2nd law of thermodynamic

Semester

3EngineeringThermodynamics

Second Law of Thermodynamics

Crated by :- Manthan Kanani

1

Babaria Institute Of Technology

MechanicalEngineering

2nd Law Of thermodynamic

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ENGINEERING THERMODYNAMICS

Introduction

A process must satisfy the first law in order to occur.

Satisfying the first law alone does not ensure that the process will take place.

Second law is useful:

provide means for predicting the direction of processes, establishing conditions for equilibrium, determining the best theoretical performance of cycles, engines

and other devices.


2nd Law Of Thermodynamic

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A cup of hot coffee does not get hotter in a cooler room.

Transferring heat to a wire will not generate electricity.

Transferring heat to a paddle wheel will not cause it to ro-tate.

These processes cannot oc-cur even though they are not in violation of the first law.


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Second Law of Thermodynamics

Kelvin-Planck statement

No heat engine can have a thermal efficiency 100 per-cent.

As for a power plant to oper-ate, the working fluid must exchange heat with the envi-ronment as well as the fur-nace.


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Heat Engines

Work can easily be converted to other forms of energy, but?

Heat engine differ considerably from one another, but all can be characterized :

o they receive heat from a high-tempera-ture source

o they convert part of this heat to work

o they reject the remaining waste heat to a low-temperature sink atmosphereo they operate on a cycle


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The work-producing device that best fit into the definition of a heat engine is the steam power plant, which is an external combustion engine.


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Thermal Efficiency

Represent the magnitude of the energy wasted in order to complete the cycle.

A measure of the performance that is called the thermal efficiency.

Can be expressed in terms of the desired output and the required input

th Desired ResultRequired Input

For a heat engine the desired result is the net work done and the input is the heat supplied to make the cycle operate.


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The thermal efficiency is always less than 1 or less than 100 percent.

thnet out

in

WQ

,

W W W

Q Qnet out out in

in net

,

where


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Applying the first law to the cyclic heat engineQ W U

W Q

W Q Q

net in net out

net out net in

net out in out

, ,

, ,

,

The cycle thermal efficiency may be written as

thnet out

in

in out

in

out

in

WQ

Q QQQQ

,

1


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A thermodynamic temperature scale related to the heat transfers between a reversible device and the high and low-temperature reservoirs by

QQ

TT

L

H

L

H

The heat engine that operates on the reversible Carnot cycle is called the Carnot Heat Engine in which its efficiency is

th revL

H

TT, 1


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Heat Pumps and Refrigerators

A device that transfers heat from a low tem-perature medium to a high temperature one is the heat pump.

Refrigerator operates exactly like heat pump except that the desired output is the amount of heat removed out of the system

The index of performance of a heat pumps or refrigerators are expressed in terms of the coefficient of performance.


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COP QW

QQ QHP

H

net in

H

H L

,

COP QWR

L

net in

,


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Carnot Cycle

Process Description

1-2 Reversible isothermal heat addition at high temperature

2-3 Reversible adiabatic expansion from high temperature to low temperature

3-4 Reversible isothermal heat rejection at low temperature

4-1 Reversible adiabatic compression from low temperature to high temperature


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Execution of Carnot cycle in a piston cylinder device


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The thermal efficiencies of actual and reversible heat engines operating between the same temperature lim-its compare as follows

The coefficients of performance of actual and reversible refrigerators operating between the same temperature limits compare as follows


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Entropy

The 2nd law states that process occur in a certain direction, not in any direction.

It often leads to the definition of a new property called entropy, which is a quantitative measure of disorder for a system.

Entropy can also be explained as a measure of the unavailability of heat to perform work in a cycle.

This relates to the 2nd law since the 2nd law predicts that not all heat provided to a cycle can be trans-formed into an equal amount of work, some heat re-jection must take place.


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Entropy Change

The entropy change during a reversible process is defined as

For a reversible, adiabatic process

dSS S

0

2 1

The reversible, adiabatic process is called an isentropic process.


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Entropy Change and Isentropic Processes

The entropy-change and isentropic relations for a process can be summarized as follows:

i. Pure substances: Any process: Δs = s2 – s1 (kJ/kgK)Isentropic process: s2 = s1

ii. Incompressible substances (liquids and solids):

Any process: s2 – s1 = cav T2/T1 (kJ/kg Isentropic process: T2 = T1


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iii. Ideal gases:

a) constant specific heats (approximate treat-ment):

s s C TT

R vvv av2 1

2

1

2

1

, ln ln

2 22 1 ,

1 1

ln lnp avT Ps s C RT P

for isentropic process

2 1

1 2.

k

s const

P vP v

for all process


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Isentropic Efficiency for Turbine


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Isentropic Efficiency for Compressor


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THANKS FOR YOUR ATTENTION