I'm simulating a car park ventilation problem. I understand we need to use the scalar transport simulation in this case.
I have few Inlets for air and CO, and few outlets. I have used the following BCs:
Fresh Air Inlets:
Volumetric flow rate: 500 L/s, scalar=0, temperature=25 DegC.
Pressure = 0 (Gauge)
CO Emission inlets:
Flow rate: 20 L/s, scalar=1, temperature=40 DegC.
For simplicity, I've assumed steady state simulation and my solver setting are:
Incompressible, enabled scalar with diffusion coeffof 0.2 cm2/s (for CO in air at STP), heat transfer with gravity Earth (0,0,-1), Advection scheme 2 (known to be good for scalar transport), KE turbulence model.
My question is regarding assigning the material properties. I have assigned a variable material and have
used piecewise linear variation of density, viscosity, conductivity and specific heat based on the Scalar (0 for air and 1 for CO).
1. how is the properties variation accounted with temperature for buoyancy effects? Density variation with scalar is accounted from the above in momentum equation.
2. is my simulation correct or any BC is missing?
Hope to get answers for the above.
I don't think you will need temperature neither gravity for CO calculation in car parks. CO has almost the same Molar mass as air, and the temperature of 40°C will dissipate at once.
I would suggest using Flow only analysis.
Thanks Riad for the info. To follow up on this, so when do you actually consider temperature effects and buoyancy for these kind of applications. Say if the exhaust CO temeprature is exceeding 70 DegC or more? If so how do you consider the density change with temperature?
You can apply temperature if you want and changing the Air material into Variable to allow density changes, it just adds some more calculations to the solver.
ΔT is the driving force for buoyancy forces, so if your model environment is extremely cold (winter scenario) then you may notice some changes. But take into consideration that the scalar has no density (it just a mixture fraction tracer). So in the output if you want to view a 100 ppm of CO, then set the viewer to >>
Scalar value = f = (100 mol CO * 28.01 g/mol CO) / ((100 mol CO * 28.01 g/mol CO)+(1,000,000 mol air * 28.97 g/mol Air)) = 9.667e-5
Read the general scalar transport equation in the Help file for more information.
Many thanks once again for the detailed explaination. I went with description in the Help where it was suggested to change the material properties variation with scalar value for a two-fluid mixing problem. So that was my initial question regarding density variation with temperature since now the density is allowed to vary with only scalar values.
Your suggestion makes it clearer that particularly for car park problems only flow simulation is good enough.
Just one more clarification regarding the ppm units. The scalar value varies from 0 to 1 (0 for 100% fresh air and 1 for 100% CO). Your suggestion for 100 ppm of CO give scalar value of f=9.6e-5. Is'nt that practically zero and would indicate fresh air? It would really be helpful if you can provide further info on this. Thanks once again.