Transcript Document 7522490

ME 152
Thermodynamics
G.A. Kallio
Dept. of Mechanical Engineering,
Mechatronic Engineering &
Manufacturing Technology
California State University, Chico
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Basic Concepts &
Definitions
Reading: Cengel & Boles,
Chapter 1
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Introduction
• Thermodynamics - science that
deals with energy, matter, and the
laws governing their interaction
– general: all engineering systems
involve energy and matter
– fundamental: based upon primitive
concepts (two primary laws)
– employs a unique vocabulary based
upon precise definitions
– initially, it appears formal and abstract,
but its significance and application will
eventually be seen
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Introduction, cont.
• Classical Thermodynamics macroscopic approach that deals
with large systems, e.g., engines,
power plants, refrigerators, etc.;
studied and used by engineers
• Statistical Thermodynamics microscopic approach that deals with
the structure and properties of matter
on an atomic/molecular level;
studied and used by physicists and
chemists
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Primary Laws of
Thermodynamics
• First Law of Thermodynamics quantitative conservation of energy
principle; energy cannot be created
nor destroyed
• Second Law of Thermodynamics places qualitative restrictions on
energy-related processes, e.g.,
direction of heat transfer, maximum
performance of power plants
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Thermodynamic
Applications
• See Figure 1-5 and class
overhead slides
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Dimensions & Units
Dimension
mass
length
time
temperature
(absolute)
force
energy
power
SI
kg
m
s
K
English
lbm
ft
s
R
N
(= 1 kgm/s2)
J
(= 1 N-m)
lbf
(= 32.174
lbm-ft/s2)
Btu
(= 778.169
lbf-ft)
hp
(= 0.7068
Btu/s)
W
(= 1 J/s)
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Basic Thermodynamic
Definitions
• System - quantity of matter or
region of space chosen for study
• Surroundings - mass or region
outside of system
• Boundary - real or imaginary
surface that separates system from
surroundings
• Closed System (Control Mass) - a
fixed quantity of mass that can only
experience energy transfer (no mass
can enter or leave); an isolated
system is a special case where no
mass or energy transfer is allowed
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Basic Thermodynamic
Definitions, cont.
• Control Volume (Open System) region of space that can experience
both energy and mass transfer across
its boundary
• Property - a characteristic of a
system that can be defined without
knowledge of the system’s history
• Extensive Property - property that
is dependent on system size
• Intensive Property - property that is
independent of system size
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Basic Thermodynamic
Definitions, cont.
• State - a condition of a system that is
fully described by properties
• Equilibrium - a state where there are
no imbalances due to mechanical,
thermal, chemical, or phase effects
• State Postulate - gives the number of
properties needed to fix the state of a
system
• Simple Compressible System - a
system where external force fields are
negligible (i.e., electrical, magnetic,
gravitational, motion, and surface
tension effects)
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Basic Thermodynamic
Definitions, cont.
• Process - a change that a system
undergoes from one equilibrium
state to another; the sequence of
states through which the system
passes is called the process path
• Quasi-equilibrium Process - a
sufficiently slow process that allows
the system to remain infinitesimally
close to equilibrium
• Cycle - a sequence of processes that
returns the system to its initial state
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Basic Thermodynamic
Definitions, cont.
• Isothermal Process - a process
where temperature remains constant
• Isobaric Process - a process where
pressure remains constant
• Isochoric Process - a process where
volume or density remains constant
• Steady-Flow Process - a control
volume process where all properties
at a fixed point remain constant with
respect to time
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Some Basic
Thermodynamic Properties
•
•
•
•
•
Energy
Density
Specific Volume
Pressure
Temperature
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Energy
• Energy is an extensive property of a
system; it is the capacity to do work
or cause change
–
–
–
–
can be stored
can be transferred
can be transformed
is always conserved
• Types of Energy
– mechanical, kinetic, potential, thermal,
electric, magnetic, chemical, nuclear,
latent, et al.
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Energy, cont.
• Macroscopic energy - forms of
energy that a system possesses as a
whole w.r.t. some external reference
frame, e.g., kinetic and potential
energies
• Microscopic energy - forms of
energy related to the molecular and
atomic structure of a system; the sum
of all microscopic forms of energy is
known as internal energy (U)
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Energy, cont.
• System energy can be stored as
– Kinetic energy, KE = ½mV2
e.g., throwing a ball
– Gravitational potential energy, PE = mgz
e.g., raising a dumbbell
– Internal energy, U = ?
e.g., heating the air in a room
• In the absence of electric, magnetic,
and surface tension effects, the total
energy (E) of a system is
E = U + KE + PE
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Energy, cont.
• Energy can be only be transferred
across a system boundary by
– work interactions, due to a force acting
through some distance
– heat transfer, due to a temperature
difference
– mass flow, due to fluid flow into or out
of a control volume
• Energy can be transformed in many
ways, e.g.,
–
–
–
–
chemical-electrical (battery)
electrical-thermal (resistor)
potential-kinetic (dropping a rock)
nuclear-thermal (nuclear reactor)
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Density and Specific
Volume
• Density (kg/m3),
m

V
• Specific Volume (m3/kg),
V 1
v 
m 
– Specific Gravity
s 

 H O @ 4C
2
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Pressure
• Fluid Pressure (N/m2)
 Fnormal 
P  lim 

Asmall
 A 
• Other units:
1 pascal (Pa) = 1 N/m2
1 kPa = 103 N/m2
1 bar = 105 N/m2
1 MPa = 106 N/m2
1 atm = 101.325 kPa
= 14.696 lbf/in2 (psi)
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Pressure, cont.
• Absolute pressure - total pressure
experienced by a fluid
• Gage pressure or vacuum pressuredifference between absolute pressure
and atmospheric pressure (usually
indicated by a measuring device):
Pgage = Pabs - Patm
Pvac = Patm - Pabs
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Pressure, cont.
• Pressure variation with depth:
P  Patm  gh
• Pascal’s principle: a force applied to
a confined fluid increases the
pressure throughout by the same
amount; since F = PA, mechanical
advantage can be developed
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Pressure Measurement
• Manometer – gravimetric device based
upon liquid level deflection in a tube
• Bourdon tube – elliptical cross-section
tube coil that straightens under under
influence of gas pressure
• Mercury barometer – evacuated glass
tube with open end submerged in
mercury to measure atmospheric
pressure
• Pressure transducer – converts
pressure to electrical signal; i) flexible
diaphragm w/strain gage ii) piezoelectric quartz crystal
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The U-tube Manometer
• Simple, accurate device for measuring
small to moderate pressure differences
• Rules of manometry:
– pressure change across a fluid column of
height h is gh
– pressure increases in the direction of
gravity
– two points at the same elevation in a
continuous static fluid have the same
pressure (Pascal’s law)
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Temperature
• Temperature (ºC or K)
– measure of a body’s “hotness” or
“coldness”
– indicative of a body’s internal energy
– used to determine when a system is in
thermal equilibrium, i.e., when all
points have the same temperature
– see zeroth law of thermodynamics,
section 1-9
– unit conversions:
K = ºC + 273.15
R = ºF + 459.67
ºF = 1.8 ºC + 32
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Temperature Measurement
• Constant-P liquid-in-glass – utilizes volume
change of mercury or alcohol in a tube
• Constant-V gas – utilizes pressure change of
hydrogen or helium
• Bimetallic strip – utilizes differential CTE of
adjoined dissimilar metals
• Thermistor, RTD – utilizes electrical
resistance of metals and semiconductors
• Thermocouple - utilizes voltage produced
from dissimilar metal junctions
• Optical pyrometer – utilizes infrared emission
spectrum
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