Athin electrical heater is wrapped around the outer surface of a long cylindrical tube whose inner surface is maintained at a temperature of 6°c. the tube wall has inner and outer radii of 24 and 78 mm, respectively, and a thermal conductivity of 10 w/m · k. the thermal contact resistance between the heater and the outer surface of the tube (per unit length of the tube) is r ′ t , c = 0.01 m ⋅ k / w . the outer surface of the heater is exposed to a fluid with t[infinity] = −10°c and a convection coefficient of h = 100 w/m2 · k. determine the heater power per unit length of tube required to maintain the heater at to = 25°c.

A3-mm-thick panel of aluminum alloy (k 177 w/m-k, c 875 j/kg-k and ? = 2770 kg/m) is finished on both sides with an epoxy coating that must be cured at or above t,-150°c for at least 5 min. the production line for the curing operation involves two steps: (1) heating in a large oven with air at ts,0-175°c and a convection coefficient of h, 40 w/m2. k, and (2) cooling in a large chamber with air at 25°c and a con- vection coefficient of he 10 w/m2.k. the heating portion of the process is conducted over a time interval te which exceeds the ime required to reach 150°c by 5 min (h = r + 300 s). the coating has an emissivity of ? = 0.8, and the temperatures of the oven and chamber walls are 175 and 25°c, respectively. if the panel is placed in the oven at an initial temperature of 25°c and removed from the chamber at a safe-to-touch tempera ture of 37°c, what is the total elapsed time for the two-step curing operation?