Engineering Thermodynamic

Topic:~ Second law of thermodynamic

(Basic concepts & Statements)

Mechanical Department

B_2Prepared By:

Kushal Panchal

Limitation of “FIRST LAW”

• The first law of thermodynamic states that a certain energy flow takes place when a system undergoes a process or change of state is possible or not.

– According to first law in ‘cyclic process’

• Work is completely converted into heat or heat is completely converted into work.

• “HEAT” & “WORK” are mutually converted into each other.

• But from experience this is NOT TRUE!

Limitation of “FIRST LAW”

• First law does not help to predict whether the certain process is possible or not.

• The first law does not give info about Direction.

• It does not provide and specify sufficient condition to process take place.

Work(W) System Heat(Q)

Hot Reservoir T1

Cold Reservoir T2

Heat Flow

Hot Reservoir T1

Cold Reservoir T2

Heat Flow

Not possible

The “SECOND LAW” of thermodynamic:

• The second low of thermodynamic gives more information about thermodynamic processes.

• Second law may be defined as– “Heat can not flow itself from colder body to a

hotter body”.

• The Second law is also used to determine the theoretical limits for the performance of mostly used engineering systems like heat engines and heat pump….

“Kelvin-Plank” statement:

• The Kelvin-Plank statement of the second law of thermodynamic is states that

– “It is impossible to for any devise as heat engine that operates on a cycle to receive heat from a single reservoir and produce net amount of work”.

• This statement means that only part of total heat absorbed by heat engine from a high temperature is converted to work, the remaining heat must be rejected at a low temperature.

Continued…..

• The heat engine as shown in fig.1is converting 100 % of heat into 100% work.

• This system is not satisfying second law.

• So, this statement can be also expressed as “No heat engine has a 100 % of thermal efficiency”.

Q=100 kj

w=100 kj

Q=100 kj

w=40kj

Q2=60 kj

Heat Source

Heat Engine

Heat Source T1

Heat Engine

Sink T2

1

“Clausius” statement

• Clausius statement of second law of thermodynamic is as below

– “It is impossible to construct a device as heat pump that operates in a cycle and produces no effect other than the transfer of heat from lower temperature to higher temperature body”.

• This statement means that heat cannot flow from cold body to hot body without any work input.

Heat Source T1 Heat Source T1

Heat sink T2

Heat sink T2

Heat pump

Heat pump

Q1=100 kj

Q2=100 kj

Q2=60 kj

Q1=100 kj

No external work

supplied

1

W=40 kj(external

work supplied)

2

Continued:

• The cop for 2 is 100/40=2.5.

• So, in other simple words this statement can be defined as

– “Heat cannot itself flow from a colder body to a hot body”.

COMPARISON

Kelvin-Plank Statement

• It is applied to ‘Heat Engine’.

• It is negative statement.

• It is based on experimental observations and no mathematical proof.

Clausius Statement

• It is applied to ‘Heat Pump’ and ‘Refrigeration’.

• It is also negative statement.

• It is based on experimental observations and no mathematical proof.

Perpetual Motion Machine PMM 2

• If the engine exchange heat only single thermal reservoir, in which heat is supplied is completely converted into an equivalent amount of work and its efficiency becomes 100 %.

• This kind of machine is known as “PMM 2”.

Thermal Reservoir

PMM 2W=Q1

Q1

Continued:

• The PMM 2 violates the second law of thermodynamic.

• Practically its IMPOSSIBLE to construct.

• The efficiency PMM 2 is • W=Q

• N is 100 % for PMM 2.

Reversible Processes and

Irreversibility's• A reversible process is one that can be executed in the reverse direction with no net change in the system or the surroundings. • At the end of a forwards and backwards reversible process, both system and the surroundings are returned to their initial states.• No real processes are reversible. • However, reversible processes are theoretically the most efficientprocesses. • All real processes are irreversible due to irreversibilities. Hence, real processes are less efficient than reversible processes.Common Sources of Irreversibility:

• Friction • Sudden Expansion and compression• Heat Transfer between bodies with a finite temperature difference.

• A quasi-equilibrium process, e.g. very slow, frictionless expansion or compression is a reversible process.

Carnot cycle and Carnot heat engine:

• A carnot cycle is hypothetical cycle consist of four process– Two reversible isothermal process and

– Two reversible adiabatic process.

• Heat is caused to flow in working fluid by application of high temperature energy source during expansion, and flow out of fluid by the application of lower temperature energy sink during compression

Assumptions made in Carnot Cycle:

I. Piston moving in the cylinder does not produce any friction.

II. The cylinder head is arranged in such a way that it can be perfect heat conductor or heat insulator.

III. The walls of cylinder and piston are consider as perfect insulator of heat.

IV. Heat transfer does not affect temperature source or sink.

V. Compression and expansion are reversible.

VI. Working is perfect gas and has constant specific heat.

Continued:• Idealized thermodynamic cycle consisting of four reversible processes

(working fluid can be any substance):• The four steps for a Carnot Heat Engine are:

Reversible isothermal expansion (1-2, TH= constant) Reversible adiabatic expansion (2-3, Q = 0, THTL) Reversible isothermal compression (3-4, TL=constant) Reversible adiabatic compression (4-1, Q=0, TLTH)

1-2 2-3 3-4 4-1

The Carnot Cycle (cont’d)Work done by the gas = PdV, i.e. area under the process curve 1-2-3.

1

2

3

34

1

Work done on gas = PdV, area under the process curve 3-4-1

subtract

Net work1

2

34

dV>0 from 1-2-3PdV>0

Since dV<0PdV<0

Carnot’s theorem:

“It is impossible to construct an engine operating between two constant

temperature reservoir can be more efficient then reversible engine operating between

the same reservoir”

Corollary of Carnot Theorem:

1) “All reversible engine operating between the two constant temperature thermal reservoir have the same efficiency.”

2) “The efficiency of any reversible heat engine operating between two thermal reservoir does not depend on nature of working fluid and depends only on the temperature of the reservoir.”

20

The Thermodynamic Temperature Scale:

A temperature scale that is independent of the properties of the substances that are used to measure temperature is called a thermodynamic temperature scale.

That is the Kelvin scale, and the temperatures on this scale are called absolute temperatures.

L

H

revL

H

T

T

Q

Q

cycles reversible For

The second Carnot principle state that the thermal efficiencies of all reversible heat engines operating between the same two reservoirs are the same.

hth, rev = f (TH,TL)

• Thank You..!

• T H E R M O D Y N A M I C S……..;)