Thus, for subsonic flow in a conver

Thus, for subsonic flow in a converging nozzle, the velocity increases and the pressure
decreases as the cross-sectional area diminishes. The maximum obtainable fluid velocity is the
speed of sound, reached at the throat. A further increase in velocity and decrease in pressure
requires an increase in cross-sectional area, a diverging section. Because of this, a converging
subsonic nozzle can be used to deliver a constant flow rate into a region of variable pressure.
Suppose a compressible fluid enters a converging nozzle at pressure PI and discharges from
the nozzle into a chamber of variable pressure P2. As this discharge pressure decreases below
PI, the flow rate and velocity increase. Ultimately, the pressure ratio P2/PI reaches a critical
value at which the velocity in the throat is sonic. Further reduction in P2 has no effect on the
conditions in the nozzle. The flow remains constant, and the velocity in the throat is sonic,
regardless of the value of P2/P1, provided it is always less than the critical value. For steam,
the critical value of this ratio is about 0.55 at moderate temperatures and pressures.