Modeling H2S diffusion...

StefaCT · February 25, 2010, 10:53

Hi there,
I'm trying to simulate a diffusion process of hydrogen sulfide in air.
I have a brick with dimension 3x4x3 and the diffusion starts from the lower face with a mass fraction of 0.005. The air is steady and so there is no velocity inside the brick. There is gravity and a little difference of temperature between upper face and lower face that mean 35°C to 25°C.
I put a velocity inlet boundary (with 0 m/s) on the lower face and a pressure outlet in upper face (with 0 pa gauge pressure). Other faces are wall.
I have probleme in convergence caused by reversed flow on boundaries.
Could anyone help me?
thank you
Stefano

s_fardisi · February 25, 2010, 18:14

Hi
Change the upper face to wall and try again.
A velocity inlet boundary with 0 m/s is the same as a wall. You shouldn't be able to fix the temperature on a pressure outlet boundary.
Try to find out why the flow goes out or comes in? Is it the hot air at 35°C producing a buoyancy driven flow? then why is this boundary right at the outlet? If you want to model a heat transfer dominated buoyancy driven flow, you should model a far distance above the location that is producing the light weight air.

StefaCT · February 26, 2010, 06:01

Hi S_fardisi, thank you for your reply.
The problem is that I have to model the diffusion of h2s inside this brick (3x4x3m). In the real case, this diffusion starts from the lower face of this brick where there is the production of h2s. Here Temperature is 25°C. Inside this brick the air is completely stedy and on the upper face Temperature is 35°C. Substantially it is a well closed at the bottom.
I modeled this situation considering a velocity inlet on the lower face to define the correct value of mass fraction of h2s and the correct temperature and zero velocity. I considered instead a pressure outlet on the upper face with temperature of 35°C and gauge pressure 0 pa. I activated the species transport with inlet diffusion and I tried both inviscid and Rk-e model but I still have convergence problems with reversed flow warning also.
Thank you.
Stefano

StefaCT · February 27, 2010, 16:08

I solved the problem thanks to activating the buoyancy model for density.
I tried both steady and unsteady simulation with and without boussinesq approach with excellent results.
Thanks S_fardisi, you turn on my light when you speak about buoyancy driven flow.
Best regards
Stefano

Uwais · March 2, 2014, 22:20

Hello StefaCT,

Is it possible for you to share how do you model and setup FLUENT for the H2s diffusion?
Your kind help is highly appreciated.
Thank you.

February 25, 2010, 10:53	Modeling H2S diffusion...	#1
StefaCT New Member Stefano UniCT Join Date: Feb 2010 Posts: 4 Rep Power: 16	Hi there, I'm trying to simulate a diffusion process of hydrogen sulfide in air. I have a brick with dimension 3x4x3 and the diffusion starts from the lower face with a mass fraction of 0.005. The air is steady and so there is no velocity inside the brick. There is gravity and a little difference of temperature between upper face and lower face that mean 35°C to 25°C. I put a velocity inlet boundary (with 0 m/s) on the lower face and a pressure outlet in upper face (with 0 pa gauge pressure). Other faces are wall. I have probleme in convergence caused by reversed flow on boundaries. Could anyone help me? thank you Stefano

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February 25, 2010, 18:14		#2
s_fardisi New Member Sina Fardisi Join Date: Feb 2010 Posts: 3 Rep Power: 16	Hi Change the upper face to wall and try again. A velocity inlet boundary with 0 m/s is the same as a wall. You shouldn't be able to fix the temperature on a pressure outlet boundary. Try to find out why the flow goes out or comes in? Is it the hot air at 35°C producing a buoyancy driven flow? then why is this boundary right at the outlet? If you want to model a heat transfer dominated buoyancy driven flow, you should model a far distance above the location that is producing the light weight air.

February 26, 2010, 06:01		#3
StefaCT New Member Stefano UniCT Join Date: Feb 2010 Posts: 4 Rep Power: 16	Hi S_fardisi, thank you for your reply. The problem is that I have to model the diffusion of h2s inside this brick (3x4x3m). In the real case, this diffusion starts from the lower face of this brick where there is the production of h2s. Here Temperature is 25°C. Inside this brick the air is completely stedy and on the upper face Temperature is 35°C. Substantially it is a well closed at the bottom. I modeled this situation considering a velocity inlet on the lower face to define the correct value of mass fraction of h2s and the correct temperature and zero velocity. I considered instead a pressure outlet on the upper face with temperature of 35°C and gauge pressure 0 pa. I activated the species transport with inlet diffusion and I tried both inviscid and Rk-e model but I still have convergence problems with reversed flow warning also. Thank you. Stefano

February 27, 2010, 16:08		#4
StefaCT New Member Stefano UniCT Join Date: Feb 2010 Posts: 4 Rep Power: 16	I solved the problem thanks to activating the buoyancy model for density. I tried both steady and unsteady simulation with and without boussinesq approach with excellent results. Thanks S_fardisi, you turn on my light when you speak about buoyancy driven flow. Best regards Stefano

March 2, 2014, 22:20		#5
Uwais New Member Muhammad Uwais Zuraidi Join Date: Mar 2014 Posts: 2 Rep Power: 0	Hello StefaCT, Is it possible for you to share how do you model and setup FLUENT for the H2s diffusion? Your kind help is highly appreciated. Thank you.