The hot combustion gases of a furnace are separated from the ambient air and its surroundings, which are at 25C, by a brick wall 0.15 m thick.The brick has a thermal conductivity of 1.2 W/m K and a surface emissivity of 0.8. Under steady-state conditions an outer surface temperature of 100C is measured. Free convection heat transfer to the air adjoining the surface is characterized by a
temperature?
T= 100C e=0.8
Ta = 25 C
k = 1.2 W/m -K
7=25* h = 20 W/m2-K Air
Answer: T = 625K
A 15-cm-diameter aluminium ball is to be heated from 80c to an average temperature of 200c. Taking the average density and specific heat of aluminium in this temperature range to be = 2700 kg/m and C p = 0.90 kJ/kg - c, respectively, determine the amount of energy that needs to be transferred to the aluminium ball. Answer: 515 kJ
A 5-m x 6-m x 8-m room is to be heated by an electrical resistance heater placed in a short duct in the room. Initially, the room is at 15c, and the local atmospheric pressure is 98 kPa. The room is losing heat steadily to the outside at a rate of 200 kJ/min. A 300-W fan circulates the air steadily through the duct and the electric heater at an average mass flow rate of 50 kg/min. The duct can be assumed to be adiabatic, and there is no air leaking in or out of the room. If it takes 18 minutes for the room air to reach an average temperature of 25c, find (a) the power rating of the electric heater and (b) the temperature rise that the air experiences each time it passes through the heater. Answer: 4.925kW, 6.2c
The inner and outer surfaces of a 4-m X 7-m brick wall of thickness 30 cm and thermal conductivity 0.69 W/m -C are maintained at temperatures of 26c and 8c, respectively. Determine the rate of heat transfer through the wall, in W. Answer: 1159W
An electrical current of 5A passing through a resistor which has a measured voltage of 6V across the resistor. The resistor is cylindrical with a diameter of 2.5cm and length of 15cm. The resistor has a uniform temperature of 9oc and the room air temperature is 20c. Assuming the heat transfer by radiation is negligible, determine the heat transfer coefficient by convection. Answer: 33.6W/m2.c
