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June 23, 2013, 10:56 
Simple gas dispersion help

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Hi all,
I am relatively new to CFD. I'm trying to validate a simple gas dispersion model in the presence of an obstacle. It is Thorney Island trial number 26. It is a transient based simulation. The gas is freon 12 (31.6% w/w) and nitrogen (68.4% w/w) and is released instantaneously 50m upwind from a small building. The gas is denser than air with relative density of 2.0. So I have enabled the species model and Ke standard model. I defined a log profile for the wind (Velocity Inlet) and ran a steady state with air only and converged. The wind speed is 1.9 m/s. The side and up boundaries are Symmetry and the outlet is set as Outflow. Then I patched the species to a cylindrical region, 50m upwind of the obstacle. I enabled gravity (9.81 m/s^2) and enabled operating density (default). I ran the transient for 250 seconds with 1 second time step. I measured the concentration at the building (mol fraction of freon12). The shape of the graph looks exactly like the experimental which is good but unfortunately the %v/v of Freon12 is exactly 10 times less than the experimental. For example, the experimental peak is 2% at 50 s. Mine is 0.18% at 50 seconds. I can't understand what's wrong. Do I need to define any more boundary conditions? Edit: It appears that I have now discovered the source of my problem. The mass fraction of Freon I was patching was too low. I know that the total volume of gas released is 2000 m^3. Therefore based on the fact: Density Freon = 1305.8 kg/m3 Density N2 = 1.135 kg/m3 Total volume = 2000 m3 Required relative density = 2 So 1.135 (1x) + 1305.8x = 2*1.225 This gives vol fraction. From there I can calculate the total mass and hence mass fraction. Although before I start the simulation, the highest density in the domain is 2 kg/m3 which is still less than the relative. How is fluent calculating the density? Last edited by masands; June 23, 2013 at 21:48. 

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