(华南理工大学)作业-答案-10车辆(给学生3-4章).doc

3-2Solutions Chapter 3Dec. 1Problem 3-46，3-48，3-61，3-73，3-84 (p156)3-46 Determine the boiling temperature in Denver.Analysis The boiling temperature changes with the pressure. The atmospheric pressure in Denver is 83.4kPa. Properties The saturation temperature of water at 75 and 100kPa are 91.76 and 99.61 C, respectively (Table A-5). Solution By linear interpolation, the boiling temperature at an atmospheric pressure 83.4kPa is 3-48 A cooking pan whose inner diameter is 20cm is covered with a 4kg lid. The local atmospheric pressure is 101kPa. Determine the boiling temperature of water in pan. Analysis The weight of the lid causes an additional pressure on the gas in pan. PA = PatmA+FProperties The saturation temperature of water at 100 and 125kPa are 99.61 and 105.97 C, respectively (Table A-5). Solution By linear interpolation, the boiling temperature at an atmospheric pressure 102.25kPa is 3-61 The properties of compressed liquid water at a specified state are to be determined using the compressed liquid tables, and also by using the saturated liquid approximation, and the results are to be compared.Analysis Compressed liquid can be approximated as saturated liquid at the given temperature. Then from Table A-4,T = 100C From compressed liquid table (Table A-7),The percent errors involved in the saturated liquid approximation are3-73 A balloon is filled with helium gas. The mole number and the mass of helium in the balloon are to be determined.Assumptions At specified conditions, helium behaves as an ideal gas. Properties The universal gas constant is Ru = 8.314 kPa.m3/kmol.K. The molar mass of helium is 4.0 kg/kmol (Table A-1). HeD = 6 m20C200 kPaAnalysis The volume of the sphere isAssuming ideal gas behavior, the mole numbers of He is determined fromThen the mass of He can be determined from3-84 The specific volume of steam is to be determined using the ideal gas relation, the compressibility chart, and the steam tables. The errors involved in the first two approaches are also to be determined. Properties The gas constant, the critical pressure, and the critical temperature of water are, from Table A-1,R = 0.4615 kPam3/kgK,Tcr = 647.1 K,Pcr = 22.06 MPaAnalysis (a) From the ideal gas equation of state,H2O10 MPa400C(b) From the compressibility chart (Fig. A-15),Thus,(c) From the superheated steam table (Table A-6), 4-3Chapter 4Dec. 5Problem 4-5，4-28，4-51，4-57，4-78 (p201)4-5 A piston-cylinder device contains nitrogen gas at a specified state. The boundary work is to be determined for the polytropic expansion of nitrogen. N2130 kPa120CProperties The gas constant for nitrogen is 0.2968 kJ/kg.K (Table A-2).Analysis The mass and volume of nitrogen at the initial state areThe polytropic index is determined fromThe boundary work is determined from4-28 An insulated rigid tank is initially filled with a saturated liquid-vapor mixture of R134a. Heat is transferred to until the pressure reach 700kPa. The mass of the refrigerant and the amount of heat are to be determined, and the process is to be shown on a P-v diagram.Assumptions 1 The tank is stationary and thus the kinetic and potential energy changes are zero. 2 The device is a rigid tank and thus there is no boundary work. Analysis We take the contents of the tank as the system. This is a closed system since no mass enters or leaves. Noting that the volume of the system is constant and thus there is no boundary work, the energy balance for this stationary closed system can be expressed asQR134aV = const. The properties of R134a are (Tables A-12)P700 kPa2160 kPa1Constant volume process, , v , R134a at 700kPa is super heated vapor. T=160C( data from Tables A-12) 4-51 The enthalpy change of nitrogen gas during a heating process is to be determined using an empirical specific heat relation, constant specific heat at average temperature, and constant specific heat at room temperature.Analysis (a) Using the empirical relation for from Table A-2c,where a = 28.9, b = -0.157110-2, c = 0.808110-5, and d = -2.87310-9. Then,(b) Using a constant cp value from Table A-2b at the average temperature of 800 K,(c) Using a constant cp value from Table A-2a at room temperature,4-57 A resistance heater is to raise the air temperature in the room from 7 to 23C within 11 min. The required power rating of the resistance heater is to be determined. Assumptions 1 Air is an ideal gas since it is at a high temperature and low pressure relative to its critical point values of -132.5K and 3.77 MPa. 2 The kinetic and potential energy changes are negligible, . 3 Constant specific heats at room temperature can be used for air. This assumption results in negligible error in heating and air-conditioning applications. 4 Heat losses from the room are negligible. 5 The room is air-tight so that no air leaks in and out during the process.Properties The gas constant of air is R = 0.287 kPa.m3/kg.K (Table A-1). Also, cv = 0.718 kJ/kg.K for air at room temperature (Table A-2). Analysis We take the air in the room to be the system. This is a closed system since no mass crosses the system boundary. The energy balance for this stationary constant-volume closed system can be expressed as456 m35CAIRWe or, The mass of air isSubstituting, the power rating of the heater becomesDiscussion In practice, the pressure in the room will remain constant during this process rather than the volume, and some air will leak out as the air expands. As a result, the air in the room will undergo a constant pressure expansion process. Therefore, it is more proper to be conservative and to use DH instead of using DU in heating and air-conditioning applications. 4-78 Stainless steel ball bearings leaving the oven at a specified uniform temperature at a specified rate are exposed to air and are cooled before they are dropped into the water for quenching. The rate of heat transfer from the ball bearing to the air is to b