What is viscosity:-
Simple meaning of viscosity is “Resistance to fluid flow”and definition is “Viscosity is quantitative measure of fluids resistance to flow”.
- Water is thin having low viscosity
- Honey is Thick having higher viscosity
Types of viscosity:-
- Newtonian: fluids Like 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. Shear thinning liquids are very commonly, but misleadingly, described as thixotropic.
- Thixotropic: Fluid which become less viscous over time when shaken.
- Rheopectic: Fluid which become more viscous over time when shaken.
- A Bingham plastic is a material that behaves as a solid at low stresses but flows as a viscous fluid at high stresses.
- A magnetorheological fluid is a type of “smart fluid” which, when subjected to a magnetic field, greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid.
It is defined in two ways:-
1) Dynamic Viscosity
2) Kinematic viscosity
Its also called absolute viscosity and it units is (Pa.s, Poise , P) and dynamic viscosity of fluid show its internal resistance to flow due to shearing force.
In dynamic viscosity tangential force is required to move one horizontal plate of a fluid with respect to another.
η= F/ [A×(u/h)]
η= τ /(u/h) N-s/m²
where :- η= Dynamic Viscosity
F= force in N
h = Height in m2
u= speed in m/s
τ= shear stress
It is ratio of absolute velocity to the density of fluid.
ν= η/ρ m²/s ; ρ= density of fluid
How to measure Viscosity
1) Capillary Viscometer
It give kinematic viscosity of fluid , and it is based on Poiseuille’s law for steady viscous flow in a pipe.
Definition:- Capillary Viscometer is used to measure viscosity of fluid by measuring the time a define volume of fluid requires to flow through a capillary tube of a specific length and width.
2) Rotational Viscometer
It is one of the most important method used to check quality of material. In Rotational viscometer spindle is immersed in fluid and viscosity is measured by sensing the torque require to rotate the spindle. The torque is proportional to the viscous drag on the spindle; thus the sample viscosity.
Under Rotational viscometer two different setup are used to measure viscosity:-
- Sprint type instrument
- Servo motor instruments
Spring type instruments
In this set-up, the system mainly uses a spring. This spring is used to measure the torque strength on the spindle. Springs in this system is marked make it easy to note and read the measurements. Usually, about three to four spring ranges exist and each particular spring range is directly related to a particular strength of viscosity.
Servo motor system
This viscometer type uses a servo meter to drive the main shaft. The spindle with the measuring rotor is attached directly to the shaft. A high resolution digital encoder measure the rotational speed .The motor current is proportional to the torque caused by viscosity of the sample under test and viscosity can be found based on rotational speed and current.
Rolling/falling ball viscometer
Rolling or falling ball viscometer measure the viscosity of Newtonian fluid and gases.It work on simple Hoppler principle which is used to measure the viscosity of Newtonian liquid by measuring the time required for a ball to fall under gravity through a simple –filled tube that is inclined at an angle .
This instrument is perform under at the inclination angle between 10 to 80 degree . If the inclination is 80 or more , the instrument is referred to as a falling ball viscometer.
F=m.g= ρ .V.g
g= acceleration of gravity(m/s2),
ρ =density (Kg/m3),
ρs=sample density (g/cm3),
tr=ball rolling time(s)
Effect of temperature on viscosity
- Viscosity of liquid fluid is decreses with increases in temperature.
- Viscosity of gaseous fluid is increases with increases in temperature.
Viscosity temperature equations is used to calculate lubricant oil viscosity at a specific temperature.
Effect of Pressure on viscosity
- Lubricants viscosity is increases with increases in pressure.
- The Barus equation:-
Viscosity –Shear relationship
- Newtonian fluid ,shear stress is remain constant.
τ = η (u/h)
- Non-Newtonian fluid , doesn’t follow the linear relation between viscosity and shear rate.
Is also called shear thinning and is associated with the thinning of the fluid as the shear rate increases.
Is also called shear duration thinning, is associated with a loss of consistency of the fluid as the duration of the fluid as the duration of shear increases.
- Oil engine with various viscosity, for high load and for high speed high viscous lubricant is required.
- In pumping operation, For high viscous fluid high power will require And for low viscous fluid low power will require
- In the operation of coating and printing