CHAPTER 7

ENTROPY

Md. Mizanur Rahman MEng(Sweden), PhD (Finland), CEng Chartered Energy Engineer (EI, UK) Certified Energy Manager School of Mechanical Engineering Universiti Teknologi Malaysia Email: mizanur@mail.fkm.utm.my

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Entropy

The quantity ∮δQ/T is the cyclic integral of the heat transfer divided by the

absolute temperature at which the heat transfer occurs.

Since the temperature TH is constant during the heat transfer QH, and TL is

constant during heat transfer QL, the integral is given by

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Thus, the quantity δQ/T is a perfect differential, since its cyclic integral is zero. We let

this perfect differential be denoted by dS, where S represents a scalar function that

depends only on the state of the system. This is a new property of a system. We shall call

this extensive property entropy

We often sketch a temperature-entropy diagram for cycles or processes of interest.

The Carnot cycle provides a simple display when plotting temperature vs. entropy.

The heat transfer during a reversible process

can be expressed in differential form

An irreversible (i.e., actual) heat engine, for example, is less

efficient than a reversible one operating between the same two thermal energy reservoirs.

Clausius inequality

That is, the cyclic integral of dQ/T is always less than or equal to zero. This inequality is valid for all cycles, reversible or irreversible

From the above equation we see that the entropy change for a reversible

process can be either positive or negative depending on whether energy is

added to or extracted from the system during the heat transfer process. For a

reversible adiabatic process the entropy change is zero.

We often sketch a temperature-entropy diagram for cycles or processes of

interest. The Carnot cycle provides a simple display when plotting temperature

vs. entropy. The change in entropy for the first process from state 1 to state 2

is

The entropy change for the reversible adiabatic process from state 2 to state 3 is zero.

For the process from state 3 to state 4 the entropy change is numerically equal to that

of the first process; the process from state 4 to state 1 is also a reversible adiabatic

process and is accompanied with a zero entropy change.

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The heat transfer during a reversible process can be expressed in differential

form as

Hence, the area under the curve in the T -S diagram represents the heat transfer during

any reversible process. The rectangular area in T-s diagram thus represents the net heat

transfer during the Carnot cycle. Since the heat transfer is equal to the work done for a

cycle, the area also represents the net work accomplished by the system during the

cycle. Here, Qnet = Wnet = ΔT ΔS.

This means that the entropy of an isolated system either remains constant or

increases. Since real processes are all irreversible, this means that

the entropy of the universe always increases in every process.

Entropy can be defined as

Entropy is an extensive property of a system and sometimes is

referred to as total entropy. Entropy per unit mass, designated s, is an intensive property and has the unit kJ/kg · K.

The entropy change of a system during a process can be

determined by integrating Eq. 7–4 between the initial and the final states:

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Since the heat transfer is equal to the work done for a cycle, the

area also represents the net work accomplished by the system

during the cycle. Here, Qnet = Wnet = ΔT ΔS.

netQ T S

Entropy and enthalpy relationship

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THE INCREASE OF ENTROPY PRINCIPLE

Consider a cycle that is made up of two processes: process 1-2, which

is arbitrary (reversible or irreversible), and process 2-1, which is

internally reversible, as shown in Figure 7–5.

From the Clausius inequality-

The second integral in the previous relation

is recognized as the entropy

change S1-S2. Therefore,

which can be rearranged as

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It can also be expressed in differential form as

Entropy is generated or created during an irreversible process,

The entropy generated during a process is called entropy generation

and is denoted by Sgen

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The entropy change is equal to the entropy transfer and entropy generation (for

a closed system)

Note that the entropy generation Sgen is always a positive quantity or zero.

For an isolated system (or simply an adiabatic closed system), the heat

transfer is zero the eq. reduces to

This equation can be expressed as the entropy of an isolated system during

a process always increases or, in the limiting case of a reversible process,

remains constant.

In other words, it never decreases. This is known as the increase of entropy

principle.

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The value of entropy at a specified state is

determined just like any other

property. In the compressed liquid and

superheated vapor regions, it can be

obtained directly from the tables at the

specified state. In the saturated mixture

region, it is determined from

The entropy change of a specified mass m (a closed system) during

a process is simply

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Entropy is commonly used as a coordinate on diagrams such as the T-s and

h-s diagrams. The general characteristics of the T-s diagram of pure

substances are shown in Fig. 7–11 using data for water.

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A process during which the entropy remains constant is called an isentropic

process. It is characterized by

It should be recognized that

a reversible adiabatic

process is necessarily

isentropic (s2 = s1),

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The T-s diagrams serve as valuable tools

for visualizing the second-law aspects of

processes and cycles, and thus they are

frequently used in thermodynamics. The T-

s diagram of water is given in the appendix

in Fig. A–9.

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Another diagram commonly used in engineering is the enthalpy-entropy

(h-s) diagram, which is quite valuable in the analysis of steady-flow

devices such as turbines, compressors, and nozzles.

In analyzing the steady flow of steam through an Adiabatic

turbine, for example, the vertical distance between the inlet

and the exit states h is a measure of the work output of the

turbine, and the horizontal distance s is a measure of the

irreversibilities associated with the process (Fig. 7–18).

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The h-s diagram is also called a Mollier diagram after the German scientist R. Mollier

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Since the heat transfer is equal to the work done for a cycle, the area also represents the

net work accomplished by the system during the cycle. Here, Qnet = Wnet = ΔT ΔS.

1Q T S

THE T -ds RELATIONS

Qnet= Q1-Q2

Q1= T2(S1-S2)

Q1= T1(S2-S1)

The differential form of the

conservation of energy equation

for a closed stationary system (a

fixed mass)

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The entropy change during a process can be determined by integrating

either of these equations between the initial and the final states.

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Constant Specific Heats (Approximate Analysis)

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Compact forms

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REVERSIBLE STEADY-FLOW WORK

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ISENTROPIC EFFICIENCIES OF STEADY-FLOW DEVICES

The ideal process that can serve as a suitable model for adiabatic

steady-flow devices is the isentropic process

Isentropic Efficiency of Turbines

The isentropic efficiency of a turbine is defined as the ratio of the actual

work output of the turbine to the work output that would be achieved if the

process between the inlet state and the exit pressure were isentropic:

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Isentropic Efficiencies of Compressors and Pumps

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Isentropic Efficiency of Nozzles

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Entropy Change of a System, Ssystem

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ENTROPY BALANCE

The entropy balance relation can be stated as:

the entropy change of a system during a process is equal to the net

entropy transfer through the system boundary and the entropy

generated within the system.

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Entropy Generation, Sgen

Entropy balance for any system undergoing any process can be

expressed more explicitly as

The entropy transfer by heat Q/T is zero for adiabatic systems, and

the entropy transfer by mass ms is zero for systems that involve no

mass flow across their boundary (i.e., closed systems).

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7–75 Air is compressed in a piston–cylinder device from 100 kPa

and 17°C to 800 kPa in a reversible, adiabatic process. Determine

the final temperature and the work done during this process,

assuming constant specific heats.

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7–104 Steam enters an adiabatic turbine at 3 MPa and 400°C with a mass flow

rate of 2 kg/s and leaves at 30 kPa. The isentropic efficiency of the turbine is

0.90. Neglecting the kinetic energy change of the steam, determine (b) the

power output of the turbine.

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7–106 Steam enters an adiabatic turbine at 7 MPa, 600°C, and 80 m/s

and leaves at 50 kPa, 150°C, and 140 m/s. If the power output of the

turbine is 6 MW, determine (a) the mass flow rate of the steam flowing

through the turbine and (b) the isentropic efficiency of the turbine.

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7. (6.16 AE) A steam turbine accepts 2 kg/s of steam at 6 MPa and 600°C

and exhausts saturated steam at 20 kPa while producing 2000 kW of work. If

the surroundings are at 30°C and the flow is steady, calculate the rate

of entropy generation.

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7–120 Air enters a compressor steadily at the ambient conditions

of 100 kPa and 22°C and leaves at 800 kPa. Heat is lost from the compressor

in the amount of 120 kJ/kg and the air experiences an entropy decrease of

0.40 kJ/kg K. Using constant specific heats, determine (a) the exit

temperature of the air, (b) the work input to the co

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