Multicomponent, Bouyancy driven flow setup

boolaly · September 10, 2013, 06:25

I am trying to simulate a dispersion of hydrogen from a source into a large space (air, 1 atm, 30degC), bouyancy is selected in the model (bouyancy reference density: 1.18kg/m3 (specified for air at 30degC)).

I am a little confused about the setup.

1. Can I use boussinesq approximation with multicomponent flow? If so how can I activate it?

2. What is the correct input for the material properties; do I use air at 25degC or Air ideal gas, H2 at ideal gas, H2 at STP? In the material properties do I need to modify the material reference temperature to that of my domain?

I need to specify a domain temperature of 30degC, but there is only an option to do this if I select Isothermal heat transfer. Later on I will specify small heat sources so Isothermal is not an option for me.

Many thanks for your help

ghorrocks · September 10, 2013, 07:33

It has been a while since I did a multicomponent simulation so am not completely sure whether you can use a boussinesq approximation with multicomponent flow. I suspect not, but could be wrong.

Personally, in my experience using full density calculation rather than the boussinesq approximation is not much slower - especially as you plan to model temperature anyway. So I would not bother about the boussinesq approximation, and all the validation you will have to do to show the model is appropriate and just use a full compressible flow model. So define air and hydrogen as compressible gases (ideal gases or using a real gas model), set the heat transfer model to total energy and use a multicomponent model for air+H2. The only thing you need for buoyancy in this case is to define the gravity vector and set a reference density.

boolaly · September 10, 2013, 10:42

Thanks for your input Glenn.

Can I specify an incompressible ideal gas and full bouyancy model? Also any idea of setting domain temperature?

Appreciate your help.

ghorrocks · September 10, 2013, 19:22

An ideal gas is compressible - if you make it incompressible it is not an ideal gas any more (but CFX will use the ideal gas laws to define the material properties at the temperature you define).

So use a compressible ideal gas is my recommendation.

The domain temperature is set by the boundary conditions and initial conditions.

September 10, 2013, 06:25	Multicomponent, Bouyancy driven flow setup	#1
boolaly New Member Join Date: Apr 2013 Posts: 3 Rep Power: 13	I am trying to simulate a dispersion of hydrogen from a source into a large space (air, 1 atm, 30degC), bouyancy is selected in the model (bouyancy reference density: 1.18kg/m3 (specified for air at 30degC)). I am a little confused about the setup. 1. Can I use boussinesq approximation with multicomponent flow? If so how can I activate it? 2. What is the correct input for the material properties; do I use air at 25degC or Air ideal gas, H2 at ideal gas, H2 at STP? In the material properties do I need to modify the material reference temperature to that of my domain? I need to specify a domain temperature of 30degC, but there is only an option to do this if I select Isothermal heat transfer. Later on I will specify small heat sources so Isothermal is not an option for me. Many thanks for your help

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September 10, 2013, 07:33		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,697 Rep Power: 143	It has been a while since I did a multicomponent simulation so am not completely sure whether you can use a boussinesq approximation with multicomponent flow. I suspect not, but could be wrong. Personally, in my experience using full density calculation rather than the boussinesq approximation is not much slower - especially as you plan to model temperature anyway. So I would not bother about the boussinesq approximation, and all the validation you will have to do to show the model is appropriate and just use a full compressible flow model. So define air and hydrogen as compressible gases (ideal gases or using a real gas model), set the heat transfer model to total energy and use a multicomponent model for air+H2. The only thing you need for buoyancy in this case is to define the gravity vector and set a reference density.

September 10, 2013, 10:42		#3
boolaly New Member Join Date: Apr 2013 Posts: 3 Rep Power: 13	Thanks for your input Glenn. Can I specify an incompressible ideal gas and full bouyancy model? Also any idea of setting domain temperature? Appreciate your help.

September 10, 2013, 19:22		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,697 Rep Power: 143	An ideal gas is compressible - if you make it incompressible it is not an ideal gas any more (but CFX will use the ideal gas laws to define the material properties at the temperature you define). So use a compressible ideal gas is my recommendation. The domain temperature is set by the boundary conditions and initial conditions.