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IA Engineering > 1P1 Thermodynamics > Flashcards

1P1 Thermodynamics Flashcards

1
Q

Define a system

A

An arbitrary gemetrical portion of the universe with fixed or movable boundaries which may contain matter or energy or both.

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2
Q

What is the difference between extensive and intensive properties?

A

Extensive, due to size of the system, depends on the mass present. E.g. Volume

Intensive, do not depend on the size of the system, include pressure and temperature.

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3
Q

What is the condition for the two property rule?

A

Intensive properties are uniform throughout.

Effecs of electricity, capillarity, gravity and magnetism can be ignored.

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4
Q

What is the two property rule?

A

For simple compressible systems at rest, two independdent intensive properties and the mass are suffucient to define the state, when the system is in equilibrium.

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5
Q

Define thermodynamics equilibrium.

A

When none of its thermodynamic properties are changing in time at a measureable rate.

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6
Q

What is quasi-equilibrium?

A

Slowly carried out, departures from equilibrium may be kept small, can be plotted on a p-v diagram.

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7
Q

What is the difference between a barometer and manometer?

A

Barometer - pressure difference between vaccuum and atmosphere.

Manometer - pressure difference

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8
Q

What is an adiabatic process?

A

Heat transfer in the process is zero

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9
Q

What is displacement work?

A

THe work done by a system as it changes volume during a quasi-equilibrium process.

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10
Q

What is entropy creation?

A

The creation of entropy due to irreversibilities.

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11
Q

What is fully resisted expansion?

A

Slow expansion where mechanical and thermodynamic equilibrium is maintained.

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12
Q

What is a process where enthalpy is constant?

A

isenthalpic

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13
Q

What is a process where specific volume is constant?

A

isochoric

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14
Q

What is a partially resisted expnasion?

A

An expansion process that does some work but is irreversible.

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15
Q

Define a perfect gas.

A

An ideal gas with constant Cv and Cp, (specific heat capacities)

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16
Q

Define a semi-perfect gas.

A

An ideal gas where Cv and Cp depend only on temperature.

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17
Q

What is a polytropic process.

A

pV^n = constant, must be quasi equilibrium

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18
Q

What is shaft work?

A

Work thatpasses through the surface of a control volume, or system boundary, by a rotating shaft.

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19
Q

Define the two specific heat capacities.

A

The heat required to raise the temperature of 1kg of a substance by 1K.

Cv = du/dT (constant volume)

Cp = dh/dT (constant pressure)

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20
Q

What is a thermal resevoir?

A

An infinitely large body which doesn’t change temperature when heat is transfered.

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21
Q

What is diffferent about non-ideal gases?

A

pV=mrT, is no longer valid.

But as it is a simple compressible system, the two property rule applied, so can use tables.

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22
Q

For polytropic process, what values of n represents which process.

A

n= gamma, isnetropic
n=1 isothermal
n= 0 isobaric
n = infinite isochoric

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23
Q

Define a reversible process.

A

a process is reversible if the system and its surroundings can be returned to their initial state.

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24
Q

What is an example of a reversible process?

A

Stretching a spring within its elastic limit

The slow fully resisted compression of expansion of a gas.

The slow discharge of a baterry at a low current

The sllow melting of ice where the temperature difference between the phases is kept infinitessimely small.

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25
Q

What is the Kelvin-Planck Statement:

A

It is impossible to construct a cyclic device whose sole effect is to produce positive work whilst receiving heat from a single thermal resevoir.

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26
Q

What is the Clausius Statement?

A

It is impossible to construct a cyclic device whose sole effect is the transfer of eat from a cooler to a hotter body.

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27
Q

Define thermodynamic efficiency.

A

net work/total heat input

28
Q

What processes are involved in the Carnot cycle? What’s its shape on the T-s diagram?

A

Isothermal expansion, adiabatic expansion

Isothermal compression,
adiabatic compression.

LLooks liek a rectangle.
Maximum efficiency of a cycle.

29
Q

What did Carnot say about reversible cycles between the same thermal resevoirs?

WHat is the result of this?

A

Must have the same efficiency.

Q1/T1 = Q2/T2

30
Q

Difference between COP for refrigorator and heat pump.

A

Refrigerator:
COP = (heat from cold space)/(work input)

Heat pump:
COP = (heat to hot space)/work input

31
Q

How is the Kelvin scale derived?

A

Based on the absolute pressure of a gas maintained at a constant volume.

32
Q

What is the Clausius inequality?

A

Sum of Qi/Ti less than or equal to 0. Or in integral form. It is an equality when the cycle is reversible.

33
Q

Define entropy.

A

Change in S = integral(dQ/T) for a reversible process

34
Q

What is the nature of the quantity entropy?

A

An extensive thermodynamic property.

35
Q

What is the heat of a reversible process in terms of temperature and entropy?

A

integral of Tds
Which is equal to pdV

area within the cycles of both graphs.

36
Q

How do you get the concept of ‘irreversible entropy’ from clausius?

A

Take a cycle with a reversible process one way and an irreversible process the other way, the ineuqliaty will show that the change in entropy is greater than integral (dQ/T) for the irreversible.

37
Q

How can you reduce entropy of a closed systeM?

A

Extract heat

38
Q

How do you represent irreversible processes on p-V or T-s diagrams?

A

With dashed lines to show the working fluid is not in equilibrium.

39
Q

If a process is adiabatic and reversible then it is?

A

Isentropic.

40
Q

What happens to an isolated syste,?

A

Isolated system - no heat input.

Entropy increases as all propertys become equal.

41
Q

When is irreversible entropy created?

A

When the properties are not in thermodynamic equilibrium. Not Fully resisted.

42
Q

What is wrong with the Carnot cycle?

A

Isothermal heat transfer is too slow.

Too skinny, susceptible to irreversibilities.

43
Q

How do you represent Carnot cycle on a T-s diagram?

A

A rectangle (isentropic, isothermal, isentropic, isothermal)

44
Q

What is the Stirling cycle?

A

External combustion engine.

Isothermal compression.
Constant volume heat addition.
Isothermal expansion at higher temperature.
Constant volume heat rejection.

45
Q

What is the difference between diesel and petrol engines?

A

Petrol is spark ignition, air and fuel mixture is introduced and ignited by a spark.

Diesel is compression ignition, air is compressed, fuel is sprayed in and ignites spontaneously.

46
Q

What is the four stroke process for a petrol engine?

A

Induction stroke (suck)
Compression stroke (squeeze)
Power Stroke (Bang)
Exhuast stroke (blow)

47
Q

What is the air standard otto cycle?

A

Isentropic compression.
Const vol heat addition (from the fuel being ignited)
Isentropic expansion
Const vol heat rejection.

48
Q

What is a typical compression ratio for a spark ignition engine?

A

Around 5-10.

49
Q

How do you find the efficiency of a spark ignition engine?

A

Use w/qin, but work this out using the heat input and heat output, which are eaiser to calculate with the temperature differences.

(q1-q2)/q1

50
Q

What is the air standard diesel cycle?

A

Isentropic compression,
constant pressure heat in,
isentropic expansion.
Constant volume heat out.

51
Q

What are the two compression ratios for the air standard diesel cycle?

A

V3/V2 = alpha (V3 is after the constant pressur eheat addition)

V1/V2 = rv (main one)

52
Q

what is the difference in efficiency between petrol and diesel engines?

A

Otto cycle has a higher efficiency for a specific compression ratio, but diesel cycle generally has higher compression rations by a factor of 2 making them more efficient.

53
Q

How does fuel injection work for SI and CI engines?

A

SI the air-fuel ratio is fixed.

CI the fuel droplets must first evaporate. Means only 80% of the oxygen can be consumed.

54
Q

How is rotational speed different for petrol and diesel?

A

Petrol has a higher rotational speed than diesel to allow for evaporation in diesel and combustion of fuel droplets.

55
Q

What factor about SI can limit their efficiencies when it comes to fuel injection?

A

Fuel has to pass through a throttle for SI, which introduces irreverseibilities. This is not the case for CI

56
Q

What is the formula for mass flow.

A

density x velocity normall to the area x area

57
Q

How do you derive the non-steady flow energy equation?

A

Consider a system within which there is a control surface.

dm enters with pi, specific energy and volume.
Then mass leaves with different pressure and specific energy after dt.

Divide by dt to get rate of changes.

Also consider shaft work output, wdot x. (other work is already included in the displacement work of the gas leaving)

use e = u+ pv

58
Q

What is true for throttling?

A

It is isenthalpic (no heat or work addition, PE and KE terms are negligible)

It is locally unsteady but globally steady. Creates irreversibilites and therefore entropy.

59
Q

How do you derive the non-steady flow entropy equation?

A

Consider a system and control surface. With mass entering (with specific entropy) as well as heat dQ and then some leaving as well.

60
Q

What can be true for isothermal flow in a pipe at velocities close to the speed of sound?

A

The flow can go against the pressure gradient?

61
Q

What is the air standard Joule cycle?

A

Isentropic compression in compressor, isobaric heat addition in the heat exchanger, isothermal expansion in the turbine, isobaric heat rejection in the heat exchanger (not used in real gas turbines as not actually a closed cycle)

Ignore all KE & PE changes.

62
Q

What is the work done/used accross a turbine/compressor?

A

cp change in T (enthalpy)

63
Q

How does a jet engine differ from a standard gas turbine?

A

Features a nozzle after the gas turbine which converts some of the remaining enthalpy into velocity which is used as momentum.

64
Q

What is true about temperatures accross compressor/turbine for a jet engine?

A

They are the same.

65
Q

What process occurs in the nozzle of a jet engine?

A

Isentropic acceleration.

IA Engineering (10 decks)

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