传热学习题解答11Word文件下载.docx

1、 which is a massless, colorless, odorless substance. It was abandoned in the middle of the nineteenth century after it was shown that there is no such thing as the caloric.1-4C The rating problems deal with the determination of the heat transfer rate for an existing system at a specified temperature

2、 difference. The sizing problems deal with the determination of the size of a system in order to transfer heat at a specified rate for a specified temperature difference. 1-5C The experimental approach （testing and taking measurements） has the advantage of dealing with the actual physical system, an

3、d getting a physical value within the limits of experimental error. However, this approach is expensive, time consuming, and often impractical. The analytical approach （analysis or calculations） has the advantage that it is fast and inexpensive, but the results obtained are subject to the accuracy o

4、f the assumptions and idealizations made in the analysis. 1-6C Modeling makes it possible to predict the course of an event before it actually occurs, or to study various aspects of an event mathematically without actually running expensive and time-consuming experiments. When preparing a mathematic

5、al model, all the variables that affect the phenomena are identified, reasonable assumptions and approximations are made, and the interdependence of these variables are studied. The relevant physical laws and principles are invoked, and the problem is formulated mathematically. Finally, the problem

6、is solved using an appropriate approach, and the results are interpreted.1-7C The right choice between a crude and complex model is usually the simplest model which yields adequate results. Preparing very accurate but complex models is not necessarily a better choice since such models are not much u

7、se to an analyst if they are very difficult and time consuming to solve. At the minimum, the model should reflect the essential features of the physical problem it represents.Heat and Other Forms of Energy1-8C The rate of heat transfer per unit surface area is called heat flux. It is related to the

8、rate of heat transfer by.1-9C Energy can be transferred by heat, work, and mass. An energy transfer is heat transfer when its driving force is temperature difference.1-10C Thermal energy is the sensible and latent forms of internal energy, and it is referred to as heat in daily life.1-11C For the co

9、nstant pressure case. This is because the heat transfer to an ideal gas is mCpT at constant pressure and mCpT at constant volume, and Cp is always greater than Cv.1-12 A cylindrical resistor on a circuit board dissipates 0.6 W of power. The amount of heat dissipated in 24 h, the heat flux, and the f

10、raction of heat dissipated from the top and bottom surfaces are to be determined.Assumptions Heat is transferred uniformly from all surfaces.Analysis （a） The amount of heat this resistor dissipates during a 24-hour period is （since 1 Wh = 3600 Ws = 3.6 kJ）（b） The heat flux on the surface of the resi

11、stor is（c） Assuming the heat transfer coefficient to be uniform, heat transfer is proportional to the surface area. Then the fraction of heat dissipated from the top and bottom surfaces of the resistor becomesDiscussion Heat transfer from the top and bottom surfaces is small relative to that transfe

12、rred from the side surface.1-13E A logic chip in a computer dissipates 3 W of power. The amount heat dissipated in 8 h and the heat flux on the surface of the chip are to be determined.Assumptions Heat transfer from the surface is uniform.Analysis （a） The amount of heat the chip dissipates during an

13、 8-hour period is（b） The heat flux on the surface of the chip is1-14 The filament of a 150 W incandescent lamp is 5 cm long and has a diameter of 0.5 mm. The heat flux on the surface of the filament, the heat flux on the surface of the glass bulb, and the annual electricity cost of the bulb are to b

14、e determined.Assumptions Heat transfer from the surface of the filament and the bulb of the lamp is uniform .Analysis （a） The heat transfer surface area and the heat flux on the surface of the filament are（b） The heat flux on the surface of glass bulb is（c） The amount and cost of electrical energy c

15、onsumed during a one-year period is1-15 A 1200 W iron is left on the ironing board with its base exposed to the air. The amount of heat the iron dissipates in 2 h, the heat flux on the surface of the iron base, and the cost of the electricity are to be determined.Analysis （a） The amount of heat the

16、iron dissipates during a 2-h period is（b） The heat flux on the surface of the iron base is（c） The cost of electricity consumed during this period is1-16 A 15 cm 20 cm circuit board houses 120 closely spaced 0.12 W logic chips. The amount of heat dissipated in 10 h and the heat flux on the surface of

17、 the circuit board are to be determined. Assumptions 1 Heat transfer from the back surface of the board is negligible. 2 Heat transfer from the front surface is uniform.Analysis （a） The amount of heat this circuit board dissipates during a 10-h period is（b） The heat flux on the surface of the circui

18、t board is1-17 An aluminum ball is to be heated from 80C to 200C. The amount of heat that needs to be transferred to the aluminum ball is to be determined. Assumptions The properties of the aluminum ball are constant.Properties The average density and specific heat of aluminum are given to be = 2,70

19、0 kg/m3 and kJ/kg.C.Analysis The amount of energy added to the ball is simply the change in its internal energy, and is determined fromwhereSubstituting,Therefore, 515 kJ of energy （heat or work such as electrical energy） needs to be transferred to the aluminum ball to heat it to 200C.1-18 The body

20、temperature of a man rises from 37C to 39C during strenuous exercise. The resulting increase in the thermal energy content of the body is to be determined.Assumptions The body temperature changes uniformly.Properties The average specific heat of the human body is given to be 3.6 kJ/kg.C.Analysis The

21、 change in the sensible internal energy content of the body as a result of the body temperature rising 2C during strenuous exercise is U = mCT = （70 kg）（3.6 kJ/kg.C）（2C） = 504 kJ1-19 An electrically heated house maintained at 22C experiences infiltration losses at a rate of 0.7 ACH. The amount of en

22、ergy loss from the house due to infiltration per day and its cost are to be determined.Assumptions 1 Air as an ideal gas with a constant specific heats at room temperature. 2 The volume occupied by the furniture and other belongings is negligible. 3 The house is maintained at a constant temperature

23、and pressure at all times. 4 The infiltrating air exfiltrates at the indoors temperature of 22Properties The specific heat of air at room temperature is = 1.007 kJ/kg.C （Table A-15）.Analysis The volume of the air in the house isNoting that the infiltration rate is 0.7 ACH （air changes per hour） and

24、thus the air in the house is completely replaced by the outdoor air 0.724 = 16.8 times per day, the mass flow rate of air through the house due to infiltration is Noting that outdoor air enters at 5C and leaves at 22C, the energy loss of this house per day is At a unit cost of $0.082/kWh, the cost o

25、f this electrical energy lost by infiltration is1-20 A house is heated from 10C to 22C by an electric heater, and some air escapes through the cracks as the heated air in the house expands at constant pressure. The amount of heat transfer to the air and its cost are to be determined.Assumptions 1 Ai

26、r as an ideal gas with a constant specific heats at room temperature. 2 The volume occupied by the furniture and other belongings is negligible. 3 The pressure in the house remains constant at all times. 4 Heat loss from the house to the outdoors is negligible during heating. 5 The air leaks out at

27、22C.Analysis The volume and mass of the air in the house are Noting that the pressure in the house remains constant during heating, the amount of heat that must be transferred to the air in the house as it is heated from 10 to 22C is determined to be Noting that 1 kWh = 3600 kJ, the cost of this ele

28、ctrical energy at a unit cost of $0.075/kWh isTherefore, it will cost the homeowner about 19 cents to raise the temperature in his house from 10 to 22C.1-21E A water heater is initially filled with water at 45F. The amount of energy that needs to be transferred to the water to raise its temperature

29、to 140F is to be determined.Assumptions 1 Water is an incompressible substance with constant specific heats at room temperature. 2 No water flows in or out of the tank during heating.Properties The density and specific heat of water are given to be 62 lbm/ft3 and 1.0 Btu/lbm.F.Analysis The mass of water in the tank isThen, the amount of heat that must be transferred to the water in the tank as it is heated from 45 to140