I am working on a project where I am trying to simulate flow through a venturi mixer.  We have a device that works, but need to improve its performance.  But I can't get Flow Simulation to correctly model it.  I am performing an internal flow simulation.  I am using Solidworks 2011 and Flow Simulation 2011.   The part consists of an orifice where a 50PSI  oxygen gas source is the inlet.  So I set the inlet pressure to Total Pressure 50 lbsft/in2.  It has a large inlet at the bottom of the part that is an atmospheric air inlet.  I left the atmospheric air inlet at default atmospheric pressure.  There is also a mixed gas outlet boundary condition that is set to atmospheric air.  I have tried changing a bunch of parameters including temperature of the inlet gas.  I have set the atmospheric boundary conditions to static pressure or atmospheric pressure. I have changed boundary conditions from laminar to turbulent and vice versa.

Here are the problems that I have encountered.

When I measure the flow of the the gas through the orifice in real life I reach a flow of 11.5 Liters/min.  In the simulation I only reach a flow of around 5.5 LPM exiting the orifice.  This also shows up in my total exit flow appearing much lower.  (I could set the flow rate coming out of the orifice, but that doesn't allow me to simulate the effects of changing the orifice, which I would like to do.)

I am also trying to simulate the volume fraction of oxygen.  Essentially, I want to know what mix ratio I am getting.  It also does not match my actual device.

I first tried to simulate the flow without an exit tube on it.  In our real-life application we have a series of tubes that the gas passes through after it exits our device.  I have simulated the flow with just one of the pieces of that tubing.  It gave me some terrible results.  It shows the gas mixture starting within the device where I have a oxygen concentration that gradually increase back to 100%.  I can't figure out where this extra oxygen is coming from.  From what I know, gas doesn't just unmix itself.  Here is an image of that simulation. (Coincidently, I performed a similar test to this where the long outlet tube seen below was smooth on the inside instead of corrugated like the actual tube). I can't believe that corrugation would really effect as much as the model is showing.  I have tried reducing the mesh, but then it has trouble modeling the gas exiting the orifice.  I tried simulating it with a localized mesh at the orifice and it also didn't work.

I have tried to break down each flow part separately to try to find the problem.

I noticed in one of the forum threads, that another user was running a flow simulation similar to this except with water.  I turns out he had to do an external flow simulation.  Is that my problem? Has anyone else run a simulation similar to this who could help?