properties of fluids
DESCRIPTION
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Kaiser Rex N. PamaBSEEn-V
Properties of Fluids
The term fluid includes both liquid and gases. The main difference between a liquid and a gas is that the volume of a liquid remains definite because it takes the shape of the surface on or in which it comes into contact, whereas a gas occupies the complete space available in the container in which it is kept.
Density
The density of a fluid is its mass per unit volume:
ρ=mv ( kgm3 )
Liquids are essentially incompressible, whereas density is highly variable in gases nearly proportional to the pressure. In general, liquids are approximately 3 orders of magnitude denser than gases at atmospheric pressure.
@20 C, 1 atm
Air WaterHydrog
enMercu
ryρ
(kg/m3)1.20 998 0.0838
13,580
Specific Weight
The specific weight of a fluid is its weight, W=mg, per unit volume. Density and specific weight are related by gravity:
Fluid AirWater
Mercury
γ(N/m3)12.0
79814
132,943
γ= ρg( Nm3 )
Specific gravity
Specific gravity is the ratio of a fluid density to a standard reference fluid, typically water at 4˚C (for liquids) and air (for gases):
sliquid=ρliquid
ρwater
sgas=ρgas
ρair
Viscosity
Viscosity is a measure of a fluid’s resistance to flow. It determines the fluid strain rate that is generated by a given applied shear stress.
µ=τdVdy
( Nsm2 )
Fluid Water EthanolMercu
rySG 1 0.78 13.6
Where:
dVdy
=Uy
τ is the shear stress,
y is the distance between
the planes
U= velocity
Kinematic Viscosity
Kinematic viscosity is the ratio of the dynamic viscosity of the fluid to its density.
v=μρ (m2
s )
Compressibility
Fluid Glycerin Water Mercury
ß (m2/N)2.1 × 10-
10 4.6 × 10-10 3.7 × 10-11
Fluid Air Water Glycerol
µ(Ns/m2)1.983 x 10-5 1 x 10-3 1 x 100
Fluid Air Water Mercury
ν(m2/s)1.46 ×
10-51.14 × 10-
6 1.145 × 10-4
Compressibility (also known as the coefficient of compressibility) is the fractional change in the volume of a fluid per unit change in pressure in a constant temperature process.
β=
−ΔVVΔ p
=1Eb
∨β=−dV
Vdp (m
2
N∨Pa−1)
Where:
ΔV = change in volume
V= original volume
∆ p=change in pressure
Eb= Bulk modulus of
elasticity