The reflectivity of aluminum coated with lead sulfate is 0.35 for radiation at wavelengths less than 3 mm and 0.95 for radiation greater than 3 mm. Determine the average reflectivity of this surface for solar radiation (T < 5800 K) and radiation coming from surfaces at room temperature (T < 300 K). Also, determine the emissivity and absorptivity of this surface at both temperatures. Do you think this material is suitable for use in solar collectors?

Consider a cuboidal furnace (l = 5 m, b = 4 m, h = 3 m), whose surfaces closely approximateblack surfaces. The base, top, and side surfaces of the furnace are maintained at uniformtemperatures of (1040) K, (840) K, and (540) K, respectively. Determine (a)the net rate of radiation heat transfer between the base and the side surfaces, (b) the net rate ofradiation heat transfer between the base and the top surface, and (c) the net radiation heattransfer from the base surface.   Steam in a heating system flows through tubes whose outer radius is r1 = (3) cm and whosewalls are maintained at a temperature of 127 °C. Circular aluminum fins (k = 180 W/m K) ofouter radius r2 = (6) cm and constant thickness t = 1 mm are attached to the tube. The spacebetween the fins is 3 mm. Heat is transferred to the surrounding air at 27 °C, with a combinedheat transfer coefficient of 60 W/m2 K. Determine the increase in heat transfer from the tubeper meter of its length as a result of adding fins

A 5 mm-thick steel firewall is suddenly exposed to a radiant heat source that can be approximated as a blackbody at 1000K. How long will it take for the surface to reach 500 K if the initial temperature of the wall is 300K?