A 25 mm diameter egg roll (k = 1 W/m degree) is roasted with the help of microwave heating. For good quality roasting, it is desired that temperature at the center of roll is maintained at 100 degree Celsius when the surrounding temperature is 25 degree Celsius. What should be the heating capacity in W/m of the microwave if the heat transfer coefficient on the surface of egg roll is 20 W/m degree? a)613.31 k W/m b) 93.31 k W/m c) 6713.31 k W/m d) 213.31 k W/m 3

Analyze the following diagram. What is the heat rate to the left side of the heater? What is the heat rate to the right side of the heater? Tin = 16°C W h = 40- T = -20°C u = 188 m2K 60mm Air 60mm 10mm 6mm 10mm 14mm 6mm 2mm 2mm Blue: Epoxy Grey: Aluminum Green: Fiberglass Yellow: Silicone Orange: Heater Blanket, 2000w Note: Treat the outer left wall as flat plates in external convection. Note: Ignore the thickness of the heater blanket.

The exhaust duct from a heater has an inside diameter of 114.3 mm with ceramic walls 6.4 mm thick. The average k=1.52 W/m· K. Outside this wall, an insulation or rock wool 102 mm thick is installed. The thermal conductivity of the rock wool is k= 0.046+ 1.56 x 104 T°C (W/m · K). The inside surface * = temperature of the ceramic is T1 = 588.7 K, and the outside surface temperature of the insulation is 311 K. Calculate the heat loss for 1.5 m of duct and the interface temperature T2 between the ceramic and the insulation. [Hint: The correct value of km for the insulation is that evaluated at the mean temperature of (T2 + T3)/2. Hence, for the first trial assume a mean temperature of, say, 448 K. Then calculate the heat loss and T2 Using this new T2, calculate a new mean temperature and proceed as before.]

A large slab of copper is initially at a uniform temperature of 90◦C. Its surfacetemperature is suddenly lowered to 30◦C. Calculate the heat-transfer rate through aplane 7.5 cm from the surface 10 s after the surface temperature is lowered.