What do you understand by thermal resistance? What are the units for thermal resistance? b) What is contact resistance? Give one example of contact resistance in practice. c) What is the overall heat transfer coefficient? What are its units and where is it used? d) What is the basis for the existence of a critical radius of insulation? How does the thermal conductivity and convection coefficient affect its value? e) Show that for a spherical shell of inside radius 1 and outside radius 2, with the convective heat transfer coefficient h on the external surface, the critical radius is given by = 2