Question

A laboratory drying oven has a composite wall made of Plane Wall 1 and Plane Wall 2 as shown above. A thin film heater (surface heat source, thickness negligible) is sandwiched between the two plane walls 1 and 2. Plane Wall 1 has a thickness L1 = 0.02 m and a thermal conductivity k1 = 0.05 W/m·K, while Plane Wall 2 has a thickness L2 = 0.01 m and a thermal conductivity k2 = 0.1 W/m·K. During steady state operation, the oven air maintains a constant temperature T∞,2 = 60 ˚C, while the heater layer is controlled at a temperature of TH = 90 ˚C. The drying oven is put inside a room where the ambient air temperature T∞,1 = 30 ˚C, and the heat transfer coefficient between the outer surface of the Plane Wall 1 and the ambient air h1 = 3 W/m2 ·K. First, assume the heat transfer coefficient between Plane Wall 2 and the oven air is h2 = 3.5 W/m2 ·K. For simplicity, assume that the drying oven has 4 side walls (each is 0.5 m long and 0.5 m high), and both the top and bottom surfaces of the oven are perfectly insulated. Consider the oven air and ambient air as an ideal gas with density ρ = 1.13 kg/m3 , specific heat capacity cp = 1000 J/kg·K, kinematic viscosity v = 15x10-6 m2 /s, thermal conductivity ka = 0.027 W/m·K, volume expansion coefficient β = 3.4x10-3 (1/K) and a Prandtl number of Pr = 0.71. Contact resistances do not occur. Additionally, all effects of thermal radiation may be neglected.

Answer 1