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engineering
mechanical engineering

Thermodynamics An Engineering Approach 8th edition Yunus A. Cengel, Michael A. Boles - Solutions

An oil pump is drawing 35 kW of electric power while pumping oil with r = 860 kg/m3 at a rate of 0.1 m3/s. The inlet and outlet diameters of the pipe are 8 cm and 12 cm, respectively. If the pressure rise of oil in the pump is measured to be 400 kPa and the motor efficiency is 90 percent, determine
A 73-percent efficient pump with a power input of 12 hp is pumping water from a lake to a nearby pool at a rate of 1.2 ft3/s through a constant-diameter pipe, the free surface of the pool is 35 ft above that of the lake. Determine the mechanical power used to overcome frictional effects in piping.
How does energy conversion affect the environment? What are the primary chemicals that pollute the air? What is the primary source of these pollutants? 2–80C What is smog? What does it consist of? How does ground-level ozone form? What are the adverse effects of ozone on human health?
What is acid rain? Why is it called a “rain”? How do the acids form in the atmosphere? What are the adverse effects of acid rain on the environment?
What is the greenhouse effect? How does the excess CO2 gas in the atmosphere cause the greenhouse effect? What are the potential long-term consequences of greenhouse effect? How can we combat this problem?
Why is carbon monoxide a dangerous air pollutant? How does it affect human health at low and at high levels?
A Ford Taurus driven 15,000 miles a year will use about 715 gallons of gasoline compared to a Ford Explorer that would use 940 gallons. About 19.7 lbm of CO2, which causes global warming, is released to the atmosphere when a gallon of gasoline is burned. Determine the extra amount of CO2 production
When a hydrocarbon fuel is burned, almost all of the carbon in the fuel burns completely to form CO2 (carbon dioxide), which is the principal gas causing the greenhouse effect and thus global climate change. On average, 0.59 kg of CO2 is produced for each kWh of electricity generated from a power
Repeat Prob. 2–85 assuming the electricity is produced by a power plant that burns coal. The average production of CO2 in this case is 1.1 kg per kWh.
Consider a household that uses 11,000 kWh of electricity per year and 1500 gallons of fuel oil during a heating season. The average amount of CO2 produced is 26.4 lbm/gallon of fuel oil and 1.54 lbm/kWh of electricity. If this household reduces its oil and electricity usage by 15 percent as a
A typical car driven 12,000 miles a year emits to the atmosphere about 11 kg per year of NO x (nitrogen oxides), which cause smog in major population areas. Natural gas burned in the furnace emits about 4.3 g of NO x per therm, and the electric power plants emit about 7.1 g of NO x per kWh of
What are the mechanisms of heat transfer?
Does any of the energy of the sun reach the earth by conduction or convection?
Which is a better heat conductor, diamond or silver?
How does forced convection differ from natural convection?
Define emissivity and absorptivity. What is Kirchhoff’s law of radiation?
What is blackbody? How do real bodies differ from a blackbody?
The inner and outer surfaces of a 5-m 6-m brick wall of thickness 30 cm and thermal conductivity 0.69 W/m €¢ °C are maintained at temperatures of 20°C and 5°C, respectively. Determine the rate of heat transfer through the wall, in W.
The inner and outer surfaces of a 0.5-cm-thick 2-m = 2-m window glass in winter are 10°C and 3°C, respectively. If the thermal conductivity of the glass is 0.78 W/m • °C, determine the amount of heat loss, in kJ, through the glass over a period of 5 h. What would your answer be if the glass
Reconsider Problem 2–96. Using EES (or other) software, investigate the effect of glass thickness on heat loss for the specified glass surface temperatures. Let the glass thickness vary from 0.2 to 2 cm. Plot the heat loss versus the glass thickness, and discuss the results.
An aluminum pan whose thermal conductivity is 237 W/m • °C has a flat bottom whose diameter is 20 cm and thickness 0.4 cm. Heat is transferred steadily to boiling water in the pan through its bottom at a rate of 500 W. If the inner surface of the bottom of the pan is 105°C, determine the
For heat transfer purposes, a standing man can be modeled as a 30-cm diameter, 170-cm long vertical cylinder with both the top and bottom surfaces insulated and with the side surface at an average temperature of 34°C. For a convection heat transfer coefficient of 15 W/m2 • °C, determine the
A 5-cm-diameter spherical ball whose surface is maintained at a temperature of 70°C is suspended in the middle of a room at 20°C. If the convection heat transfer coefficient is 15 W/m2 • C and the emissivity of the surface is 0.8, determine the total rate of heat transfer from the ball.
Reconsider Problem 2–100. Using EES (or other) software, investigate the effect of the convection heat transfer coefficient and surface emissivity on the heat transfer rate from the ball. Let the heat transfer coefficient vary from 5 to 30 W/m2 • °C. Plot the rate of heat transfer against the
Hot air at 80°C is blown over a 2-m x 4-m flat surface at 30°C. If the convection heat transfer coefficient is 55 W/m2 • °C, determine the rate of heat transfer from the air to the plate, in kW.
A 1000-W iron is left on the ironing board with its base exposed to the air at 20°C. The convection heat transfer coefficient between the base surface and the surrounding air is 35 W/m2 €¢ °C. If the base has an emissivity of 0.6 and a surface area of 0.02 m2, determine the
A thin metal plate is insulated on the back and exposed to solar radiation on the front surface. The exposed surface of the plate has an absorptivity of 0.6 for solar radiation. If solar radiation is incident on the plate at a rate of 700 W/m2 and the surrounding air temperature is 25°C,
Reconsider Problem 2–104. Using EES (or other) software, investigate the effect of the convection heat transfer coefficient on the surface temperature of the plate. Let the heat transfer coefficient vary from 10 to 90 W/m2 • °C. Plot the surface temperature against the convection heat transfer
A 5-cm-external-diameter, 10-m-long hot-water pipe at 80°C is losing heat to the surrounding air at 5°C by natural convection with a heat transfer coefficient of 25 W/m2 • °C. Determine the rate of heat loss from the pipe by natural convection, in kW.
The outer surface of a spacecraft in space has an emissivity of 0.8 and an absorptivity of 0.3 for solar radiation. If solar radiation is incident on the spacecraft at a rate of 1000 W/m2, determine the surface temperature of the spacecraft when the radiation emitted equals the solar energy
Reconsider Problem 2–107. Using EES (or other) software, investigate the effect of the surface emissivity and absorptivity of the spacecraft on the equilibrium surface temperature. Plot the surface temperature against emissivity for solar absorbtivities of 0.1, 0.5, 0.8, and 1, and discuss the
A hollow spherical iron container whose outer diameter is 20 cm and thickness is 0.4 cm is filled with iced water at 0°C. If the outer surface temperature is 5°C, determine the approximate rate of heat loss from the sphere, and the rate at which ice melts in the container.
The inner and outer glasses of a 2-m x 2-m double pane window are at 18°C and 6°C, respectively. If the 1-cm space between the two glasses is filled with still air, determine the rate of heat transfer through the window, in kW.
Two surfaces of a 2-cm-thick plate are maintained at 0°C and 100°C, respectively. If it is determined that heat is transferred through the plate at a rate of 500 W/m2, determine its thermal conductivity.
Consider a vertical elevator whose cabin has a total mass of 800 kg when fully loaded and 150 kg when empty. The weight of the elevator cabin is partially balanced by a 400-kg counterweight that is connected to the top of the cabin by cables that pass through a pulley located on top of the elevator
Consider a homeowner who is replacing his 25-yearold natural gas furnace that has an efficiency of 55 percent. The homeowner is considering a conventional furnace that has an efficiency of 82 percent and costs $1600 and a high efficiency furnace that has an efficiency of 95 percent and costs $2700.
Wind energy has been used since 4000 BC to power sailboats, grind grain, pump water for farms, and, more recently, generate electricity. In the United States alone, more than 6 million small windmills, most of them under 5 hp, have been used since the 1850s to pump water. Small windmills have been
Repeat Prob. 2–114 for an average wind velocity of 25 km/h.
The energy contents, unit costs, and typical conversion efficiencies of various energy sources for use in water heaters are given as follows: 1025 Btu/ft3, $0.012/ft3, and 55 percent for natural gas; 138,700 Btu/gal, $1.15/gal, and 55 percent for heating oil; and 1 kWh/kWh, $0.084/kWh, and 90
A homeowner is considering these heating systems for heating his house: Electric resistance heating with $0.09/kWh and 1 kWh = 3600 kJ, gas heating with $1.24/therm and 1 therm = 105,500 kJ, and oil heating with $1.25/gal and 1 gal of oil = 138,500 kJ. Assuming efficiencies of 100 percent for the
A typical household pays about $1200 a year on energy bills, and the U.S. Department of Energy estimates that 46 percent of this energy is used for heating and cooling, 15 percent for heating water, 15 percent for refrigerating and freezing, and the remaining 24 percent for lighting, cooking, and
The U.S. Department of Energy estimates that up to 10 percent of the energy use of a house can be saved by caulking and weather stripping doors and windows to reduce air leaks at a cost of about $50 for materials for an average home with 12 windows and 2 doors. Caulking and weather-stripping every
The U.S. Department of Energy estimates that 570,000 barrels of oil would be saved per day if every household in the United States lowered the thermostat setting in winter by 6°F (3.3°C). Assuming the average heating season to be 180 days and the cost of oil to be $40/barrel, determine how much
Consider a TV set that consumes 120 W of electric power when it is on and is kept on for an average of 6 hours per day. For a unit electricity cost of 8 cents per kWh, determine the cost of electricity this TV consumes per month (30 days).
The pump of a water distribution system is powered by a 15-kW electric motor whose efficiency is 90 percent. The water flow rate through the pump is 50 L/s. The diameters of the inlet and outlet pipes are the same, and the elevation difference across the pump is negligible. If the pressures at the
In a hydroelectric power plant, 100 m3/s of water flows from an elevation of 120 m to a turbine, where electric power is generated. The overall efficiency of the turbine–generator is 80 percent. Disregarding frictional losses in piping, estimate the electric power output of this plant.
The demand for electric power is usually much higher during the day than it is at night, and utility companies often sell power at night at much lower prices to encourage consumers to use the available power generation capacity and to avoid building new expensive power plants that will be used only
A diesel engine with an engine volume of 4.0 L and an engine speed of 2500 rpm operates on an air–fuel ratio of 18 kg air/kg fuel. The engine uses light diesel fuel that contains 750 ppm (parts per million) of sulfur by mass. All of this sulfur is exhausted to the environment where the sulfur is
Leaded gasoline contains lead that ends up in the engine exhaust. Lead is a very toxic engine emission. The use of leaded gasoline in the United States has been unlawful for most vehicles since the 1980s. However, leaded gasoline is still used in some parts of the world. Consider a city with 10,000
A 2-kW electric resistance heater in a room is turned on and kept on for 30 min. The amount of energy transferred to the room by the heater is (a) 1 kJ (b) 60 kJ (c) 1800 kJ (d) 3600 kJ (e) 7200 kJ
On a hot summer day, the air in a well-sealed room is circulated by a 0.50-hp fan driven by a 65 percent efficient motor. (Note that the motor delivers 0.50 hp of net shaft power to the fan.) The rate of energy supply from the fan motor assembly to the room is (a) 0.769 kJ/s (b) 0.325 kJ/s (c)
A fan is to accelerate quiescent air to a velocity to 12 m/s at a rate of 3 m3/min if the density of air is 1.15 kg/m3, the minimum power that must be supplied to the fan is (a) 248 W (b) 72 W (c) 497 W (d) 216 W (e) 162 W
A 900-kg car cruising at a constant speed of 60 km/s is to accelerate to 100 km/h in 6 s. The additional power needed to achieve this acceleration is (a) 41 kW (b) 222 kW (c) 1.7 kW (d) 26 kW (e) 37 kW
The elevator of a large building is to raise a net mass of 400 kg at a constant speed of 12 m/s using an electric motor. Minimum power rating of the motor should be (a) 0 kW (b) 4.8 kW (c)47 kW (d) 12 kW (e) 36 kW
Electric power is to be generated in a hydroelectric power plant that receives water at a rate of 70 m3/s from an elevation of 65 m using a turbine–generator with an efficiency of 85 percent when frictional losses in piping are disregarded the electric power output of this plant, is? (a) 3.9 MW
A 75-hp compressor in a facility that operates at full load for 2500 h a year is powered by an electric motor that has an efficiency of 88 percent. If the unit cost of electricity is $0.06/kWh, the annual electricity cost of this compressor is (a) $7382 (b) $9900 (c) $12,780 (d) $9533 (e) $8389
Consider a refrigerator that consumes 320 W of electric power when it is running. If the refrigerator runs only one quarter of the time and the unit cost of electricity is $0.09/kWh, the electricity cost of this refrigerator per month (30 days) is (a) $3.56 (b) $5.18 (c) $8.54 (d) $9.28 (e)
A 2-kW pump is used to pump kerosene (r = 0.820 kg/L) from a tank on the ground to a tank at a higher elevation. Both tanks are open to the atmosphere, and the elevation difference between the free surfaces of the tanks is 30 m. The maximum volume flow rate of kerosene is (a) 8.3 L/s (b) 7.2 L/s
A glycerin pump is powered by a 5-kW electric motor. The pressure differential between the outlet and the inlet of the pump at full load is measured to be 211 kPa. If the flow rate through the pump is 18 L/s and the changes in elevation and the flow velocity across the pump are negligible, the
A 10-cm high and 20-cm wide circuit board houses on its surface 100 closely spaced chips, each generating heat at a rate of 0.08 W and transferring it by convection to the surrounding air at 40°C. Heat transfer from the back surface of the board is negligible. If the convection heat transfer
A 50-cm-long, 0.2-cm-diameter electric resistance wire submerged in water is used to determine the boiling heat transfer coefficient in water at 1 atm experimentally. The surface temperature of the wire is measured to be 130°C when a wattmeter indicates the electric power consumption to be 4.1 kW.
A 3-m2 hot black surface at 80°C is losing heat to the surrounding air at 25°C by convection with a convection heat transfer coefficient of 12 W/m2 • °C, and by radiation to the surrounding surfaces at 15°C. The total rate of heat loss from the surface is (a) 1987 W (b) 2239 W (c) 2348 W
Heat is transferred steadily through a 0.2-m thick 8 m X 4 m wall at a rate of 1.6 kW. The inner and outer surface temperatures of the wall are measured to be 15°C to 5°C. The average thermal conductivity of the wall is (a) 0.001 W/m • °C (b) 0.5 W/m • °C (c) 1.0 W/m • °C (d) 2.0
The roof of an electrically heated house is 7-m long, 10-m wide, and 0.25-m thick. It is made of a flat layer of concrete whose thermal conductivity is 0.92 W/m • °C. During a certain winter night, the temperatures of the inner and outer surfaces of the roof are measured to be 15°C and 4°C,
Is iced water a pure substance? Why?
What is the difference between saturated liquid and compressed liquid?
What is the difference between saturated vapor and superheated vapor?
Is there any difference between the intensive properties of saturated vapor at a given temperature and the vapor of a saturated mixture at the same temperature?
Is there any difference between the intensive properties of saturated liquid at a given temperature and the liquid of a saturated mixture at the same temperature?
Is it true that water boils at higher temperatures at higher pressures? Explain.
If the pressure of a substance is increased during a boiling process, will the temperature also increase or will it remain constant? Why?
Why are the temperature and pressure dependent properties in the saturated mixture region?
What is the difference between the critical point and the triple point?
Is it possible to have water vapor at -10°C?
A househusband is cooking beef stew for his family in a pan that is (a) uncovered, (b) covered with a light lid, and (c) covered with a heavy lid. For which case will the cooking time be the shortest? Why?
How does the boiling process at supercritical pressures differ from the boiling process at sub critical pressures?
In what kind of pot will a given volume of water boil at a higher temperature: a tall and narrow one or a short and wide one? Explain.
A perfectly fitting pot and its lid often stick after cooking, and it becomes very difficult to open the lid when the pot cools down. Explain why this happens and what you would do to open the lid.
It is well known that warm air in a cooler environment rises. Now consider a warm mixture of air and gasoline on top of an open gasoline can. Do you think this gas mixture will rise in a cooler environment?
In 1775, Dr. William Cullen made ice in Scotland by evacuating the air in a water tank. Explain how that device works, and discuss how the process can be made more efficient.
Does the amount of heat absorbed as 1 kg of saturated liquid water boils at 100°C have to be equal to the amount of heat released as 1 kg of saturated water vapor condenses at 100°C?
Does the reference point selected for the properties of a substance have any effect on thermodynamic analysis? Why?
What is the physical significance of hfg? Can it be obtained from a knowledge of hf and hg? How?
Is it true that it takes more energy to vaporize 1 kg of saturated liquid water at 100°C than it would at 120°C?
What is quality? Does it have any meaning in the superheated vapor region?
Which process requires more energy: completely vaporizing 1 kg of saturated liquid water at 1 atm pressure or completely vaporizing 1 kg of saturated liquid water at 8 atm pressure?
Does hfg change with pressure? How?
Can quality be expressed as the ratio of the volume occupied by the vapor phase to the total volume? Explain.
In the absence of compressed liquid tables, how is the specific volume of a compressed liquid at a given P and T determined?
Complete this table for H2O:
Reconsider Prob. 3–26. Using EES (or other) software, determine the missing properties of water. Repeat the solution for refrigerant-134a, refrigerant-22, and ammonia.
Complete this table for H2O: T, °F P, psia u, Btu/lbm Phase description 300 782 40 Saturated liquid 500 120 400 400
Reconsider Prob. 3–28E Using EES (or other) software, determine the missing properties of water. Repeat the solution for refrigerant-134a, refrigerant- 22, and ammonia.
Complete this table for H2O
Complete this table for refrigerant-134a:
Complete this table for refrigerant-134a:
Complete this table for refrigerant-134a:
Complete this table for H2O:
Complete this table for H2O:
A 1.8-m3 rigid tank contains steam at 220°C. One third of the volume is in the liquid phase and the rest is in the vapor form. Determine (a) the pressure of the steam, (b) the quality of the saturated mixture, and (c) the density of the mixture.

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