VISCOSITY.ppt

Viscosity
SUNIL PRABHAKAR
SR. No. 08458

Introduction
 Viscosity is a quantitative measure of a fluid’s
resistance to flow.
Dynamic (or Absolute) Viscosity:
 The dynamic viscosity(η) of a fluid is a measure of the
resistance it offers to relative shearing motion.
η= F/ [A×(u/h)]
η= τ /(u/h) N-s/m²
Kinematic Viscosity :
 It is defined as the ratio of absolute viscosity to the
density of fluid.
ν= η/ρ m²/s ; ρ= density of fluid

Viscosity Measurements
Capillary Viscometers
 It gives the ‘kinematic viscosity’ of the fluid. It is based
on Poiseuille’s law for steady viscous flow in a pipe.

Rotational Viscometers
 These viscometer give the value of the ‘dynamic
viscosity’.
 It is based on the principle that the fluid whose viscosity
is being measured is sheared between two surfaces.
 In these viscometers one of the surfaces is stationary
and the other is rotated by an external drive and the
fluid fills the space in between.
 The measurements are conducted by applying either a
constant torque and measuring the changes in the
speed of rotation or applying a constant speed and
measuring the changes in the torque.
 There are two main types of these viscometers: rotating
cylinder and cone-on-plate viscometers

Rotating cylinder viscometer

Cone-on-plate viscometer

Effects of temperature
 The viscosity of liquids decreases with increase the
temperature.
 The viscosity of gases increases with the increase the
temperature.

 The lubricant oil viscosity at a specific temperature can
be either calculated from the viscosity - temperature
equation or obtained from the viscosity-temperature
ASTM chart.
Viscosity-Temperature Equations

fig: Viscosity-temperature characteristics of
selected oils

Viscosity index
 An entirely empirical parameter which would accurately
describe the viscosity- temperature characteristics of
the oils.
 The viscosity index is calculated by the following
formula:
VI = (L - U)/ (L - H) * 10
where ,
VI is viscosity index
U is the kinematic viscosity
of oil of interest
L and H are the kinematic
viscosity of the reference oils
Fig . Shows the evaluation of viscosity index

Effects of pressure
 Lubricants viscosity increases with pressure.
 For most lubricants this effect is considerably
largest than the other effects when the pressure is
significantly above atmospheric.
 The Barus equation :

Viscosity - shear relationship
 For Newtonian fluids, shear stress linearly vary with
the shear rate as shown in Figure. Viscosity is
constant for this kind of fluid.
τ = η (u/h)
 Non Newtonian fluid doesn’t
follow the linear relation
between viscosity and shear rate.

Viscosity – shear relationship
Pseudoplastic Behaviour
 Pseudoplastic or shear thinning and is associated with
the thinning of the fluid as the shear rate increases.
Thixotropic Behaviour
 Thixotropic or shear duration thinning, is associated
with a loss of consistency of the fluid as the duration of
shear increases.
 The opposite of this behavior is
known as inverse thixotropic.

Applications
 Selection of lubricants for various purpose.
- we can choose an optimum range of viscosity for
engine oil.
- for high load and also for speed operation high
viscous lubricants is required.
 In pumping operation
- for high viscous fluid high power will require.
- for low viscous fluid low power will require.
 In making of blend fuel
- less viscous fuels easy to mix.
 In the operation of coating and printing.

References
 ‘Engineering Tribology’ by “Gwidon W. stachowiak” and
“Andrew W. Batchelor”.
 ‘Engineering Tribology’ by “J. A. Williams”.
 www.wikipedia.org
 www.google.com

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