Buoyancy reference density for combustion simulation

brajh11 · February 20, 2014, 07:32

I am trying to do a simulation for gas phase combustion in CFX.

I am not sure which is the most adequate Buoyancy reference density for such a kind of flow.

Is Buoyancy reference density must be set up for standard conditions (273 K and 1 atm)?

ghorrocks · February 20, 2014, 18:04

No. The reference condition should be set to a representative condition in your model. If the burner is running at 3 bar and 1000K then use that as the reference condition.

The reference condition is used to reduce round off errors. So you want to make the deviation from the reference condition a minimum across the entire simulation.

JuPa · February 22, 2014, 18:33

This thread is probably a good place to ask:

I'm doing phase change simulations (flash evaporation) with continuous liquid (water) and continuous vapour (water vapour) phases.

Should the buoyancy reference density be close to the density of the continuous liquid (water) or continuous vapour (water vapour)?

ghorrocks · February 23, 2014, 06:35

This is discussed in the documentation. The recommendation is to use the density of the lighter fluid.

brajh11 · February 23, 2014, 07:40

ghorrocks: I know that the Buoyancy reference density should be set to a representative condition in CFD model.

In my CFD model for combustion simulation density of flue gases vary from 1.287 kg/m3 (at 273 K) to 0.234 kg/m3 (at 1500 K). I have tried to define Buoyancy reference density as a expression which is temperature dependent but it is not allowed. CFX said:The parameter 'Buoyancy Reference Density' is defined to be Single Valued but it depends on the field variable: T

Any idea how to fix this problem?

JuPa · February 23, 2014, 11:35

Read up in the theory guide on how CFX deals with buoyancy - ergo I don't think it's a good idea to have it set as a variable which depends on Temperature.

I think the best option to take is set it to the mean density in your domain.

ghorrocks · February 23, 2014, 16:54

If the reference condition is a variable then it is not much of a reference, is it.....

The reference condition needs to be a constant. Use the mean density, as Mr CFD says. If you are worried about it, do a sensitivity analysis - try a range of values and see if it makes a difference. If you have a stable solution it should make no difference.

February 20, 2014, 07:32	Buoyancy reference density for combustion simulation	#1
brajh11 New Member Bostjan Join Date: Apr 2009 Posts: 24 Rep Power: 17	I am trying to do a simulation for gas phase combustion in CFX. I am not sure which is the most adequate Buoyancy reference density for such a kind of flow. Is Buoyancy reference density must be set up for standard conditions (273 K and 1 atm)?

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February 20, 2014, 18:04		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	No. The reference condition should be set to a representative condition in your model. If the burner is running at 3 bar and 1000K then use that as the reference condition. The reference condition is used to reduce round off errors. So you want to make the deviation from the reference condition a minimum across the entire simulation.

February 22, 2014, 18:33		#3
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	This thread is probably a good place to ask: I'm doing phase change simulations (flash evaporation) with continuous liquid (water) and continuous vapour (water vapour) phases. Should the buoyancy reference density be close to the density of the continuous liquid (water) or continuous vapour (water vapour)?

February 23, 2014, 06:35		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	This is discussed in the documentation. The recommendation is to use the density of the lighter fluid.

February 23, 2014, 07:40		#5
brajh11 New Member Bostjan Join Date: Apr 2009 Posts: 24 Rep Power: 17	ghorrocks: I know that the Buoyancy reference density should be set to a representative condition in CFD model. In my CFD model for combustion simulation density of flue gases vary from 1.287 kg/m3 (at 273 K) to 0.234 kg/m3 (at 1500 K). I have tried to define Buoyancy reference density as a expression which is temperature dependent but it is not allowed. CFX said:The parameter 'Buoyancy Reference Density' is defined to be Single Valued but it depends on the field variable: T Any idea how to fix this problem?

February 23, 2014, 11:35		#6
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	Read up in the theory guide on how CFX deals with buoyancy - ergo I don't think it's a good idea to have it set as a variable which depends on Temperature. I think the best option to take is set it to the mean density in your domain.

February 23, 2014, 16:54		#7
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	If the reference condition is a variable then it is not much of a reference, is it..... The reference condition needs to be a constant. Use the mean density, as Mr CFD says. If you are worried about it, do a sensitivity analysis - try a range of values and see if it makes a difference. If you have a stable solution it should make no difference.