4. Liquid hydrogen is flowing through an insulated metal pipe (k=25W/mK, inner diameter D1=2 cm, outer diameter D0=3cm, length L=10m ), where the ambient air temperature is 25C. The convective heat transfer of liquid hydrogen and ambient air is 200 and 55W/m2.K, respectively. Find the thickness of the pipe insulation (k=0.4W/m.K) in order to keep liquid hydrogen temperature at 310C. Outer temperature of the pipe (between the pipe and the insulation) is 0C