2 Replies Latest reply on Aug 21, 2017 8:24 AM by Peter Francis

    Flowrate calculation error

    Lukas Müller

      Hi,

      right now, I am running an internal FlowSim-Simulation, where the air circulates inside a bunch of parallel pipes and then coming back to the fan. I like to have the total flowrate through the fan and all the single flowrates inside the parallel pipes.

      Of course, the total flowrate through the fan must be the sum of all the single flowrates. But the result shows, that there is a difference of round about 25% (the total flowrate through the fan is increased about 25 %).

      I don`t see any net-failures and there is no "leckage".

      So where is this different coming from?

       

      My setup is quite simple: static pressure at the fan-area, and nothing else.

      Goals: surface-goals of flowrate inside the parallel pipes and the fan-area. Global goals (min/max/average) of static pressure

       

      Thanks in advance!

        • Re: Flowrate calculation error
          Amit Katz

          First thing I would do is change the flow rate goals from volume flow rate to mass flow rate. Volume can change with density, but mass does not.

           

          If you are still having an discrepancy with mass flow rates, then there's definitely something wrong. Maybe you have vortices crossing your goal surfaces which are giving your wrong readings.

          • Re: Flowrate calculation error
            Peter Francis

            Hi Lukas,

            Give the following a try , this will ensure that Axial fans I'm assuming they are axial) behave in the correct manner within Flow Simulation

             

            Fan Pressure Type - Pfa free blowing

            REASON - This is because that although fan curves are written as static pressure, most normally the pressure difference is actually calculated using the total inlet pressure and outlet static pressure - this is what the free blowing pressure measures. It is easier to measure the total pressure at the inlet as it is approximately equal to the environment pressure in the suction chamber.

             

            Fan Inlet Geometry - Simplify if possible , but make sure the area calculation is accurate

             

            Fan Outlet Geometry - Simplify if possible , but make sure the area calculation is accurate

             

            Initial Mesh Settings

                Narrow Channel Refinement - 10 cells across a narrow channel (the default is normally 5)

             

            REASON - This will help when the fan outlet is in close proximity to the walls (~ 1/2 Diameter)

             

                Thin Wall Resolution - Turned ON

             

            If using automatic grid settings define the min wall thickness and the min gap distance so that thin wall and narrow channel work correctly. Define them as the smallest dimension for the thin wall and gap that you are interested in, this will ensure the Narrow Channel Refinement and Thin Wall Resolution functions operate correctly.

             

            Goals - Set goals for the fan - for a typical free blowing set the following

             

                Surface goal for total pressure at inlet face

                Surface goal for static at outlet face

                Equation goal for Pfa equation goal using the above ( SG Av Static Outlet Pressure - SG Av Total Inlet Pressure )

                Surface goals for volume flow rate and mass flow rate at relevant points, for easy checking once the model is solved.

               

            Ensure that you check the mass balance in and out of the system ..... it should be the same (or as close as possible )

             

            Hope this helps

             

            Peter