MOVING BOUNDARY WORK - جامعة نزوى...• Examine the moving boundary work or P dV work...
Transcript of MOVING BOUNDARY WORK - جامعة نزوى...• Examine the moving boundary work or P dV work...
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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
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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.
Wb is positive for expansion
Wb is negative for compression
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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 is equal, in magnitude, to the work
done during a quasi-equilibrium expansion or
compression process of a closed system.
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5 kPa
400 kPa
v, m3/kg
P,
kPa
400
Boundary Work for a constant-pressure
Boundary Work for a constant-pressure
• Example 4-2
A frictionless piston-cylinder device contains 5 kg of steam at 400 kpa and 200oC. Heat is now transferred to the steam until the temperature reaches 250oC. If the piston is not attached to a shaft and its mass is constant . Determine the work done by the steam during this process.
• Solution
steam in a piston cylinder device is heated and the temperature rises at constant pressure. The boundary work done is to be determined.
Analysis:
Assumption:
Even though it isn’t explicitly stated, the pressure of the steam within the cylinder remains constant during this process since both the atmospheric pressure and the weight of the piston remain constant. Therefore, this is a constant-pressure process and, from equation 4-2 (Refer to the book).
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What is the boundary
work for a constant-
volume process?
Boundary Work for a constant-Volume
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Boundary Work for an Isothermal Compression Process
• Example 4-3
• A piston-cylinder device initially contains 0.4 m3 of air at 100 kPa and 80oC. The air is now compressed to 0.1 m3 in such a way that the temperature inside the cylinder remains constant. Determine the work done during this process.
• Solution
air in a piston-cylinder device is compressed isothermally. The boundary work done is to be determined.
Analysis a sketch of the system and the P-V diagram of the process shown in Fig 4-8.
Assumption: at specified conditions, air can be considered to be an ideal gas since it is a high temperature and low pressure relative to its critical-point values To,
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Boundary Work for an Isothermal Compression Process
Apply the equation of boundary work related to isothermal compression of ideal gas
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SPECIFIC HEATS Specific heat at constant volume, cv: The
energy required to raise the temperature of
the unit mass of a substance by one degree
as the volume is maintained constant.
Specific heat at constant pressure, cp: The
energy required to raise the temperature of
the unit mass of a substance by one degree
as the pressure is maintained constant.
Constant-
volume and
constant-
pressure specific
heats cv and cp
(values are for
helium gas).
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• The equations are valid for any
substance undergoing any process.
• cv and cp are properties.
• cv is related to the changes in internal
energy and cp to the changes in
enthalpy.
• A common unit for specific heats is
kJ/kg·°C or kJ/kg·K. Are these units
identical?
True or False?
cp is always greater than cv
Formal definitions of cv and cp.
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Summary
• Moving boundary work
Wb for an isothermal process
Wb for a constant-pressure process
Wb for a polytropic process
• Specific heats
Constant-pressure specific heat, cp
Constant-volume specific heat, cv