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engineering
mechanical engineering

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

Determine the maximum amount of load, in kg, the balloon described in Prob. 1–101 can carry. Answer: 520.5 kg
The pressure in a steam boiler is given to be 92 kgf/cm2. Express this pressure in psi, kPa, atm, and bars.
The basic barometer can be used as an altitude measuring device in airplanes. The ground control reports a barometric reading of 753 mm Hg while the pilot’s reading is 690 mm Hg. Estimate the altitude of the plane from ground level if the average air density is 1.20 kg/m3.
The lower half of a 10-m-high cylindrical container is filled with water (r = 1000 kg/m3) and the upper half with oil that has a specific gravity of 0.85. Determine the pressure difference between the top and bottom of the cylinder.
A vertical, frictionless piston–cylinder device contains a gas at 250 kPa absolute pressure. The atmospheric pressure outside is 100 kPa, and the piston area is 30 cm2. Determine the mass of the piston.
A pressure cooker cooks a lot faster than an ordinary pan by maintaining a higher pressure and temperature inside. The lid of a pressure cooker is well sealed, and steam can escape only through an opening in the middle of the lid. A separate metal piece, the petcock, sits on top of this opening and
A glass tube is attached to a water pipe, as shown in Fig. P1€“109, if the water pressure at the bottom of the tube is 115 kPa and the local atmospheric pressure is 92 kPa, determine how high the water will rise in the tube, in m. Take the density of water to be 1000 kg/m3.
The average atmospheric pressure on earth is approximated as a function of altitude by the relation Patm = 101.325 (1 = 0.02256z 5.256, where Patm is the atmospheric pressure in kPa and z is the altitude in km with z = 0 at sea level. Determine the approximate atmospheric pressures at Atlanta (z =
When measuring small pressure differences with a manometer, often one arm of the manometer is inclined to improve the accuracy of reading. (The pressure difference is still proportional to the vertical distance and not the actual length of the fluid along the tube.) The air pressure in a circular
Consider a U-tube whose arms are open to the atmosphere. Now equal volumes of water and light oil (r = 49.3 lbm/ft3) are poured from different arms. A person blows from the oil side of the U-tube until the contact surface of the two fluids moves to the bottom of the U-tube, and thus the liquid
Intravenous infusions are usually driven by gravity by hanging the fluid bottle at sufficient height to counteract the blood pressure in the vein and to force the fluid into the body. The higher the bottle is raised, the higher the flow rate of the fluid will be.(a) If it is observed that the fluid
A gasoline line is connected to a pressure gage through a double-U manometer, as shown in Fig. P1€“114, if the reading of the pressure gage is 370 kPa determine the gage pressure of the gasoline line.
Repeat Prob. 1€“114 for a pressure gage reading of 180 kPa.
A water pipe is connected to a double-U manometer as shown in Fig. P1€“116E at a location where the local atmospheric pressure is 14.2 psia, determine the absolute pressure at the center of the pipe.
It is well-known that the temperature of the atmosphere varies with altitude. In the troposphere, which extends to an altitude of 11 km, for example, the variation of temperature can be approximated by T = T0 - bz, where T0 is the temperature at sea level, which can be taken to be 288.15 K, and B =
The variation of pressure with density in a thick gas layer is given by P = Cpn, where C and n are constants. Noting that the pressure change across a differential fluid layer of thickness dz in the vertical z-direction is given as dP = - pg dz, obtain a relation for pressure as a function of
Pressure transducers are commonly used to measure pressure by generating analog signals usually in the range of 4 mA to 20 mA or 0 V-dc to 10 V-dc in response to applied pressure. The system whose schematic is shown in Fig P1€“119 can be used to calibrate pressure transducers. A rigid
Consider a fish swimming 5 m below the free surface of water. The increase in the pressure exerted on the fish when it dives to a depth of 45 m below the free surface is (a) 392 Pa (b) 9800 Pa (c) 50,000 Pa (d) 392,000 Pa (e) 441,000 Pa
The atmospheric pressures at the top and the bottom of a building are read by a barometer to be 96.0 and 98.0 kPa. If the density of air is 1.0 kg/m3, the height of the building is (a) 17 m (b) 20 m (c) 170 m (d) 204 m (e) 252 m
An apple loses 4.5 kJ of heat as it cools per °C drop in its temperature. The amount of heat loss from the apple per °F drop in its temperature is (a) 1.25 kJ (b) 2.50 kJ (c) 5.0 kJ (d) 8.1 kJ (e) 4.1 kJ
Consider a 2-m deep swimming pool. The pressure difference between the top and bottom of the pool is (a) 12.0 kPa (b) 19.6 kPa (c) 38.1 kPa (d) 50.8 kPa (e) 200 kPa
At sea level, the weight of 1 kg mass in SI units is 9.81 N. The weight of 1 lbm mass in English units is (a) 1 lbf (b) 9.81 lbf (c) 32.2 lbf (d) 0.1 lbf (e) 0.031 lbf
During a heating process, the temperature of an object rises by 20°C. This temperature rise is equivalent to a temperature rise of (a) 20°F (b) 52°F (c) 36 K (d) 36 R (e) 293 K
A river flowing steadily at a rate of 240 m3/s is considered for hydroelectric power generation. It is determined that a dam can be built to collect water and release it from an elevation difference of 50 m to generate power. Determine how much power can be generated from this river water after the
A person gets into an elevator at the lobby level of a hotel together with his 30-kg suitcase, and gets out at the 10th floor 35 m above. Determine the amount of energy consumed by the motor of the elevator that is now stored in the suitcase.
In what forms can energy cross the boundaries of a closed system?
When is the energy crossing the boundaries of a closed system heat and when is it work?
What is an adiabatic process? What is an adiabatic system?
A gas in a piston–cylinder device is compressed, and as a result its temperature rises. Is this a heat or work interaction?
A room is heated by an iron that is left plugged in. Is this a heat or work interaction? Take the entire room, including the iron, as the system.
A room is heated as a result of solar radiation coming in through the windows. Is this a heat or work interaction for the room?
An insulated room is heated by burning candles. Is this a heat or work interaction? Take the entire room, including the candles, as the system.
What are point and path functions? Give some examples.
What is the caloric theory? When and why was it abandoned?
A car is accelerated from rest to 85 km/h in 10 s, would the energy transferred to the car be different if it were accelerated to the same speed in 5 s?
Lifting a weight to a height of 20 m takes 20 s for one crane and 10 s for another. Is there any difference in the amount of work done on the weight by each crane?
Determine the energy required to accelerate an 800- kg car from rest to 100 km/h on a level road.
Determine the energy required to accelerate a 1300- kg car from 10 to 60 km/h on an uphill road with a vertical rise of 40 m.
Determine the torque applied to the shaft of a car that transmits 450 hp and rotates at a rate of 3000 rpm.
Determine the work required to deflect a linear spring with a spring constant of 70 kN/m by 20 cm from its rest position.
The engine of a 1500-kg automobile has a power rating of 75 kW. Determine the time required to accelerate this car from rest to a speed of 100 km/h at full power on a level road. Is your answer realistic?
A ski lift has a one-way length of 1 km and a vertical rise of 200 m. The chairs are spaced 20 m apart, and each chair can seat three people. The lift is operating at a steady speed of 10 km/h. Neglecting friction and air drag and assuming that the average mass of each loaded chair is 250 kg,
Determine the power required for a 2000-kg car to climb a 100-m-long uphill road with a slope of 30° (from horizontal) in 10 s(a) At a constant velocity,(b) From rest to a final velocity of 30 m/s, and(c) From 35 m/s to a final velocity of 5 m/s, disregard friction, air drag, and rolling
A damaged 1200-kg car is being towed by a truck. Neglecting the friction, air drag, and rolling resistance determine the extra power required (a) For constant velocity on a level road, (b) For constant velocity of 50 km/h on a 30° (from horizontal) uphill road, and (c) To accelerate on a
Portable electric heaters are commonly used to heat small rooms. Explain the energy transformation involved during this heating process.
Consider the process of heating water on top of an electric range. What are the forms of energy involved during this process? What are the energy transformations that take place?
What is the difference between the macroscopic and microscopic forms of energy?
What is total energy? Identify the different forms of energy that constitute the total energy.
List the forms of energy that contribute to the internal energy of a system.
How are heat, internal energy, and thermal energy related to each other?
What is mechanical energy? How does it differ from thermal energy? What are the forms of mechanical energy of a fluid stream?
Consider a river flowing toward a lake at an average velocity of 3 m/s at a rate of 500 m3/s at a location 90 m above the lake surface determine the total mechanical energy of the river water per unit mass and the power generation potential of the entire river at that location.
Electric power is to be generated by installing a hydraulic turbine–generator at a site 120 m below the free surface of a large water reservoir that can supply water at a rate of 1500 kg/s steadily. Determine the power generation potential.
At a certain location, wind is blowing steadily at 10 m/s. Determine the mechanical energy of air per unit mass and the power generation potential of a wind turbine with 60-m-diameter blades at that location. Take the air density to be 1.25 kg/m3.
A water jet that leaves a nozzle at 60 m/s at a flow rate of 120 kg/s is to be used to generate power by striking the buckets located on the perimeter of a wheel. Determine the power generation potential of this water jet.
Two sites are being considered for wind power generation. In the first site, the wind blows steadily at 7 m/s for 3000 hours per year, whereas in the second site the wind blows at 10 m/s for 2000 hours per year. Assuming the wind velocity is negligible at other times for simplicity determines which
For a cycle, is the net work necessarily zero? For what kind of systems will this be the case?
On a hot summer day, a student turns his fan on when he leaves his room in the morning. When he returns in the evening, will the room be warmer or cooler than the neighboring rooms? Why? Assume all the doors and windows are kept closed.
What are the different mechanisms for transferring energy to or from a control volume?
Water is being heated in a closed pan on top of a range while being stirred by a paddle wheel. During the process, 30 kJ of heat is transferred to the water, and 5 kJ of heat is lost to the surrounding air. The paddle-wheel work amounts to 500 N €¢ m. Determine the final energy of the
A vertical piston–cylinder device contains water and is being heated on top of a range. During the process, 65 Btu of heat is transferred to the water and heat losses from the side walls amount to 8 Btu. The piston rises as a result of evaporation, and 5 Btu of work is done by the vapor.
A classroom that normally contains 40 people is to be air-conditioned with window air-conditioning units of 5- kW cooling capacity. A person at rest may be assumed to dissipate heat at a rate of about 360 kJ/h there are 10 lightbulbs in the room, each with a rating of 100 W the rate of heat
The lighting requirements of an industrial facility are being met by 700 40-W standard fluorescent lamps. The lamps are close to completing their service life and are to be replaced by their 34-W high-efficiency counterparts that operate on the existing standard ballasts. The standard and high
The lighting needs of a storage room are being met by 6 fluorescent light fixtures, each fixture containing four lamps rated at 60 W each. All the lamps are on during operating hours of the facility, which are 6 AM to 6 PM 365 days a year. The storage room is actually used for an average of 3 h a
A university campus has 200 classrooms and 400 faculty offices. The classrooms are equipped with 12 fluorescent tubes, each consuming 110 W, including the electricity used by the ballasts. The faculty offices, on average, have half as many tubes. The campus is open 240 days a year. The classrooms
Consider a room that is initially at the outdoor temperature of 20°C. The room contains a 100-W lightbulb, a 110-W TV set, a 200-W refrigerator, and a 1000-W iron. Assuming no heat transfer through the walls, determine the rate of increase of the energy content of the room when all of these
A fan is to accelerate quiescent air to a velocity of 10 m/s at a rate of 4 m3/s. Determine the minimum power that must be supplied to the fan. Take the density of air to be 1.18 kg/m3
Consider a fan located in a 3 ft = 3 ft square duct. Velocities at various points at the outlet are measured, and the average flow velocity is determined to be 22 ft/s. Taking the air density to 0.075 lbm/ft3 estimate the minimum electric power consumption of the fan motor.
A water pump that consumes 2 kW of electric power when operating is claimed to take in water from a lake and pump it to a pool whose free surface is 30 m above the free surface of the lake at a rate of 50 L/s. Determine if this claim is reasonable.
The driving force for fluid flow is the pressure difference, and a pump operates by raising the pressure of a fluid (by converting the mechanical shaft work to flow energy). A gasoline pump is measured to consume 5.2 kW of electric power when operating. If the pressure differential between the
The 60-W fan of a central heating system is to circulate air through the ducts. The analysis of the flow shows that the fan needs to raise the pressure of air by 50 Pa to maintain flow. The fan is located in a horizontal flow section whose diameter is 30 cm at both the inlet and the outlet.
At winter design conditions, a house is projected to lose heat at a rate of 60,000 Btu/h. The internal heat gain from people, lights, and appliances is estimated to be 6000 Btu/h. If this house is to be heated by electric resistance heaters, determine the required rated power of these heaters in kW
An escalator in a shopping center is designed to move 30 people, 75 kg each, at a constant speed of 0.8 m/s at 45° slope. Determine the minimum power input needed to drive this escalator. What would your answer be if the escalator velocity were to be doubled?
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, compute the total rate of heat transfer from the ball.
Consider a 1400-kg car cruising at constant speed of 70 km/h. Now the car starts to pass another car, by accelerating to 110 km/h in 5 s. Determine the additional power needed to achieve this acceleration. What would your answer be if the total mass of the car were only 700 kg?
What is mechanical efficiency? What does a mechanical efficiency of 100 percent mean for a hydraulic turbine?
How is the combined pump–motor efficiency of a pump and motor system defined? Can the combined pump–motor efficiency be greater than either the pump or the motor efficiency?
Define turbine efficiency, generator efficiency, and combined turbine–generator efficiency.
Can the combined turbine-generator efficiency be greater than either the turbine efficiency or the generator efficiency? Explain.
Consider a 3-kW hooded electric open burner in an area where the unit costs of electricity and natural gas are $0.07/kWh and $1.20/therm, respectively. The efficiency of open burners can be taken to be 73 percent for electric burners and 38 percent for gas burners. Determine the rate of energy
A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is replaced by a high-efficiency 75-hp motor that has an efficiency of 95.4 percent. Determine the reduction in the heat gain of the room due to higher efficiency under full-load conditions.
A 90-hp (shaft output) electric car is powered by an electric motor mounted in the engine compartment. If the motor has an average efficiency of 91 percent, determine the rate of heat supply by the motor to the engine compartment at full load.
A 75-hp (shaft output) motor that has an efficiency of 91.0 percent is worn out and is to be replaced by a high efficiency motor that has an efficiency of 95.4 percent. The motor operates 4368 hours a year at a load factor of 0.75. Taking the cost of electricity to be $0.08/kWh, determine the
The steam requirements of a manufacturing facility are being met by a boiler whose rated heat input is 3.6 x 106 Btu/h. The combustion efficiency of the boiler is measured to be 0.7 by a hand-held flue gas analyzer. After tuning up the boiler, the combustion efficiency rises to 0.8. The boiler
Reconsider Prob. 2–60E Using EES (or other) software, study the effects of the unit cost of energy and combustion efficiency on the annual energy used and the cost savings. Let the efficiency vary from 0.6 to 0.9, and the unit cost to vary from $4 to $6 per million Btu. Plot the annual energy
An exercise room has eight weight-lifting machines that have no motors and four treadmills each equipped with a 2.5-hp (shaft output) motor. The motors operate at an average load factor of 0.7, at which their efficiency is 0.77. During peak evening hours, all 12 pieces of exercising equipment are
Consider a classroom for 55 students and one instructor, each generating heat at a rate of 100 W. Lighting is provided by 18 fluorescent lightbulbs, 40 W each, and the ballasts consume an additional 10 percent. Determine the rate of internal heat generation in this classroom when it is fully
A room is cooled by circulating chilled water through a heat exchanger located in a room. The air is circulated through the heat exchanger by a 0.25-hp (shaft output) fan. Typical efficiency of small electric motors driving 0.25-hp equipment is 54 percent. Determine the rate of heat supply by the
Electric power is to be generated by installing a hydraulic turbine–generator at a site 70 m below the free surface of a large water reservoir that can supply water at a rate of 1500 kg/s steadily. If the mechanical power output of the turbine is 800 kW and the electric power generation is 750
At a certain location, wind is blowing steadily at 12 m/s. Determine the mechanical energy of air per unit mass and the power generation potential of a wind turbine with 50- m-diameter blades at that location. Also determine the actual electric power generation assuming an overall efficiency of 30
Reconsider Prob. 2–66. Using EES (or other) software, investigate the effect of wind velocity and the blade span diameter on wind power generation. Let the velocity vary from 5 to 20 m/s in increments of 5 m/s, and the diameter vary from 20 to 80 m in increments of 20 m, tabulate the results, and
A wind turbine is rotating at 15 rpm under steady winds flowing through the turbine at a rate of 42,000 kg/s. The tip velocity of the turbine blade is measured to be 250 km/h. If 180 kW power is produced by the turbine, determine (a) The average velocity of the air and (b) The conversion
Water is pumped from a lake to a storage tank 20 m above at a rate of 70 L/s while consuming 20.4 kW of electric power. Disregarding any frictional losses in the pipes and any changes in kinetic energy, determine(a) The overall efficiency of the pump€“motor unit and(b) The pressure
A geothermal pump is used to pump brine whose density is 1050 kg/m3 at a rate of 0.3 m3/s from a depth of 200 m, For a pump efficiency of 74 percent, determine the required power input to the pump. Disregard frictional losses in the pipes, and assume the geothermal water at 200 m depth to be
Consider an electric motor with a shaft power output of 20 kW and an efficiency of 88 percent. Determine the rate at which the motor dissipates heat to the room it is in when the motor operates at full load. In winter, this room is normally heated by a 2-kW resistance heater. Determine if it is
Large wind turbines with blade span diameters of over 100 m are available for electric power generation. Consider a wind turbine with a blade span diameter of 100 m installed at a site subjected to steady winds at 8 m/s. Taking the overall efficiency of the wind turbine to be 32 percent and the air
A water pump delivers 3 hp of shaft power when operating. If the pressure differential between the outlet and the inlet of the pump is measured to be 1.2 psi when the flow rate is 8 ft3/s and the changes in velocity and elevation are negligible, determine the mechanical efficiency of this pump.
Water is pumped from a lower reservoir to a higher reservoir by a pump that provides 20 kW of shaft power. The free surface of the upper reservoir is 45 m higher than that of the lower reservoir. If the flow rate of water is measured to be 0.03 m3/s, determine mechanical power that is converted to
A 7-hp (shaft) pump is used to raise water to an elevation of 15 m. If the mechanical efficiency of the pump is 82 percent, determine the maximum volume flow rate of water.
A hydraulic turbine has 85 m of elevation difference available at a flow rate of 0.25 m3/s and its overall turbine– generator efficiency is 91 percent. Determine the electric power output of this turbine.

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