flow rate in the case of subsonic and supersonic flow ( CD NOZZLE)

aero2020

Hi every body :

Are you ok with me that in the absence of leakage , also in the case of isentropic flow ( no friction , no drop in total pressure etc.) the mass flow rate is constant between inlet and outlet ? that means : Ae me (dot) =As ms (dot)?

My problem : i have found in a book what follows : (it is not clear for me )

1/ after a shock in the throat , the flow becomes subsonic , it is isentropic so the Mach number at the exit of the divergent remains the same as the Mach calculated before the shock .

my question is why ? is this conclusion was deduced from the mass flow conservation ? I don't see the physics behind it .

Remark : M=cte that means A/Ac=cte

2/ We have a shock at the divergent and the flow is considered sonic at the throat .We can so suppose that the flow rate at the outlet is the same ( as this found before the shock ) !

It is not clear ! At my opinion , the shock is irreversible so the flow rate can not be the same as we have a shock in the divergent ! Conclusion : it is not possible to consider ( in the presence of a shock) the same flow rate (found before the shock)

Can you help me please !

Kind regards

(sorry for english mistakes 😊)

rahkumar

Hello,

1. In a CD nozzle, when the flow accelerates in the converging section such that the Mach number at the throat increases to be become sonic (M=1), the Mach number at the throat cannot exceed this condition. Even if the flow in the converging section is further accelerated, the Mach number at the throat remains constant (M=1).
2. To answer generally, if it is an isentropic flow, then mass flow in = mass flow out.

aero2020

Hi freinds ;

In otherwise , my question was :

Consider a compressible flow in a CD nozzle ( regardless an eventual change in the flow regime) :

In which case :

1/ The Mach remains constant ( in some section A/Ac)?

2/ The flow rate remains constant across some section A/Ac ?

Thank you very much for your assistance

rahkumar

Hello,

1. In a CD nozzle, when the flow accelerates in the converging section such that the Mach number at the throat increases to be become sonic (M=1), the Mach number at the throat cannot exceed this condition. Even if the flow in the converging section is further accelerated, the Mach number at the throat remains constant (M=1).
2. To answer generally, if it is an isentropic flow, then mass flow in = mass flow out.

aero2020

Ok thank you very much