课程名称︰热传学 课程性质︰ 课程教师︰马小康 开课学院：工学院 开课系所︰机械系 考试日期（年月日）︰2011/1/11 考试时限（分钟）：100 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : (15%) Problem (1) The variation of the spectral absorptivity of a surface with wavelength is given in the following Figure. Determine the average absorptivity, reflectivity , and emissivity of the surface for radiation that originates from a source at T=2500K. Also, determine the average emissivity of this surface at T=2800K. (λα) │ │ 0.7┼ ┌─── │ │ 0.2┼─┘ └─┴─── 2 λ,(μm) (20%) Problem (2) In Chapter 4, you may solve exact solution of one-dimensional transient conduction problem by the Separation of Variable method. (a) Does the Separation of Variable method be used to slove two-dimensional steady state conduction problem without heat generation? (5%) (b) Please write the boundary conditions and try to solve the general solution. (15%) Assumptions: (1) The f1(x) is not zero, and f2(y)=f3(x)=f4(y)=0 (2) All the heat transfer properties are assuming constan. y │f1(x) b├───┐ f4(y)│ │f2(y) └───┴── f3(x) a x (15%) Problem (3) A large water droplet falls in the geated plate, please (a) plot the heat transfer rate (q/A) of the water droplet with (Tdroplet-T0). (b) plot the life time of the water droplet with Tdroplet. (c) explain the above phenomena. Assume Tdroplet is the water droplet temperatur T0 is the heated plate temperature (15%) Problem (4) You have probably noticed that water vapor that condenses on a canned drink slides down, clearing the surface for further condensation. Therefore, condensation in this case can be considered to be dropwise. Determine the condensation heat transfer coefficient on a cold canned drink at 2℃ that is placed in a large container filled with saturated steam at 95℃ (Table A-9). Assumptions: The heat transfer coefficient relation for dropwise condensation that was developed for copper surfaces is also applicable for aluminum surfaces. [Textbook problem 10-91] (20%) Problem (5) Consider two infinite plates of thicknesses L1 and L2 separated by a very thin electrical heater. The electrical energy dissipated by the heater per unit surface area and per unit time is q". Fluid at temperature Tf is in contace with the surface of each plate. The heat transfer coefficient between plate 1 and the fluid is h1 and between plate 2 and the fluid is h2. The thermal conductivities are k1 and k2. h2 ┌───────┐ ─ │ 2 │ L2 └───────┘ ─ ∞ <------- ／＼／＼／＼／＼ ------->∞ ┌───────┐ ─ │ 1 │ L1 └───────┘ ─ h1 Group A Group B ￣￣￣￣ ￣￣￣￣ 1. q" 9. Tmax (maximum temperature of system) 2. k1 10. Tw1 (interface between 1 and fluid) 3. k2 (k2>K1) 11. Tw2 (interface between 2 and fluid) 4. L1 12. q1" (heat transfer trough 1 to fluid) 5. L2 (L1>L2) 13. q2" (heat transfer trough 2 to fluid) 6. h1 7. h2 (h2>h1) 8. Tf Increase 9.Tmax 10.Tw1 11.Tw2 12.q1" 13.q2" ￣￣￣￣ 1. q" ╴╴╴ ╴╴╴ ╴╴╴ ╴╴╴ ╴╴╴ 3. k2 (k2>k1) ╴╴╴ ╴╴╴ ╴╴╴ ╴╴╴ ╴╴╴ 4. L1 ╴╴╴ ╴╴╴ ╴╴╴ ╴╴╴ ╴╴╴ 7. h2 (h2>h1) ╴╴╴ ╴╴╴ ╴╴╴ ╴╴╴ ╴╴╴ *use S: same or I: Increase or D: Decrease (20%) Problem (6) The radiator in an automobile is a cross-flow heat exchanger (UAs=10 kW/K) that uses air (Cp=1.00 kJ/kg˙K) to cool the engine-coolant fluid (Cp=4.00 kJ/kg˙K). The engine fan draws 30℃ air through this radiator at a rate of 10 kg/s while the coolant pump circulates the engine coolant at a rate of 5 kg/s. The coolant enters this radiator 80℃. Under these conditions, the effectiveness of the radiator is 0.4 . Determine (a) the outlet temperature of the air and (b) the rate of heat transfer between the two fluids [Textbook problem 11-85] --

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