FLUID PROPERTIES
• To define the state of matter any instant we
require properties like Pressure Temperature,
Volume. (P,V,T)
• All other properties add more information to
the state of any particle at an instant taken.
• Eg: Viscosity, thermal conductivity, modulus of
elasticity, thermal expansion coefficient etc…..
• The list goes on…
Intensive Vs Extensive Properties
• Intensive Properties: Those that are
independent of the mass of the system.
• Eg: mass, temperature, pressure etc….
• Extensive Properties: Those that are
dependent on the mass of the system.
• Eg: total mass, density etc..
Density and Specific Gravity
• Density: is mass per unit volume, or the mass
or any substance occupying unit volume.
ρ = m/V
Unit: kg/m3
• Specific Volume: Volume per unit mass, the
inverse of density,
• v = V/m = 1/ ρ
• Specific Gravity: It is relative density, the ratio
of density of a substance to density of a
standard fluid (air, water )
SG = ρ / ρ (water, air)
• Unitless property
• Specific Weight: The weight of a substance
occupying unit volume.
ϒ = ρg
Unit : N/m3
Vapor Pressure
• Saturation temperature : temperature at which
phase change occurs for a constant pressure
• Saturation pressure Psat: pressure at which phase
change occurs for a constant temperature
• Vapor pressure Pv : is the pressure exerted by its
vapor in equilibrium with its liquid at given
temperature.
• Pv = Psat
Cavitation
• Pressure of any liquid in a liquid flow system can
drop below the vapor pressure of the liquid at given
temperature which causes it to vaporize and for
bubbles.
• these bubbles formed combine and can form big
bubbles which explodes at a later point causing
erosion or bursting of pipes. This is called
CAVITATION
Physical properties:
Surface tension
Viscosity
Vapor pressure
Compressibility and Bulkmodulus
Capillarity
Surface Tension
• A molecule in the interior of a liquid is under attractive forces in all
directions and the vector sum of these forces is zero. But a molecule S at
the surface of a liquid is acted by a net inward cohesive force that is
perpendicular to the surface. Hence it requires work to move molecules
to the surface against this opposing force, and surface molecules have
more energy than interior ones.
The surface tension (s sigma) of a liquid is the work that must be done to bring
enough molecules from inside the liquid to the surface to form one new unit area
of that surface (J/m2 = N/m). Historically surface tensions have been reported in
handbooks in dynes per centimeter (1 dyn/cm = 0.001 N/m).
Surface tension is the tendency of the surface of a liquid to behave like
a stretched elastic membrane.
There is a natural tendency for liquids to minimize their surface area.
For this reason, drops of liquid tend to take a spherical shape in order
to minimize surface area. For such a small droplet, surface tension will
cause an increase of internal pressure p in order to balance the surface
force.