Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Z thermodynamics > Chapter 6 > Flashcards

Chapter 6 Flashcards

1
Q

Heat Engines

A

The devices that convert heat to work

  1. They receive heat from a high-temperature source
  2. They convert part of this heat to work (usually in the form of a rotating shaft)
  3. They reject the remaining waste to a low-temperature sink
  4. They operate on a cycle
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

High-temperature source

A

Solar energy, oil furnace, nuclear reactor, etc.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Low-temperature sink

A

The atmosphere, rivers, etc.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Heat engines and other cycle devices usually involve a fluid to and from which heat is transferred while undergoing a cycle. This fluid is called the _____

A

Working fluid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Work can always be converted to ___ directly and completely

A

Heat

The reverse is not true

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Qin

A

Amount of heat supplied to steam in boiler from a high-temperature source (furnace)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Qout

A

Amount of heat reject from steam in condenser to a low-temperature sink

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Wout

A

Amount of work delivered by steam as it expands in turbine

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Win

A

Amount of work required to compress water to boiler pressure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Steam power plant

A

A portion of the work output of a heat engine is consumed internally to maintain continuous operation

Wnet,out = Wout-Win (kJ)

Wnet,out = Qin-Qout (kJ)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Thermal efficiency

A

Net work output/total heat input

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

The most efficient heat engines

A

Reject almost one-half of the energy they receive as waste heat

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Can we save Qout?

A

In a steam power plant, the condenser is the device where large quantities of waste heat is rejected to rivers, lakes, or the atmosphere

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Can we not just take the condenser out of the plant and save all that waste energy?

A

No - without a heat rejection process in a condenser, the cycle cannot be completed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

A heat-engine cycle cannot be completed without…

A

Rejecting some heat to a low-temperature sink

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

The Second Law of Thermodynamics: Kelvin Planck Statement

A

It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work

No heat engine can have a thermal efficiency of 100%, or as for a power plant to operate, the working fluid must exchange heat with the environment as well as the furnace

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

The impossibility of having a 100% efficient heat engine

A

Limitation that applies to both the idealized and the actual heat engines

Not due to friction or other dissipating effects

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

The transfer of heat from a low-temperature medium to a high-temperature one requires special devices called _____

A

Refrigerators

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Refrigerators

A

Cyclic devices

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

The working fluid used in the refrigeration cycle is called a _____

A

Refrigerant

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

The most frequently used refrigeration cycle is the

A

Vapor-compression refrigeration cycle

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

In a household refrigerator, the freezer compartment where heat is absorbed by the refrigerant serves as the _____

A

Evaporator

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

In a household refrigerator, the coils usually behind the refrigerator where heat is dissipated to the kitchen air serve as the ______

A

Condenser

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Basic components of a refrigeration system

A

Expansion valve, condenser, compressor, evaporator

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

The efficiency of a refrigerator is expressed in terms of the

A

Coefficient of performance (COP)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

The objective of a refrigerator is to…

A

Remove heat (Ql) from the refrigerated space (cooled space)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

Can the value of COPr be greater than unity (1)?

A

Yes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

Can the value of COPhp be lower than unity?

A

No

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What does COPhp = 1 represent?

A

Violates 2nd law (Ql = 0)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

The objective of a heat pump

A

Supply heat Qh into the warmer space

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

Most heat pumps in operation today have a seasonally averaged COP of ____

A

2 to 3

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

Most existing heat pumps use ____ as the heat source in winter

A

The cold outside air

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

Heat pump efficiency in cold climates

A

Drops considerably when temperatures are below the freezing point

Geothermal (ground-source) HP that use the ground as the heat source can be used

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

Geothermal HP

A

More expensive to install but more efficient

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

_____ are basically refrigerators whose refrigerated space is a room or a building instead of the food compartment

A

Air conditioner

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

The COP of a refrigerator decreases with…

A

Decreasing refrigeration temperature

Not economical to refrigerate to a lower temperature than needed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

The amount of heat removed from the cooled space in Btu’s for 1 Wh of electricity consumed

A

Energy Efficiency Rating (EER)

EER = 3.412 COPr

38
Q

The work supplied to a heat pump is used…

A

To extract energy from the cold outdoors and carry it into the warm indoors

39
Q

The Second Law of Thermodynamics: Clausius Statement

A

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer or heat from a lower-temperature body to a higher-temperature body

States that a refrigerator cannot operate unless its compressor is driven by an external power source, such as an electric motor

40
Q

Kelvin-Planck and the Clausius statements

A

Are equivalent in their consequences, and either statement can be used as the expression of the second law of thermodynamics

Any device that violates the Kelvin-Planck statement also violates the Clausius statement, and vice versa

41
Q

A cult of hot coffee

A

Does not get hotter in a cooler room

42
Q

Transferring heat to a wire

A

Will not generate electricity

43
Q

Transferring heat to a paddle wheel

A

Will not cause it to rotate

44
Q

Major uses of the second law

A
  1. The second law may be used to identify the direction of processes
  2. The second law also asserts that energy has quality as well as quantity
  3. The second law is also used in determining the theoretical limits for the performance of commonly used engineering systems, such as heat engines and refrigerators, as well as predicting the degree of completion of chemical reactions
45
Q

Second law - process

A

Occur in a certain direction and not in the reverse direction

Must satisfy both the 1st and 2nd laws of thermodynamics to proceed

46
Q

Bodies with relatively large thermal masses can be modeled as _____

A

Thermal energy reservoirs

47
Q

Thermal energy capacity

A

Mass * specific heat

48
Q

A hypothetical body with a relatively large thermal energy capacity that can supply or absorb final amounts of heat without undergoing any change in temperature is call a _____

A

Thermal energy reservoir or just a reservoir

49
Q

In practice, large bodies of water such as oceans, lakes, and rivers as well as the atmospheric air can be modeled accurately as thermal energy reservoirs because…

A

Of their large thermal energy storage capabilities or thermal masses

50
Q

Source

A

Supplies energy in the form of heat

51
Q

Sink

A

Absorbs energy

52
Q

Any device that violates the first or second law

A

Perpetual-motion machine

53
Q

A device that violates the first law (by creating energy)

A

PMM1

54
Q

A device that violates the second law

A

PMM2

55
Q

A process that can be reversed without leaving any trance on the surroundings

A

Reversible process

Deliver the most and consume the least work

56
Q

A process that is not reversible

A

Irreversible process

57
Q

All the processes occurring in nature are ____

A

Irreversible

58
Q

Why are we interested in reversible processes?

A
  1. They are easy to analyze

2. They serve as idealized models (theoretical limits) to which actual processes can be compared

59
Q

The facts that cause a process to be irreversible are called ________

A

Irreversibilities

60
Q

Irreversibilities include:

A

Friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical reactions

61
Q

Heat transfer through a temperature difference is ____

A

Irreversible

62
Q

If no irreversibilities occur within the boundaries of the system during the process

A

Internally reversible process

63
Q

If no irreversibilities occur outside the system boundaries

A

Externally reversible

64
Q

It involves no irreversibilities within the system or its surroundings

A

Totally reversible process

65
Q

A totally reversible process involves:

A

No heat transfer through a finite temperature difference, no nonquasi-equilibrium changes, and no friction or other dissipative effects

66
Q

A reversible process involves no ____

A

Internal and external irreversibilities

67
Q

Reversible Isothermal Expansion

A

Th = constant

68
Q

Reversible Adiabatic Expansion

A

Temperature drops from TH to TL

69
Q

Reversible Isothermal Compression

A

TL = constant

70
Q

Reversible Adiabatic Compression

A

Temperature rises from TL to TH

71
Q

The Reversed Carnot Cycle

A

The Carnot heat-engine cycle is a totally reversible cycle

Therefore, all the processes that comprise that it can be reversed, in which case it becomes the Carnot refrigeration cycle

72
Q

The Carnot Principles

A
  1. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs
  2. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same
73
Q

All reversible heat engines operating between the same two reservoirs have the ____

A

Same efficiency (2nd Carnot principle)

74
Q

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

A

Thermodynamic Temperature scale

75
Q

Thermodynamic Temperature Scale is called the ____

A

Kelvin Scale

76
Q

The temperatures on the Thermodynamic Temperature Scale are called _____

A

Temperatures

77
Q

Efficiency > reverse efficiency (heat engine)

A

Impossible heat engine

78
Q

Efficiency < reversible efficiency (heat engine)

A

Irreversible heat engine

79
Q

Efficiency = reversible efficiency (heat engine)

A

Reversible heat engine

80
Q

_____ the most efficient of all heat engines operating between the same high- and low-temperature reservoirs

A

Carnot heat engine

81
Q

Can we use C unit for temperature?

A

No

82
Q

How do you increase the thermal efficiency of a Carnot heat engine?

A

Decrease TL

Increase TH

83
Q

How do you increase the thermal efficiency for actual heat engines

A

Decrease QL

Increase QH

84
Q

How do you increase the COP of a Carnot refrigerator or heat pump?

A

Increase TL

Decrease TH

85
Q

How do you increase the COP of actual refrigerators or heat pumps?

A

Increase QL

Decrease QH

86
Q

COPr < COPr, rev

A

Irreversible refrigerator

87
Q

COPr = COPr, rev

A

Reversible refrigerator

88
Q

COPr > COPr, rev

A

Impossible refrigerator

89
Q

The COP of a reversible refrigerator or heat pump is the…

A

Maximum theoretical value for the specified temperature limits

90
Q

The COPs of both the refrigerators and heat pumps ____ as TL ____

A

Decrease, decrease

Requires more work to absorb heat from lower-temperature media

Z thermodynamics (11 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions