basic concepts
TRANSCRIPT
Thermodynamics & Heat
Engines
Basic Concepts
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
• Application areas of thermodynamics• Steam power plant
• I.C.Engine
• Refrigerator and air conditioning
• Gas turbine
• Compressor etc.
Schematic of a Carnot refrigerator
• Microscopic and macroscopic view of thermodynamics• Macro- Large scale
• Micro – small scale
Macroscopic Microscopic
Attention is focused on certain quantity of
matter without considering the activity
occurred at molecular level
Matter consituting the system is
considered to comprise a large no. of
discrete particles called molecules.
A few properties are required to describe the
system such as P,V ,T etc and these can be
perceived by senses and measured by
available instruments. Example Expansion of
gases in a I.C. engine
Large no. of variables are required to
describe the system such as position, KE,
Velocity, P,V, T etc. It is very difficult to
measure these quantities with help of
available instruments.Example KTG
Requires Simple mathematical formulae to
analyse the system
Requires Advanced statistical and
mathematical formulae to analyse the
system
Known as statistical thermodynamics Known as Classical thermodynamics
System Mass Energy Example
Closed System × √ gas filled in a cylinder
Open System √ √ compressor, turbine, or nozzle
Isolated System × ×
gas filled in a cylinder but with
insulation
TYPES OF THERMODYNAMIC SYSTEM
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-
• 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
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.
• 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
Quasi Static Vs Non Quasi StaticQuasi- Almost slow, or infinitely slow
Quasi static Non Quasi Static1. Infinitely slowness is the characteristic
of process and all the intermediate
change in states are equilibrium with each
other.
1. Nature of process is very fast and
there is no equilibrium with intermediate
change of states.
2. Path (1-2) of process can easily be
defined due to all the change in states
are in equilibrium , hence process can
be drawn on graph paper with firm line.
2. Path of process (1-2) can not be
easily defined due to existence of non
equilibrium change in states, hence can
be drawn on graph paper with dotted
line.
3. Processes are reversible in nature. It
means it is possible to attain the initial
states by eliminating the effect.
3. Processes are irreversible in natute.
It means it is not possible to attain the
initial states by eliminating the effect
4. Example: Expansion of gases behind
the pistion against infinitely small
weigthts.
4. Example: Expansion of gases
behind the pistion against a single
weigtht.
Example : Compression process in piston –cylinder arrangement
Reversible & Irreversible process
Reversible Process Irreversible Process1. It is possible to attain the initial states
after eliminating the effects introduced to
obtain the final state.
1. It is not possible to attain the initial
states after eliminating the effects
introduced to obtain the final state.
Initial state will always be different in
reverse process
2. All the quasi static processes are
reversible in nature .
2. All the non quasi static processes
are reversible in nature .
3.Process will become reversible by
eliminating the causes of irrversibility i.e.
resisted expansion of gases, no internal
molecular friction or external friction
3. Causes of irrversibility: (a) Internal
friction between molecules (b) Free
expansion of gases ( c) Paddle wheel
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
process or no change in entropy for
reversible process( ds =0)
4. Clausius inequality
∮
dQ/T< 0 for
cyclic process or change in entropy for
irreversible process( ds ≠0)
• No spontaneous change in macroscopic property (i.e. isolated system)
• Conditions for thermodynamic equilibrium
• Mechanical equilibrium ( No pressure gradient withinthe system and also between system & surroundingsi.e.δΡ=0, or no unbalance force)
• Chemical equilibrium (No transfer of mass by anychemical process across the boundary of system i.e.diffusion and no unbalanced chemical reaction withinthe system)
• Thermal equilibrium ( No transfer of heat across theboundary of system when it is separated from universeby means of Diathermic wall- that allows the heat orδT=0)
Thermodynamics Equilibrium
Concept of Continuum
• In concept of continuum matter within the system isassumed to be continuous and distributed uniformly.
• Importance- Used for defining the pressure and density
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
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)
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.
Example
Solution:
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