Fluids

Flow from 4E+05 Pa to 5 Pa inside the closed domain

• alokansys
Subscriber

Hey Ansys guys,

I am stuck in a problem of simulating a flow from very high 4 bar pressure to a tank having vacuum of 5 Pa.

I am using density based solver with pressure ( 4bar as inlet condition), gas (N2) as ideal gas, courant number of 5. I am not giving any outlet BC because i just want to see how tank is getting filled, want to know about shock waves and how is the velocity at nozzles.

Solution method and all are set at default.

To make the region inside the tank at 5 Pa, I am patching it with 5 Pa.

getting divergence in solutions.

Pls help, let me know if it required ill upload geometry and BCs.

PS:: If there is any other way than let me know, I just want to have constant supply of 4 bar at inlet and want it to flow into vacuum chamber (5Pa).

• DrAmine
Ansys Employee

• alokansys
Subscriber

Hey, really thankful for your time.

Screen shots of almost everything is attached. Waiting for your reply. Thanks!

• DrAmine
Ansys Employee

Add a sensible Gauge Pressure in the initialization as well as at the boundary in order to flow started. Are you using FMG Initialization? Why only inviscid flow? Which Mach numbers are you expecting?

• alokansys
Subscriber

Supersonic/initial gauge pressure is what making me confuse. Pls suggest what initial gauge should I give and how I can give outlet Bc as pressure, I have read somewhere we can not give both inlet and outlet as pressure boundary condition. If we can pls suggest.

Standard initialization has been done. expected mach number is in between 1-2 at nozzle ( inlet). Inviscid flow has been considered to simplify Naviers equation to eulers ( which I thought may be the reason for divergence) . I am new to ansys and also to Fluid Mech, Pls suggest proper BCs and solver.

Thank you !

• DrAmine
Ansys Employee

So if it is supersonic then you need to provide the right gauge pressure. Read in the documentation to know about that. You need to know either the stagnation pressure or Mach number to get the right static pressure or static pressure and stagnation to have the right mach number at inlet. At the outlet you need to provide the pressure outside. If the flow is supersonic at outlet your input will be neglected.

• alokansys
Subscriber

In experiments at inlet we are giving constant supply of 4 bar Nitrogen gas, so this is the static pressure (initial/supersonic gauge pressure) as my operating condition is 0 Pa. Then to define gauge total pressure at inlet I need to use total pressure formula as given in 7.3-4 in fluent guide. For this I need to calculate Mach number.

Mach number I am calculating based on my experimental inlet volumetric flow rate (i.e 500 sccm). Velocity that i am getting from here is 0.03 m/s, I am confused here since speed is very less fluid is not supersonic at 4 bar inlet. But when fluid will flow from 4 bar to 5 Pa it will be supersonic with sudden expansion (joule thomson expansion). Pls correct me . it may be a very naive question, actually i am confused with all this.

based on 0.031 m/s velocity mach number is in range of 8e-05. From here my static gauge pressure and total pressure is same. Total pressure that i am getting is 400000.0002 Pa.

• DrAmine
Ansys Employee

You need to get you velocity out of the density and specific flow rate at your inlet boundary at the temperature you hate at inlet. Once you have that you calculate the sound speed at the same static temperature and then you will have the Mach number. Once you have that you can then say if you supersonic inlet or not. So if it is subsonic the value you enter at the inlet for static gauge pressure is ignored (or at least will be used if you say you want to initialize from inlet). An expansion can be supersonic or subsonic.

Even if I am not a fan it for compressible flow: you can try using mass-flow inlet at your inlet

To do a JT you need to use real gas and not ideal gas model

• alokansys
Subscriber

1. 500 sccm flow rate 1.12 kg/m3 density at STP, then for 4 bar pressure velocity that I am getting is 0.04 m/s. This is the velocity at inlet.

2. But when flow is moving from inlet (4bar) to region of 5 Pa. Definitely gas will fill the vacuum very fast. So my question is what velocity I need to calculate for Mach number, Whether is it corresponding to 4 bar or is it corresponding to 5 Pa.. I have doubts on it pls help, if there is any paper regarding that please suggest.

Can you please elaborate your point on using mass-flow inlet at your inlet how to use that ? I know mass flow rate inlet, initial pressure of 5 Pa inside the chamber and I know outlet pressure corresponding to this mass flow rate.

Really thankful to such beautiful Ansys community for help.

• DrAmine
Ansys Employee

You need to provide the input at inlet so forget at first the changes which your flow will experience. The values at the boundaries are the ones which Fluent or any other solver does not know.

Again: The value of Gauge Pressure you provide at pressure inlet or mass flow inlet is only used for supercritical /supersonic conditions at Inlet.