What do you mean by slit pore? If you mean parallel plates, my previous answer still holds:
>> Is there a way to use lb fluid in slitpore? While I tried this the simuation gave error that lbboundary does not accept the slitpore parameter.>Yes. Read the User's Guide about lbboundary wall
My suggestion to implement your boundary geometry in the C code was meant for more complicated geometries.
>> Is there a way to simulate lb fluid in only specific custom defined boundaries? Like a U-tube.
>Yes. You might be able to construct your channel from several cylinder boundaries. If not you would have to implement a distance function for your boundary shape in the C code.