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Thermodynamics > Chapter 7 > Flashcards

Chapter 7 Flashcards

1
Q

Clausius equality

A

The equality in the Clausius inequality holds for totally or just internally reversible cycles and the inequality for the irreversible ones

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2
Q

____ is an extensive property of a system

A

Entropy

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3
Q

The entropy change between two specified states…

A

Is the same whether the process is reversible or irreversible

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4
Q

Internally reversible isothermal heat transfer process

A

This equation is particularly useful for determining the entropy changes of thermal energy reservoirs

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5
Q

The net change in volume (a property) during a cycle is always ____

A

Zero

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6
Q

The increase of entropy principle

A

The equality holds for an internally reversible process and the inequality for an irreversible process

Some entropy is generated or created during an irreversible process, and this generation is due entirely to the presence of irreversibilities

The entropy generation Sgen is always a positive quantity or zero

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7
Q

The entropy change of an isolated system…

A

The sum of the entropy changes of its components, and is never less than zero

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8
Q

A system and its surroundings form an ______

A

Isolated system

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9
Q

Sgen > 0

A

Irreversible process

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10
Q

Sgen = 0

A

Reversible process

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11
Q

Sgen < 0

A

Impossible process

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12
Q

The entropy change of a system can be _____

A

Negative

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13
Q

Can entropy generation (Sgen) be negative?

A

No

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14
Q

Entropy

A
  1. Processes can occur in a direction only, not in any direction.
  2. Entropy is a nonconserved property and there is no such thing as the conservation of entropy principle
  3. The performance of engineering systems is degraded by the presence of irreversibilities and entropy is a measure of the magnitudes of the irreversibilities during that process. It is also used to establish criteria for the performance of engineering devices
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15
Q

Processes can occur in a direction only, not in any direction

A

A process must proceed in the direction that complies with the increase of entropy principle (Sgen >= 0)

A process that violates this principle is impossible

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16
Q

Entropy is a non-conserved property

A

Entropy is conserved during the idealized reversible processes only and increases during all actual processes

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17
Q

Entropy change of pure substances

A

Entropy is a property, and thus the value of entropy of a system is fixed once the state of the system is fixed

Determined from the tables

18
Q

A process during which the entropy remains constant

A

Isentropic process

19
Q

Isentropic process

A

During an internally reversible, adiabatic process

Appears as a vertical line segment on a T-s diagram

20
Q

On a T-s diagram, the area under the process curve represents…

A

The heat transfer for internally reversible processes

21
Q

Mollier diagram

A

The h-s diagram

22
Q

For adiabatic steady-flow devices, h-s diagrams

A

The vertical distance (change in h) is a measure of work

The horizontal distance (change in s) is a measure of irreversibilities

23
Q

The level of molecular disorder (entropy) of a substance _______ as it melts or evaporates

A

Increases

24
Q

A pure crystalline substance at absolute zero temperature is in perfect order and its entropy is zero

A

The third law of thermodynamics

25
Q

Disorganized energy

A

Does not create must useful effect, no matter how large it is

26
Q

In the absence of friction, raising a weight by a rotating shaft…

A

Does not create any disorder (entropy) and energy is not degraded during this process

27
Q

The paddle-wheel work done on a gas…

A

Increases the level of disorder (entropy) of the gas and energy is degraded during this process

28
Q

During a heat transfer process, the net entropy ____

A

Increases

Hot body - decreases

Cold body - increases

29
Q

T ds Relations

A

Valid for both reversible and irreversible processes and for both closed and open systems

30
Q

Liquids and solids can be approximated as _____ since their specific volumes remain nearly constant during a process

A

Incompressible substances

31
Q

For solids and liquids, an isentropic process is also ____

A

Isothermal

32
Q

A reversible turbine delivers more work than an irreversible one if…

A

Both operate between the same end states

33
Q

The adiabatic compression requires the ____ work

A

Maximum

34
Q

The isothermal compression requires the ____ work

A

Minimum

35
Q

The gas is compression in stages and cooled between each stage by passing it through a heat exchanger called an ____

A

Intercooling

36
Q

To minimize compression work during two-stage compression, the pressure ratio across each stage of the compressor…

A

Must be the same

37
Q

Isentropic process

A

Involves no irreversibilities

Serves as the ideal process for adiabatic devices

38
Q

Can you use isentropic efficiency for a non-adiabatic compressor?

A

No

39
Q

Can you use isothermal efficiency for an adiabatic compressor?

A

Yes

40
Q

Compressors are sometimes intentionally cooled to…

A

Minimize the work input

41
Q

Mechanisms of Entropy Transfer

A
  1. Heat transfer

2. Mass flow

42
Q

Entropy transfer by work

A

0

Thermodynamics (11 decks)

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