4. A schematic diagram of a recently
installed steam power plant is shown in
Fig1.1. High pressure superheated steam
leaves the drum and enters the turbine. The
steam expands in the turbine and in doing so
does work, which enables the turbine to
drive the electric generator.
國立成功大學工程科學系
5. The steam, now at low pressure, exits
the turbine and enters the heat exchanger,
where heat is transferred from the steam
(causing it to condense) to the cooling
water. The pressure of the condensate
leaving the condenser is increased in the
pump, enabling it to return to the steam
generator for reuse.
國立成功大學工程科學系
7. When a conventional power plant is
viewed as a whole, as shown in Fig1.4,
fuel and air enter the power plant and
products of combustion leave the unit.
There is also a transfer of heat to the
cooling water, and work is done in the
form of the electrical energy leaving the
power plant.
國立成功大學工程科學系
8. The overall objective of a power plant
is to convert the availability of the fuel
work (in the form of electrical energy) in
the most efficient manner, taking
consideration cost, space, and
environmental concerns.
國立成功大學工程科學系
11. A simple vapor-compression refrigeration
cycle is shown schematically in Fig. 1.6. The
refrigerant enters the compressor as a slightly
superheated vapor at a low pressure. It then
leaves the compressor and enters the
condenser as a vapor at some elevated pressure,
where the refrigerant is condensed as heat is
transferred to cooling water or to the
surroundings.
國立成功大學工程科學系
12. The refrigerant then leaves the
condenser as a high-pressure liquid. The
pressure of the liquid is decreased as it
flows through the expansion valve, and is
vaporized in the evaporator as heat is
transferred from the refrigerated space.
This vapor then reenters the compressor.
國立成功大學工程科學系
15. The basic operation of a gas turbine is
similar to that of the steam power plant,
except that air is used instead of water.
Fresh atmospheric air flows through a
compressor that brings it to a high
pressure. Energy is then added by
spraying fuel into the air and igniting it so
the combustion generates a high-
temperature flow.
16. This high-temperature, high-pressure
gas enters a turbine, where it expands
down to the exhaust pressure, producing
a shaft work output and (or) discharging
the exhaust gases at high velocity in the
process.
19. Fig 1.12 shows a simplified schematic
diagram of a liquid-propellant rocket. The
oxidizer and fuel are pumped through the
injector plate into the combustion
chamber where combustion takes place at
high pressure.
國立成功大學工程科學系
20. The high-pressure, high-temperature,
products of combustion expand as they
flow through the nozzle, and as a result
they leave the nozzle with a high velocity.
The momentum change associated with
this increase in velocity gives rise to the
forward thrust on the vehicle.
國立成功大學工程科學系
21. Thermodynamics plays a vital role
in the analysis, development, and
design of all power-producing
systems, refrigerative equipments,
including reciprocating internal-
combustion engines and gas turbine.
國立成功大學工程科學系
22. Thermodynamics
Chapter 2
Some Concepts and Definitions
國立成功大學工程科學系
23. Thermodynamics:
the science of energy and entropy.
Thermodynamics:
the science deals with heat and work and
those properties of substances that bear a
relation to heat and work.
國立成功大學工程科學系
24. Basis of thermodynamics:
Experiments –
They have been formalized into the 1st,
2nd, and 3rd laws of thermodynamics.
國立成功大學工程科學系
25. 2.1
Thermodynamics
system and the control volume
國立成功大學工程科學系
26. System:
A thermodynamic system comprises a
device or combination of devices
containing a quantity of matter that is
being studied.
國立成功大學工程科學系
27. Open system (control volume):
a properly selected region in space so that it
contains the matter and devices inside a
control surface.
Closed system (control mass):
when a control surface is closed to mass flow,
no mass can cross its boundary.
Isolated system:
when the energy (e.g. heat or work) is not
allowed to cross the boundary.
國立成功大學工程科學系
32. Microscopic:
there are molecules for a 25 of
monatomic gas at atmospheric conditions.
Thus, we must deal with equations at
least (3 position coordinates and 3 velocity
components). It’s a quite hopeless
task.
國立成功大學工程科學系
33. Macroscopic
▲ statistical approach (kinetic theory or
statistical mechanics)
▲ classical thermodynamics
▲ Time-averaged influence (gross or average
effects) of many molecules which can be
measured (or experienced).
國立成功大學工程科學系
34. Continuum concept of the macroscopic view
This continuum model is valid when the
characteristic length of the system under
consideration is much larger than the mean
free path (λ) of the molecules. In other words,
we are always concerned with volumes that are
very large compared to molecules dimensions.
For example, these are molecules of in
1 at 1atm, . λ of air at atmospheric
condition in ~0.1 μm
國立成功大學工程科學系
36. Phase : a q u a n t i t y o f ma t t e r t h a t i s
homogeneous throughout.
State: a state is identified or described by
certain observable macroscopic properties e.g.
pressure, temperature).
Properties: a property is defined as any
quantity that depend on the state of the system
and is independent of the path (i.e. the prior
history).
國立成功大學工程科學系
37. Equilibrium: it implies a state of balance.
Thermal equilibrium: same temperature.
Mechanical equilibrium: same pressure.
Thermodynamic equilibrium: when a system
is in eq’m regarding all possible changes of
state
國立成功大學工程科學系
38. Intensive properties:
properties independent of the mass.
Extensive properties:
properties dependent on the mass.
國立成功大學工程科學系
40. Process:
the path of the succession of states
through which the system passes.
國立成功大學工程科學系
41. Figure 2.3 Example of a system that may undergo a quasi-equilibrium
process.
國立成功大學工程科學系
42. Q: how to describe the state of a system during a
process when equilibrium does not exist?
A: Quasi-equilibrium process: the deviation
from thermodynamic equilibrium states is
infinitesimal.
國立成功大學工程科學系
43. For non-equilibrium processes, we are
limited to the system before the process
occurs and after the process is completed
and equilibrium is restored.
An isothermal process is a constant –
temperature process, an isobaric (constant-
pressure) process.
國立成功大學工程科學系
44. A thermodynamic cycle:
a system in a given initial state goes through a
number of different change of state on
processes and returns to its initial state.
國立成功大學工程科學系
46. They are related by Newton’s second law of
motion
F = ma (Metric SI system)
Where basic units are mass: kilogram (kg)
length :meter (m)
time :second (s)
force :Newton (N)
1N=1kg
國立成功大學工程科學系
47. Table 2.1
Do not confuse “weight” with “mass”, the former is a
concept of a force.
國立成功大學工程科學系
48. Example 2.1
What is the weight of a one kg mass at an
altitude where the local acceleration of gravity
is 9.75 m/ ?
Solution:
Weight is the force acting on the mass, which from
Newton’s second law is
F = mg = 1kg9.75 m/ [1N /kg m] = 9.75 N
50. Energy can be stored within a system and
can be transferred (as heat or work for
example) from one system to another.
Later in § 5.2
E = U (internal energy) + KE + PE
國立成功大學工程科學系
51. Identify energy from the molecular viewpoint
1. Intermolecular potential energy: forces
between molecules.
2. Molecular kinetic energy: translational
velocities of individual molecules.
3. Intermolecular energy: associated with the
molecular and atomic structure and related
force.
國立成功大學工程科學系
52. Take a diatomic molecule (e.g. ) for
example: there is translational energy as
molecules may travel in 3 directions, there is
rotational energy as molecules may rotate
about X and Y axes, and there is vibrational
energy as molecules vibrate along the Y axis.
國立成功大學工程科學系
53. Figure 2.4 The coordinate system for a diatomic molecule.
國立成功大學工程科學系
54. Although this course is on the
macroscopic viewpoint, it is helpful to
have microscopic perspective in mind
to better understand basic concepts of
thermodynamics.
國立成功大學工程科學系
57. specific volume, : volume per unit mass (intensive
property)
density, ρ: mass per unit volume (kg/ )
the smallest volume for which the
mass can be considered a continuum.
國立成功大學工程科學系
58. Figure 2.6 The continuum limit for the specific volume.
國立成功大學工程科學系
59. The continuum assumption simply means that
physical properties of fluid are distributed
continuously throughout space. Every point in
space has limit values for properties such as
v elo city, d en sity, p ressu re, e tc. Th is
assumption is valid when
國立成功大學工程科學系
60. where
L: some characteristic length of the flow
field of interest.
λ: mean free path (i.e., the average distance
a molecule travels before colliding with
another molecule.)
A commonly used volume unit is the liter (L).
1L = (the SI unit for volume is )
國立成功大學工程科學系
62. pressure, P: the normal component
of force per unit area
國立成功大學工程科學系
63. The SI unit for pressure :
1 Pa (pascal) = 1N/
Two other units: 1 bar = Pa = 0.1 MPa
1atm (standard atmosphere) = 101325 Pa
國立成功大學工程科學系
64. Figure 2.9 Thebalance of forces on a movable boundary relates to
inside gas pressure.
國立成功大學工程科學系
65. In most eases, we are concerned with
absolute pressure. Most pressure and
vacuum gauges, however, read difference
betw een the absolute pres sure and
atmospheric pressure existing at the gauge.
This is called “gauge pressure”. Pressure
differences are usually measured with a
manometer.
國立成功大學工程科學系
67. Figure 2.12 Examples of pressure measurement using a column
of fluid.
國立成功大學工程科學系
68. Since points A and B are at the same elevation,
(2.2)
For distinguishing between absolute and gauge
pressures, the term pascal will refer to
absolute pressure. Any gauge pressure will be
indicated as in (2.2)
國立成功大學工程科學系
70. It states that when two bodies have
equality of temperature with a third body,
they in turn have equality of temperature
with each other. It is the basis of
temperature measurement.
國立成功大學工程科學系
72. The scale used for measuring temperature in SI
units is the Celsius scale, It is based on two
fixed, easily duplicated points, the ice point
and the steam point. They are designated 0 and
100 on the Celsius scale.
The absolute scale related to the Celsius scale
is the Kelvin scale, K.
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