What is the diffrent between the BC of symmetry and Pressure Farfield?
 

I want to setup a Film cooling over a flat plate with a row of holes in Fluent. When i set the Bc i wanted to use the pressure far field as a Bc. I let the input on the farfield on default and only got divergence, but when i only change the pressure far field to symmetry, it completely convergent. What is the big difference between those two bcs?

The symmetry boundary condition is like a putting a slip-wall at the boundary. Nothing can cross the wall. It is different than a wall in that it imposes a zero normal gradients constraint for all variables (where as you can have a slip wall with imposed temperature). Nothing flows across a symmetry plane.

The farfield pressure allows flow out and in. It is different than a mass-flow or velocity inlet because you don't specify the mass-flow or velocity. You specify the conditions of the farfield. You also don't impose constraints on the pressure. It takes some math to show that if the stuff continues to flow outward and move along characteristics (or if the stuff flows in from these characteristics), eventually it should reach the farfield conditions (the farfield pressure, temperature, and velocity). The farfield BC doesn't directly impose constraints on pressure or velocity, thus it is a soft/weak boundary condition.

Btw after you get a converged result with the symmetry plane, can you switch to a farfield pressure and get a converged result? If not, you've probably messed up your settings somewhere.