Thermodynamics

Assuming the argon remaining inside the tank has undergonea reversible, adiabatic process, determine the final mass inthe tank. Answer: 2.46 kgARGON4 kg30°C450 kPaFIGURE P7–787–79 Reconsider Prob. 7–78. Using EES (or other)software, investigate the effect of the final pressureon the final mass in the tank as the pressure varies from450 to 150 kPa, and plot the results.7–80E Air enters an adiabatic nozzle at 60 psia, 540°F, and200 ft/s and exits at 12 psia. Assuming air to be an ideal gaswith variable specific heats and disregarding any irreversibilities,determine the exit velocity of the air.7–81 Air enters a nozzle steadily at 280 kPa and 77°C witha velocity of 50 m/s and exits at 85 kPa and 320 m/s. Theheat losses from the nozzle to the surrounding medium at20°C are estimated to be 3.2 kJ/kg. Determine (a) the exittemperature and (b) the total entropy change for this process.7–82 Reconsider Prob. 7–81. Using EES (or other)software, study the effect of varying the surroundingmedium temperature from 10 to 40°C on the exittemperature and the total entropy change for this process, andplot the results.7–83 A container filled with 45 kg of liquid water at 95°Cis placed in a 90-m 3 room that is initially at 12°C. Thermalequilibrium is established after a while as a result of heattransfer between the water and the air in the room. Usingconstant specific heats, determine (a) the final equilibriumtemperature, (b) the amount of heat transfer between theRoom90 m 312°CWater45 kg95°CFIGURE P7–83Chapter 7 | 407water and the air in the room, and (c) the entropy generation.Assume the room is well sealed and heavily insulated.7–84 Air at 800 kPa and 400°C enters a steady-flow nozzlewith a low velocity and leaves at 100 kPa. If the air undergoesan adiabatic expansion process through the nozzle, whatis the maximum velocity of the air at the nozzle exit, in m/s?7–85 An ideal gas at 100 kPa and 27°C enters a steady-flowcompressor. The gas is compressed to 400 kPa, and 10 percentof the mass that entered the compressor is removed for someother use. The remaining 90 percent of the inlet gas is compressedto 600 kPa before leaving the compressor. The entirecompression process is assumed to be reversible and adiabatic.The power supplied to the compressor is measured to be 32kW. If the ideal gas has constant specific heats such that c v 0.8 kJ/kg K and c p 1.1 kJ/kg K, (a) sketch the compressionprocess on a T-s diagram, (b) determine the temperatureof the gas at the two compressor exits, in K, and (c) determinethe mass flow rate of the gas into the compressor, in kg/s.7–86 A constant-volume tank contains 5 kg of air at 100kPa and 327°C. The air is cooled to the surroundings temperatureof 27°C. Assume constant specific heats at 300 K.(a) Determine the entropy change of the air in the tank duringthe process, in kJ/K, (b) determine the net entropy changeof the universe due to this process, in kJ/K, and (c) sketch theprocesses for the air in the tank and the surroundings on asingle T-s diagram. Be sure to label the initial and final statesfor both processes.Reversible Steady-Flow Work7–87C In large compressors, the gas is frequently cooledwhile being compressed to reduce the power consumed bythe compressor. Explain how cooling the gas during a compressionprocess reduces the power consumption.7–88C The turbines in steam power plants operate essentiallyunder adiabatic conditions. A plant engineer suggests toend this practice. She proposes to run cooling water throughthe outer surface of the casing to cool the steam as it flowsthrough the turbine. This way, she reasons, the entropy of thesteam will decrease, the performance of the turbine willimprove, and as a result the work output of the turbine willincrease. How would you evaluate this proposal?7–89C It is well known that the power consumed by a compressorcan be reduced by cooling the gas during compression.Inspired by this, somebody proposes to cool the liquidas it flows through a pump, in order to reduce the power consumptionof the pump. Would you support this proposal?Explain.7–90 Water enters the pump of a steam power plant as saturatedliquid at 20 kPa at a rate of 45 kg/s and exits at 6 MPa.Neglecting the changes in kinetic and potential energies andassuming the process to be reversible, determine the powerinput to the pump.