8 Replies Latest reply on Sep 24, 2013 7:50 AM by Sjoerd Post

    Trouble with inlet vs outlet flow

    Sjoerd Post

      Hello,

       

      Currently i am trying to simulate a airconditioning in a room. I am interested in a couple of paramaters such as velocity at certain points.

       

      I made this setup to start a flow simulation:

      A internal analysis.

      fluid:     Air     Pre-defined

      I kept the other parts of the wizard as they are.

       

      Boundary conditions:

      Inlet:     Volume flow      Uniform     0.033333m³/s     (120m³/h)

      Thermodynamic parameters:     Only changed the temperature to 289.15 K

       

      Outlet flow:     Pressure opening     Enviroment pressure

       

      I am interested in the velocity in a couple of points in the air conditioning box.

      I calculated this by hand and i did not get the same value as SolidWorks gave me.

       

      When i make point parameters on the outlet LID, the velocity is not the same as on the the inlet LID.

      Because the LID's are from the same diameter, i supposed the volume flow of the inlet was the same as the volume flow of the outlet.

      Assuming there Solidworks is calculation without leakage this is supposed to be the same volume flow.

       

      Can somebody maybe tell me what i am doing wrong here?

      Maybe i am thinking in a whole wrong direction.

       

      Attached is a picture of the current situation.

       

      Already a big thanks for helping!

        • Re: Trouble with inlet vs outlet flow
          Chris Michalski

          How much different are the inlet and outlet flow?  Solidworks bases calculations on mass so it verifies the inlet and outlet mass are the same.  If the pressure changes the density it will induce some difference between the inlet and outlet.

            • Re: Trouble with inlet vs outlet flow
              Sjoerd Post

              I have tried it with inlet volume flow and with inlet mass flow.

              In both situations i kept the outlet as a environment pressure.

              When i specify the outlet as a mass flow outlet, with the same properties as the inlet, i get the following error when running the simulation:

              It is not possible to calculate the internal task, if there are mass/volume flow openings but no pressure boundary conditions defined.

               

              Maybe it is an easy answer, but i am new in the world of flow simulations

                • Re: Trouble with inlet vs outlet flow
                  Chris Michalski

                  You cannot define the inlet and outlet flows (either volume or mass) because Flow needs to iterate to determine one of them through a mass balance.

                   

                  And if you look at the "surface parameters" for the outlet, it doesn't sum to the same mass flow as the inlet? 

                    • Re: Trouble with inlet vs outlet flow
                      Sjoerd Post

                      You are correct, it does sum the same mass flow.

                      The difference in pressure is only 2.56 Pa!

                      I assume this low difference wil not cause the difference in speed?

                       

                      I noticed the surface area [m^2] of the outlet is 0.0215  while de diameter is 100 when i measure this with the evaluate tool.

                      At the inlet the surface area is, as it supposed to be, 0.0075 m².

                        • Re: Trouble with inlet vs outlet flow
                          Chris Michalski

                          You may need to refine the mesh at the outlet so that it more accurately determines the area.  If it has a coarse mesh it will not accurately resolve the cross-section and as a result the velocities will be inaccurate.

                           

                          When you look at the mesh is it the same size and shape as the exit opening?

                            • Re: Trouble with inlet vs outlet flow
                              Sjoerd Post

                              Project.bmp

                              This is a picture of the situation right now. I noticed at the inlet, the meshing is definetly finer!

                              The mesh at the outlet doesn't have any difference from the rest of the meshing in the room.

                              How do you refine a mesh? With Solidworks Simulation i know how to refine a mesh, but i don't seem to find a option like this in flow simulation?

                                • Re: Trouble with inlet vs outlet flow
                                  Jared Conway

                                  1. i would recommend posting your model, it will make troubleshooting easier

                                  2. on mesh refinement, look at the tutorial on mesh refinement, it will show you a LOT of the options that are available

                                  3. mass flow rate is conserved so that is a good sign, if your model doesn't match reality, i would recommend posting a description of you problem, how you went about calculating, what assumptions are inherent to those equations and calculations and then your problem setup and the assumptions that are inherent there. overall, the software is validated, you can see that in the validation examples. when simulation doesn't match real life, it is almost always bad assumptions or boundary conditions. don't just look at individual locations, look at the solution as a whole and also look at multiple parameters. if youre still concerned, run some variance tests to see how the system behaves. if the trends are correct, it is almost likely boundary conditions or assumptions that are bad.

                                    • Re: Trouble with inlet vs outlet flow
                                      Sjoerd Post

                                      Thanks a lot guys!

                                       

                                      The problem i found was due to the refinement of the mesh. I did refine it at the outlet and i noticed the surface parameters of the volume flow came almost the same as the inlet flow.

                                      Right now i am refining the mesh until i got the results i am fine with.

                                       

                                      Thanks a lot! I would set this question as answered!