There are a few problems here.
First one is you may not have contact between the two fluid streams. You should have a wall and wall:shadow pair between the zones. As you're talking about add frozen that's DesignModeler so you want to create a multibody part before meshing. To create the geometry go through the tutorials as there are several approaches that are equally valid.
I can't work out whether there's a solid casing around the larger channel or not.
Next problem is the mesh. You need more resolution in the zones.
Finally, for the CFD part, you need a longer inlet & outlet for the larger channel. Have a look at the Learning courses on here, but you really want to move the inlet & outlet to some diameters from the area of interest.