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mechanical engineering

Questions and Answers of Mechanical Engineering

Hot combustion gases enter the nozzle of a turbojet engine at 260 kPa, 747°C, and 80 m/s, and they exit at a pressure of 85 kPa. Assuming an isentropic efficiency of 92 percent and treating the
Refrigerant-134a is expanded adiabatically from 100 psia and 100°F to a saturated vapor at 10 psia. Determine the entropy generation for this process, in Btu/lbm·R.
Oxygen enters an insulated 12-cm-diameter pipe with a velocity of 70 m/s. At the pipe entrance, the oxygen is at 240 kPa and 20°C; and, at the exit, it is at 200 kPa and 18°C. Calculate the rate at
Nitrogen is compressed by an adiabatic compressor from 100 kPa and 25°C to 600 kPa and 290°C. Calculate the entropy generation for this process, in kJ/kg·K.
Air enters a compressor steadily at the ambient conditions of 100 kPa and 22°C and leaves at 800 kPa. Heat is lost from the compressor in the amount of 120 kJ/kg and the air experiences an entropy
Steam enters an adiabatic turbine steadily at 7 MPa, 500°C, and 45 m/s, and leaves at 100 kPa and 75 m/s. If the power output of the turbine is 5 MW and the isentropic efficiency is 77 percent,
In an ice-making plant, water at 0°C is frozen at atmospheric pressure by evaporating saturated R-134a liquid at -16°C. The refrigerant leaves this evaporator as a saturated vapor, and the
Water at 20 psia and 50°F enters a mixing chamber at a rate of 300 lbm/min where it is mixed steadily with steam entering at 20 psia and 240°F. The mixture leaves the chamber at 20 psia and
Steam is to be condensed on the shell side of a heat exchanger at 150°F. Cooling water enters the tubes at 60°F at a rate of 44 lbm/s and leaves at 73°F. Assuming the heat exchanger to be
A well-insulated heat exchanger is to heat water (cp = 4.18 kJ/kgˆ™°C) from 25 to 60°C at a rate of 0.50 kg/s. The heating is to be accomplished by geothermal water (cp = 4.31 kJ/kgˆ™°C)
An adiabatic heat exchanger is to cool ethylene glycol (cp = 2.56 kJ/kg∙°C) flowing at a rate of 2 kg/s from 80 to 40°C by water (cp = 4.18 kJ/kg∙°C) that enters at 20°C and leaves at 55°C.
A well-insulated, thin-walled, double-pipe, counter-flow heat exchanger is to be used to cool oil (cp = 2.20 kJ/kg∙°C) from 150°C to 40°C at a rate of 2 kg/s by water (cp = 4.18 kJ/kg∙°C)
In a dairy plant, milk at 4°C is pasteurized continuously at 72°C at a rate of 12 L/s for 24 hours a day and 365 days a year. The milk is heated to the pasteurizing temperature by hot water
An ordinary egg can be approximated as a 5.5-cmdiameter sphere. The egg is initially at a uniform temperature of 8°C and is dropped into boiling water at 97°C. Taking the properties of the
Chickens with an average mass of 2.2 kg and average specific heat of 3.54 kJ/kg∙°C are to be cooled by chilled water that enters a continuous-flow-type immersion chiller at 0.5°C and leaves at
Carbon-steel balls (Ï = 7833 kg/m3 and cp = 0.465 kJ/kgˆ™°C) 8 mm in diameter are annealed by heating them first to 900°C in a furnace and then allowing them to cool
In a production facility, 1.2-in-thick, 2-ft × 2-ft square brass plates (ρ = 532.5 lbm/ft3 and cp = 0.091 Btu/lbm∙°F) that are initially at a uniform temperature of 75°F are heated by passing
Long cylindrical steel rods (Ï = 7833 kg/m3 and cp = 0.465 kJ/kg·°C) of 10-cm diameter are heat treated by drawing them at a velocity of 3 m/min through a 7-m-long oven
The inner and outer surfaces of a 4-m × 10-m brick wall of thickness 20 cm are maintained at temperatures of 16°C and 4°C, respectively. If the rate of heat transfer through the wall is 1250 W,
A frictionless piston-cylinder device contains saturated liquid water at 40-psia pressure. Now 600 Btu of heat is transferred to water from a source at 1000°F, and part of the liquid vaporizes at
Steam enters a diffuser at 20 psia and 240°F with a velocity of 900 ft/s and exits as saturated vapor at 240°F and 100 ft/s. The exit area of the diffuser is 1 ft2. Determine (a) The mass flow rate
Steam enters an adiabatic nozzle at 2 MPa and 350°C with a velocity of 55 m/s and exits at 0.8 MPa and 390 m/s. If the nozzle has an inlet area of 7.5 cm2, determine (a) The exit temperature and (b)
Steam expands in a turbine steadily at a rate of 40,000 kg/h, entering at 8 MPa and 500°C and leaving at 40 kPa as saturated vapor. If the power generated by the turbine is 8.2 MW, determine the
A hot-water stream at 70°C enters an adiabatic mixing chamber with a mass flow rate of 3.6 kg/s, where it is mixed with a stream of cold water at 20°C. If the mixture leaves the chamber at 42°C,
Liquid water at 200 kPa and 15°C is heated in a chamber by mixing it with superheated steam at 200 kPa and 150°C. Liquid water enters the mixing chamber at a rate of 4.3 kg/s, and the chamber is
A 0.18-m3 rigid tank is filled with saturated liquid water at 120°C. A valve at the bottom of the tank is now opened, and one-half of the total mass is withdrawn from the tank in the liquid form.
An iron block of unknown mass at 185°F is dropped into an insulated tank that contains 0.8 ft3 of water at 70°F. At the same time, a paddle wheel driven by a 200-W motor is activated to stir the
Compressed air is one of the key utilities in manufacturing facilities, and the total installed power of compressed-air systems in the United States is estimated to be about 20 million horsepower.
The compressed-air requirements of a plant at sea level are being met by a 90-hp compressor that takes in air at the local atmospheric pressure of 101.3 kPa and the average temperature of 15°C and
A 150-hp compressor in an industrial facility is housed inside the production area where the average temperature during operating hours is 25°C. The average temperature of outdoors during the same
The compressed-air requirements of a plant are being met by a 100-hp screw compressor that runs at full load during 40 percent of the time and idles the rest of the time during operating hours. The
The compressed-air requirements of a plant are being met by a 90-hp screw compressor. The facility stops production for one hour every day, including weekends, for lunch break, but the compressor is
The compressed-air requirements of a plant are met by a 150-hp compressor equipped with an intercooler, an aftercooler, and a refrigerated dryer. The plant operates 6300 h/yr, but the compressor is
The 1800-rpm, 150-hp motor of a compressor is burned out and is to be replaced by either a standard motor that has a full-load efficiency of 93.0 percent and costs $9031 or a high-efficiency motor
The space heating of a facility is accomplished by natural gas heaters that are 85 percent efficient. The compressed air needs of the facility are met by a large liquid-cooled compressor. The coolant
The compressors of a production facility maintain the compressed-air lines at a (gage) pressure of 700 kPa at 1400-m elevation, where the atmospheric pressure is 85.6 kPa. The average temperature of
The energy used to compress air in the United States is estimated to exceed one-half quadrillion (0.5 × 1015) kJ per year. It is also estimated that 10 to 40 percent of the compressed air is lost
A heat engine whose thermal efficiency is 35 percent uses a hot reservoir at 1100 R and a cold reservoir at 550 R. Calculate the entropy change of the two reservoirs when 1 Btu of heat is transferred
A refrigerator with a coefficient of performance of 4 transfers heat from a cold region at -20°C to a hot region at 30°C. Calculate the total entropy change of the regions when 1 kJ of heat
It has been suggested that air at 100 kPa and 25°C can be cooled by first compressing it adiabatically in a closed system to 1000 kPa and then expanding it adiabatically back to 100 kPa. Is this
1-1bm of air at 10 psia and 70°F is contained in a piston-cylinder device. Next, the air is compressed reversibly to 100 psia while the temperature is maintained constant. Determine the total amount
Can saturated water vapor at 200 kPa be condensed to a saturated liquid in an isobaric, closed system process while only exchanging heat with an isothermal energy reservoir at 90°C?
A 100-lbm block of a solid material whose specific heat is 0.5 Btu/lbm·R is at 80°F. It is heated with 10 lbm of saturated water vapor that has a constant pressure of 20 psia. Determine the final
A horizontal cylinder is separated into two compartments by an adiabatic, frictionless piston. One side contains 0.2 m3 of nitrogen and the other side contains 0.1 kg of helium, both initially at
A piston-cylinder device contains air that undergoes a reversible thermodynamic cycle. Initially, air is at 400 kPa and 300 K with a volume of 0.3 m3 Air is first expanded isothermally to 150 kPa,
A piston-cylinder device initially contains 15 ft3 of helium gas at 25 psia and 70°F. Helium is now compressed in a polytropic process (PVn = constant) to 70 psia and 300°F. Determine (a) The
A piston-cylinder device contains steam that undergoes a reversible thermodynamic cycle. Initially the steam is at 400 kPa and 350°C with a volume of 0.3 m3. The steam is first expanded isothermally
A 0.8-m3 rigid tank contains carbon dioxide (CO2) gas at 250 K and 100 kPa. A 500-W electric resistance heater placed in the tank is now turned on and kept on for 40 min after which the pressure of
Helium gas is throttled steadily from 400 kPa and 60°C. Heat is lost from the helium in the amount of 1.75 kJ/kg to the surroundings at 25°C and 100 kPa. If the entropy of the helium increases by
Air enters the evaporator section of a window air conditioner at 100 kPa and 27°C with a volume flow rate of 6 m3/min. The refrigerant-134a at 120 kPa with a quality of 0.3 enters the evaporator
Refrigerant-134a enters a compressor as a saturated vapor at 160 kPa at a rate of 0.03 m3/s and leaves at 800 kPa. The power input to the compressor is 10 kW. If the surroundings at 20°C experience
Air at 500 kPa and 400 K enters an adiabatic nozzle at a velocity of 30 m/s and leaves at 300 kPa and 350 K. Using variable specific heats, determine(a) The isentropic efficiency,(b) The exit
3-kg of helium gas at 100 kPa and 27°C are adiabatically compressed to 900 kPa. If the isentropic compression efficiency is 80 percent, determine the required work input and the final temperature of
An inventor claims to have invented an adiabatic steady-flow device with a single inlet-outlet that produces 230 kW when expanding 1 kg/s of air from 1200 kPa and 300°C to 100 kPa. Is this claim
You are to expand a gas adiabatically from 3 MPa and 300°C to 80 kPa in a piston-cylinder device. Which of the two choices - air with an isentropic expansion efficiency of 90 percent or neon with an
An adiabatic capillary tube is used in some refrigeration systems to drop the pressure of the refrigerant from the condenser level to the evaporator level. R-134a enters the capillary tube as a
Determine the work input and entropy generation during the compression of steam from 100 kPa to 1 MPa in (a) an adiabatic pump and (b) an adiabatic compressor if the inlet state is saturated liquid
Air is compressed steadily by a compressor from 100 kPa and 20°C to 1200 kPa and 300°C at a rate of 0.4 kg/s. The compressor is intentionally cooled by utilizing fins on the surface of the
A completely reversible heat engine operates with a source at 1500 R and a sink at 500 R. If the entropy of the sink increases by 10 Btu/R, how much will the entropy of the source decrease? How much
Air is compressed steadily by a compressor from 100 kPa and 17°C to 700 kPa at a rate of 5 kg/min. Determine the minimum power input required if the process is (a) Adiabatic and (b) Isothermal.
Air enters a two-stage compressor at 100 kPa and 27°C and is compressed to 625 kPa. The pressure ratio across each stage is the same, and the air is cooled to the initial temperature between the
Steam at 6 MPa and 500°C enters a two-stage adiabatic turbine at a rate of 15 kg/s. 10 percent of the steam is extracted at the end of the first stage at a pressure of 1.2 MPa for other use. The
Refrigerant-134a at 140 kPa and -10°C is compressed by an adiabatic 1.3-kW compressor to an exit state of 700 kPa and 60°C. Neglecting the changes in kinetic and potential energies, determine (a)
An adiabatic air compressor is to be powered by a direct-coupled adiabatic steam turbine that is also driving a generator. Steam enters the turbine at 12.5 MPa and 500°C at a rate of 25 kg/s and
Reconsider Prob. 7-194. Using EES (or other) software, determine the isentropic efficiencies for the compressor and turbine. Then use EES to study how varying the compressor efficiency over the range
Air is expanded in an adiabatic turbine of 85 percent isentropic efficiency from an inlet state of 2200 kPa and 300°C to an outlet pressure of 200 kPa. Calculate the outlet temperature of air and
Air is expanded in an adiabatic turbine of 90 percent isentropic efficiency from an inlet state of 2800 kPa and 400°C to an outlet pressure of 150 kPa. Calculate the outlet temperature of air, the
To control the power output of an isentropic steam turbine, a throttle valve is placed in the steam line supplying the turbine inlet, as shown in the figure. Steam at 6 MPa and 400°C is supplied
Two rigid tanks are connected by a valve. Tank A is insulated and contains 0.3 m3 of steam at 400 kPa and 60 percent quality. Tank B is uninsulated and contains 2 kg of steam at 200 kPa and
Air is compressed by a 15-kW compressor from P1 to P2. The air temperature is maintained constant at 25°C during this process as a result of heat transfer to the surrounding medium at 20°C.
A 1200-W electric resistance heating element whose diameter is 0.5 cm is immersed in 40 kg of water initially at 20°C. Assuming the water container is well-insulated, determine how long it will take
A 5-ft3 rigid tank initially contains refrigerant-134a at 60 psia and 100 percent quality. The tank is connected by a valve to a supply line that carries refrigerant-134a at 140 psia and 80°F. The
A passive solar house that is losing heat to the outdoors at 3°C at an average rate of 50,000 kJ/h is maintained at 22°C at all times during a winter night for 10 h. The house is to be heated by 50
A 15-ft3 steel container that has a mass of 75 lbm when empty is filled with liquid water. Initially, both the steel tank and the water are at 120°F. Now heat is transferred, and the entire system
In order to cool 1-ton of water at 20°C in an insulated tank, a person pours 80 kg of ice at -5°C into the water. Determine (a) The final equilibrium temperature in the tank and (b) The entropy
One ton of liquid water at 80°C is brought into a well-insulated and well-sealed 4-m Ã— 5-m Ã— 7-m room initially at 22°C and 100 kPa. Assuming constant specific heats
A well-insulated 4-m × 4-m × 5-m room initially at 10°C is heated by the radiator of a steam heating system. The radiator has a volume of 15 L and is filled with superheated vapor at 200 kPa and
An insulated piston-cylinder device initially contains 0.02 m3 of saturated liquid-vapor mixture of water with a quality of 0.1 at 100°C. Now some ice at -18°C is dropped into the cylinder.
Consider a 50-L evacuated rigid bottle that is surrounded by the atmosphere at 95 kPa and 27°C. A valve at the neck of the bottle is now opened and the atmospheric air is allowed to flow into the
(a) Water flows through a shower head steadily at a rate of 10 L/min. An electric resistance heater placed in the water pipe heats the water from 16 to 43°C. Taking the density of water to be 1
Heat in the amount of 100 kJ is transferred directly from a hot reservoir at 1200 K to a cold reservoir at 600 K. Calculate the entropy change of the two reservoirs and determine if the increase of
Using EES (or other) software, determine the work input to a multistage compressor for a given set of inlet and exit pressures for any number of stages. Assume that the pressure ratio across each
The inner and outer surfaces of a 2-m × 2-m window glass in winter are 10°C and 3°C, respectively. If the rate of heat loss through the window is 3.2 kJ/s, determine the amount of heat loss, in
The inner and outer glasses of a 2-m Ã— 2-m double-pane window are at 18°C and 6°C, respectively. If the glasses are very nearly isothermal and the rate of heat transfer through
A hot-water pipe at 80°C is losing heat to the surrounding air at 5°C at a rate of 2200 W. Determine the rate of entropy generation in the surrounding air, in W/K.
Consider the turbocharger of an internal combustion engine. The exhaust gases enter the turbine at 450°C at a rate of 0.02 kg/s and leave at 400°C. Air enters the compressor at 70°C and
When the transportation of natural gas in a pipeline is not feasible for economic reasons, it is first liquefied using nonconventional refrigeration techniques and then transported in super-insulated
A constant volume tank filled with 2 kg of air rejects heat to a heat reservoir at 300 K. During the process the temperature of the air in the tank decreases to the reservoir temperature. Determine
Consider two bodies of identical mass m and specific heat c used as thermal reservoirs (source and sink) for a heat engine. The first body is initially at an absolute temperature T1 while the second
A heat engine receives heat from a constant volume tank filled with 2 kg of air. The engine produces work that is stored in a work reservoir and rejects 400 kJ of heat to a heat reservoir at 300 K.
In Prob. 7-21, assume that the heat is transferred from the cold reservoir to the hot reservoir contrary to the Clausius statement of the second law. Prove that this violates the increase of entropy
For an ideal gas with constant specific heats show that the compressor and turbine isentropic efficiencies may be written asThe states 1 and 2 represent the compressor inlet and exit states and the
Starting with the Gibbs equation dh = Tds + vdP, obtain the expression for the change in enthalpy of an ideal gas having constant specific heats during the isentropic process Pvk = constant.
An initially empty rigid vessel is filled with a fluid from a source whose properties remain constant. Determine the entropy generation if this is done adiabatically and without any work, and the
The temperature of an ideal gas having constant specific heats is given as a function of specific entropy and specific volume as T(s,v) = Av1-k exp(s/cv) where A is a constant. For a reversible,
An ideal gas undergoes a reversible, steady-flow process in which pressure and volume are related by the polytropic equation Pvn = constant. Neglecting the changes in kinetic and potential energies
The polytropic or small stage efficiency of a compressor Î·ˆž,C is defined as the ratio of the actual differential work done on the fluid to the isentropic differential work
Steam is compressed from 6 MPa and 300°C to 10 MPa isentropically. The final temperature of the steam is (a) 290°C (b) 300°C (c) 311°C (d) 371°C (e) 422°C
An apple with an average mass of 0.12 kg and average specific heat of 3.65 kJ/kg·°C is cooled from 25°C to 5°C. The entropy change of the apple is (a) -0.705 kJ/K (b) -0.254 kJ/K (c) -0.0304
A piston-cylinder device contains 5 kg of saturated water vapor at 3 MPa. Now heat is rejected from the cylinder at constant pressure until the water vapor completely condenses so that the cylinder

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