Significance of Critical Pressure Ratio

*Consider a nozzle which connects vessel V_A and V_B. The vessel V_A contains steam at high pressure P_1. The vessel V_B contains steam but its pressure P₂ can be varied based on requirements.

*Initially the pressure P₂ be equal to the pressure P₁ in vessel V_A. By that time, there is no flow through nozzle.

*If the pressure P₂ is gradually reduced in vessel V_B, the discharge through the nozzle increases gradually.

*The pressure P₂, at which the discharge is maximum is called critical pressure.

*If the pressure P₂ is reduced further, the discharge through the nozzle will not increase and it remains at constant level.

* Hence it is confirmed that, at critical pressure ratio, the flow will be maximum.

*The velocity of steam at critical pressure is equal to sonic velocity. This condition will exist at throat cross section of the Convergent divergent nozzle.

Note;

*For convergent – Divergent nozzle, the flow of steam in the convergent portion of the nozzle is subsonic. i.e., the velocity of steam in the convergent section is less than the sound velocity.

*The steam flows in the divergent part of the nozzle is supersonic, i.e., the velocity of the steam in divergent portion is greater than velocity of sound.