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# Not sure how to define my flow conditions

Question asked by Karlis Banis on Oct 15, 2014
Latest reply on Mar 6, 2016 by Mitch Seymour

Hello, I am trying to simulate a flow bench, simply because I did not find any other way in flow simulation which could provide me with the same or similar CFM readings as a real flow bench.

At the moment it is very simple, it is just a pipe with atmospheric pressure opening at one end, a flow orifice in the middle, and the other end is where the problem lives.

What I need is basically a surface between the atmospheric end and the flow orifice. I need this surface to move volume of air like a fan if you will. And what I need to make sure of is that this "fan" will create a pressure drop of 12 inches of water below atmospheric. I need this exact test pressure at the first section of my bench to proceed with the calculations. This is how a real flow bench would work.

Then I could simply define the other end as environment pressure and let the air from the fan escape.

Some of you might say why build a virtual flow bench when I can just calculate the CFM of my engine head directly from flow simulation. If any of you could tell me why am I getting 25 CFM where I should be seeing 200 CFM, please go ahead and tell me, I will abandon the virtual flow bench project right away. And yes, I DO need CFM values similar to a real bench.

I have attached two pictures, one is my virtual simplified flow bench, and the other one is a CFD analysis of my engine head.

Over here I have highlighted the piston top where I inserted a surface parameter and picked volume flow rate. It showed me 25 CFM. I have valid information that this head reads close to 200 CFM on a real bench.

Here is my flow bench. I have highlighted the surface which needs to emulate a fan. If this is not possible, maybe someone can suggest me how else can I define conditions that would create me 12 inches of water pressure difference from the inlet to the highlighted surface and let the air continue down and out the pipe? Thank you.

Also, I am sorry for this extremely long post but I might have a stupid question. How is it that the mass flow rate is different from one end to the other?

My surface parameters tell me that there are 0.23 pounds of air entering my bench every second, but only 0.20 pounds exiting it. WHERE does the rest mass go? Or is solidworks flow simulation really inaccurate?

Thanks to everyone who stopped by to read my problem. I am desperate of this. I really need it to work so please help me.