notes 5 pure substances
TRANSCRIPT
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Lec 5: Thermodynamic
properties, Pvtbehavior
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For next time: Read: 3-5
HW 3 due Sept 17
Outline: Buoyancy and stability
Pure substances and processes Property diagrams for pure substances
Important points:
How to calculate point of action of hydrostaticload
The general shape of the property diagrams
How to solve problems using the propertydiagrams
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Properties - Introduction
We have discussed extensive propertiessuch as U, m, and V (for volume) whichdepend on the size or extent of a system,
and Intensive properties such as u, v, T, and P
which are independent of system extent(engineers are ambivalent about
nomenclature for pressure, sometimesusing p and P interchangeably)
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Important questions...
How many properties are needed to definethe state of a system?
How do we obtain those properties?
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For a simple system,
We may write: p = p(v,T)
or perhaps: v = v(p,T).
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Pure
Puremeans of uniform and invariablechemical composition (but more thanone molecular type is allowed). Thisallows a single phase of air to be a puresubstance.
All our substances will be pure. We will
drop the use of the word. When werefer to a simple system we mean onefilled with a pure substance--a simple,pure system.
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For a simple, pure substance
y1 = f(y2,y3), or
p = p(v,T), v = v(p,T) and T = T(p,v)
What do these equations define, in space?
Equations used to relate properties arecalled equations of state
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Ideal gas law is a simpleequation of state
RTPvM
RR
u
Ru = universal gas constant
m = mass
n = number of moles
M = molar mass or molecular weight
mRTPV TnRPVu
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Behavior of real substances
Lets consider substances that do not obeythe perfect gas law
certainly not as a solid
certainly not as a liquid
sometimes not very well as a gas
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Phase Change Process of a PureSubstance
P=1 atmT=20C
P=1 atmT=100C
P=1 atmT=100C
P=1 atmT=250C
P=1 atmT=100C
liquid liquid liq/vap
vapor vapor
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Consider a constant pressureprocess (of water)
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T-v Diagram
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You should be able to discusscharacteristics of the following:
Liquid to liquid/vapor to vapor transition(begin with a constant pressureprocess).
Single phase regions--liquid, vapor,solid.
Two-phase regions--liquid/vapor andsolid/vapor.
Melting--solid to liquid (freezing)
vaporization--liquid to vapor(liquefaction)
sublimation--solid to vapor
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Notice the triple-stateline. Along this line all
three states exist inequilibrium
For water, the triple point is at273.16 K (32.018 F) and 0.6113 kPa
(0.0887 psia).
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Vapor Dome - regionencompassing the two-phase, vapor-liquidequilibrium region
Saturated vapor line
Saturated liquid line
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Phase Diagram (PT-coordinates)
P
T
Solid Phase
Region
Liquid
Phase
Region
Superheated Gas
Phase Region
Melting
Curve
Triple
State
Critical
State
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GAS @ gGAS
State d
Weight
LIQUID
GAS
Weight
LIQUID @ a
P
T
g
P
T
g
d
P
T
g
d
a
Superheated
Vapor
CompressedLiquid
Constant Temperature Process
Q
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P
T
b
Gas @ bGas @ b
Q
GAS
STATE f
LIQUID
Q
GASLIQUID
Q
P
T
bf
P
T
bfa
SuperheatedVapor
SubcooledLiquid
Constant Pressure Process
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P
v
SuperheatedRegion
SaturationRegion
SubcooledRegion
Critical Point
Sat. Vapor
Line
Sat. LiquidLine
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Saturation temperature
Temperature at which a phase changetakes place at a given pressure.
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Saturation pressure
Pressure at which a phase change takesplace at a given temperature.
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TEAMPLAY
Discuss what happens when water boils onthe stove at your residence. Start with apan of water at 70 F, 1 atm pressure.
QContinued onnext slide.
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TEAMPLAY (CONTINUED)
What is the pressure at various timesduring the entire process?
Does the temperature of the water
change?
If the vapor (steam) were contained inan elastic container, what would happen
as heat continued to be added after allthe liquid disappeared.
Sketch pV and TV diagrams.
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Saturation properties
Along thesaturated liquidline, properties areidentified by the
subscript f
Along thesaturated vaporline, properties areidentified by the
subscript g
Both sets of properties can be found in thetemperature and pressure tables in your
books appendices.Both tables give the same data.
There are different tables for different
substances.
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Temperaturetable (also
known as asaturationtable)
Table A-4
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Pressure table(also known
as asaturationtable)
Table A-5
[T o p ope ties a e not
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[Two properties are notindependent in the vapor dome
(the two-phase region)]
The temperature and pressure areuniquely related. Knowing a T defines theP and vice versa.
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TEAMPLAY
Find, for Refrigerant 134a, the followingproperties: the saturation pressure ata saturation temperature of -10 F.
and find for the same substance thesaturation temperature at a pressure of0.06 MPa.
Make sure everyone in your group
understands how to do this.