Boundary condition
I have a 3D transport problem and want to simplied into 2D model. The problem is that there is a small tube (R2) located inside a large tube (R1) (not at center). The small tube will become the cylinderical annulus whose center is the same of large tube.The gas out of annulus will mix with the material at large tube, which also has gas flowing. So when I simplied this problem to 2D, I can only and must only keep large tube and the annulus there. If I keep the small tube, it will seperate the connected area for flow in large tube (Actually in 2D sense, the effect of this small tube should be neglected since it is not 360o symmetric in term of the axis of large tube). So at the top edge of annulus, the upside of this edge should be the wall for the fluid in large tube. The downside of this edge should be velocity inlet, which is determined by the flow rate from small tube and the annulus cross area. I tried lots of methods, but can not figure out the boundary condiction for the edges of annulus.
inlet 1  symmetric axis   ! ! ! ! ! ! !    w  _1_  a  !!!  l  !!! annulus l  !!!      2     ! !!!! ! ! !    !: flow direction the inside one is cylindrical annulus, whose center is as same as large tube. the left edge is wall of large tube, and right edge is the centerline. I seperated the whole domain to 2 faces: one for large tubes without annulus, and the other is annulus. But there is error on set the boundary condition for the edge 2, i.e. the outlet of annulus. I also tried to use a very small dummy face at the top of annus with all other edges are walls, but not work. Anyone can help me? Or I must configure it in 3D. Your help is greatly appreciated. My email is vculiu@yahoo.com Thanks in advance. 
problem scheme (last one not working)
inlet 1  symmetric axis
 !!!!!!!!!!!!!!!!! ................. ... _1___........ ...!!!!........ ...!!!!.annulus ...!!!!........ ... 2........ ................. ................. .................   ! is flow direction. .. dots are nothing but used for format since this forum does not recognize contionuous spacebars the inside one is cylindrical annulus, whose center is as same as large tube. the left edge is wall of large tube, and right edge is the centerline. 
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