Thermodynamics (Properties of pure substance)

thermodynamics

1. 1
PROPERTIES OF PURE SUBSTANCES
Introduce the concept of a pure substance.
Discuss the physics of phase-change processes.
Illustrate the P-v, T-v, and P-T property diagrams and P-v-T
surfaces of pure substances.
Demonstrate the procedures for determining
thermodynamic properties of pure substances from tables
of property data.
Describe the hypothetical substance “ideal gas” and the
ideal-gas equation of state.
Apply the ideal-gas equation of state in the solution of
typical problems.
Introduce the compressibility factor, which accounts for the
deviation of real gases from ideal-gas behavior.
Present some of the best-known equations of state.
The objectives of Today’s Learningare to:

2. 2
PROPERTIES OF PURE SUBSTANCES
A substance that has a fixed chemical composition throughout is
called a pure substance. Water, nitrogen, helium, and carbon
dioxide, for example, are all pure substances.
A pure substance does not have to be of a single chemical
element or compound, however. A mixture of various chemical
elements or compounds also qualifies as a pure substance as
long as the mixture is homogeneous.
Airfor example, is a mixture of several gases, but it is often
considered to be a pure substance because it has a uniform
chemical composition
However, a mixture of oil and water is not a pure substance.
Since oil is not soluble in water, it can be collected on top of the
water forming two chemically dissimilar regions.
Pure Substance:

3. 3
PROPERTIES OF PURE SUBSTANCES
A mixture of two or more phases of a pure substance is still a
pure substance as long as the chemical composition of all
phases is the same.
A mixture of ice and liquid water, for example, is a pure
substance because both phases have the same chemical
composition.
A mixture of liquid air and gaseous air, however, is not a pure
substance since the composition of liquid air is different from
the composition of gaseous air, and thus the mixture is no
longer chemically homogeneous.
Why ?
This is due to different components in air condensing at different
temperatures at a specified pressure.
Pure Substance:

4. 4
PHASE-CHANGE PROCESSES OF PURE SUBSTANCES
There are many practical situations where two
phases of a pure substance coexist in equilibrium.
Water exists as a mixture of liquid and vapor in the
boiler and the condenser of a steam power plant.
The refrigerant turns from liquid to vapor in the
freezer of a refrigerator.
The arrangement of atoms in different phases: (a) molecules are at relatively
fixed positions in a solid, (b) groups of molecules move about each other in
the liquid phase, and (c) molecules move about at random in the gas phase.

5. 5
PHASE-CHANGE PROCESSES OF PURE SUBSTANCES
Plasma is astate of matterin which an ionised gaseous
substance becomes highly electrically conductiveto
the point that long-rangeelectric and magnetic fields
dominate the behavior of the matter.

6. 6
PHASE-CHANGE PROCESSES OF PURE SUBSTANCES
Plasma is an electrically neutral medium of
unbound positive and negative particles (i.e. the
overall charge of a plasma is roughly zero).
Although these particles are unbound, they are
not ‘free’ in the sense of not experiencing forces.
Moving charged particles generate an electric
current within a magnetic field, and any
movement of a charged plasma particle affects
and is affected by the fields created by the other
charges.

7. 7
Saturation Temperature and Saturation Pressure
At a given pressure, the temperature at which a pure
substance changes phase is called the saturation
temperature Tsat. Likewise, at a given temperature,
the pressure at which a pure substance changes
phase is called the saturation pressure P sat.

8. 8
Saturation Temperature and Saturation Pressure
At a given pressure, the temperature at which a pure
substance changes phase is called the saturation
temperature Tsat. Likewise, at a given temperature,
the pressure at which a pure substance changes
phase is called the saturation pressure P sat.

9. 9
Supercritical Fluids
At a given pressure, the
temperature at which a
pure substance changes
phase is called the
saturation temperature
Tsat. Likewise, at a
given temperature, the
pressure at which a pure
substance changes
phase is called the
saturation pressure P
sat. At supercritical pressures (P Pcr), there
is no distinct phase-change (boiling)
process.

10. 10
Enthalpy—A Combination Property
The combination u + Pvis frequently encountered
in the analysis of control volumes.
H = U + PV (kJ)
Enthalpy (from the Greek word enthalpien, which means to heat)

11. 11
Saturated Liquid and Saturated Vapor States
Vf=Specific volume of vapor and
saturated liquid
Vg=Specific volume of saturated
vapor
Vfg= Vg-Vf

12. 12
Lets Solve Togather
Temperature of Saturated Vapor in a Cylinder
A piston–cylinder device contains 2 ft3 of
saturated water vapor at 50-psia pressure.
Determine the temperature and the mass
of the vapor inside the cylinder.

13. 13
Lets Solve Togather
Volume and Energy Change during
Evaporation
A mass of 200 g of saturated liquid water is
completely vaporized at a constant pressure
of 100 kPa. Determine (a) the volume
change and (b) the amount of energy
transferred to the water.

14. 14
Working fluids
oForourstudyofthermodynamics,wewillclassifythe
workingfluidsintotwocategories:
Idealgasandrealfluidsuchassteamorliquidwater.
oPropertiesofliquidandgaseouswaterareplottedin
threetables.Oneforsubcooledregion,secondfor
saturatedregion,andthirdforsuperheatedregion.

15. 15
Steam tables
Propertiesofliquidandgaseouswaterareplotted
inthreetables.Oneforsubcooledregion,second
forsaturatedregion,andthirdforsuperheated
region.
Youmayneedsingle-ordouble-interpolation
whileusingsuchtables.

16. 16
A part of saturated steam table: temp. table [1]

17. 17
A part of saturated steam table: press. table [1]

18. 18
A part of superheated table [1]

19. 19
T-sdiagram of water [1]

20. 20
Mollier (h-s) diagram of water [1]