Working with an energymodel

Deranda · May 25, 2016, 05:56

Hi,

I start working with an energymodel for my simulation right now, just to get a boundary temperature for the bai-gosman collision model.
However, if I use the regular molecular weight of air (28.96 g/mol) the calculated density is 0.9 kg/m³. but it should be 8.238 kg/m³ (at 8.5bar and 86°C)
If I calculate the density with the thermodynamic equation of state
p*v=m*R*T
-> density = p/(T*R_L/M)
The calculated density is 8.238 kg/m³. Now I changed the molecular wight of air until the density fits. The result is 246 g/mol. To me it looks like the universal gas constant is different to 8.31 J/(mol*K)

Would be nice if you share your experiences with me

best regards

marmot · May 25, 2016, 06:11

Perhaps before changing MW, when you say density is that the cell-center value, using cell-center temperature, but perhaps you are looking at wall temperature and calculating a density based on this. Just an idea to check this,

Deranda · May 25, 2016, 10:02

well I've chosen 359.15K (86°C) as the inlet, outlet, boundary and initial temperature

marmot · May 25, 2016, 11:25

OK, did you set your reference pressure to 8.5bar and initialize with 0 static pressure, or did you initialize the pressure to be at 8.5bar, leaving the reference pressure at default, this could also be a reason,

May 25, 2016, 05:56	Working with an energymodel	#1
Deranda Member Nils Join Date: Nov 2015 Posts: 59 Rep Power: 10	Hi, I start working with an energymodel for my simulation right now, just to get a boundary temperature for the bai-gosman collision model. However, if I use the regular molecular weight of air (28.96 g/mol) the calculated density is 0.9 kg/m³. but it should be 8.238 kg/m³ (at 8.5bar and 86°C) If I calculate the density with the thermodynamic equation of state pv=mRT -> density = p/(TR_L/M) The calculated density is 8.238 kg/m³. Now I changed the molecular wight of air until the density fits. The result is 246 g/mol. To me it looks like the universal gas constant is different to 8.31 J/(mol*K) Would be nice if you share your experiences with me best regards

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May 25, 2016, 06:11		#2
marmot Senior Member kevin alun Join Date: Sep 2011 Location: Germany Posts: 106 Rep Power: 14	Perhaps before changing MW, when you say density is that the cell-center value, using cell-center temperature, but perhaps you are looking at wall temperature and calculating a density based on this. Just an idea to check this,

May 25, 2016, 10:02		#3
Deranda Member Nils Join Date: Nov 2015 Posts: 59 Rep Power: 10	well I've chosen 359.15K (86°C) as the inlet, outlet, boundary and initial temperature

May 25, 2016, 11:25		#4
marmot Senior Member kevin alun Join Date: Sep 2011 Location: Germany Posts: 106 Rep Power: 14	OK, did you set your reference pressure to 8.5bar and initialize with 0 static pressure, or did you initialize the pressure to be at 8.5bar, leaving the reference pressure at default, this could also be a reason,