ejector for purging system of fired furncae

wadoud31 · November 28, 2018, 11:10

Hi all,

I am trying to simulate an existing device(ejector) used to purge an fired furnace, but the issue is how we must define the boundary conditions.
I tried the boundary conditions below:
Reference pressure: 1 atm
Inlet of ejector: static pressure 7 bar and velocity 48 m/s
In the bottom of the furnace I tried 2 options:
1 inlet of furnace with a static pressure 0 atm
2 as opening with a static pressure 0atm
Out of the furnace I tried 2 options:
1 outlet of furnace with a static pressure 0 atm
2 as opening with a static pressure 0atm
The simulation did not converge.
NB: the geometry and the conditions is like the site (real case).
I need your help and your advice; thank you in advance.

best regards,

wadoud31 · November 29, 2018, 11:28

Hi all,

after trying a lot of different setups, it seems that I need some help with the boundary conditions of my problem. I'm looking for a stable (steady state?) solution
I am using ANSYS CFX 14.5.
thanks, I am very grateful

AliE · November 29, 2018, 18:45

Hi, since nobody is replying, I will attempt an answer for this.

First of all you should post you question in cfx section here in cfd-online forum. Somebody skilled with the software can be helpful.

In my opinion your problem is that you are trying to fix both the pressure and the velocity at inlet. Up to my knowledge you can do 3 things:
-specify the velocity and leave the pressure free to adjust using zero gradient (known as velocity inlet)
- fix the pressure and the velocity direction (not the magnitude), in this case velocity will be calculated in response to the pressure assigned
-fix the total pressure and the velocity direction

Try if one of these works for you.

wadoud31 · December 1, 2018, 09:34

dear ALIE, thanks for your advice, I try this simulation to know the mass flow rate of air at the bottom in the furnace, I used the knonw boundary conditions wich are:
1 the speed, mass flow rate and the pressure at the inlet of the ejector.
2 at the bottom of the furmace (atmospheric openning), we have only the atmospheric pressur, normaly is the inlet of atmospheric air.
3 at th top (duct) of furnace is the same (atmospheric openning), normaly is the exit (out).
at the and of this simulation we must confirm the mass flow rate at the bottom;

thank you very much

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November 29, 2018, 11:28		#2
wadoud31 Member wadoud Join Date: Nov 2010 Posts: 67 Rep Power: 15	Hi all, after trying a lot of different setups, it seems that I need some help with the boundary conditions of my problem. I'm looking for a stable (steady state?) solution I am using ANSYS CFX 14.5. thanks, I am very grateful

November 29, 2018, 18:45		#3
AliE Senior Member Join Date: Dec 2017 Posts: 153 Rep Power: 8	Hi, since nobody is replying, I will attempt an answer for this. First of all you should post you question in cfx section here in cfd-online forum. Somebody skilled with the software can be helpful. In my opinion your problem is that you are trying to fix both the pressure and the velocity at inlet. Up to my knowledge you can do 3 things: -specify the velocity and leave the pressure free to adjust using zero gradient (known as velocity inlet) - fix the pressure and the velocity direction (not the magnitude), in this case velocity will be calculated in response to the pressure assigned -fix the total pressure and the velocity direction Try if one of these works for you.

December 1, 2018, 09:34		#4
wadoud31 Member wadoud Join Date: Nov 2010 Posts: 67 Rep Power: 15	dear ALIE, thanks for your advice, I try this simulation to know the mass flow rate of air at the bottom in the furnace, I used the knonw boundary conditions wich are: 1 the speed, mass flow rate and the pressure at the inlet of the ejector. 2 at the bottom of the furmace (atmospheric openning), we have only the atmospheric pressur, normaly is the inlet of atmospheric air. 3 at th top (duct) of furnace is the same (atmospheric openning), normaly is the exit (out). at the and of this simulation we must confirm the mass flow rate at the bottom; thank you very much