SCHOOL of ENGINEERING  

BENG2 APPLIED THERMODYNAMICS (THER 205)

Heat Transfer 1

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1. Calculate the heat loss rate per square meter through a 150mm thick concrete wall where the inner surface temperature is 21°C and the outer surface temperature is -1°C. Assume the thermal conductivity of concrete is 1.5 W/mK. [220 W/m²]

2. A steel pipe 100mm OD and with a wall thickness of 10mm carries saturated steam at 120°C. It is insulated with a 25mm thick layer of polyurethane foam. Calculate the heat loss rate per meter length of pipe from the inside surface of the pipe to the outside surface if the outside surface temperature is 28°C. How much steam condenses to water every hour because of this heat loss? [49.88 W/m, 0.082 kg/h]

Assume l _steel = 49 W/mK and l _foam = 0.035 W/mK.

3. An 80mm diameter pipe is to be lagged (insulated) with two types of insulant in two layers each 20mm thick. One type of insulant has a thermal conductivity = 0.05 W/mK and the other has a thermal conductivity = 0.072 W/mK.Which insulant should be applied first for maximum insulating effect? [l=0.05 W/mK]

4. A radiant bar heater consumes 1.2 kW of electrical energy. Its surface temperature is 890°C. Find the proportion of the total heat transfer which is by radiation assuming it is in black surroundings at 25°C and its: emissivity, e = 0.95, diameter=10mm, length = 320mm. [82%]

5. The combustion chamber of an operating gas turbine has a surface area of 1.5m², and a surface temperature of 597°C. If the engine is run in a large test house with walls at 12°C, calculate the radiant heat loss from the combustion chamber. Assume the emissivity of the surface of the combustion chamber is 0.18. [8670W]

6. The earth's surface receives 1000W/m² of radiation from the sun when the sun is directly overhead. If 300W/m² is absorbed by the atmosphere calculate the temperature of the sun. Assume the sun is a black body 1.39x10⁶ km in diameter at a distance of 151x10⁶ km from the earth. [5725K]