$20.$ A gas$-$liquid system in a mini channel $($diameter $=1$ mm$)$ operates with gas and liquid phase velocities of $15$ m$/$s and $2$ m$/$s$,$ respectively. Find the drift velocity and then compute the void fraction using a modified drift flux model where the coefficient is adjusted by $15\%.$

$21.$ For a liquid$-$liquid flow in a pipe of $20$ mm diameter, with the dispersed phase volume flow rate being $0\mathrm{.}001$ m$/$s and the continuous phase flow rate being $0\mathrm{.}004$ m$/$s$,$ first calculate the volume fraction of the dispersed phase. Subsequently, determine the drift velocity of the dispersed phase.