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