newtonian and non newtonian behaviour of fluids
TRANSCRIPT
Newtonian and non-newtonian
behavior of fluidsGroup Members
Waheed Ahmed (2k11-ChE-09)
Adnan Rafi (2k11-ChE-16)
Ahmed Haroon (2k11-ChE-23)
Shahzad Ali Zahid (2k11-ChE-49)
Fluids which obey the Newton's law of viscosity are called as Newtonian fluids
Newton's law of viscosity is given by
= shear stress
= viscosity of fluid
dv/dy = shear rate, rate of strain or velocity gradient
Newtonian fluids
dy
dv
Examples of Newtonian fluids
All gases and most liquids which have simpler molecular formula and low molecular weight such as
Water
Benzene
ethyl alcohol
CCl4
Hexane
and most solutions of simple molecules are Newtonian fluids
Non-Newtonian fluids
Fluids which do not obey the Newton's law of
viscosity are called as non-Newtonian fluids
η is the apparent viscosity and is not constant
for non-Newtonian fluids
dy
dv
Apparent Viscosity
If the viscosity is influenced by the shear rate,
it is important to specify that the values are
different from the constant ones of an ideally
viscous fluid. The values obtained are
'apparent viscosity' or 'apparent shear viscosity'
values
Examples of Non-Newtonian fluids
Generally non-Newtonian fluids are complex
mixtures such as
slurries
Pastes
Gels
polymer solutions etc
Various non-Newtonian Behaviors
Time-independent:
Time-dependent
Viscoelastic
Time-Independent Fluid Behavior
• Fluids for which the rate of shear at any point
is determined only by the value of the shear
stress at that point at that instant; these fluids
are variously known as “time independent”,
“purely viscous. these fluids may be further
subdivided into three different types:
• Shear-thinning or pseudoplastics
• Visco-plastics
• Shear-thickening or dilatant
Time-Independent Fluid Behavior
A shear-thinning or pseudoplastic substance is
characterized by an apparent viscosity that
decreases with increasing shear rate the rate of
decrease of the apparent viscosity is not the
same for each fluid
1. Shear thinning or pseudoplastic fluids
The Power-Law or Ostwald-De
Waele Model
The relationship between shear stress and shear rate for pseudoplastic fluids can often be approximated by a straight line over a limited range of shear rate, and hence this part of the flow curve can be described by the power-law expression
τyx =m(˙γyx)n
Where n and m, known as the power-law index and the fluid consistency coefficient,for a Newtonian fluid=1 and for a pseudoplasticsubstance n<1
flow curves for different types of non-
Newtonian fluids
Pseudoplastics
Flow of pseudoplastics is consistent
with the random coil model of polymer
solutions and melts. At low stress, flow
occurs by random coils moving past
each other w/o coil deformation. At
moderate stress, the coils are deformed
and slip past each other more easily. At
high stress, the coils are distorted as
much as possible and offer low
resistance to flow.
Pseudoplastic (Shear thinning) Foods
Shear stress
Shear rate
Examples:
•Banana puree
• Orange juice concentrate
• Oyster sauce
Shear Thinning Behavior
Shear thinning behavior is often a result of:
Orientation of non-spherical particles in the direction of
flow. An example of this phenomenon is the pumping of
fiber slurries
Orientation of polymer chains in the direction of flow and
breaking of polymer chains during flow. An example is
polymer melt extrusion
Deformation of spherical droplets to elliptical droplets in
an emulsion. An industrial application where this
phenomenon can occur is in the production of low fat
margarine
Breaking of particle aggregates in suspensions. An
example would be stirring paint
yxBB
yx 0 forB
yx 0
0yx forB
yx 0
Often the two model parameters t0B and mB are treated as curve fitting
constants, even when there is no true yield stress
2. Viscoplastic Fluid BehaviorViscoplastic fluids behave as if they have a yield stress (t0). Until t0is exceeded they do not appear to flow. A Bingham plastic fluid has
a constant plastic viscosity
3. Shear-thickening or Dilatant Fluid BehaviorEq. (*) is applicable with n>1.
Viscosity increases with shear stress. Dilatant: shear thickening
fluids that contain suspended solids. Solids can become close
packed under shear
Dilatant (Shear thickening) Foods
Shear stress
Shear rate
Examples:
• Liquid Chocolate
• 40% Corn starch solution
The apparent viscosity of a fluid changes with
time as the fluid is continuously sheared
Thixotropic
Rheopectic
Time-dependent Fluid Behavior
Thixotropic
If the apparent viscosity decreases with time
e.g. paints, cream, aqueous iron oxide gels,
some drilling mud's
Result of a break down in the microstructure of
the material as shearing continues
This happens when the sheer is exceeded of a
limit
It leads to non-linear stress-strain behavior
Thixotropy in a Cement Paste
Typical experimental data showing
thixotropic behavior in red mud
suspension
Rheopectic
Is the rare property of some non-newtonian
fluids if the apparent viscosity increases with
time
The longer the fluid undergoes shearing force,
the higher its viscosity
Examples are gypsum pastes and printer inks
Also termed as negative thixotropy
Rheopectic behavior in a saturated
polyester
Qualitative shear stress–shear rate
behavior for thixotropic and rheopectic
materials
Time dependent behaviors
Non - newtonian
Time independent Time dependent
A EC D F GB
_ _
Rheological curves of Time - Independent and Time – Dependent Liquids
++
Visco-elastic Fluid Behavior
A visco-elastic fluid displays both elastic and
viscous properties. A true visco-elastic fluid
gives time dependent behavior
Examples
steel or aluminum
Quartz
Some phenomena in viscoelastic
materials are
If the stress is held constant, the strain
increases with time
If the strain is held constant, the stress
decreases with time
During rolling, frictional resistance occurs
Types of viscoelasticity
Linear viscoelasticity
o It usually happens when the deformations are
Small
Nonlinear viscoelasticity
o Usually happens when the deformations are
Large
Measuring viscoelasticity
broadband viscoelastic spectroscopy (BVS)
resonant ultrasound spectroscopy (RUS)
Thanks