Consider a $3-m$ diameter spherical tank that is initially filled with liquid hydrogen at $-253C$ and $1$ atm $.$ The tank is exposed

to ambient air at $30C$ with a combined convection and radiation heat transfer coefficient of $50\frac{W}{m^2}.C.$ The temperature

of the thin$-$shelled spherical tank is observed to be almost the same as the temperature of the hydrogen inside. Determine

the rate of evaporation of the liquid hydrogen in the tank as a result of the heat transfer to ambient air if the tank is

a$.$ not insulated

b$.$ insulated with $5-cm-$thick superinsulation which has an effective thermal conductivity of $0\mathrm{.}00005\frac{W}{m}.k.$

Heat of vaporization of liquid hydrogen is $446k\frac{J}{k}g$ at its boiling temperature of $-253C.$