THERMODYNAMIC PROCESSES AND PROPERTIES CLASS NOTES

THERMODYNAMIC PROCESSES

AND PROPERTIES CLASS

NOTES  

Any thermodynamic system works on

a closed repeatable cycle. Each cycle

consists of four processes. In these

processes, there is transfer of heat and

work into the system or out of the system.

Normally processes have one

variable as constant. Adiabatic process in

which there is no heat exchange.

 Analysis use properties. Every

thermodynamic device use one fluid

or the other. The combination of the

thermodynamic fluid and device is such to

give maximum efficiency.

The equation of state is a relation between

thermodynamic properties for ideal gas.

 Simple equation of state is not

applicable for thermodynamic properties

  with real gases. There is a need of

more mathematical relations for the real

gases. Fundamental thermodynamic

properties temperature, pressure and

volume are measurable. Derived

thermodynamic properties namely Internal

energy, enthalpy and entropy cannot be

measured.  Analysis of thermodynamic

systems use these properties. For the

determination of these properties, there is

a need to develop thermodynamic

relations.

Analysis of power producing cycles is

done with thermodynamics and transport

properties. Power consuming cycles

use these properties too.

 THERMODYNAMIC PROCESSES

(a)    Temperature constant process -ISO-THERMAL

(b)    Pressure constant process-ISOBARIC

(c)     Enthalpy constant process -ISEN-THALPIC (h=constant)

(d)    Entropy constant process -ISEN-TROPIC (s=constant)

(e)    Volume constant process -ISO-CHORIC (V=constant)

(f)      No heat exchange -ADIABATIC PROCESS

A thermodynamic cycle will have four processes.

Carnot cycle consists of two iso-thermal and two isentropic processes.

 Thermodynamic and Transport Properties

THERMODYNAMIC PROPERTIES

Fundamental Properties- these are measurable properties.

       a.  Temperature

   b. Pressure

       c. Volume

        i. Specific volume of saturated liquid

         ii. Saturated vapor specific volume

          iii. Specific volume of super heated vapor

Derived Properties- These are non-measurable properties.

Enthalpy

  1. Specific enthalpy of saturated liquid

  2.  Enthalpy(specific) of saturated vapor

  3. Specific enthalpy of super heated vapor

Entropy

  1. Specific entropy of saturated liquid

  2.  Entropy(specific) of saturated vapor

  3.  Entropy(specific) of super heated vapor

 

INTERNAL ENERGY

Internal Energy is the sum of kinetic and potential energy. These energies are of random translation, rotational and vibratory motion of the atoms of the system. Its symbol for 1 kg is ‘u’ for 1 kg mass.  U is symbol for m kg mass. Its units are kJ. From the First Law of Thermodynamics, the internal energy of a system increases with rise of temperature.

dU = Q – W= heat supplied into the system—work done by the system.

ENTROPY

Entropy is measure of disorder. More is the disorder, more will be entropy. Mathematically δV/V is a measure of disorder. It will increase in an expansion process and vice versa.  Take entropy zero with respect to a datum. Various processes use difference of entropies. Take entropy of water zero at 0 C. Based on this, calculate the values of entropy. Similar is the case of refrigerants. Take entropy of each refrigerant zero at –40 C.  Calculate entropy values at other temperatures and tabulate. Temperature-entropy charts are mad from the tabulated data. Units of specific entropy are k J / kg K.  The symbols for specific entropy is ‘s’. S is total entropy for m kg. Read Specific entropy (k J/ kg K) values from tables and charts.

Enthalpy

It is sum of internal energy and flow work.

Mathematically

h = u+ p v

Transport properties are

  1. Specific heat of liquid

  2.  Vapor specific heat

  3. Thermal conductivity of liquid

  4.  Conductivity(thermal) of vapor

  5. Viscosity of liquid

  6. Vapor viscosity

 

Origin of transport properties

  1.  Internal energy comes from the First Law of Thermodynamics

  2. Entropy comes from the Second Law of Thermodynamics. Measurement of entropy comes from the Third Law of Thermodynamics.

  3. Definition of enthalpy comes from the total heat content which the sum of internal energy and the flow work.

SPECIFIC AND TOTAL PROPERTIES

(a) Specific property

Properties for 1 kg mass are specific properties.  Refrigerant charts and tables have specific properties.

(i) specific entropy (s),

(ii)  Enthalpy (specific) (h),

(iii) specific volume (v),

(iv) Humidity(specific)  (w).

Specific properties are for

(i) Subcooled liquid

(ii) Saturated liquid

(iii) Wet vapor

(iv) Dry saturated vapor

(v) Superheated vapor

(vi) gas

The symbol of specific property is small alphabet.

2(b). Total properties

Total property is equal to specific property multiplied by mass (m). This represented by the corresponding capital alphabet symbol like H=m h, S=m s, V=m v.

Thermodynamic relations used in the derivation of derived properties

1.  (a)  f(x, y, z) =0, 

  1.   (∂x/∂y)z (∂y/∂z)x(∂z/∂x)y = -1

  2. (∂y/∂x)z(∂x/∂y)z=(∂y/∂z)x(∂z/∂y)x =(∂z/∂x)y(∂x/∂z)y=1

     (b)Z=z(x, y),

  1. y =y(z, x) and x=x(y, z)     z = z(x, y)

  2. dz = (∂z/∂x)y dx + (∂z/∂y)x dy

  1. For a continuous function

          ∂2z/∂x∂y =∂2z/∂y∂x

  1. Thermodynamic characteristic functions

(i) Internal energy(U)

(ii) Helmholtz energy (A)

(iii) enthalpy (H)

(iv) Gibb’s free energy(G)

These functions use thermodynamic parameters.

     4. Thermodynamic parameters

(i) Temperature (T)

(ii) Entropy (S)

(iii) Pressure (p)

(iv) Volume (V)

5. Reversible process

δq and δw use properties of the system.

δqrev = T ds  and δwrev = p dv

 For a reversible process

du = T ds—p dv

This relation combines the First and Second Law.

6. Enthalpy

For a unit mass, h = u + P v

For mass m,  H = U + PV

7. Helmholtz energy at a constant temperature

A =U-TS

8. Gibb’s free energy

G = U-TS + pV

REFERENCES

  1. Chemical Thermodynamics, D.J.G. Ives, University Chemistry, MacDonald Technical and Scientific, 1971

  2. Elements of Statistical Thermodynamics (2nd Edition), L.K. Nash, Addison-Wesley, 1974

  3. Thermal Physics (2nd Edition), Kittel, Charles & Kroemer, Herbert (1980).

  4. Encyclopedia of Physics (2nd Edition, W. H. Freeman Company. McGraw Hill,”), C.B. Parker, 1994

  5. Thermodynamics – an Engineering Approach , Cengel, Yunus A., & Boles, Michael A, McGraw Hill, 2002

  6. Statistical Physics (2nd Edition), F. Mandl, Manchester Physics, John Wiley & Sons, 2008

  7. Thermodynamics, From Concepts to Applications (2nd Edition), A. Shavit, C. Gutfinger, CRC Press (Taylor and Francis Group, USA), 2009