Rarefied Flow through sudden expansion
 

im doing a flow simulation in FLUENT for rarefied flow through sudden expansion channels of circular c/s..its a slip laminar flow..
with 2D axissym model and with mass flow inlet bc and pressure outlet bc..i define press at the outlet and compute the press at the inlet side..but the press which im getting shows a very large varitaion upto 50-60% with the expterimental values..can anybody help me regarding this?? what can be done to get the proper values??
THe mass flow rate is as low as 1e-08kg/s to 1e-06kg/s and pressure values in the range of 10-500 Pa as the flow varies..