东北大学2001级《工程热力学》期末试题(2003年6月27日星期五)

东北大学 2001 级工程热力学期末试题（2003 年 6 月 27 日星期五） Questions of Semester Examination about Engineering Thermodynamics 答题在答题纸上进行，写清题号，不必抄题。答题语言随意。 1. Write out the Following Concepts and Laws: (1) open system; (5%) (2) Carnot cycle; (5%) (3) The Kelvin-Planck statement of the second law of thermodynamics; (5%) (4) The Laval nozzle; (5%) (5) Ideal gas; (5%) (6) The thermal efficiency of a heat engine. (5%) 2. Example 1-10 The piston of a vertical piston-cylinder device containing a gas has a mass of 60kg and a cross- sectional area of 0.04 m2, as shown in Fig. 1-66. The local atmosphere pressure is 0.97bar (0.097MPa), and the gravitational acceleration is 9.81 m/s2. (a) Determine the pressure inside the cylinder. (b) If some heat is transferred to the gas and its volume is doubled, do you expect the pressure inside the cylinder to change? (10%) 3. Example 2-10 Mass of Air in a Room Determine the mass of the air in a room whose dimensions are 4m5m6m at 100kPa and 25. (10%) 4. Example 4-9 Deceleration of Air in a Diffuser Air at 10 and 80 kPa enters the diffuser of a jet engine steadily with a velocity of 200m/s. The inlet area of the diffuser is 0.4m2. The air leaves the diffuser with a velocity that is very small compared with the inlet velocity. Determine (a) the mass flow rate of the air and (b) the temperature of the air leaving the diffuser. (15%) 5. Example 5-5 Heating a House by a Heat Pump A heat pump is used to meet the heating requirements of a house and maintain it at 20. On a day when the outdoor air temperature drops to -2, the house is estimated to lose heat at a rate of 80,000kJ/h. If the heat pump under these conditions has a COP(Coefficient of Performance) of 2.5, determine (a) the power consumed by the heat pump, and (b) the rate at which heat is absorbed from the cold outdoor air, and (c) If we were to use an electric resistance heater instead, how many times we would have to supply the entire 80,000kJ/h to the resistance as electric energy? (15%) 6. Example 9-4 The Ideal Reheat Rankine Cycle Patm=0.97bar Patm m=60kg A=0.04m2 P=? P W=mg Figure 1-66 Schematic for Example 1-10, and the free-body diagram of the piston. Consider a steam power plant operating on the ideal reheat Rankine cycle. Steam enters the high-pressure turbine at 15MPa and 600 and is condensed in the condenser at a pressure of 10kPa. If the moisture content of the steam at the exit of the low-pressure turbine is not to exceed 10.4 percent, determine (a) the pressure at which the steam should be reheated and (b) the thermal efficiency of the cycle. Assume the steam is reheated to the inlet temperature of the high-pressure turbine. (20%) Clew: wpump, in= =v(p2-p1), because the water is incompressible.21d We known: p=10kPa s=0.6493kJ/(kgK) s“=8.1502kJ/(kgK) h=191.83kJ/kg h“=2584.63kJ/kg v=0.00101m3/kg v“=14.676m3/kg p=5.0MPa T=600 s=7.2586kJ/(kgK) h=3665.4kJ/kg p=4.0MPa T=600 s=7.370kJ/(kgK) h=3674.4kJ/kg p=3.0MPa T=600 s=7.5084kJ/(kgK) h=3681.5kJ/kg p=15.0MPa T=600 s=6.6776kJ/(kgK) h=3582.3kJ/kg p=5.0MPa T=408 s=6.6776kJ/(kgK) h=3216.6kJ/kg p=4.0MPa T=375.5 s=6.6776kJ/(kgK) h=3154.3kJ/kg p=3.0MPa T=332.8 s=6.6776kJ/(kgK) h=3074.8kJ/kg Answers 1. (1) open system; (5%) (2) Carnot cycle; (5%) (3) The Kelvin-Planck statement of the second law of thermodynamics; (5%) (4) The Laval nozzle; (5%) (5) Ideal gas; (5%) (6) 2. A gas is contained in a vertical cylinder with a heavy piston. The pressure inside the cylinder and the effect of volume change on pressure are to be determined. Assumptions: Friction between the piston and the cylinder is negligible. Analysis: (a) The gas pressure in the piston-cylinder device depends on the atmospheric pressure and the weight of the piston. Drawing the free-body diagram of the piston an shown in Fig. 1-66 and balancing the vertical forces yield pA=patmA+W Solving for P and substituting, p=patm+ mg =0.97105+ =111715Pa04.8196 =1.12bar (b) The volume change will have no effect on the free-body diagram draw in part (a), and therefore the pressure inside the cylinder will remain the same. 3. The mass of air in a room is to be determined. Analysis: A sketch of the room is given in Fig. 2-48. Air at specified conditions can be treated as an ideal gas. From Table A-1, the gas constant of air is R=0.287kPam3/kgK, 东北大学 2001 级工程热力学期末试题 （2003 年 6 月 27 日星期五，小语种） 答题时不必抄题，要写清题号。 一. 填空：（每空 2 分，共 40 分） 1. 煤炭、石油、天然气等都是 次能源，而电、煤气、柴油等都是 次能 源。 2. 典型的热力系统有： 系统、 系统、 系统和 系 统。 3. 热力学第一定律是 定律在热现象上的应用。 4. h= +pv，w= w t+( )，q=h+ 。 5. 的分子是些弹性的、不占据体积的质点，分子相互之间没有 。 6. pv= RT 是 方程，其中 R 为 。pV m= RmT 中的 Rm 是 ，单位是 。 7. 逆向（制冷）循环是消耗一定量的 使热能从 处传送到 处的循 环。 8. 热力学第二定律的开尔文说法可以表述为第二类 是不可能造成的。 二. 计算题：（ 30 分） 一般供热锅炉炉内温度最高为 12001300，按 1500K 计；采暖 季节室外温度大都处于-10 左右，按 260K 计；室内温度达到 16即为合格，按 290K 计。假定建筑供热指标为 50W/m2，若供热 锅炉产热 6MW，请问可为多大面积的建筑 物供暖？孤立系统熵增是多少？ 若采用另外一个系统：热电联产加热 泵装置。其中，电厂排热温度取 360K，热 泵排热取 310K，热泵吸热温度 250K。电 厂效率和热泵供热系数各取相应卡诺值的 一半，锅炉产热仍取 6MW， 。那么这套系 统又可为多大面积的建筑物供暖？ 三. 计算题：（ 15 分） 某定压加热燃气轮 机理想循环中，压气机进口工质温度 T1=300K，压力 p1=1bar，增压比 =6，涡轮 进口工质温度 T3=1000K。设工质可视为空 气，并取定值比热 cp0=1.004kJ/(kgK)，k=1.4。请计算循环中的加热量、放热量、循环 功与热效率。 四. 计算题：（ 15 分） 氧气 O2 由 t1=40、p 1=0.1MPa 被定温压缩到 p2=0.4MPa。 锅 炉 150K电 厂 排 热 36热 泵 排 热 采 暖 建 筑 物 290K热 泵 吸 热 5环 境 6热 电 联 产 热 泵 装 置 二题图 热电联产加热泵装置 （1）试计算压缩每 1kg 氧气所消耗的技术功； （2）如果按绝热过程压缩，初始状态和终压与上述相同，试计算压缩每 1kg 氧气 消耗的技术功； （3）比较两种情况技术功的大小。 参考答案： 一. 一； 二； 闭口（封闭） ； 开口（开放） ； 绝热； 孤立； 能量 守恒与转换； u； pv； wt； 理想气体； 作用力； 理想气体状态； 气体常数； 通用气体常数； J/(kmolK)或 J/(molK)、kJ/(