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Prof. Siyoung Jeong
Thermodynamics I
MEE2022-02
Spring 2014
Chapter 1
Introduction
Fundamentals of Thermodynamics
Thermal Engineering Lab.
Chapter 1. Introduction
2
• Thermodynamics : Science of energy and
entropy
- Science of heat and work and properties related
to heat and work
• Basis for diverse scientific disciplines
- Physics, chemistry, biology, etc.
• Applied to various fields
- Mechanical engineering, chemical engineering,
aerospace engineering, etc.
Thermal Engineering Lab.
Chapter 1. Introduction
3
1.1 A Thermodynamic system and the control volume
• Schematic diagram of a steam power plant
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Chapter 1. Introduction
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• Schematic diagram of a refrigerator
Thermal Engineering Lab.
Chapter 1. Introduction
• Thermodynamic system
- System / Surroundings
- Control mass (no mass flow)
- Control volume (more general)
- Open / closed systems
- System boundary / Control surface
- Isolated system : No influence from surroundings
5
Control mass Control volume
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Chapter 1. Introduction
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1.2 Macroscopic versus microscopic points of view
• Macroscopic vs. Microscopic
25 mm
Atmospheric P, T
10^20 atoms
6*10^20 equations!
Classical Thermodynamics Statistical Thermodynamics
Continuum
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Chapter 1. Introduction
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1.3 Properties and state of a substance
• Phase : Gas, liquid, solid
• A substance may exist in in various states (T, p, etc.).
• Thermodynamic property : observable & macroscopic
- Intensive : pressure, temperature, density, etc.
- Extensive : mass, volume, etc.
- Specific property : property/mass,
intensive property, ex) specific volume
• Thermodynamic equilibrium - Thermal equilibrium : T
- Mechanical equilibrium : P
- Chemical equilibrium : μ (chemical potential)
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Chapter 1. Introduction
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1.4 Processes and cycles
• Process : Path of state change
• States during a process?
- Quasi-equilibrium process : Defined ideally but
applied to many practical cases
- Nonequilibrium process
• Iso-X process : Constant X
- Isothermal : T=const.
- Isobaric (isopiestic) : P=const.
- Isochoric : V=const.
• Cycle : Initial state = Final state
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Chapter 1. Introduction
9
1.5 Units for mass, length, time, and force
• SI units : Metric International System
- Time : second [s]
- Length : meter [m]
- Mass : kilogram [kg]
cf) mole [mol] : 12 g of carbon-12
- Force : Newton [N]
• English Engineering Units
- ft, in, lbm, lbf, …
Thermal Engineering Lab.
Chapter 1. Introduction
Ex. 1.1
What is the weight of a 1 kg mass at an altitude where the local acceleration of gravity is 9.75 m/s2?
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Chapter 1. Introduction
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1.6 Specific volume and density
• v , Specific volume = volume / mass [m3/kg]
r , Density = mass / volume [kg/m3]
• Continuum concept :
• Molal property : [m3/kmol] , [kmol/m3]
m
Vv
VV
lim
v r
Both intensive properties }
Thermal Engineering Lab.
Chapter 1. Introduction
Ex. 1.2 A 1 m3 container, shown in Fig. 1.9, is filled with
0.12 m3 of granite, 0.15 m3 of sand, and 0.2m3 of
liquid 25℃ water; the rest of the volume, 0.53 m3,
is air with a density of 1.15 kg/m3. Find the
overall (average) specific volume and density.
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Thermal Engineering Lab.
Chapter 1. Introduction
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1.7 Pressure
• Continuum concept
• Unit : 1 Pa = 1 N/m2
1 bar = 105 Pa = 0.1 MPa
1 atm = 101325 Pa ~ 1 bar
A
FP n
AA
lim
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Chapter 1. Introduction
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• Absolute vs. gauge pressure
gHPPP
gHPP
AgHAPmgAPAP
B
B
B
r
r
r
0
0
00
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Chapter 1. Introduction
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0
00
Fluid with a constant density:
Fluid with a varying density: ( )
B
H
B
P P P gH
P P P dP z gdz
r
r
0P P
P
P dP
BP P
0z
z
z dz
z H
Thermal Engineering Lab.
Chapter 1. Introduction
Ex. 1.3 The hydraulic piston/cylinder system shown in
Fig. 1.11 has a cylinder diameter of D = 0.1 m
with a piston and rod mass of 25 kg. The rod has
a diameter of 0.01 m with an outside atmospheric
pressure of 101 kPa. The inside hydraulic fluid
pressure is 250 kPa. How large a force can the
rod push with in the upward direction?
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Thermal Engineering Lab.
Chapter 1. Introduction
Ex. 1.4
A mercury barometer located in a room at 25 ℃ has a height of 750 mm. What is the atmospheric
pressure in kPa?
Ex. 1.5
A mercury (Hg) manometer is used to measure the pressure in a vessel as shown in Fig. 1.13. The
mercury has a density of 13,590 kg/m3, and the height difference between the two columns is
measured to be 24 cm. We want to determine the pressure inside the vessel.
17
Thermal Engineering Lab.
Chapter 1. Introduction
Ex. 1.6
What is the pressure at the bottom of the 7.5 m tall storage tank of fluid at 25 ℃ , shown in Fig.
1.15? Assume that the fluid is gasoline with atmospheric pressure 101 kPa on the top surface.
Repeat the question for the liquid refrigerant R-134a when the top surface pressure is 1 MPa.
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Thermal Engineering Lab.
Chapter 1. Introduction
Ex. 1.7
A piston/cylinder with a cross-sectional area of 0.01 m2 is connected with a hydraulic line to another
piston/cylinder with a cross-sectional area of 0.05 m2. Assume that both chambers and the line are
filled with hydraulic fluid of density 900 kg/m3 and the larger second piston/cylinder is 6 m higher
up in elevation. The telescope arm and the buckets have hydraulic piston/cylinders moving them, as
seen in Fig. 1.16. With an outside atmospheric pressure of 100 kPa and a net force of 25 kN on the
smallest piston, what is the balancing force on the second larger piston?
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Chapter 1. Introduction
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1.8 Energy
• Microscopic view
- Intermolecular (potential) energy
- Molecular kinetic energy
- Intramolecular energy
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Chapter 1. Introduction
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1.9 Equality of temperature
• “Sense of hotness or coldness”
However, difficult to rigorously define temperature
• Two objects in thermal contact for a long time
→ No change → thermal equilibrium
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Chapter 1. Introduction
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1.10 The zeroth law of thermodynamics
• Zeroth law
TA = Tthermometer
TB = Tthermometer
∴ TA = TB
Basis of temperature measurement
Thermal Engineering Lab.
• Fahrenheit, Celsius
- Celsius: formerly know as centigrade
• Absolute temperatures
K = °C + 273.15
R = F + 459.67
Fahrenheit Celsius
Ice pt 32 0
Steam pt 212 100
Chapter 1. Introduction
1.11 Temperature scales
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Chapter 1. Introduction
1.12 Engineering applications
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Chapter 1. Introduction
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