backflow volume fraction in pressure outlet
 

in multiphase modeling, the backflow volume fraction should be defined at the pressure outlet, Is it right that the value being set as "1" means the second phase total reflux?
what about the value being set as "0.5"?
What should we do if we want exit of one of species in outlet?

You should set it to what is the most realistic for your problem. If you have, say, a column with a gas-filled headspace, then backflow should be 100% gas, Just regard it as an additional inlet - in case the flow would be inward here, what would be flowing back in?