$2)$ Air $(\backslash$rho $=1\mathrm{.}22$ kg$/$m$3,=1\mathrm{.}9$x$10-5$ Pa$.$s$)$ flows in a fixed bed with a diameter of $1$ m and a length of $3$ meters. The fixed bed is filled with spherical solids with a diameter of $5$ mm and a density of $1200$ kg$/$m$3.$ If the bulk density inside the bed is $800$ kg$/$m$3$ and the pressure drop across the bed is $120$ kPa, calculate the air flow rate.