Learning Forum

[S.Shrestha-4]

Hi!

I am getting mach at inlet of nozzle higher than at throat of nozzle. I am simulating the nozzle compressible flow with transient pressure based solver and partial slip wall conditions. Temperature at inlet is around 500 K for water vapor as fluid. Each time step is converging, but I am still getting this. What might be the reason? If someone could help me figure this out, it would be really helpful.

kind regards,

 

[Essence]

Hello,

You mean to say you are getting Mach number at throat lesser than that of the inlet. The inlet conditions are provided by the user. Therefore, based on the inlet BCs, the solver will solve the equations. What is the Mach number at the inlet?

You need to understand that everytime you use converging nozzle, the velocity will not be increased. This holds good only for subsonic (Ma < 1) flows. But when you have supersonic or sonic flows (Ma >= 1), using convergent nozzle will decrease the velocity. To accelerate the sonic or supersonic flows, you need to use divergent nozzles.

I would suggest you to refer convergent-divergent nozzles for more information.

[S.Shrestha-4]

Yes, the nozzle is a conveging-diverging nozzle. I am expecting mach to be 1 at throat. My Inlet condition is subsonic and pressure inlet with pressure ~ 1 bar and pressure outlet=30 Pa. Transient simulation is converging at each time step. But it results in Mach at inlet around 1.44 while at throat mach is around 0.5 which is very wrong. 

Could you suggest what might be wrong here?

Thanks!

 

[Essence]

What Mach number are you expecting at the outlet? I suspect either the pressure ratio or the area ratio is incorrect.

 

[S.Shrestha-4]

From calculation, I am expecting about mach 3. Also, my pressure contour looks like this. Somehow the inlet pressure boundary has not propagated and expansion happens in the inlet region, may be thats the reason? But why is this happening?

[Essence]

I see the throat area too small. Is the area ratio correct? If you need Ma = 3 at the outlet, then according to the isentropic relations, P/Po = 0.02722, A/A = 4.234 and P/P* = 0.0515. But you have applied P/Po = 30/10^5 = 0.0003.

[S.Shrestha-4]

Hi! Area ratio is correct. Since it's meant to operate in vacuum, I put the ambient pressure at the outlet far from exit = 30 pa. Is there any other way to incorporate this? As that does not seem to work?

 

[Essence]

You can try patching up the values. The image implies solver has not figured out the flow field yet.