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How to use volumetric flow (m^3/s) - outlet boundary condition

Sid_20854

Hello all,

I am new to the Altiar ecosystem and trying to model a basic binary flow diffusion/multi-species problem wherein air and gas are mixing. The simplified system is represented as a box with two inlets and one outlet. For the first inlet, I have flow rate data of air (100% air and 0% gas) entering the system at a specific temperature and pressure. For the outlet, I have details of the volumetric flow rate (m3/s) along with the volume fraction of the mixture (%ages of air and gas). The objective is to predict the flow rate & volume fraction at the second inlet.

To summarise:

Inlet 1 - Air

Vol. flow rate: known
volume fraction: known (100% air)
Pressure: known
Temperature: known

Outlet: Mixture of air & gas

Vol. flow rate: known
Volume fraction: known
Pressure: to be computed
Temperature: to be computed

Inlet 2 - Mixture of air & gas

Vol. flow rate: to be computed
volume fraction: to be computed
pressure: to be computed
temperature - known

I have a couple of questions regarding this and any advice/recommendation from the best practices perspective is also welcomed

Given that the system is a fully compressible one (gasses), I am unable to find multi-species options (they're greyed out) when I use the (mildly) compressible solver/option in the flow ribbon while it is available for the incompressible flow. Can someone suggest if this is the limitation of the current solver or if there is some other way to go about it? Any information would be a great help.
Irrespective of the multispecies issue with the compressible flow, I thought I would model just a simple box with 2 inlets and 1 outlet of air in AcuSolve. To this extent, I have managed to find my way to the vol. flow rate boundary condition for the inlet condition, but I am unable to find one for the outlet boundary. Can someone please point me to specify the flow rate at the outlet please?
Any thoughts on what boundary condition should i use for Inlet 2 would be helpful too. I was thinking of assuming average pressure for inlet 2 but then if there is any other way to go about it, I would prefer them.

Thanks,

Sid

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acupro

First - you shouldn't be using mildly compressible or compressible, unless you expect transonic or supersonic flows. The standard N-S solver will be used, but with variable density like ideal gas. This will work to Mach 0.7 or so, and probable even higher if there are no shocks. What maximum Mach number do you expect?

This tutorial is a good starting point:
https://help.altair.com/hwcfdsolvers/acusolve/topics/tutorials/acu/acu_4400_intro_sl_r.htm

There might be too many unknowns in your system description. What is the physical real-world situation you are trying to model? Can you describe that in detail - including any images?

Sid_20854

Thank you for the response, Acupro. It seems quite helpful!

I will pen down my approach to the problem as of now:

1. I am using AcuSolve (HW CFD as pre-processor) and Mach No. is expected to be well below 0.7. Based on your recommendation, I will use the 'incompressible' option (that is what I assume you mean by the standard Navier Stokes solver?) with variable density.

2. The physical system is sort of a ventilation problem wherein an (air-)gas (toxic/flammable/harmful) mixture is being diluted by supplying fresh air and is being cleared out of the system via the outlet.

Therefore, the listed (in the original post):

Inlet 1 - represents the supply of fresh air from the fan (at a specific temperature and pressure)

Inlet 2 - represents the inlet of the toxic air-gas mixture (at a specific temperature and pressure)

Outlet - represents the diluted outlet to the environment (a certain level of dilution needs to be achieved before it is safe to release in the environment)

To start with a simple system (without baffles or moving parts to aid mixing/dilution), the schematics of the system is attached with two inlets at the bottom and one outlet at the top

Boundary conditions:

1. Volumetric flow rate at inlet 2 (fresh air) is available along with pressure and temperature data. I can either use this directly (m3/s) or convert it into mass flow rate (kg/s)

2. Volumetric flow rate at Outlet: is available along with the volume fraction of the gas (based on dilution needs). I am not sure if this flow rate can be used as an outlet boundary condition (any thoughts?). I am thinking of using pressure (atm) as the boundary constraint. I can try to convert it into the mass flow rate (kg/s) based on the combined density of the mixture (not sure if it is a homogenous mixture - assumption) and use it to compute the mass flow rate at inlet 2 (outlet - inlet 1)

3. Computed mass flow rate at Inlet 1

If you have any other example of gas transport or air-gas mixing in Acusolve or Simlab (pre-pro), it'll be helpful.

Thanks

acupro

Thank you for the response, Acupro. It seems quite helpful!

I will pen down my approach to the problem as of now:

1. I am using AcuSolve (HW CFD as pre-processor) and Mach No. is expected to be well below 0.7. Based on your recommendation, I will use the 'incompressible' option (that is what I assume you mean by the standard Navier Stokes solver?) with variable density.

2. The physical system is sort of a ventilation problem wherein an (air-)gas (toxic/flammable/harmful) mixture is being diluted by supplying fresh air and is being cleared out of the system via the outlet.

Therefore, the listed (in the original post):

Inlet 1 - represents the supply of fresh air from the fan (at a specific temperature and pressure)

Inlet 2 - represents the inlet of the toxic air-gas mixture (at a specific temperature and pressure)

Outlet - represents the diluted outlet to the environment (a certain level of dilution needs to be achieved before it is safe to release in the environment)

To start with a simple system (without baffles or moving parts to aid mixing/dilution), the schematics of the system is attached with two inlets at the bottom and one outlet at the top

Boundary conditions:

1. Volumetric flow rate at inlet 2 (fresh air) is available along with pressure and temperature data. I can either use this directly (m3/s) or convert it into mass flow rate (kg/s)

2. Volumetric flow rate at Outlet: is available along with the volume fraction of the gas (based on dilution needs). I am not sure if this flow rate can be used as an outlet boundary condition (any thoughts?). I am thinking of using pressure (atm) as the boundary constraint. I can try to convert it into the mass flow rate (kg/s) based on the combined density of the mixture (not sure if it is a homogenous mixture - assumption) and use it to compute the mass flow rate at inlet 2 (outlet - inlet 1)

3. Computed mass flow rate at Inlet 1

If you have any other example of gas transport or air-gas mixing in Acusolve or Simlab (pre-pro), it'll be helpful.

Thanks

I didn't see any attachment.

I think you'll have better success if you consider the two inlets as the knowns - using a standard outflow BC at the outlet. Then adjust the flow rate and concentration at inlet two (over various runs) to get your desired behavior at the outlet.

If you know the flow rate at inlet 1 and the flow rate at the outlet - that gives you the flow rate at inlet 2 - then you're just left with the concentration as the unknown at inlet 2, which you can vary over several runs to see what happens at the outlet.

Sid_20854

My bad! I have attached the image to this response.

Thanks for the response! I will give it a go and see how it pans out.

Sid

ventilation_box.png