Chapter_5_lecture.pdf

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Thermodynamics: An Engineering Approach, 6 th Edition
Yunus A. Cengel, Michael A. Boles
McGraw-Hill, 2008
Chapter 5
MASS AND ENERGY
ANALYSIS OF CONTROL
VOLUMES
Created by Mehmet Kanoglu
Modified by Kai Ming Kiang
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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Objectives
•
Develop the conservation of mass principle.
•
Apply the conservation of mass principle to various systems
including steady- and unsteady-flow control volumes.
•
Apply the first law of thermodynamics as the statement of the
conservation of energy principle to control volumes.
•
Identify the energy carried by a fluid stream crossing a control
surface as the sum of internal energy, flow work, kinetic energy,
and potential energy of the fluid and to relate the combination of
the internal energy and the flow work to the property enthalpy.
•
Solve energy balance problems for common steady-flow devices
such as nozzles, compressors, turbines, throttling valves, mixers,
heaters, and heat exchangers.
•
Apply the energy balance to general unsteady-flow processes with
particular emphasis on the uniform-flow process as the model for
commonly encountered charging and discharging processes.
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CONSERVATION OF MASS
Conservation of mass : Mass, like energy, is a conserved property,
and it cannot be created or destroyed during a process.
Closed systems : The mass of the system remain constant during a
process.
Control volumes : Mass can cross the boundaries, and so we must
keep track of the amount of mass entering and leaving the control
volume.
Mass is conserved even during chemical reactions.
Mass m and energy E can be converted to each other according to
where c is the speed of light in a vacuum, which is c = 2.9979 × 10 8 m/s.
The mass change due to energy change is absolutely negligible.
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Mass and Volume Flow Rates
Definition of
average velocity
Volume flow rate
Mass flow
rate
The average velocity V avg is
defined as the average speed
through a cross section.
The volume flow rate is the
volume of fluid flowing through a
cross section per unit time.
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Conservation of Mass Principle
The conservation of mass principle for a control volume : The net mass transfer
to or from a control volume during a time interval ∆ t is equal to the net change
(increase or decrease) in the total mass within the control volume during ∆ t .
General conservation of mass
General conservation of mass in rate form
or
Conservation of mass principle
for an ordinary bathtub.
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