A thick wall made of refractory bricks (k = 1.0 W/m·K and α = 5.08 × 10-7 m2 /s) has a uniform initial temperature of 15°C. The wall surface is subjected to uniform heat flux of 20 kW/m2 . Using EES (or other) software, investigate the effect of heating time on the temperature at the wall surface and at x = 1 cm and x = 5 cm from the surface. Let the heating time vary from 10 to 3600 s, and plot the temperatures at x = 0, 1, and 5 cm from the wall surface as a function of heating time.

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.]

2. The wall of a furnace is made up of a material which has thermal conductivity of 1.65 W/m•C and a thicknesses 250 mm. The inner side of the furnace wall is exposed to hot gases at 1000 °C with a convection coefficient of 25 W/m2°C and the inside the surface is at 800 °C. The air outside the furnace is at 25°C with a convection coefficient of 12 W/m2°C. a. Draw a clear diagram representing the system b. Determine the temperature of the outside surface of the furnace C. The rate of heat loss from the furnace.

Example • The cylinder is made from aluminum with diameter and thickness of 5 cm and 1 cm respectively. The wall temperature in the inside of cylinder is 50°C, while the temperature of the air is 30°C. the coefficient of convection in the air is 10 W/m2-C. Calculate the heat loss from the cylinder per unit length! • Calculate the thickness of the insulation so heat loss from cylinder will decrease 50% of initial condition. Thermal conductivity of the isolator (k) : 0.5 W/m°C state.pptx 7 MB VIEW Tampilkan semua