used.

Coefficient of Viscosity an! Temperatre&

' change in o$ may produce as much as 0 > change in viscosity. 's a general rule it can%e stated that the temperature should %e kept constant.

Types of Viscometers&

ome of the important viscosity meters or simply viscometers are given %elow.

. 5apillary !iscometer

3. ay%olt?s !iscometer

@. :otameter Type !iscometer

Capillary Viscometer

=f the different methods of viscosity measurement for liquids the classical method due to

oiseuille is still the standard and the viscometer %ased on this technique is known as the

capillary viscometer. #hen the liquid flows through a tu%e of small diameter 3: as shown

in figure %elowA

5onsider for an annular flow channel of radial thickness dr at a distance r from the central

line, the viscosity equation is given %y,



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#here, v B !elocityA B ressure difference across the tu%e of length l

Integrating in the limits r / r: and corresponding v / v0.

!olume flow rate,

Cence a%solute viscosity is given %y,

The capillary method has %een successfully used in refineries for measuring viscosities of

petroleum products.

The schematic arrangement is shown in figure %elow. This is a continuous method. Theliquid is forced at a constant velocity through the capillary (friction tu%e). The pressure

drops across the ends of the tu%e is measured %y a pneumatic force %alance type

Differential ressure Transmitter (DT). The indicator can %e directly cali%rated in

viscosity. This method can measure viscosity from 01/2 poise to 2111 poise. It is usa%le at

high static pressures and is suita%le for remote indication and control.



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Say#olt3s Viscometer&

ay%olt?s viscometer works upon the same principle of operation of capillary viscometer,%ut this method measures kinematic viscosity. The schematic set up of the method is

shown in figure %elow.

It consists of a graduated tu%e with a capillary at the %ottom. The tu%e placed in a constant

temperature %ath is filled with the liquid and a measured quantity of liquid is drained

trough the capillary. $or varying viscosity, the time required should vary. The time t

required to drain 0 cc of the liquid indicates viscosity and is denoted %y ay%olt num%er.

'n empirical equation relating kinematic viscosity and time is as given %elow.

Cere ' and ; are constants. ' is having a value of 3F and ;, G00 for ay%olt?s viscometer.



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The following formulas may %e used to convert centistrokes (ct units) to

appro&imate ay%olt universal seconds (< units).

$or < values %etween @3 and 00

$or < values greater than 00

Rotameter Type Viscometer&

In this type, rotameter %o%s may %e designed to %e viscosity sensitive. Therefore with a

rotameter in the line of flow, viscosity can %e measured if flow rate is constant. $igure

%elow shows a scheme of the method.

' rotameter with two %o%s, one sensitive and another immune to viscosity is mounted in a

%ypass line through which the flow is regulated %y a suita%le regulator. ;y ad*usting the

regulator, the flow rate inde& is set at the inde& mark. The other %o% will indicate viscosity.If the range is required to %e changed, the flow rate can %e changed and the scale readings

can then %e related to the viscosity %y empirical cali%ration.

This method is commonly used up to a viscosity of 211 centipoises(cp), %ut the range is

e&tendi%le. It measures viscosity and also checks consistency at high static pressure.



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CONSIST*NCY

Hany fluids undergo continuous deformation with the application of shearing stress. If the

force B flow relation is linear, the fluid is 1ewtonian. $or non B 1ewtonian fluids, the force B

flow relation is not only nonlinear %ut changes from material to material. #hen continuous

deformation occurs, the fluid tries to oppose this with a frictional resistance. This

resistance can %e measured in terms of consistency. 5onsistency of 1ewtonian fluid is

called viscosity.

Consistency measrement is also important in industry particularly in the manufacturing

processes. ome commonly used methods are

. :otating !ane 5onsistency Heter

3. =scillating Type 5onsistency Heter

Rotating Vane Consistency 4eter

In paper or food processing industries, online consistency control is done %y suspending an

agitator wheel in the flow %o& and driving it %y an electric motor. The method is similar to

the one descri%ed earlier.

#hen the consistency of the material changes, there is a change in torque of the driving

motor, which can %e measured %y either pneumatic or electrical methods. ;y measuring

the power required to drive the agitator motor, consistency can also %e measured. The

wattmeter is empirically cali%rated.

The schematic circuit is shown in figure %elow.

Oscillating Type Consistency 4eter

It consists of two concentric cylinders. The inner cylinder is given an a&ial sinusoidal

motion through a mechanical drive rod as shown. The fluid in the annular space gets a

shearing force and the motion to the inner cylinder will %e transmitted to the outer

cylinder %ecause of the metal %ellows. The magnitude of this transmission will depend on



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the consistency of the fluid flowing through the instrument. The magnitude of transducer

outputs are compared for the consistency. This also requires empirical cali%ration.

This system is schematically shown in figure %elow

viscosity consistency

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