# Boundary condition help needed in ventilation problem

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 June 21, 2016, 07:41 Boundary condition help needed in ventilation problem #1 New Member   Neha Sinha Join Date: Jun 2016 Posts: 4 Rep Power: 10 Dear Experts, I am analysing a biotechnological equipment( a container) in which ventilation is required in order to reduce the biological contamination suspended in the container volume. I have modelled biological contaminant as an additional variable. Initially the container is filled with Argon with contaminant at certain known ppm level. At t=0, supply of fresh argon starts. Inlet flow rate is fixed at 10lpm. Outlet is maintained at slight negative pressure ( negative half inch of water column). Experimentally I have online sampler that samples the contamination ppm passing through the outlet. In computational model, I have calculated this outlet contamination value by plotting expression areaAve(Contaminant)@outlet. My computational results are close to experimental results. Both are decreasing trend from the same initial ppm value as expected. But there is slight difference in slope, which has resulted in deviation of about 25% (lesser) from the experimental result at the end of half an hour. At inlet, flow sensor is situated near to the pressure regulator(output half kg), I suspect due to compressibility the inlet velocity is more that what I have calculated from the flowmeter value of 10lpm. I changed to Ideal Gas Argon. My inlet pressure and outlet mass flow rate boundary condition is failing. My pressure-pressure boundary conditions are failing( I read somewhere pressure-pressure condition works in compressible flow). Sorry for the long post.... but I am stuck....please suggest

 June 22, 2016, 02:37 Please help in setting boundary condition in compressible flow #2 New Member   Neha Sinha Join Date: Jun 2016 Posts: 4 Rep Power: 10 Sorry I think I didn't put my problem in precise way. Give me one more chance. Regarding the same system, my problem is : 1. If driving force is inlet pressure (here 0.6 kg by pressure regulator), and I have assumed outlet mass flow rate same as inlet ( I have flowmeter at inlet and not at outlet) why my run fails. 2. I do know the pressure at inlet and at outlet, can it be solved for compressible flow(runs are failing). Thanks in advance Neha

 June 22, 2016, 02:45 #3 Senior Member   Maxim Join Date: Aug 2015 Location: Germany Posts: 415 Rep Power: 12 please provide screenshots of your setup, your CCL and your out-file with the complete error message of the failed runs.

 June 27, 2016, 02:29 Boundary conditions in compressible flow #4 New Member   Neha Sinha Join Date: Jun 2016 Posts: 4 Rep Power: 10 Sorry I am afraid I will not be able to upload the screenshots as the simulations are carried out in a lab where there is no internet connection. I am summarizing the problem based on memory. I just needed some expert to confirm that inlet pressure and outlet pressure boundary condition in compressible flow can give correct results. Thanks in advance

 June 27, 2016, 07:06 #5 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,789 Rep Power: 143 Read the section in the documentation on well-posed boundary conditions (CFX Modelling guide, section 2.3.2 "Recommended configurations of boundary conditions"). This will answer your question and give you a useful background in the area.

 June 27, 2016, 07:55 #6 New Member   Neha Sinha Join Date: Jun 2016 Posts: 4 Rep Power: 10 Thanks Sir

 Tags additional variable, cfx, compressible flow, hvac, ventilation