[karakoca]

when i try to nozzle analysis the mass flux result which ansys give me is not same mine. for example;
Pinlet=600kPa Vinlet=19 m/s Tinlet=432K Ainlet=0.0095m2 fluid is steam

when i calculated mass flux with these values m=ro*V*A=0.56 kg/s

but ansys give me this.




Why is this happening?
Is there anyone who can help me?

[LuckyTran]

What are your boundary conditions?

[karakoca]

pressure inlet pressure (total) =600.5kPa temperature (total)=432.1 600kPa supersonic pressure

[karakoca]

pressure outlet

[LuckyTran]

Originally Posted by karakoca

pressure inlet pressure (total) =600.5kPa temperature (total)=432.1 600kPa supersonic pressure

With these settings, you do not know beforehand that the massflow rate at the inlet. I.e. you do not know that Pinlet is 600 kPa or that Vinlet is 19 m/s. You specify total properties, not static ones. This is not a mistake on your end, it is the right way to do things.

However, you have no way of knowing that the mass flow rate will be 0.56 kg/s.

[karakoca]

but mass flow is calculated m=ro*V*A this formula
and i know the ro(density), A(area) and V(velocity) so i can calculated mass flow.
i define total properties and supersonic pressure
supersonic pressure is the static pressure?

[LuckyTran]

Supersonic static pressure is the static pressure only if the inlet is supersonic. I doubt you are supersonic with a velocity of 19 m/s. If your inlet is not supersonic, that setting does nothing and you specify only a total pressure.

Furthermore, rho*V*A gives the mdot only if you are using a weighted V, which you would have to get from the simulation. Again, you don't have the means of determining the mdot beforehand.

[karakoca]

ok i know wrong about supersonic pressure
i know my inlet flow is subsonic then what i must choose inlet and outlet boundary condition
thank u for helping by the way

[LuckyTran]

It's up to you what boundary conditions you want to use.

I am merely pointing out that shouldn't compare your reasonable CFD results to some numbers you made up to try and find fault with what you have done. There is a reasonably small mass imbalance that can probably be improved with better convergence. Otherwise, what's the issue?

[karakoca]

i have convergence-divergence nozzle so the flow is supersonic in divergence section
as i know in supersonic flow analysis, inlet boundary condition is pressure inlet
but if inlet flow is subsonic can i use the mass flow inlet

[LuckyTran]

Absolutely

But then of course you cannot specify the inlet stagnation pressure because that would over-constrain the problem.

[karakoca]

while i use mass flow inlet there is supersonic pressure section there what am i suppose to write that pressure

[LuckyTran]

You can put just about anything. Again, that cell is only used for initialization or if the inlet becomes supersonic (which can happen during iteration). Put anything that makes sense because once your solution is converged it plays no role.

[karakoca]

ok thank u very much

[FMDenaro]

When I worked on compressible flow with subsonic inlet, I found that the best method for my code was to let mass flow rate to adjust according to the fact that one BC can not be prescribed since one characteristic comes from the interior.

[karakoca]

ok then I try to mass flow inlet boundary condition
thank u Denaro

[juliom]

I always prescribe all the conditions at the inlet and I predict them all at the outlet. For inlet I prescribe velocities, temperature and densities. Pressure is a headache in compressible flows.

[FMDenaro]

Originally Posted by juliom

I always prescribe all the conditions at the inlet and I predict them all at the outlet. For inlet I prescribe velocities, temperature and densities. Pressure is a headache in compressible flows.

This is correct for supersonic inflow/outflow...