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Unformatted text preview: Assignment 7
Entropy D44 A wellinsulated rigid tank contains 2 kg of a saturated liquid—vapor mixture of
water at 100 kPa. Initially, threequarters of the mass is in the liquid phase. An electric
resistance heater placed in the tank is now turned on and kept on until all the liquid in the
tank is vaporized. Determine the entropy change of the steam during this process. Answer: 8.0962 kJ/K D45 A 50—kg copper block initially at 80°C is dropped into an insulated tank that contains
120 L of water at 25°C. Determine the ﬁnal equilibrium temperature and total entropy change for this process.
Answers: 27 C, 0.218 kJ/K D46 A 0.5m3 insulated rigid tank contains 0.9 kg of carbon dioxide at 100 kPa. Now
paddlewheel work is done on the system until the pressure in the tank rises to 120 kPa.
Determine the entropy change of carbon dioxide during this process in kJ/K. Assume constant speciﬁc heats.
Answer: 0.108 kJ/K D47 Air is compressed steadily by a 5kW compressor from 100 kPa and 17°C to 600
kPa and 167°C at a rate of 1.6 kg/min. During this process, some heat transfer takes
place between the compressor and the surrounding medium at 17°C. Determine the rate of entropy change of air during this process.
Answer: 0.0025 kW/K D48 Liquid water enters a IOkW pump at lOO—kPa pressure at a rate of 5 kg/s.
Determine the highest pressure the liquid water can have at the exit of the pump. Neglect
the kinetic and potential energy changes of water, and take the speciﬁc volume of water
to be 0.001 m3/kg. Answer: 2100kPa D49 Helium gas is compressed from 14 psia and 70°F to 120 psia at a rate of 5 ft3/s.
Determine the power input to the compressor, assuming the compression process to be
isentropic. Answer: 44.5 Btu/s D50 Steam enters an adiabatic turbine at 8 MPa and 500°C with a mass ﬂow rate of 3
kg/s and leaves at 30 kPa. The isentropic efﬁciency of the turbine is 0.90. Neglecting the
kinetic energy change of the steam, determine (a) the temperature at the turbine exit and
(b) the power output of the turbine. Answers: (a) 691°C, (b) 3052 kW D51 Combustion gases enter an adiabatic gas turbine at 1540°F and 120 psia and leave at
60 psia with a low velocity. Treating the combustion gases as air and assuming an
isentropic efﬁciency of 86 percent, determine the work output of the turbine. Answer: 75.2 Btu/lbm D52 A wellinsulated heat exchanger is to heat water [Cp= 4.20 kJ/(kg*°C)] from 25°C to
60°C at a rate of 0.2 kg/s. The heating is to be accomplished by geothermal water [Cp=
4.31 kJ/(kg*°C)] available at 140°C at a mass flow rate of 0.3 kg/s. The inner tube is
thinwalled and has a diameter of 0.8 cm. Determine (a) the rate of heat transfer and (b) the rate of entropy generation in the heat exchanger.
Answers: (a) 29.26 kJ/s, (b) 0.0197 kW/K D53 Steam expands in a turbine steadily at a rate of 25,000 kg/h, entering at 8 MPa and
450°C and leaving at 50 kPa as saturated vapor. If the power generated by the turbine is
4 MW, determine the rate of entropy generation for this process. Assume the surrounding medium is at 25°C.
Answer: 8.38 kW/K D54 Liquid water at 200 kPa and 20°C is heated in a chamber by mixing it with
superheated steam at 200 kPa and 300°C. Liquid water enters the mixing chamber at a
rate of 2.5 kg/s, and the chamber is estimated to lose heat to the surrounding air at 25°C at
a rate of 600 kJ/min. If the mixture leaves the mixing chamber at 200 kPa and 60°C,
determine (a) the mass ﬂow rater of the superheated steam and (b) the rate of entropy generation during this mixing process.
Answers: (a) 0.152 kg/s, (b) 0.297 kW/K College of Engineering “mum _7ﬁ UNIVERSITY
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This note was uploaded on 04/12/2009 for the course MEEG 2403 taught by Professor Davis during the Spring '09 term at Arkansas.
 Spring '09
 Davis

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