5 Replies Latest reply on Jan 2, 2015 12:34 PM by Jared Conway

    Slip Flow in Microchannels-Solidworks Flow Simulation

    Andrew Price

      Good day everyone,

       

      I'm working on simulating a rectangular Microchannel in Solidworks flow simulation. This channel is about 0.001" tall and 0.02" wide and 0.2" long. My flow path mesh is very detailed, # cells LengthXWidthXHeight (100x50x50). I have ran N2, He and Ar (built in, Ideal Gas). I always receive an error if I do not choose ideal gas since I'm outside the pressure range (P<Pmin). I'm looking for mass flow rate over a variety of Inlet and Outlet pressures (both environment pressures). Right now I'm focusing on one back pressure (50 torr) and varying the inlet pressure to find mass flow. Increasing mesh density results in less flow.

       

      Inlet Pressure Range

      300 torr to 2000 torr

       

      Outlet Pressure Range

      10 torr to 400 torr (currently just 50 torr)

       

      Temp

      25C

       

      Surface Roughness

      0.125 micrometer (global)

       

      Gases

      N2, Ar, He (all ideal gas)

       

      Flow in all simulations is lower than experimental (anywhere from 5% to 40%). My main concern is that He is flowing less than N2, which is not correct.

       

      I'm thinking this has something to do with slip flow. I have done my Knudsen calculations and we are in the slip flow regime. Is there a way to tell the flow simulation to have slip flow besides "ideal wall". Ideal wall gives extremely high flow. Will making the surface roughness close to zero (~0.000001 micrometer) give me more of a slip flow calculation? I think "real wall" means that it will be a no-slip assumption.

       

      This may just be a limitation of flow simulation, but I was wondering if anyone found a way. I might just have to play with the surface roughness and add in a real gas model for these gases.

       

      On a side note, Is it possible to speed up convergence by creating a two fluid subdomains? For example, before my flow path have it start at 1000 torr and then at the outlet of the flow path have it start at 50 torr? Otherwise the model spends a lot of time just filling up the inlet volume..

        • Re: Slip Flow in Microchannels-Solidworks Flow Simulation
          Jared Conway

          in reverse

           

          you can create local initial conditions to start the solution at a particular value in different locations

           

          can you elaborate on slip flow? ideal wall is no slip, roughness can add roughness, the only other reference to slip flow that i've seen is in non neutonian fluid definition

           

          you mention you're getting an error, is that when you are using real gas? is there a reason you want to use ideal gas?

            • Re: Slip Flow in Microchannels-Solidworks Flow Simulation
              Andrew Price

              The legendary Mr. Conway, thank you for the response.

               

              I will play with the fluid sub-domain more to figure out partial domains with different initial conditions. It usually selects the entire fluid domain when I select a lid (inlet/outlet BC) since they are connected via micro channel. The ability to have the Inlet pressure side start at 1000 torr and the outlet pressure side start at 50 torr would save a lot of time.

               

              Ideal wall has slip flow but it also doesn't have the appropriate pressure drop along my flow channel. No-slip is a reasonable assumption at certain pressures and gases, but when the molecules of gas collide more with the wall than other molecules they tend to 'bounce' or 'roll' along the wall instead of 'stick' thus resulting in higher flow through a microchannel. Calculating the Knudsen number, Kn= (small lambda)/L; where small lambda is the fluid's mean free path and the L is the physical length scale of the flow channel(in my case the hydraulic or effective diameter ~=2*(height of channel)). The mean free path for each gas can be approximated by  Kb*T/(sqrt(2)*pi*d^2*p)   [from left to right: Boltzmann Constant, Temperature, hydraulic diameter and pressure]. From these calculations I'm in the slip regime of navier-stokes and flow simulation's no-slip results are lower than expected. I suspect the no-slip assumption to be the reason. I may be able to decrease the surface roughness until the flow matches.

               

              I receive an error/warning when using the built-in Nitrogen 'real gas'. Which states "Real gas parameters (pressure and/or temperature) are outside of the definitional domain of substance properties. P<Pmin"

              I just started running the real gas N2 again so I will let it run over night and see if the warning goes away.

              I only use ideal gas because it doesn't give me any warnings and partly because there are more gases available.

                • Re: Slip Flow in Microchannels-Solidworks Flow Simulation
                  Jared Conway

                  I will play with the fluid sub-domain more to figure out partial domains with different initial conditions. It usually selects the entire fluid domain when I select a lid (inlet/outlet BC) since they are connected via micro channel. The ability to have the Inlet pressure side start at 1000 torr and the outlet pressure side start at 50 torr would save a lot of time.

                   

                  >you may need to create bodies to apply the local initial conditions to, sorry, should have been more clear about that

                   

                  I receive an error/warning when using the built-in Nitrogen 'real gas'. Which states "Real gas parameters (pressure and/or temperature) are outside of the definitional domain of substance properties. P<Pmin"

                  I just started running the real gas N2 again so I will let it run over night and see if the warning goes away.

                  I only use ideal gas because it doesn't give me any warnings and partly because there are more gases available.

                   

                  >see the technical reference, real gas has limits, you need to work within them or use an ideal gas.

                   

                  Ideal wall has slip flow but it also doesn't have the appropriate pressure drop along my flow channel. No-slip is a reasonable assumption at certain pressures and gases, but when the molecules of gas collide more with the wall than other molecules they tend to 'bounce' or 'roll' along the wall instead of 'stick' thus resulting in higher flow through a microchannel. Calculating the Knudsen number, Kn= (small lambda)/L; where small lambda is the fluid's mean free path and the L is the physical length scale of the flow channel(in my case the hydraulic or effective diameter ~=2*(height of channel)). The mean free path for each gas can be approximated by  Kb*T/(sqrt(2)*pi*d^2*p)   [from left to right: Boltzmann Constant, Temperature, hydraulic diameter and pressure]. From these calculations I'm in the slip regime of navier-stokes and flow simulation's no-slip results are lower than expected. I suspect the no-slip assumption to be the reason. I may be able to decrease the surface roughness until the flow matches.

                   

                  >your workaround sounds reasonable but it sounds like you should be making sure that you aren't breaking the continuum requirement of flow simulation. should be a good article on this in the solidworks KB. nothing can be done about that unfortunately, inherent functionality.

              • Re: Slip Flow in Microchannels-Solidworks Flow Simulation
                Andrew Price

                Thank you. Can you supply a link to the KB article on continuum?