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Heat Transfer Essay

The University of Sydney Aerospace, Mechanical and Mechatronic Engineering MECH3260 Thermal Engineering Heat Transfer Quiz 3 2007 Time: 40 minutes Answer ONE question only. Question 1 Consider the cylindrical receiver in a solar thermal power plant shown below. The receiver is made of opaque material and has a diameter D = 8m and length L = 14m. At a particular time, the heliostats direct a concentrated solar flux of q”S = 80 kW/m2 onto the receiver. The ambient air has a temperature T( = 300K and there is no wind. Under these conditions, the surface of the receiver has a temperature of Ts = 700K, an emissivity of ( = 0. and a solar absorptivity (S = 0. 8. i) Calculate the heat loss from the vertical surface of the receiver due to convection. Assume effects of curvature are negligible. ii) Calculate the heat loss from the vertical surface of the receiver due to radiative emission. Neglect irradiation from the surroundings. iii) Determine the collector efficiency. iv) If a wind started to blow, what would happen to the surface temperature of the receiver? Would you expect the collector efficiency to increase, decrease or remain the same? Explain your answers. (Note – assume all other conditions remain the same. ) [pic] Question 2

The roof of a refrigerated truck compartment is of composite construction, consisting of a layer of foamed urethane insulation (t2 = 40mm, ki = 0. 03W/mK) sandwiched between aluminium alloy panels (t1 = 6mm, kp = 200W/mK). The length and width of the roof are L = 12m and W = 4m respectively. The temperature of the refrigerated space within the truck is –15? C. The solar absorptivity and the emissivity of the outer surface are (S = 0. 3 and ( = 0. 7. Consider conditions for which the truck is moving at a speed of V = 90km/h, the air temperature is T( = 30? C and the solar irradiation is GS = 900W/m2.

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Assume turbulent flow over the entire roof. Take the average convection coefficient on the inner surface of the roof to be 0. 5W/m2K. i) Derive an equation for the average temperature of the outer surface Ts,o in the form and hence show that Ts,o ( 302K = 29? C. Assume a film temperature of 300K and neglect irradiation from surroundings. (No marks if your equation is not in the above form) ii) Determine the corresponding heat load imposed on the refrigeration system. iii) The average convection coefficient assumed for the inner surface of the roof is relatively low. Explain why this is so. [pic] ———————– [pic] [pic]

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