what actually is the 'zero pressure outlet b. c.'
 

I am simulating transient flow in a small pipe. I used transient vel as inflow B.C. and tried to use a transient pressure as outlet b. c. but the solver failed to converge.

then I set the outlet pressure to zero, the solver converged. I did another experiment, I set the outflow pressure as 80mmHg, and reference pressure as 120mmHg, again converge was difficult.

Could anyone explain what actually is the 'zero pressure outlet b. c' please?

Umm, you define the pressure and set it to zero. Isn't that obvious?

Is your simulation compressible? Are there other inlets or outlets? Can you describe what you are modelling?

it's about blood flow in a vessel of 3mm in diameter, flow velocity <1m/s.

why should I set the outflow pressure as 0, instead of 80mmHg (much more difficult to converge), which is a much more sensible choice?

You should set the outlet pressure to zero relative pressure. You should then adjust the reference pressure so you get the correct absolute pressure. Is absolute pressure important in your simulation? Is your fluid compressible or have material properties dependent on absolute pressure? If not then the pressure level does not matter.

Usually, similar BC's for inlet/outlet would cause convergence issues. Try using pressure at inlet and flow at outlet, or velocity/pressure, or other combination.