Fluid bulk modulus: addition of air

We can express the bulk modulus with the volume V as:

This gives for the fluid:

Or if we apply the previous relation to the {liquid+vapor} and to the air, we obtain:

The air volumetric fraction Φ is defined as:

And we can write the bulk modulus as

Equation 17: Fluid bulk modulus at P and T

With:

Case	Fluid bulk modulus B_fluid
Case 1:	P > PHvap
Case 2:	PLvap < P < PHvap
Case 3:	P < PLvap