I have a cylindrical volume of water. I want to define a smaller cylinder (hypothetical) inside this volume just to define a higher thermal condition inside it uptil a certain time (eg. t=6000 secs) (By an UDF to define this smaller vol as an energy source. At t>6000 I want to stop this source and see the transient temperature profile.

For this I created the larger volume and then subtracted (and retained) the smaller one. I meshed these 2 volumes.

Please note that both the volumes are basically the same continuum, but with different thermal conditions. The faces for the smaller volume are therfore defined as interior.

I exported the mesh to FLUENT 5.3 But it gives error that the faces of the smaller volume cannot be changed to interior ( because there is only one adjacent cell zone).

But I carved the smaller vol out of the larger one. How come there is only one adjacent cell zone?

If you are using gambit, probably you have 2 coincident surfaces; if you can't delete one of these try with merge faces (virtual forced). Bye.

You may instead of subtracting want to split the big cylinder using the smaller cylinder as the split tool.

It worked. Thanks.