Iscentropic Flow of a vapor or gas through a nozzle

First law of thermodynamics:

	(if )

where is enthalpy drop across the nozzle
Again we know, Tds = dh - νdp

For the isentropic flow, dh = νdp

or,
or,	(20.1)

Assuming that the pressure and volume of steam during expansion obey the law pνⁿ = constant, where n is the isentropic index

		(20.2)

Now, mass flow rate

Therefore, the mass flow rate at the exit of the nozzle

=

(20.3)

The exit pressure, p₂ determines the for a given inlet condition. The mass flow rate is maximum when,

For maximum ,

(20.4)

n =	1.4,	for diatomic gases
		for super saturated steam
		for dry saturated steam
		for wet steam with dryness fraction x

For ,     (50%drop in inlet pressure)

If we compare this with the results of sonic properties, as described in the earlier section, we shall observe that the critical pressure occurs at the throat for Ma = 1. The critical pressure ratio is defined as the ratio of pressure at the throat to the inlet pressure, for checked flow when Ma = 1