Chapter_4_lecture.pdf

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Thermodynamics: An Engineering Approach, 6 th Edition
Yunus A. Cengel, Michael A. Boles
McGraw-Hill, 2008
Chapter 4
ENERGY ANALYSIS OF
CLOSED SYSTEMS
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
• Examine the moving boundary work or P dV work commonly
encountered in reciprocating devices such as automotive engines
and compressors.
• Identify the first law of thermodynamics as simply a statement of
the conservation of energy principle for closed (fixed mass)
systems.
• Develop the general energy balance applied to closed systems.
• Define the specific heat at constant volume and the specific heat at
constant pressure.
• Relate the specific heats to the calculation of the changes in
internal energy and enthalpy of ideal gases.
• Describe incompressible substances and determine the changes in
their internal energy and enthalpy.
• Solve energy balance problems for closed (fixed mass) systems
that involve heat and work interactions for general pure
substances, ideal gases, and incompressible substances.
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MOVING BOUNDARY WORK
Moving boundary work ( P dV work) :
The expansion and compression work
in a piston-cylinder device.
Quasi-equilibrium process :
A process during which the system
remains nearly in equilibrium at all
times.
W b is positive → for expansion
W b is negative → for compression
The work associated
with a moving
boundary is called
boundary work.
A gas does a
differential
amount of work
δ W b as it forces
the piston to
move by a
differential
amount ds .
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The boundary
work done
during a process
depends on the
path followed as
well as the end
states.
The area under the process
curve on a P - V diagram
represents the boundary work.
The net work done
during a cycle is the
difference between
the work done by
the system and the
work done on the
system.
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Polytropic, Isothermal, and Isobaric processes
Polytropic process: C , n (polytropic exponent) constants
Polytropic
process
Polytropic and for ideal gas
When n = 1
(isothermal process)
Constant pressure process
What is the boundary
work for a constant-
volume process?
Schematic and
P-V diagram for
a polytropic
process.
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