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3-1
Solutions Manual
for
Fundamentals of Thermal Fluid Sciences
5th Edition
Yunus A. Çengel, John M. Cimbala, Robert H. Turner
McGraw-Hill, 2017
Chapter 3
ENERGY, ENERGY TRANSFER, AND
GENERAL ENERGY ANALYSIS
PROPRIETARY AND CONFIDENTIAL
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3-2
Forms of Energy
3-2C The internal energy of a system is made up of sensible, latent, chemical and nuclear energies. The sensible internal
energy is due to translational, rotational, and vibrational effects.
3-4C The mechanical energy is the form of energy that can be converted to mechanical work completely and directly by a
3-5C Hydrogen is also a fuel, since it can be burned, but it is not an energy source since there are no hydrogen reserves in
3-6C In electric heaters, electrical energy is converted to sensible internal energy.
3-8E The total kinetic energy of an object is given is to be determined.
Analysis The total kinetic energy of the object is given by
/sft 037,25
Btu/lbm 1
2
)ft/s 50(
2
2
2
V
3-3
3-9E The total potential energy of an object is to be determined.
Analysis Substituting the given data into the potential energy expression gives
2
/sft 037,25
Btu/lbm 1
3-10 A person with his suitcase goes up to the 10th floor in an elevator. The part of the energy of the elevator stored in the
suitcase is to be determined.
3-11 A hydraulic turbine-generator is to generate electricity from the water of a large reservoir. The power generation
potential is to be determined.
3-5
3-14 Two sites with specified wind data are being considered for wind power generation. The site better suited for wind
power generation is to be determined.
3-7
Energy Transfer by Heat and Work
3-17C The form of energy that crosses the boundary of a closed system because of a temperature difference is heat; all
other forms are work.
3-19C It is a work interaction.
3-21C It is a heat interaction since it is due to the temperature difference between the sun and the room.
3-23 The power produced by an electrical motor is to be expressed in different units.
Analysis Using appropriate conversion factors, we obtain
mN 1
J/s 1
3-8
3-24E The power produced by a model aircraft engine is to be expressed in different units.
PROPRIETARY MATERIAL. © 2017 McGraw-Hill Education. Limited distribution permitted only to teachers and educators for course preparation. If
you are a student using this Manual, you are using it without permission.
3-36C Energy can be transferred to or from a control volume as heat, various forms of work, and by mass transport.
3-38 Water is heated in a pan on top of a range while being stirred. The energy of the water at the end of the process is to
be determined.
3-39E Water is heated in a cylinder on top of a range. The change in the energy of the water during this process is to be
determined.
3-13
3-40E The heat loss from a house is to be made up by heat gain from people, lights, appliances, and resistance heaters. For a
3-14
3-42 A water pump is claimed to raise water to a specified elevation at a specified rate while consuming electric power at a
specified rate. The validity of this claim is to be investigated.
3-43 A classroom is to be air-conditioned using window air-conditioning units. The cooling load is due to people, lights,
and heat transfer through the walls and the windows. The number of 5-kW window air conditioning units required is to be
determined.
kW/unit 5
3-16
3-46E A fan accelerates air to a specified velocity in a square duct. The minimum electric power that must be supplied to
the fan motor is to be determined.
3-47 The fan of a central heating system circulates air through the ducts. For a specified pressure rise, the highest possible
average flow velocity is to be determined.
3-18
3-50 A car cruising at a constant speed to accelerate to a specified speed within a specified time. The additional power
needed to achieve this acceleration is to be determined.
Energy Conversion Efficiencies
3-51C The combined pump-motor efficiency of a pump/motor system is defined as the ratio of the increase in the
mechanical energy of the fluid to the electrical power consumption of the motor,
inelect,
pump
inelect,
fluidmech,
inelect,
inmech,outmech,
motorpumpmotor-pump W
W
W
E
W
EE
The combined pump-motor efficiency cannot be greater than either of the pump or motor efficiency since both pump and
motor efficiencies are less than 1, and the product of two numbers that are less than one is less than either of the numbers.
3-52C The turbine efficiency, generator efficiency, and combined turbine-generator efficiency are defined as follows:
||
fluid thefromextracted energy Mechanical
output energy Mechanical
fluidmech,
outshaft,
turbine E
W
inshaft,
outelect,
generator input power Mechanical
output power Electrical
W
W
|| fluidmech,
outelect,
outmech,inmech,
outelect,
generatorturbinegen-turbine E
W
EE
W
