hi how to initialize the volume fraction of secondary phase in volume of fluid method?

You can just initialize one phase and then patch the other(s).

solve/initialize/patch

ok thanks, can we declare boundary conditions on both sides i.e inlet and outlet as one single phase and in between as mixture of both( two phases). if yes , how?

When you set the bc then you have to set it for

- the mixture (values for both phases) - 1´st phase (values just for 1´st phase) - 2´nd phase (values just for 2´nd phase)

You can switch between the panels with the bar on the bottom of the bc - window ( default window -> mixture )

hope that helps