Basic concepts


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Basic concepts

  1. 1. Thermodynamics & Heat Engines Basic Concepts
  2. 2. Thermodynamics• Thermodynamics= therme + dynamis• Latin word therme means = heat• Dynamis means = power or forces causing motion so, overall meaning of thermodynamics is heat–power or force interaction between system and surrounding. for example It is based upon general observation and those may be formulated in form of thermodynamic law as –• Zeroth law of thermodynamics• First law of thermodynamics• Second law of thermodynamics
  3. 3. • Application areas of thermodynamics • Steam power plant • I.C.Engine • Refrigerator and air conditioning • Gas turbine • Compressor etc.
  4. 4. Schematic of a Carnot refrigerator
  5. 5. • Microscopic and macroscopic view of thermodynamics • Macro- Large scale • Micro – small scaleMacroscopic MicroscopicAttention is focused on certain quantity of Matter consituting the system ismatter without considering the activity considered to comprise a large no. ofoccurred at molecular level discrete particles called molecules.A few properties are required to describe the Large no. of variables are required tosystem such as P,V ,T etc and these can be describe the system such as position, KE,perceived by senses and measured by Velocity, P,V, T etc. It is very difficult toavailable instruments. Example Expansion of measure these quantities with help ofgases in a I.C. engine available instruments.Example KTGRequires Simple mathematical formulae to Requires Advanced statistical and mathematical formulae to analyse theanalyse the system systemKnown as statistical thermodynamics Known as Classical thermodynamics
  6. 6. SYSTEM- Definite region in space on which attention isconcentrated for investigation of the thermodynamicproblems i.e. heat, work transfer, etc. It may be classified onthe basis of transfer of mass & energy as indicated in table- TYPES OF THERMODYNAMIC SYSTEMSystem Mass Energy ExampleClosed System × √ gas filled in a cylinderOpen System √ √ compressor, turbine, or nozzle gas filled in a cylinder but withIsolated System × × insulation
  7. 7. • Homogeneous System • Quantity of matter is homogeneous throughout in chemical and physical structure i.e. system in a single phase• Pure substance • Substance that is homogeneous and invariable in chemical composition i.e. combustion product, atmospheric air
  8. 8. Thermodynamic Properties, Processes and Cycles• Properties • Characteristics by which physical condition of any system can easily be defined , is known as property. • Two types- • Intensive ( Independent of mass example pressure, temperature, density, composition, viscosity, thermal conductivity) • Extensive ( depends on mass examples- energy, enthalpy , entropy, volume etc.) • Check for a property- dP= Mdx + Ndy would be a thermodynamic property if its differential is exact i.e. • Specific quantity = Absolute / Mass and denoted by small letters.Applicable for quantities depending upon the mass like, internal energy, enthalpy, heat, work, volume etc.
  9. 9. • State • If any system have definite values of properties , it is known as definite state . Properties are the state variables of any system• Change in state • Any change in property will lead to change in state.• Path • Locus of all change of states is known as path.• Process • When path is completely defined , it becomes one process • Process may be reversible or irreversible in nature. • Reversible: it is possible to attain the initial states by eliminating the effects. For example quasi static process ( reversible process)• Cycle • Final state of any process is identical with the initial state , it becomes one cycle.• State, change in states, path, process, and cycles can be described on a diagram that is drawn between property vs property as shown
  10. 10. Quasi Static Vs Non Quasi Static Quasi- Almost slow, or infinitely slow Quasi static Non Quasi Static1. Infinitely slowness is the characteristic 1. Nature of process is very fast andof process and all the intermediate there is no equilibrium with intermediatechange in states are equilibrium with eachother. change of states.2. Path (1-2) of process can easily be 2. Path of process (1-2) can not bedefined due to all the change in states easily defined due to existence of non equilibrium change in states, hence canare in equilibrium , hence process can be drawn on graph paper with dottedbe drawn on graph paper with firm line. line.3. Processes are reversible in nature. It 3. Processes are irreversible in natute.means it is possible to attain the initial It means it is not possible to attain thestates by eliminating the effect. initial states by eliminating the effect4. Example: Expansion of gases behind 4. Example: Expansion of gasesthe pistion against infinitely small behind the pistion against a singleweigthts. weigtht.
  11. 11. Example : Compression process in piston –cylinder arrangement
  12. 12. Reversible & Irreversible process Reversible Process Irreversible Process1. It is possible to attain the initial states 1. It is not possible to attain the initialafter eliminating the effects introduced to states after eliminating the effectsobtain the final state. introduced to obtain the final state. Initial state will always be different in reverse process2. All the quasi static processes are 2. All the non quasi static processesreversible in nature . are reversible in nature .3.Process will become reversible by 3. Causes of irrversibility: (a) Internaleliminating the causes of irrversibility i.e. friction between molecules (b) Freeresisted expansion of gases, no internal expansion of gases ( c) Paddle wheelmolecular friction or external friction work- Braking action causing the conversion of mechanical work in form of heat., it is not possible to aobtain the ∮ motion of wheel by supplying the same ∮ amount of heat to wheel.4. Clausius inequality dQ/T=0 for cyclic 4. Clausius inequality dQ/T< 0 forprocess or no change in entropy for cyclic process or change in entropy forreversible process( ds =0) irreversible process( ds ≠0)
  13. 13. Thermodynamics Equilibrium• No spontaneous change in macroscopic property ( i.e. isolated system)• Conditions for thermodynamic equilibrium • Mechanical equilibrium ( No pressure gradient within the system and also between system & surroundings i.e.δΡ=0, or no unbalance force) • Chemical equilibrium (No transfer of mass by any chemical process across the boundary of system i.e. diffusion and no unbalanced chemical reaction within the system) • Thermal equilibrium ( No transfer of heat across the boundary of system when it is separated from universe by means of Diathermic wall- that allows the heat or δT=0)
  14. 14. Concept of Continuum• In concept of continuum matter within the system is assumed to be continuous and distributed uniformly.• Importance- Used for defining the pressure and density
  15. 15. Pressure• Definition : P = Normal Force / Cross sectional area• Units: Height of liquid ( 760 mm of Hg), N/m2, Pascal, Bar, Torr etc.• One atmospheric pressure= 1.01325 N/m2
  16. 16. Pascal’s LawThe pressure is the same at all points on a horizontal plane in a given fluidregardless of geometry, provided that the points are interconnected by thesame fluid. (see figure below)
  17. 17. Measurement- Pressure• U tube manometer- Used for measurement of pressure.• For same liquid equation of pressure can be written very easily as: take +ve sign if it is desired to obtain the pressure at lower level as shown in diagram below.
  18. 18. Example
  19. 19. Solution:
  20. 20. Review Questions & Problems• Book Engineering thermodynamics by P K Nag , (Ed. Third )P. No. 15 Review questions section Q. No.1.1 , 1.4 to 1.17• Problems( P.No. 16, Q.No. 1.5, 1.6, 1.8, 1.9)• Questions/ Problems given in assignment no. 1