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Chem Eng - Y2.S1 > Thermodynamics - Heat Engines and Refrigerators > Flashcards

Thermodynamics - Heat Engines and Refrigerators Flashcards

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1
Q

What is thermodynamics?

A

The science of the relationship between heat, work, temperature, and energy. In broad terms, thermodynamics deals with the transfer of energy from one place to another and from one form to another.

The key concept is that heat is a form of energy corresponding to a definite amount of mechanical work.

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2
Q

What is sensible heat?

A

Sensible heat is literally the heat that can be felt. It is the energy moving from one system to another that changes the temperature rather than changing its phase.

Temperature changes, not phase.

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3
Q

What is latent heat?

A

The heat required during a phase change, without causing a change in temperature.

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4
Q

What’s thermal equilibrium?

A

When the temperature gradient between the system and its surroundings is small or heat flows very slowly.

Primitive or basic definition:

𝛿Q = C dT (C is absolute heat capacity)

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5
Q

What is work?

A

The transfer of ordered energy, involving the bulk displacement of matter as a result of an applied force.

W = Fx

Work or mechanical equilibrium is when there is at most an infinitesimal force imbalance within the system, or between the system and its surroundings, leading to the displacement.

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6
Q

Are work and heat exact or inexact?

A

Inexact

How the transfer takes place must be specified.

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7
Q

What happens if system pressure, P is equal to external pressure, P ex?

A

We have work, or mechanical equilibrium.

Otherwise the system pressure will not be uniform or single-valued (it will be higher in some places than others) and we can no longer write δ W = – P dV.

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8
Q

What’s the equation for equilibrium pressure-volume work?

A

δW = - Pex*dV

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9
Q

What are gas power cycles?

A

Cycles involving heat engines operating over a power cycle where the working fluid remains a gas/vapour.

Performance measured by thermal efficiency, eta.
n = W net out/ Qin

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10
Q

What’s internal reversibility?

A

When there’s no irreversible processes (friction, fast expansion, …) within boundaries of system.

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11
Q

What’s external reversibility?

A

When there’s no irreversible processes between system and surroundings (e.g., heat transfer across a finite temp. gradients).

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12
Q

What’s total reversibility?

A

Internal and external reversibility

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13
Q

What’s the Carnot cycle?

What are the 4 steps?

A

A totally reversible cycle (most efficient cycle operating between two temperatures)

Four reversible steps:
1-2 isothermal expansion 
2-3 isentropic expansion 
3-4 isothermal compression 
4-1 isentropic compression

Can have closed or flow cycles.

It is internally and externally reversible.

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14
Q

How do most gas power cycles operate?

A

By internal combustion

In IC engines, products are exhausted and replaced by fresh fuel and air.
• Such open cycles are not thermodynamic cycles since working fluid is not returned to its initial state.

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15
Q

What’s a reciprocating engine?

A

Reciprocating engines are basically piston-cylinder devices.

A piston reciprocates between top dead centre and bottom dead centre in a cylinder.

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16
Q

What are the air standard assumptions?

A 
• Working fluid is air.
• Air is ideal gas.
• Closed cycle.
• Combustion modelled as a heat
addition from an external source.
• Exhaust modelled as heat rejection
that restores air to its original state.

Cold-air-standard assumption (CAS) is that heat capacities are constant at the values for 25 °C (298 K).

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17
Q

What is the heat in an isochoric process equal to?

A

The change in interval energy.

U = Q + W

W = 0 for constant volume (as dV = 0) therefore dU = dQ

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18
Q

How is enthalpy defined/ calculated?

A

H = U + pV

dH = dU + pdV + VdP

Since U = Q + W

dH = dQ + dW + pdV + VdP

And since W = - PdV

dH = dQ + VdP

For isobaric, dH = dQ

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19
Q

What are the features of a reciprocating engine?

A

They’re piston-cylinder devices.
A piston reciprocates between the top dead centre and bottom dead centre in a cylinder.

The difference between top and bottom dead dead centres is called the stroke.
The width of the cylinder is the bore.

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20
Q

What are the two ignition methods of a reciprocating engine?

A
  • spark-ignition (SI) engines combustion initiated by a spark.
  • compression-ignition (CI) engines combustion initiated by pressure.
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21
Q

What’s the compression ratio (in a reciprocating engine)?

A

r = V max / V min

= max volume / min volume

= V bdc / V tdc
(dead centres)

= (displacement volume + clearance volume) / clearance volume

= (Vd + Vc)/Vc

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22
Q

What’s mean effective pressure, MEP?

A

The cylinder pressure that would produce the same amount of work as produced in the actual cycle.

For a given Vd (displacement volume) MEP is a measure of net work per cycle.

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23
Q

What’s the Otto cycle?

A

A description of what happens to a mass of gas as it is subjected to changes of pressure, temperature, volume, addition of heat, and removal of heat.

The mass of gas that is subjected to those changes is called the system. The system, in this case, is defined to be the fluid (gas) within the cylinder.

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24
Q

What are the stages of the Otto cycle?

A

1-2: isentropic (Δs=0) compression
Stroke 1 - compression/squeeze

2-3: isochoric (isometric, Δv=0) heat addition
Stroke 2 - power/bang (part 1)

3-4: isentropic (Δs=0) expansion
Stroke 2 - power/bang (part 2)

4-1: isochoric (isometric, Δv=0) heat rejection
Stroke 3 - exhaust/blow
Stroke 4 - intake/suck

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25
Q

Within the Otto cycle, how are the isochoric heat transfers calculated?

A

Q in = u₃ - u₂ = Cv * (T₃ - T₂)

Q out = u₄ - u₁ = Cv * (T₄- T₁)

26
Q

How is efficiency of the Otto cycle calculated?

A

n = W net,out / Q in

= 1 - Q out / Q in

= 1 - (T₄- T₁)/(T₃ - T₂)

= 1 - 1/r ^ (k - 1)

27
Q

How are the ratios of T1/T2 and T4/T3 calculated for isentropic processes involving ideal gases?

A

T₁ / T₂ = (V₂ / V₁) ^ (k - 1)

T₄ / T₃ = (V₃ / V₄) ^ (k - 1)

Where k = Cp / Cv

28
Q

What are the issues with the Otto cycle?

A

Otto efficiency increases with r (compression ratio) and k (Cp/Cv) for constant heat capacity

  • high r leads to auto-ignition (knock) which damages engines.
    Anti-knock (leaded petrol) can be used but has environmental implications
  • Engine gases always contain large molecules which reduce k, thus reducing Otto efficiency
  • Otto efficiency is reduced at higher temperatures, where k is low
  • two stroke cycle can be used instead, but is less efficient than four stroke
29
Q

What are features of Diesel engines?

A

They use compression ignition, CI

Air is compressed in compression stroke and fuel spray is injected near the top dead centre

Compression induces autoignition and combustion continues through the first part of the power stroke.

Diesel engines avoid engine knock (unlike spark ignition) and can reach higher compression ratios.

Lower grade fuel can be used. This reduces efficiency as the heat capacity ratio is lower.

30
Q

What are the stages of the diesel cycle?

A

1-2: Isentropic/adiabatic compression

2-3: Isobaric heat addition

3-4: Adiabatic/isentropic expansion

4-1: Isenchoric heat rejection

31
Q

How is efficiency of a Diesel engine determined?

A

n = 1 - ( 1/r^(k-1) * [(rc^k -1)/k*(rc - 1)]

Where rc is the cut off ratio, v3/v2

k is heat capacity ratio and r is pressure ratio

32
Q

How does Diesel engine efficiency change with increasing cut off ratios (v3/v2)?

A

As cut off ratio (rc) is increased (for the same compression ratio) diesel efficiency decreases.

33
Q

What are features of gas turbines?

A

There’re another type of gas power system

Ambient air is compressed (supercharged) to higher p and T.

The high p and T air is mixed with fuel and combusts. 
Resulting gas (now at even high T) enters a turbine and expands to atmospheric P to produce power.

They’re open cycle devices.

Some turbine power is used to drive a compressor.

34
Q

What’s the Brayton cycle?

A

A thermodynamic cycle that describes the workings of a constant-pressure heat engine.

Used for closed cycle steady-flow gas turbine models.

Combustion is represented as isobaric heat addition from an external source.

Exhaust is represented as isobaric heat rejection to a sink.

Compression and expansion are isentropic.

Air standard assumptions used

35
Q

What are the stages of the Brayton cycle?

A

1-2: Reversible, asiabatic compression (in compressor)

2-3: Isobaric heat addition (combustion)

3-4: Reversible, adiabatic expansion (in turbine)

4-1: Isobaric heat rejection (exhaust)

36
Q

How are compressor and turbine work calculated when analysing the Brayton cycle?

A

W compressor = H2 - H1

W turbine = H3 - H4

where H is enthalpy

37
Q

How are net work and back-work ratio calculated when analysing the Brayton cycle?

A

Net work = turbine work - compressor work

Back-work ratio = compressor work / turbine work

38
Q

How is Brayton cycle efficiency calculated?

A

n = 1 - [1 / rp ^((k-1)/k) ]

Where rp is the pressure ratio, p2/p1 and k is heat capacity ratio, cp/cv

39
Q

How is heat capacity ratio calculated?

A

k = Cp / Cv

40
Q

What are issues with gas turbines?

A

Deviations from Brayton cycle

  • pressure drops in heat transfer
  • more compressor work
  • less turbine work

Increased turbine inlet (firing) temperature
- materials selection and cooling turbine components affected

41
Q

What are features of vapour power cycles?

A

Working fluid will be liquid, vapour or a mix of both at different stages of the cycle.

Common working fluid is water/steam

Basically, compressed fluid is heated from an external source. The fluids energy is then transferred to a turbine.

They usually work on closed cycles (unlike gas power cycles)

The ideal vapour power cycle is the Rankine cycle.

42
Q

What are the stages of the Rankine vapour cycle?

A

1-2: Isentropic compression. Water enters pump at state 1 and is compressed isentropically to state 2 (pump work)

2-3: Isobaric heat addition in boiler. Compressed water at state 2 is converted to superheated steam (state 3) in a boiler (steam generation)by heat transfer (Q in) from an external source.

3-4: Isentropic expansion in turbine. Superheated steam at state 3 enters a turbine and expands isentropically to state 4 (liquid-vapour mix), producing work (turbine work) by rotating a shaft.

4-1: Isobaric heat rejection to condenser.
Remaining vapour in state 4 is condensed to liquid at state 1 by heat transfer (Q out) in a condenser to either water (wet-cooling) or air (dry-cooling)

43
Q

What does CHP represent?

A

Combined heat and power. Some heat from the boiler, turbine and condenser are used as process heat.

44
Q

What does CCGT represent?

A

Combined cycle gas turbine.

Hot exhaust from gas turbine directed to boiler in steam power plant.

45
Q

What is a refrigerator?

A

A device where heat is transferred from a cold environment to a warm environment by the input of work.

Work is needed as heat transfer is not spontaneous.

46
Q

What’s a refrigerant?

A

The working fluid within a refrigerator.

47
Q

How does a refrigerator differ from a heat pump?

A

Refrigerator - removes heat from cold environment

Heat pump - supplies heat to a warm environment

48
Q

What are the layers of the Earths atmosphere? (Bottom to top)

A

Troposphere

Stratosphere

Mesosphere

Thermosphere

49
Q

What’s tropospheric ozone?

A
  • Ozone is a highly reactive and oxidising molecule
  • Tropospheric ozone is generated as a by‐product of photochemical oxidation of organic pollutants (photochemical smog)
  • Ozone in the lower atmosphere is a pollutant associated with plant and lung damage
50
Q

What’s a Dobson unit?

A

A measure of the amount of ozone in the atmosphere above any given area.
1 DU ~ 2.7 x 1016 ozone molecules cm^-2

51
Q

What types of UV radiation are present in the stratospheric ozone?

A

UV‐A: wavelengths between 320-400 nm.
Harmless. Passes through atmosphere and reaches the ground

UV-B: wavelengths between 280-320 nm.
Harmful but mostly absorbed by ozone.

UV-C: wavelengths between 200-280 nm.
Very harmful but mainly absorbed by oxygen to form ozone.

52
Q

What are examples of ozone depleting substances?

A

CFC refrigerants

Halon fire extinguishers

Brominates pesticides

NO2 from aircraft exhaust

53
Q

How is Ozone continually being formed and broken down?

A

By free radical substitution reactions

54
Q

How can vapour power cycle efficiency be improved?

What are the issues?

A

• Decrease condenser pressure, p1 = p4.
This decreases T1 = T4.
! T can’t go below temp of condenser cooling water.

• Increase temp of feed gas to turbine, T3.
This also increases steam quality, x.
! But too high T3 affects turbine materials.

  • Increase feedwater temp to boiler , T2 (feedwater heating or regeneration) by bleeding steam from turbine to mix with feedwater.
  • Increasing boiler pressure, p2 = p3, tends to increase efficiency but reduces steam quality, x, in turbine.
  • Can increase x by reheating.
55
Q

What are properties of the Stratosphere?

A
  • Thetemperatureinversionofthe stratospherepreventsmixingwiththe loweratmosphere
  • Onlymoleculeswithalongatmospheric lifetimewilldiffuseacrossthetropopause
  • Onceinthestratospheretheystaythere foralongtime
56
Q

What are the typical compression ratios, r, for an ideal Otto cycle?

A

r = 7 to 10

57
Q

How is Otto efficiency, n, affected by increasing specific heat ratio, k and compression ratio, r?

A

Efficiency increases with increasing k and r

58
Q

What are typical compression ratios, r, for Diesel engines?

A

r = 12 to 23

59
Q

What are typical pressure ratios, rp, for an ideal Brayton cycle?

A

rp = 5 to 20

60
Q

What are issues of high compression ratios, r, for Otto cycles?

A

It leads to auto ignition (knock) which damages the engine.

Efficiency is also reduced at higher temperatures, where k is low.

61
Q

How is the total mass of air in a thermodynamic cycle calculated (considering specific volume)?

A

Find specific volume, v, e.g. from ideal gas equation.

Mass = total displacement volume / specific volume

m = Vd / v

Chem Eng - Y2.S1 (6 decks)

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