工程热力学第三版答案【英文】第5章

5-7 Air is expanded and is accelerated as it is heated by a hair dryer of constant diameter. The percent increase in the velocity of air as it flows through the drier is to be determined. Assumptions Flow through the nozzle is steady. Properties The density of air is given to be 1.20 kg/m3 at the inlet, and 1.05 kg/m3 at the exit.  Analysis There is only one inlet and one exit, and thus . Then, Therefore, the air velocity increases 26.3% as it flows through the hair drier. 5-14 A smoking lounge that can accommodate 15 smokers is considered. The required minimum flow rate of air that needs to be supplied to the lounge and the diameter of the duct are to be determined. Assumptions  Infiltration of air into the smoking lounge is negligible. Properties The minimum fresh air requirements for a smoking lounge is given to be 30 L/s per person. Analysis The required minimum flow rate of air that needs to be supplied to the lounge is determined directly from The volume flow rate of fresh air can be expressed as Solving for the diameter D and substituting,  Therefore, the diameter of the fresh air duct should be at least 26.8 cm if the velocity of air is not to exceed 8 m/s. 5-20 An air compressor compresses air. The flow work required by the compressor is to be determined. Assumptions 1 Flow through the compressor is steady. 2 Air is an ideal gas. Properties The gas constant of air is R = 0.287 kPam3/kgK (Table A-1). Analysis Combining the flow work expression with the ideal gas equation of state gives 5-21 Steam is leaving a pressure cooker at a specified pressure. The velocity, flow rate, the total and flow energies, and the rate of energy transfer by mass are to be determined. Assumptions 1 The flow is steady, and the initial start-up period is disregarded. 2 The kinetic and potential energies are negligible, and thus they are not considered. 3 Saturation conditions exist within the cooker at all times so that steam leaves the cooker as a saturated vapor at 20 psia. Properties The properties of saturated liquid water and water vapor at 20 psia are vf = 0.01683 ft3/lbm, vg = 20.093 ft3/lbm, ug = 1081.8 Btu/lbm, and hg = 1156.2 Btu/lbm (Table A-5E). Analysis (a) Saturation conditions exist in a pressure cooker at all times after the steady operating conditions are established. Therefore, the liquid has the properties of saturated liquid and the exiting steam has the properties of saturated vapor at the operating pressure. The amount of liquid that has evaporated, the mass flow rate of the exiting steam, and the exit velocity are (b) Noting that h = u + Pv and that the kinetic and potential energies are disregarded, the flow and total energies of the exiting steam are  Note that the kinetic energy in this case is ke = V2/2 = (34.1 ft/s)2 /2 = 581 ft2/s2 = 0.0232 Btu/lbm, which is very small compared to enthalpy. (c) The rate at which energy is leaving the cooker by mass is simply the product of the mass flow rate and the total energy of the exiting steam per unit mass,  Discussion The numerical value of the energy leaving the cooker with steam alone does not mean much since this value depends on the reference point selected for enthalpy (it could even be negative). The significant quantity is the difference between the enthalpies of the exiting vapor and the liquid inside  (which is hfg) since it relates directly to the amount of energy supplied to the cooker. 5-30 Air is decelerated in an adiabatic diffuser. The velocity at the exit is to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Air is an ideal gas with constant specific heats. 3 Potential energy changes are negligible. 4 There are no work interactions. 5 The diffuser is adiabatic. Properties The specific heat of air at the average temperature of (20+90)/2=55°C =328 K is cp = 1.007 kJ/kgK (Table A-2b). Analysis There is only one inlet and one exit, and thus . We take diffuser as the system, which is a control volume since mass crosses the boundary. The energy balance for this steady-flow system can be expressed in the rate form as Solving for exit velocity, 5-38 R-134a is decelerated in a diffuser from a velocity of 120 m/s. The exit velocity of R-134a and the mass flow rate of the R-134a are to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Potential energy changes are negligible. 3 There are no work interactions. Properties From the R-134a tables (Tables A-11 through A-13) and Analysis (a)  There is only one inlet and one exit, and thus . Then the exit velocity of R-134a is determined from the steady-flow mass balance to be (b) We take diffuser as the system, which is a control volume since mass crosses the boundary. The energy balance for this steady-flow system can be expressed in the rate form as Substituting, the mass flow rate of the refrigerant is determined to be It yields            5-46 Steam expands in a turbine. The change in kinetic energy, the power output, and the turbine inlet area are to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Potential energy changes are negligible. 3 The device is adiabatic and thus heat transfer is negligible. Properties From the steam tables (Tables A-4 through 6) and Analysis (a) The change in kinetic energy is determined from (b) There is only one inlet and one exit, and thus . We take the turbine as the system, which is a control volume since mass crosses the boundary. The energy balance for this steady-flow system can be expressed in the rate form as Then the power output of the turbine is determined by substitution to be (c)  The inlet area of the turbine is determined from the mass flow rate relation, 5-50 Air is compressed at a rate of 10 L/s by a compressor. The work required per unit mass and the power required are to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Kinetic and potential energy changes are negligible. 3 Air is an ideal gas with constant specific heats. Properties The constant pressure specific heat of air at the average temperature of (20+300)/2=160°C=433 K is cp = 1.018 kJ/kg·K (Table A-2b). The gas constant of air is R = 0.287 kPam3/kgK (Table A-1). Analysis (a) There is only one inlet and one exit, and thus . We take the compressor as the system, which is a control volume since mass crosses the boundary. The energy balance for this steady-flow system can be expressed in the rate form as Thus,    (b) The specific volume of air at the inlet and the mass flow rate are Then the power input is determined from the energy balance equation to be 5-65 Steam is throttled by a well-insulated valve. The temperature drop of the steam after the expansion is to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Kinetic and potential energy changes are negligible. 3 Heat transfer to or from the fluid is negligible. 4 There are no work interactions involved. Properties The inlet enthalpy of steam is (Tables A-6), Analysis There is only one inlet and one exit, and thus . We take the throttling valve as the system, which is a control volume since mass crosses the boundary. The energy balance for this steady-flow system can be expressed in the rate form as since . Then the exit temperature of steam becomes 5-84 Two streams of cold and warm air are mixed in a chamber. If the ratio of hot to cold air is 1.6, the mixture temperature and the rate of heat gain of the room are to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Kinetic and potential energy changes are negligible. 3 There are no work interactions. 4 The device is adiabatic and thus heat transfer is negligible. Properties The gas constant of air is R = 0.287 kPa.m3/kg.K. The enthalpies of air are obtained from air table (Table A-17) as h1  = h @280 K  = 280.13 kJ/kg h2  =  h @ 307 K = 307.23 kJ/kg hroom  =  h @ 297 K = 297.18 kJ/kg Analysis (a) We take the mixing chamber as the system, which is a control volume since mass crosses the boundary. The mass and energy balances for this steady-flow system can be expressed in the rate form as Mass balance: Energy balance: Combining the two gives      Substituting, h3 = (280.13 +2.2 307.23)/3.2 = 298.76 kJ/kg From air table at this enthalpy, the mixture temperature is T3  = T @ h = 298.76 kJ/kg  = 298.6 K = 25.6C (b) The mass flow rates are determined as follows The rate of heat gain of the room is determined from The negative sign indicates that the room actually loses heat at a rate of 4.93 kW. 5-102 A room is to be heated by an electric resistance heater placed in a duct in the room. The power rating of the electric heater and the temperature rise of air as it passes through the heater are to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant specific heats at room temperature. 3 Kinetic and potential energy changes are negligible. 4 The heating duct is adiabatic, and thus heat transfer through it is negligible. 5 No air leaks in and out of the room. Properties The gas constant of air is 0.287 kPa.m3/kg.K (Table A-1). The specific heats of air at room temperature are cp = 1.005 and cv = 0.718 kJ/kg·K  (Table A-2). Analysis (a) The total mass of air in the room is We first take the entire room as our system, which is a closed system since no mass leaks in or out. The power rating of the electric heater is determined by applying the conservation of energy relation to this constant volume closed system: Solving for the electrical work input gives (b) We now take the heating duct as the system, which is a control volume since mass crosses the boundary. There is only one inlet and one exit, and thus . The energy balance for this adiabatic steady-flow system can be expressed in the rate form as Thus, 5-107 R-134a is condensed in a condenser. The heat transfer per unit mass is to be determined. Assumptions 1 This is a steady-flow process since there is no change with time. 2 Kinetic and potential energy changes are negligible. 3 There are no work interactions. Analysis We take the pipe in which R-134a is condensed as the system, which is a control volume. The energy balance for this steady-flow system can be expressed in the rate form as The enthalpies of R-134a at the inlet and exit of the condenser are (Table A-12, A-13). Substituting, 5-112 Helium flows from a supply line to an initially evacuated tank. The flow work of the helium in the supply line and the final temperature of the helium in the tank are to be determined. Properties The properties of helium are R = 2.0769 kJ/kg.K, cp = 5.1926 kJ/kg.K, cv = 3.1156 kJ/kg.K (Table A-2a). Analysis The flow work is determined from its definition but we first determine the specific volume Noting that the flow work in the supply line is converted to sensible internal energy in the tank, the final helium temperature in the tank is determined as follows Alternative Solution: Noting the definition of specific heat ratio, the final temperature in the tank can also be determined from which is practically the same result. 5-119 A rigid tank initially contains superheated steam. A valve at the top of the tank is opened, and vapor is allowed to escape at constant pressure until the temperature rises to 500C.  The amount of heat transfer is to be determined. Assumptions 1 This is an unsteady process since the conditions within the device are changing during the process, but it can be analyzed as a uniform-flow process by using constant average properties for the steam leaving the tank. 2 Kinetic and potential energies are negligible. 3 There are no work interactions involved. 4 The direction of heat transfer is to the tank (will be verified). Properties The properties of water are (Tables A-4 through A-6) Analysis We take the tank as the system, which is a control volume since mass crosses the boundary. Noting that the microscopic energies of flowing and nonflowing fluids are represented by enthalpy h and internal energy u, respectively, the mass and energy balances for this uniform-flow system can be expressed as Mass balance:        Energy balance: The state and thus the enthalpy of the steam leaving the tank is changing during this process.  But for simplicity, we assume constant properties for the exiting steam at the average values. Thus, The initial and the final masses in the tank are Then from the mass and energy balance relations, 5-131 An insulated piston-cylinder device with a linear spring is applying force to the piston. A valve at the bottom of the cylinder is opened, and refrigerant is allowed to escape. The amount of refrigerant that escapes and the final temperature of the refrigerant are to be determined. Assumptions 1 This is an unsteady process since the conditions within the device are changing during the process, but it can be analyzed as a uniform-flow process assuming that the state of fluid leaving the device remains constant. 2 Kinetic and potential energies are negligible. Properties The initial properties of R-134a are (Tables A-11 through A-13) Analysis We take the tank as the system, which is a control volume since mass crosses the boundary. Noting that the microscopic energies of flowing and nonflowing fluids are represented by enthalpy h and internal energy u, respectively, the mass and energy balances for this uniform-flow system can be expressed as Mass balance:        Energy balance: The initial mass and the relations for the final and exiting masses are Noting that the spring is linear, the boundary work can be determined from