Compressibility, Bulk Modulus & Cavitations

♦Compressibility and Bulk Modulus♦

● The property by virtue of which fluids undergoes a change in volume under the action of external pressure is called compressibility 

● Compressibility decreases with increase of pressure and bulk modulus increases with the increase of pressure.

● Bulk modulus (K) is defined as the ratio of change in pressure to corresponding volumetric strain.

K = - dp / (dV / V₀) 

k = 1/β

where

K = Bulk Modulus of Elasticity (Pa, N/m²)

β= Compressibility (m²/N)

dp = change in pressure  (Pa, N/m²)

dV = change in volume (m³)

V₀ = Initial volume (m³)

♦Cavitations♦

● The liquid pressure in liquid flow systems drops below the vapour pressure  at some location , results in vaporization of liquid. 

● For example, water at 10*c will convert into vapour and form bubble at locations where the pressure drops below 1.23 kPa the bubbles collapse as they  are swept away from the low-pressure regions, generating highly destructive, extremely high pressure waves.

● This phenomenon, which is a common cause for drop in performance and even the erosion of impeller blades, is called cavitation and it is an important consideration in the design of hydraulic turbines and pumps.

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