lectures-chapter1.pdf
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650:351 Thermodynamics
Chapter 1
Mahsa Mortazavi
Email: [email protected]
Oce hours: Tuesday, 2:00 - 3:30 pm
Summer 2015
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Overview of Course
Conservation of mass Conservation of momentum Conservation of energy Equation of state Property tables and charts
All problems in this course are solved using the above
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Fundamental Units
All physical processes involve units The fundamental units in the SI system are
Dimension Unit
Length meter (m)
Mass kilogram (kg)
Time second (s)
Temperature kelvin (K)
Electric current ampere (A)
Amount of light candela (cd)
Amount of matter mole (mol)
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Fundamental Units
The fundamental units in the English system are
Dimension Unit
Length foot (ft)
Mass slug (slug)
Time second (s)
Temperature rankine (R)
Electric current ampere (A)
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Fundamental Units
The fundamental units can be converted between English and SI
1 foot = 0.3048 meters
1 slug = 14.5939029 kilograms
1 deg R = 0.55556 deg K
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Fundamental Units
Derived unitsUnit English SI
Force pound force (lbf) Newton (Nt)
Energy British Thermal Unit (BTU) Joule (J)
Power Horsepower (HP) Watt (W)
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Fundamental Units
Derived units can be converted between English and SI1 lbf = 4.44822162 Nt
1 BTU = 1055.05585 J
1 HP = 745.699872 W
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Fundamental Units
What is pound mass (lbm) ?One lbm is the amount of mass that weighs one lbf on earth
Since weight is the force of gravity on an objectweight = 1 lbm gravity1 lbf = 1 lbm 32.17 ft/s21 slugft/s2 = 1 lbm 32.17 ft/s2
and therefore
1 lbm =1
32.17slug
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Fundamental Units
The dierences between lbm and lbf and slug are important Failure to understand the dierences can lead to an error of nearlytwo orders of magnitude and thus . . .
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Fundamental Units
The dierences between lbm and lbf and slug are important Failure to understand the dierences can lead to an error of nearlytwo orders of magnitude and thus . . .
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Systems and Control Volumes
A system is a quantity of matter or region of space A closed system comprises a xed amount of mass An open system is a properly selected region of space
Closed system
Closed system
moving boundary
Open system
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State and Equilibrium
Thermodynamics deals with equilibrium states An equilibrium state represents a condition wherein there are nounbalanced forces or potentials in the system
Thermal equilibrium implies that the temperature is the samethroughout the system
Mechanical equilibrium implies that the net forces and moments onthe system are zero
Phase equilibrium implies that the mass of each phase (e.g., solid,liquid and gas) remains constant
Chemical equilibrium implies that the chemical composition of thesystem remains constant
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State Postulate
Once a sucient number of properties of a system are specied, theremaining properties can be determined
As an example, consider the ideal gas equation
pV = nRT
For a given volume V, knowing the pressure p, and temperature T issucient to determine
1
the number of moles n
The State Postulate
The state of a simple compressible system
is completely specied by
two independent, intensive properties
1R = 8.31447 kJ/kmoleK is the Universal Gas Constant
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Processes and Cycles
The change of a system from one state to another is a process The series of states through which a system passes from its initial tonal state is the path
The path may be quasi-equilibrium or non-equilibrium
Processes
p V diagram
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Steady Flow Processes
During a steady ow process, the ow variables at a xed locationdo not change in time; however, they may change with position
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Zeroth Law of Thermodynamics
Two bodies in thermal equilibrium with a third body are in thermalequilibrium with each other (i.e., all have the same temperature)
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Pressure
Pressure is the average normal force on a small element of uid,either at rest or in motion
The SI unit of pressure is the pascal (Pa)1 Pa = 1 Nt/m2
There are several other units for pressureUnit Denition
bar 10
5
Pa
atm 101, 325 Papsi 6, 894.75 Pa
The actual pressure is the absolute pressure The gage pressure is dened asp
gage
= p patm
where p
atm
is 1 bar (1 atm)
What does the pressure gage for an automobile tire measure ?
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Variation of Pressure with Depth
In a uid at rest the pressure varies linearly with depthdp
dz
= g
where g = 9.8 m/s2, is the uid density and z is measured positiveupwards
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Variation of Pressure with Depth
Under hydrostatic (i.e., uid at rest) conditions, the pressure is thesame at all positions on a horizontal plane regardless of geometry,
provided that the points are interconnected by the same uid
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Pressure Measurement Devices
The barometer measures the atmospheric pressure provided that thegap at the top of the tube is a vacuum
p
C
+ gh = pB
= patm
thus p
atm
= gh if pC
= 0
Note: The pressure on the open uid surface is p
atm
. Since point B
and the open surface are connected by the same uid, p
B
= patm
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Manometer
The manometer measures pressure in the same manner as thebarometer
p
1
= p2
= patm
+ gh
since the open end of the tube is subject to p
atm
The specic gravity of a uid is dened bySG = uid
/water
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Manometer
Note that pA
= pB
(Why ?)
Therefore assuming the uid is the same tube is at rest
p
1
+ 1
g (a+ h) = p2
+ 1
ga+ 2
gh
and thus
p
1
p2
= (2
1
) gh
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