Rob
Ansys Employee

If the inlet is bubbly then the mass fraction will vary with time, and you'd need to allocate position to the bubble phase to see the bubbles. That won't work with VOF as the model doesn't allow for that. Multifluid VOF (an Eulerian model option) will give more flexibility, but the boundary condition won't be trivial. 

Upstream length needs to be sufficient to prevent the inlet value forcing the solution at the point of interest. With bubbly flow that could be further upstream than you think as the phase regime might try and change in the pipe. Have a look at turbulence damping as that could be important in this case. 

Bubbles aren't distributed as such, you need to set the phase value with location and time. That's probably going to need a UDF, and is going to get complicated.