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mechanical engineering

Questions and Answers of Mechanical Engineering

A boiler delivers steam at 10 MPa, 550C to a two-stage turbine as shown in Fig. 11.17. After the first stage, 25% of the steam is extracted at 1.4 MPa for a process application and returned
Consider an ideal air-standard Brayton cycle in which the air into the compressor is at 100 kPa, 20C, and the pressure ratio across the compressor is 12:1. The maximum temperature in the
Repeat Problem 11.46, but assume variable specific heat for the air, table A.7.
An ideal regenerator is incorporated into the ideal air-standard Brayton cycle of Problem 11.46. Find the thermal efficiency of the cycle with this modification.
A Brayton cycle inlet is at 300 K, 100 kPa and the combustion adds 670kJ/kg. The maximum temperature is 1200 K due to material considerations. What is the maximum allowed compression ratio? For this
A large stationary Brayton cycle gas-turbine power plant delivers a power output of 100 MW to an electric generator. The minimum temperature in the cycle is 300 K, and the maximum temperature is 1600
Repeat Problem 11.50, but assume that the compressor has an isentropic efficiency of 85% and the turbine an isentropic efficiency of 88%.
Repeat Problem 11.51, but include a regenerator with 75% efficiency in the cycle.
A gas turbine with air as the working fluid has two ideal turbine sections, as shown in Fig. P11.53, the first of which drives the ideal compressor, with the second producing the power output. The
The gas-turbine cycle shown in Fig P11.54 is used as an automotive engine. In the first turbine, the gas expands to pressure P5, just low enough for this turbine to drive the compressor. The gas is
Repeat Problem 11.54, but assume that the compressor has an efficiency of 82%, that both turbines have efficiencies of 87%, and that the regenerator efficiency is 70%.
Repeat the questions in Problem 11.54 when we assume that friction causes pressure drops in the burner and on both sides of the regenerator. In each case, the pressure drop is estimated to be 2% of
Consider an ideal gas-turbine cycle with two stages of compression and two stages of expansion. The pressure ratio across each compressor stage and each turbine stage is 8 to 1. The pressure at the
Repeat Problem 11.57, but assume that each compressor stage and each turbine stage has an isentropic efficiency of 85%. Also assume that the regenerator has an efficiency of 70%.
A gas turbine cycle has two stages of compression, with an intercooler between the stages. Air enters the first stage at 100 kPa, 300 K. The pressure ratio across each compressor stage is 5 to 1, and
A two-stage air compressor has an intercooler between the two stages as shown in Fig. P11.60, the inlet state is 100 kPa, 290 K, and the final exit pressure is 1.6 MPa. Assume that the constant
Repeat Problem 11.60 when the intercooler brings the air to T3320 K. The corrected formula for the optimal pressure is P2 = [P1P4 (T3/T1) n/ (n-1)] 1/2 see Problem 9.131, where n is
Consider an ideal air-standard Ericsson cycle that has an ideal regenerator as shown in Fig. P11.62, the high pressure is 1 MPa and the cycle efficiency is 70%. Heat is rejected in the cycle at a
An air-standard Ericsson cycle has an ideal regenerator. Heat is supplied at 1000C and heat is rejected at 20C. Pressure at the beginning of the isothermal compression process is 70
Consider an ideal air-standard cycle for a gas-turbine, jet propulsion unit, such as that. The pressure and temperature entering the compressor are 90 kPa, 290 K. The pressure ratio across the
The turbine in a jet engine receives air at 1250 K, 1.5 MPa. It exhausts to a nozzle at 250 kPa, which in turn exhausts to the atmosphere at 100 kPa. The isentropic efficiency of the turbine is 85%
Repeat Problem 11.64, but assume that the isentropic compressor efficiency is 87%, the isentropic turbine efficiency is 89%, and the isentropic nozzle efficiency is 96%.
Consider an air standard jet engine cycle operating in a 280K, 100 kPa environment. The compressor requires a shaft power input of 4000 kW. Air enters the turbine state 3 at 1600 K, 2 MPa, at the
A jet aircraft is flying at an altitude of 4900 m, where the ambient pressure is approximately 55 kPa and the ambient temperature is 18C. The velocity of the aircraft is 280 m/s, the
Air flows into a gasoline engine at 95 kPa, 300 K. The air is then compressed with a volumetric compression ratio of 8; 1. In the combustion process 1300 kJ/kg of energy is released as the fuel
A gasoline engine has a volumetric compression ratio of 9. The state before compression is 290 K, 90 kPa, and the peak cycle temperature is 1800 K. Find the pressure after expansion, the cycle net
To approximate an actual spark-ignition engine consider an air-standard Otto cycle that has a heat addition of 1800 kJ/kg of air, a compression ratio of 7, and a pressure and temperature at the
Repeat Problem 11.71, but assume variable specific heat. The ideal gas air tables, Table A.7, are recommended for this calculation (and the specific heat from Fig. 5.10 at high temperature).
A gasoline engine takes air in at 290 K, 90 kPa and then compresses it. The combustion adds 1000 kJ/kg to the air after which the temperature is 2050 K. Use the cold air properties (i.e. constant
Answer the same three questions for the previous problem, but use variable heat capacities.
When methanol produced from coal is considered as an alternative fuel to gasoline for automotive engines, it is recognized that the engine can be designed with a higher compression ratio, say 10
It is found experimentally that the power stroke expansion in an internal combustion engine can be approximated with a polytropic process with a value of the polytropic exponent n somewhat larger
In the Otto cycle all the heat transfer qH occurs at constant volume. It is more realistic to assume that part of qH occurs after the piston has started its downward motion in the expansion stroke.
A diesel engine has a compression ratio of 20:1 with an inlet of 95 kPa, 290 K, state 1, with volume 0.5 L. The maximum cycle temperature is 1800 K. Find the maximum pressure, the net specific work
A diesel engine has a bore of 0.1 m, a stroke of 0.11 m and a compression ratio of 19:1 running at 2000 RPM (revolutions per minute). Each cycle takes two revolutions and has a mean effective
At the beginning of compression in a diesel cycle T = 300 K, P = 200 kPa and after combustion (heat addition) is complete T = 1500 K and P = 7.0 MPa. Find the compression ratio, the thermal
Consider an ideal air-standard diesel cycle in which the state before the compression process is 95 kPa, 290 K, and the compression ratio is 20. Find the maximum temperature (by iteration) in the
Consider an ideal Sterling-cycle engine in which the state at the beginning of the isothermal compression process is 100 kPa, 25°C the compression ratio is 6, and the maximum temperature in the
An air-standard Sterling cycle uses helium as the working fluid. The isothermal compression brings helium from 100 kPa, 37°C to 600 kPa. The expansion takes place at 1200 K and there is no
Consider an ideal air-standard Sterling cycle with an ideal regenerator. The minimum pressure and temperature in the cycle are 100 kPa, 25°C, the compression ratio is 10, and the maximum temperature
The air-standard Carnot cycle was not shown in the text; show the T–s diagram for this cycle. In an air-standard Carnot cycle the low temperature is 280 K and the efficiency is 60%. If the pressure
Air in a piston/cylinder goes through a Carnot cycle in which TL 26.8C and the total cycle efficiency is 2/3. Find TH, the specific work and
Consider an ideal refrigeration cycle that has a condenser temperature of 45C and an evaporator temperature of 15C; determine the coefficient of performance of this
The environmentally safe refrigerant R-134a is one of the replacements for R-12 in refrigeration systems. Repeat Problem 11.87 using R-134a and compare the result with that for R-12.
A refrigerator using R-22 is powered by a small natural gas fired heat engine with a thermal efficiency of 25%. The R-22 condenses at 40C and it evaporates at-20C and the cycle is
A refrigerator with R-12 as the working fluid has a minimum temperature of 10C and a maximum pressure of 1 MPa. Assume an ideal refrigeration cycle as in Fig. 11.32. Find the specific
A refrigerator in a meat warehouse must keep a low temperature of -15C and the outside temperature is 20C. It uses R-12 as the refrigerant which must remove 5 kW from the cold space.
A refrigerator with R-12 as the working fluid has a minimum temperature of 10C and a maximum pressure of 1 MPa. The actual adiabatic compressor exit temperature is 60C. Assume
Consider an ideal heat pump that has a condenser temperature of 50C and an evaporator temperature of 0C, determine the coefficient of performance of this heat pump for the working
The air conditioner in a car uses R-134a and the compressor power input is 1.5 kW bringing the R-134a from 201.7 kPa to 1200 kPa by compression. The cold space is a heat exchanger that cools
A refrigerator using R-134a is located in a 20C room. Consider the cycle to be ideal, except that the compressor is neither adiabatic nor reversible. Saturated vapor at -20C enters the compressor,
A small heat pump unit is used to heat water for a hot-water supply. Assume that the unit uses R-22 and operates on the ideal refrigeration cycle. The evaporator temperature is 15C and the
The refrigerant R-22 is used as the working fluid in a conventional heat pump cycle. Saturated vapor enters the compressor of this unit at 10C; its exit temperature from the compressor is
In an actual refrigeration cycle using R-12 as the working fluid, the refrigerant flow rate is 0.05 kg/s. Vapor enters the compressor at 150 kPa, 10C, and leaves at 1.2 MPa,
Consider a small ammonia absorption refrigeration cycle that is powered by solar energy and is to be used as an air conditioner. Saturated vapor ammonia leaves the generator at 50C, and
The performance of an ammonia absorption cycle refrigerator is to be compared with that of a similar vapor-compression system. Consider an absorption system having an evaporator temperature of
A heat exchanger is incorporated into an ideal air-standard refrigeration cycle, as shown in Fig. P11.101. It may be assumed that both the compression and the expansion are reversible adiabatic
Repeat Problem 11.101, but assume an isentropic efficiency of 75% for both the compressor and the expander.
Repeat Problems 11.101 and 11.102, but assume that helium is the cycle working fluid instead of air. Discuss the significance of the results.
A binary system power plant uses mercury for the high-temperature cycle and water for the low-temperature cycle, as shown in Fig. 11.39. The temperatures and pressures are shown in the corresponding
A Rankine steam power plant should operate with a high pressure of 3 MPa, a low pressure of 10 kPa, and the boiler exit temperature should be 500C. The available high-temperature source is
A simple Rankine cycle with R-22 as the working fluid is to be used as a bottoming cycle for an electrical generating facility driven by the exhaust gas from a Diesel engine as the high temperature
For a cryogenic experiment heat should be removed from a space at 75 K to a reservoir at 180 K. A heat pump is designed to use nitrogen and methane in a cascade arrangement (see Fig. 11.41), where
A cascade system is composed of two ideal refrigeration cycles. The high-temperature cycle uses R-22. Saturated liquid leaves the condenser at 40C, and saturated vapor leaves the heat
Consider an ideal dual-loop heat-powered refrigeration cycle using R-12 as the working fluid, as shown in Fig. P11.109. Saturated vapor at 105°C leaves the boiler and expands in the turbine to the
Find the availability of the water at all four states in the Rankine cycle described in Problem 11.12. Assume that the high-temperature source is 500C and the low temperature reservoir is at
The effect of a number of open feedwater heaters on the thermal efficiency of an ideal cycle is to be studied. Steam leaves the steam generator at 20 MPa, 600C, and the cycle has a condenser
Find the availability of the water at all the states in the steam power plant described in Problem 11.36. Assume the heat source in the boiler is at 600C and the low-temperature reservoir is
The power plant shown in Fig. 11.40 combines a gas-turbine cycle and a steam turbine cycle. The following data are known for the gas-turbine cycle. Air enters the compressor at 100 kPa, 25C, the
For Problem 11.105, determine the change of availability of the water flow and that of the air flow. Use these to determine a second law efficiency for the boiler heat exchanger.
One means of improving the performance of a refrigeration system that operates over a wide temperature range is to use a two-stage compressor. Consider an ideal refrigeration system of this type that
A jet ejector, a device with no moving parts functions as the equivalent of a coupled turbine-compressor unit. Thus, the turbine compressor in the dual-loop cycle of could be replaced by a jet
A steam power plant, as shown in Fig. 11.3, operating in a Rankine cycle has saturated vapor at 600 lbf/in 2 leaving the boiler. The turbine exhausts to the condenser operating at 2 lbf/in 2. Find
Consider a solar-energy-powered ideal Rankine cycle that uses water as the working fluid. Saturated vapor leaves the solar collector at 350 F, and the condenser pressure is 1 lbf/in.2. Determine the
A supply of geothermal hot water is to be used as the energy source in an ideal Rankine cycle, with R-134a as the cycle working fluid. Saturated vapor R-134a leaves the boiler at a temperature of 180
Do Problem 11.119 with R-22 as the working fluid.
The power plant is modified to have a superheated section following the boiler so the steam leaves the super heater at 600 lbf/in 2, 700 F. Find the specific work and heat transfer in each of the
Consider a simple ideal Rankine cycle using water at a supercritical pressure. Such a cycle has a potential advantage of minimizing local temperature differences between the fluids in the steam
Consider an ideal steam reheat cycle in which the steam enters the high-pressure turbine at 600 lbf/in 2, 700 F, and then expands to 120 lbf/in.2. It is then reheated to 700 F and expands to 2 lbf/in
A closed feedwater heater in a regenerative steam power cycle heats 40 lbm/s of water from 200 F, 2000 lbf/in.2 to 450 F, 2000 lbf/in 2, the extraction steam from the turbine enters the heater at 500
Consider an ideal steam regenerative cycle in which steam enters the turbine at 600 lbf/in 2, 700 F, and exhausts to the condenser at 2 lbf/in 2. Steam is extracted from the turbine at 120 lbf/in 2
Consider an ideal combined reheat and regenerative cycle in which steam enters the high-pressure turbine at 500 lbf/in 2, 700 F, and is extracted to an open feedwater heater at 120 lbf/in.2 with exit
A steam power cycle has a high pressure of 500 lbf/in 2 and a condenser exit temperature of 110 F. The turbine efficiency is 85%, and other cycle components are ideal. If the boiler superheats to
The steam power cycle has an isentropic efficiency of the turbine of 85% and that for the pump it is 80%. Find the cycle efficiency and the specific work and heat transfer in the components.
Steam leaves a power plant steam generator at 500 lbf/in 2, 650 F, and enters the turbine at 490 lbf/in.2, 625 F. The isentropic turbine efficiency is 88%, and the turbine exhaust pressure is 1.7
In one type of nuclear power plant, heat is transferred in the nuclear reactor to liquid sodium. The liquid sodium is then pumped through a heat exchanger where heat is transferred to boiling water.
A boiler delivers steam at 1500 lbf/in.2, 1000 F to a two-stage turbine. After the first stage, 25% of the steam is extracted at 200 lbf/in 2 for a process application and returned at 150 lbf/in 2,
A large stationary Brayton cycle gas-turbine power plant delivers a power output of 100000 hp to an electric generator. The minimum temperature in the cycle is 540 R, and the maximum temperature is
An ideal regenerator is incorporated into the ideal air-standard Brayton cycle of Problem 11.132. Calculate the cycle thermal efficiency with this modification.
Consider an ideal gas-turbine cycle with two stages of compression and two stages of expansion. The pressure ratio across each compressor stage and each turbine stage is 8 to 1. The pressure at the
Repeat Problem 11.134, but assume that each compressor stage and each turbine stage has an isentropic efficiency of 85%. Also assume that the regenerator has an efficiency of 70%.
An air-standard Ericsson cycle has an ideal regenerator as shown in Fig. P11.62. Heat is supplied at 1800 F and heat is rejected at 68 F. Pressure at the beginning of the isothermal compression
The turbine in a jet engine receives air at 2200 R, 220 lbf/in 2. It exhausts to a nozzle at 35 lbf/in 2, which in turn exhausts to the atmosphere at 14.7 lbf/in 2. The isentropic efficiency of the
Air flows into a gasoline engine at 14 lbf/in 2, 540 R. The air is then compressed with a volumetric compression ratio of 8; 1. In the combustion process 560 Btu/lbm of energy is released as the fuel
To approximate an actual spark-ignition engine consider an air-standard Otto cycle that has a heat addition of 800 Btu/lbm of air, a compression ratio of 7, and a pressure and temperature at the
In the Otto cycle all the heat transfer qH occurs at constant volume. It is more realistic to assume that part of qH occurs after the piston has started it’s downwards motion in the expansion
It is found experimentally that the power stroke expansion in an internal combustion engine can be approximated with a polytropic process with a value of the polytropic exponent n somewhat larger
A diesel engine has a bore of 4 in., a stroke of 4.3 in. and a compression ratio of 19:1 running at 2000 RPM (revolutions per minute). Each cycle takes two revolutions and has a mean effective
At the beginning of compression in a diesel cycle T = 540 R, P = 30 lbf/in2 and the state after combustion (heat addition) is 2600 R and 1000 lbf/in 2. Find the compression ratio, the thermal
Consider an ideal air-standard diesel cycle where the state before the compression process is 14 lbf/in 2, 63 F and the compression ratio is 20. Find the maximum temperature (by iteration) in the

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