Thermodynamics

258 | Thermodynamics5–15 Air enters a 28-cm diameter pipe steadily at 200 kPaand 20°C with a velocity of 5 m/s. Air is heated as it flows,and leaves the pipe at 180 kPa and 40°C. Determine (a) thevolume flow rate of air at the inlet, (b) the mass flow rate ofair, and (c) the velocity and volume flow rate at the exit.Air200 kPa20°C5 m/sQFIGURE P5–15180 kPa40°C5–16 Refrigerant-134a enters a 28-cm diameter pipesteadily at 200 kPa and 20°C with a velocity of 5 m/s. Therefrigerant gains heat as it flows and leaves the pipe at 180kPa and 40°C. Determine (a) the volume flow rate of therefrigerant at the inlet, (b) the mass flow rate of the refrigerant,and (c) the velocity and volume flow rate at the exit.5–17 Consider a 300-L storage tank of a solar water heatingsystem initially filled with warm water at 45°C.Warm water is withdrawn from the tank through a 2-cmdiameter hose at an average velocity of 0.5 m/s while coldwater enters the tank at 20°C at a rate of 5 L/min. Determinethe amount of water in the tank after a 20-minute period.Assume the pressure in the tank remains constant at 1 atm.Answer: 212 kgCold water20°C5 L/min300 L45°CFIGURE P5–17Flow Work and Energy Transfer by MassWarm water45°C0.5 m/s5–18C What are the different mechanisms for transferringenergy to or from a control volume?5–19C What is flow energy? Do fluids at rest possess anyflow energy?5–20C How do the energies of a flowing fluid and a fluidat rest compare? Name the specific forms of energy associatedwith each case.5–21E Steam is leaving a pressure cooker whose operatingpressure is 30 psia. It is observed that the amount of liquid inthe cooker has decreased by 0.4 gal in 45 minutes after thesteady operating conditions are established, and the crosssectionalarea of the exit opening is 0.15 in 2 . Determine(a) the mass flow rate of the steam and the exit velocity,(b) the total and flow energies of the steam per unit mass, and(c) the rate at which energy is leaving the cooker by steam.5–22 Refrigerant-134a enters the compressor of a refrigerationsystem as saturated vapor at 0.14 MPa, and leaves assuperheated vapor at 0.8 MPa and 60°C at a rate of 0.06 kg/s.Determine the rates of energy transfers by mass into and outof the compressor. Assume the kinetic and potential energiesto be negligible.5–23 A house is maintained at 1 atm and 24°C, andwarm air inside a house is forced to leave the house at a rateof 150 m 3 /h as a result of outdoor air at 5°C infiltratinginto the house through the cracks. Determine the rate ofnet energy loss of the house due to mass transfer. Answer:0.945 kW5–24 Air flows steadily in a pipe at 300 kPa, 77°C, and 25m/s at a rate of 18 kg/min. Determine (a) the diameter of thepipe, (b) the rate of flow energy, (c) the rate of energy transportby mass, and (d) also determine the error involved inpart (c) if the kinetic energy is neglected.Steady-Flow Energy Balance:Nozzles and Diffusers5–25C How is a steady-flow system characterized?5–26C Can a steady-flow system involve boundary work?5–27C A diffuser is an adiabatic device that decreases thekinetic energy of the fluid by slowing it down. What happensto this lost kinetic energy?5–28C The kinetic energy of a fluid increases as it is acceleratedin an adiabatic nozzle. Where does this energy comefrom?5–29C Is heat transfer to or from the fluid desirable as itflows through a nozzle? How will heat transfer affect thefluid velocity at the nozzle exit?5–30 Air enters an adiabatic nozzle steadily at 300 kPa,200°C, and 30 m/s and leaves at 100 kPa and 180 m/s. Theinlet area of the nozzle is 80 cm 2 . Determine (a) the massflow rate through the nozzle, (b) the exit temperature of theair, and (c) the exit area of the nozzle. Answers: (a) 0.5304kg/s, (b) 184.6°C, (c) 38.7 cm 2P 1 = 300 kPaT 1 = 200°CV 1 = 30 m/sA 1 = 80 cm 2AIRFIGURE P5–30P 2 = 100 kPaV 2 = 180 m/s