Is the reason you can’t do this because there is only one surface?
If so, you need to open the geometry in SpaceClaim then use Split Body on the Design tab and click on the edges at the ends of each bend. Once you have done that, you will have 13 bodies instead of 1 body. In SpaceClaim, click on the Workbench tab and click the Share button to cause the mesh to be connected across the 12 coincident edges.
Now when you open the geometry in Mechanical, you can select just the six bends and type N to put them in a Named Selection.
Note that you are picking the surface body. The body has two sides: Top and Bottom. When you create a Boundary Condition and use the Named Selection, you will get a chance to specify Top or Bottom. One of those will be the inside of the tube.
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