11 Replies Latest reply on Dec 2, 2014 2:48 AM by Jared Conway

    How exactly does SW Flow predict fan swirling, and How does the user replicate this

    Hayden Krause

      I am trying to estimate the swirling effect of an internal fan.  I have questions on two aspects of this:

       

      1. What is the free body diagram of a particle of air leaving the fan when swirl is defined?
      2. How does one predict the fans swirling effect without experimental or complex CDF analysis?

       

       

      I have answered both, however my answers are incomplete and/or may be incorrect and need checking.  So below is what I know for each question – please comment below, provide links, share experience. 

       

      1.   What is the free body diagram of a particle of air leaving the fan when swirl is defined?

       

       

      The image below defines the velocity components acting on one particle of air leaving the face of the fan (defined as a boundary condition).  Relating this image to SW Flow sim.  The fluid exiting the fan will have a normal component (defined by the fan curve definition).  If a user wants to add swirling effects he/she can do so by adding the angular velocity (omega) and radial velocity (Vr) components.  Angular velocity can be converted from tangential by Vt = rw.

       

      FBD.png

       

      2. How does one predict the fans swirling effect without experimental or complex CDF analysis?

       

      So far I have a simple method for calculating swirling but I don’t think it is very good.  As far as I am aware, SWFlow Sim makes its swirling predictions using this method (You can get SWFlow to make the swirl predictions for you if you setup your fan using “Axial” definition instead of “Fan Curve” definition - though precicely what is happening is hidden from the user).  The following equations are used to calculate Angular Velocity (Omega).

       

      EQ.png

       

      I have attached a spreadsheet with this calculation made for a 40x40x10 DC fan.  All information is provided except efficiency data for the fan.  This can be estimated to be between 10% and 30% accoring to sources I have found (but I am not sure). 

       

      I am not sure what efficiency this relates to (electrical, mechanical ....).

       

      I am sure this is not the complete picture for a good estimation of swirling because this does not predict radial velocities.  If anyone can elaborate on what I have got - that would be great.  Or indeed if they can correct me - then please feel free!  I realise that this must be a crude method of estimating fan swirl, but this coupled with a sensitivity analysis should be enough for most people. 

       

      Any papours on this subject would also be good if any1 knows of any. 

       

      Hayden

        • Re: How exactly does SW Flow predict fan swirling, and How does the user replicate this
          Jared Conway

          what exactly are you trying to do? and how much effect do you think the swirl is going to have on your analysis? seems like like a lot of effort for little return but since you didn't describe your goal, its hard to answer.

           

          a couple of thoughts:

          1. have you checked your assumptions in flow? you should be able to get the components of velocity to test your theory

          2. in the long run, physical tests are going to be needed. is that in your plan for this project?

          • Re: How exactly does SW Flow predict fan swirling, and How does the user replicate this
            Bill McEachern

            While I appreciate the rigor you might be over thinking this one, though that depends on what you are after. In my expereicne the swirl dies out after a couple rotor diameters so you might want to consider that, convince yourself (don't believe me). Check out the fan models in the engineering database. Some of the modesl have an RPM designations and I believe it is used to designate the swirls at pretty much the design speed. I have the electronic cooling module and it has a lot more fan models than the regular instal so you may or may nothave them. Still you can use the models and simulate fans off a data sheet or a fan you know.

            • Re: How exactly does SW Flow predict fan swirling, and How does the user replicate this
              Eric Ryan

              I am glad I am not the only one perplexed by the fan swirl settings.  I have experimented with it in my flow studies.  I have gleened a basic understanding from tutorials, the help files, and comparing to the Axial fan settings, but  have wanted to get a better understanding of how these settings are intended to be used.  It is also interesting to learn how others are using it. 

               

              The effect of the swirl seems to depend on how your model is set up.  I have seen a few technical papers that looked at internal electronic enclosures where the fan was blowing cooling air over heat sinks or a set of circuit boards.  Mostly they just did CFD studes with and without swirl, then compared the results to lab tests.   I don't recall seeing anything that explained how they calculated the swirl.  If I remember right, they were using Flowtherm for a couple of them.

               

              The fan swirl definitely has an effect in my studies. They are also of an electronic enclosure where the internal air flows around and through a variety of components.   Using swirl seems to more closely match our experimental tests. 

               

              Sorry I can't add more to the explanation.  I am definitely not a flow expert.  Maybe someone from Solidworks can add to the discussion, if they actually understand it.

               

              Good luck,

               

              Eric

              • Re: How exactly does SW Flow predict fan swirling, and How does the user replicate this
                David Paulson

                Hayden,

                 

                I would understand from your post that you are using a pre-defined fan in your simulation and are trying to set the swirl parameter to make the simulation more reasonable.  I would suggest that you model the actual axial fan impeller that you are using and then use a rotating region to create the swirl that the fan develops.  Following is one that I did several years ago using a Multi-Wing 7Z axial fan of about 26 inch diameter:

                 

                Fan Flow Test 3 (4).jpg

                From the above you can see the swirl as the air leaves the fan, but it dissipates after one fan diameter or so.  As there is no input for swirl using this approach, I would think that only the fan geometry could produce the result shown.