编程代考 Practical 1: Modes of Heat Transfer

Practical 1: Modes of Heat Transfer
Complete Q1-5 during the practical.
Ensure that you attempt Q3 before the next lecture.
1. Modes of Heat Transfer: Engineers need to be able to consider real-life situations, identify the key processes occurring and describe these mathematically in a manner which allows questions to be answered quantitatively. For each of the situations below:

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 Draw a diagram identifying the transfers of heat
 Is the heat transfer occurring at steady state, or is it unsteady?
 Define the modes of heat transfer operating
 Which mode do you think will be dominant? Why?
 If convection is present, is it natural, forced, boiling or condensing?
Situations:
a) Liquid pitch (30oC) is pumped through an insulated pipeline in a smelter, where air temperature is 5oC and wind speed is 20 kph.
b) Anelectrickettleisboiledtomakeacupoftea.
c) A Brisbane house is uninsulated and unshaded, with a corrugated iron roof. It is
midday, midsummer, and the air conditioner is operating.
d) A motorbike is driven from Gladstone to Brisbane (approx 6 hour journey). The
engine is cooled by air passing over the engine.
e) Cooling water from a power station (25oC) is released into a river (22oC).
f) An operator in a casting house opens a furnace door, and experiences the heat
given off by molten metal at 800oC.
g) Aneskyispackedwithfood,beerandiceforacampingtrip,thenleftinthesun
2. One side of a brick wall is at 40oC, the other side is at 20oC. The wall is 20cm thick. What is the rate of heat transfer through the wall? Thermal conductivity of brick is 0.72 Wm-1K-1.
3. An aluminium and copper block are clamped together. The cross-section of the blocks is 200cm by 200cm. Both plates are 10cm thick. Heat is being transferred through the plates, from the outside of the copper, across the interface and through the aluminium at a rate of 1MW. The outside (hottest) face of the copper block is at 500oC.
a) What is the temperature at the interface between the copper and aluminum? b) Whatisthetemperatureontheoutsidefaceofthealuminium?
(Thermal conductivity of copper & aluminium are approximately 380 and 230 WK-1m-1 respectively in the range 0-500oC)

4. A rectangular black barbeque (BBQ) plate has dimensions 40 cm by 60 cm. In Brisbane, air at 25oC blows over the BBQ, and the convective heat transfer coefficient is 12 Wm-2K-1. If the BBQ plate is maintained at 250oC,
a) What is the rate of heat loss from the plate due to convection?
b) Whatistherateofheatlossfromtheplateduetoradiation?
c) Assuming that all energy input is lost as convection and radiation from the
surface of the plate, what power required to keep the BBQ plate at 250oC?
5. Consider the same BBQ plate from Q4, in Canberra on a windy day. The air temperature is 10oC and above the BBQ plate h=30 Wm-2K-1.
a) Assuming the same rate of energy input to the BBQ, as calculated above, determine the steady state temperature of the BBQ plate in Canberra on a windy day [note: you may need to use a computer to solve]
b) Calculatetherateofheatlossduetoconvectionandradiationunderthese conditions. Compare to the Brisbane BBQ. How will this effect cooking on the BBQ in the two different cities?

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