# Slip Flow in Microchannels-Solidworks Flow Simulation

Question asked by Andrew Price on Dec 15, 2014
Latest reply on Jan 2, 2015 by Jared Conway

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..