ENGR 2213 Thermodynamics

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ENGR 2213 Thermodynamics

• F. C. Lai
• School of Aerospace and Mechanical
• Engineering
• University of Oklahoma

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Example 4
Determine the temperature of water at a state
of P 0.5 MPa and v 0.45 m3/kg.
Table A-5, P 0.5 MPa, vg 0.3749 m3/kg
v gt vg, ? superheated vapor
Table A-6, P 0.5 MPa,
T v 200 0.4249 250
0.4744
v 0.45
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Example 4 (continued)
Determine the temperature of water at a state
of P 0.5 MPa and v 0.45 m3/kg.
T v 200 0.4249 250
0.4744
v 0.45
T 225.3 ºC
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Example 5
Determine the specific volume of water at 80 ºC
and 5 MPa.
Table A-4, T 80 ºC, Psat 47.39 kPa
P gt Psat, ? compressed liquid
Table A-5, P 5 MPa, Tsat 263.99 ºC
T lt Tsat, ? subcooled liquid
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Example 5 (continued)
Determine the specific volume of water at 80 ºC
and 5 MPa.
Table A-7, P 5 MPa and T 80 ºC
v 0.0010268 m3/kg
Table A-4, T 80 ºC
vf 0.0010291 m3/kg
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Example 5 (continued)
For compressed liquid, if you cannot find the
data in Table A-7, use the saturated water table
(Table A-4) as an approximation.
When use the saturated water table as
an approximation, look up properties based on the
temperature, not the pressure.
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Work
Work is the energy transfer associated with a
force acting through a distance.
To evaluate the integral, it is necessary to
know how the force varies with the displacement.
The value of W depends on the process path and
not just the initial and final states of the
system.
Work is not a property of a system or its
surroundings
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Work
The notion of work at a state has no meaning.
Sign Convention W gt 0, if work is done by the
system W lt 0, if work is done on the system
For a simple compressible system, dW F ds PA
ds P dV
Expansion dV gt 0
dW gt 0
Compression dV lt 0
dW lt 0
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Work
For a simple compressible system,
To perform the integration, it requires a
relationship between the pressure and volume.
The work done can be presented by the area under
the curve of pressure versus volume.
P
1
2
W
V
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Example
Air contained in a piston-cylinder assembly
undergoes an expansion process (pVn
constant). Given p1 300 kPa, V1 0.1 m3
p2 ? V2 0.2 m3 Find W ? if
(a) n 1.5, (b) n 1.0, (c) n 0.
pVn constant
p1 V1n p2V2n C
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Example (continued)
(c) n 0, p constant, p1 p2 W
p1(V2 V1) 300 (0.2 0.1) 30 kJ
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Example (continued)
(b) n 1, pV constant, p1V1 p2V2 C
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Other Types of Work
Shaft Work
Electric Work
Spring Work
Stretching a Liquid Film
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Heat
Heat is the form of energy that is transferred
between two systems (or a system and its
surroundings) by virtue of a temperature
difference.
The direction of heat transfer is always from the
higher temperature body to the lower
temperature body.
Once the temperature equality is established, the
heat transfer stops.
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Heat
Sign Convention Q gt 0, if heat is transferred to
the system Q lt 0, if heat is transferred from the
system
The sign convention for heat transfer is just
the reverse of that adopted for work.
Like work, heat is not a property. The notion of
heat at a state has no meaning.
Q 0
Q 0 adiabatic process.
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Heat and Work
Heat Fluid Joseph Black (1760) ? Heat is fluid
like substance.
Caloric Theory Antoine Lavoisier (1789) ? Heat
is fluid like substance called the caloric. ? It
is massless, colorless, odorless, and
tasteless. ? It cannot be created or destroyed.
Creation of Heat Benjamin Thompson (1798) ? It
is possible to create any quantity of heat from
mechanical work by means of friction.
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Heat and Work
Equivalence of Heat and Work ? It was first
formulated by Robert Mayer (1842). ? He also
calculated the conversion factor based on
the experimental data provided by Gay-Lussac. ?
The finding was refused to publish in Annalen
der Physik. ? The result was finally
published in Annalen der Chemie und Pharmazie.
? James P. Joule also independently determined
the equivalence of heat and work (1843).
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Similarities between Heat and Work
? Heat and Work are boundary phenomena Both
are recognized at the boundaries of the
system.
? Heat and Work are transient phenomena Both
are associated with a process, not a state.
? Heat and Work are not properties Systems
possess energy, but not heat or work.
? Heat and Work are functions of path Their
value depends on the path followed during a
process as well as the end states.
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Heat Transfer
? Conduction Energy exchange takes place by
the kinetic motion or direct impact of
molecules.
Fouriers Law
Q Heat transfer rate k Thermal conductivity
A Area normal to the direction of heat flow
The minus sign is inserted so that the 2nd Law of
Thermodynamics is satisfied.
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Heat Transfer
Mechanism of Heat Conduction
? Gases - Collisions
? Liquids - Collisions
? Solids - Lattice vibration and transport by
free electrons
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Heat Transfer
? Convection Energy exchange takes place as
a consequence of the relative motion of fluid.
Newtons Cooling Law
Q Heat transfer rate h Heat transfer coefficient
Heat transfer coefficient is not a property. It
is determined by experiments
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Heat Transfer
Type of Heat Convection
? Forced convection if the fluid motion is
artificially induced
? Natural convection if the fluid motion is
set up by buoyancy effects resulting from
density difference caused by temperature
variation in the fluid
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Heat Transfer
? Radiation Energy exchange takes place by
emission and absorption of electromagnetic
waves.
Stefan-Boltzmann Law of Thermal Radiation
s Stefan-Boltzmann constant, 5.67x10-8 W/m2K4
This equation is valid only for thermal radiation
and it applies only to blackbodies.
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Heat Transfer
Thermal Radiation
? For real bodies
e emissivity, 0 e 1
? Radiation exchange
FG geometric view factor