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MECH 453 > Section 5 - Combustion Theory > Flashcards

Section 5 - Combustion Theory Flashcards

(Quiz 2 Content)

1
Q

Common H-C fuel groups

A
  1. paraffins (CnH2n+2 - methane, ethane, propane, etc.)
  2. Olefins (CnH2n)
  3. Acetylenes (CnH2n-2)
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2
Q

Main alcohols used in engine fuels

A

Ethanol, methanol, butanol

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3
Q

Crude oil contains ______ H-C compounds

A

25000

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4
Q

Purpose of refining?

A

Separate crude oil via distillation, then chemically process into fuels and other products. Still is used to heat sample, so boiling off lighter components are condensed + recovered

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5
Q

Fractions

A

Group of compounds that boil off between two temps

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6
Q

Order of fractions as they leave the still

A

Naptha > distillate > gas oil > residual oil (subdivided using light, middle, heavy)

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7
Q

T/F: Light virgin naptha cannot be used as gasoline

A

F: opposite is true

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8
Q

Gasoline is a blend of ____ distillates with a range of bpts between ___ and ___ºC

A

H-C

25-225ºC

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9
Q

What molecule is often used to simulate the properties of gasoline?

A

Octane

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10
Q

There are ___ isomers of octane

A

18

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11
Q

Requirement of the Clean Air Act Amendment (1990) by the U.S.

A

U.S. required to use reformulated gas (RFG) year-round IOT reduce ozone by requiring a minimum O2 content of 2% by weight and max benzene content of 1%

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12
Q

As part of the reformulated gasoline program, sulfur is restricted to ___ ppm.

A

31

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13
Q

Sources for diesel fuel

A

Petroleum diesel
Synthetic diesel
Biodiesel

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14
Q

List reasons why alternative fuels have become very attractive

A
  1. Occasional high cost of oil
  2. Accessibility to oil reserves
  3. Environmental concerns such as global warming
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15
Q

Why are alcohols recieving a lot of attention?

A

Can be synthesized biologically

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16
Q

T/F: Since there is O2 in fuel, combustion of alcohols produces more CO than fossil fuel combustion, but less CO2.

A

F: produces more CO2, less CO

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17
Q

Drawbacks of alcohol vs. fossil fuels (6)

A
  • Lower E density than gas
  • Corrosive to fuel systems (meth > eth > but)
  • Poor cold temp start up due to low vapour P
  • Toxicity (meth)
  • Use of food crops drives up world prices of food
  • Production of eth is very E intensive and expensive
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18
Q

Other alternative fuels

A

Liquid petroleum gas (LPG) (mainly used on fork lifts and golf carts, mix of propane and butane, stored as liquid in steel tanks)

Dimethyl ether: good diesel fuel bc good auto-ignition quality

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19
Q

Gaseous alternative fuels

A

Hydrogen
Natural gas
Producer gas

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20
Q

Drawback of gaseous alternative fuels

A

Gaseous state at 25ºC and thus have low E density. Engines that use them have low volumetric efficiencies

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21
Q

Issues with H2 as a gaseous fuel?

A
  • Safety (very reactive and volatile, only need 5% mixed with air and you can ignite)
  • Lack of distribution structure (how would we transport H2 if it’s unsafe?)
  • Production E (not found in nature but produced from natural gas or electrolysis)
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22
Q

Features of natural gas as a gaseous fuel alternative (5)

A
  • Cleaner burning than gas
  • Lower E content than LPG
  • Used for power gen gas turbines and large SI engines
  • Low soot, NOx if burnt lean
  • Can’t be compression-ignited
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23
Q

For combustion, dry air is taken to be composed of ___ O2 and ___ N2 by volume

A

21%

79%

24
Q

For every mol of O2, there are ___ moles N2

A

3.76

25
Q

Amount of water in moist air at temp T is specified by the _______ and _______.

A

Specific humidity (omega = m_w/m_air)

Relative humidity (Phi = P_w/P_sat(T))

26
Q

Chemical equation for complete combustion (sufficient O2 available)

A

[FUEL] + aO2 –> bCO2 + cH2O

27
Q

T/F: If products are at “low” temperature, N2 is not significantly affected by the reaction and is considered inert.

A

T

28
Q

Complete rxn equation for a general H-C fuel with air is:

A

[FUEL] + a(O2 + 3.76N2) –> bCO2 + cH2O + dN2

29
Q

Fuel lean

A

Excess air for combustion

30
Q

Combustion rxn equation with excess air:

A

[CaHb] + gamma(a+b/4)(O2 + 3.76N2) –> aCO2 + b/2 H2O + dN2 + eO2

31
Q

Combustion rxn equation with fuel-rich mixture:

A

[CaHb] + gamma(a+b/4)(O2 + 3.76N2) –> aCO2 + b/2 H2O + dN2 + eCO + fH2

32
Q

Equivalence ratio (phi)

A

Indicates if mix is stoichiometric, fuel-lean, or fuel-rich

phi = (A/F)s/(A/F)mix

OR

phi = (F/A)mix/(F/A)s

33
Q

Conditions for equiv ratio

A

Stoichiometric: phi = 1
Fuel lean: phi < 1
Fuel rich: phi > 1

34
Q

What is gamma in the rxn equation?

A

1 / phi

35
Q

T/F: 110% stoichiometric air is equal to 110% excess air

A

F: 110% stoic = 110% theor = 10% excess

36
Q

Heat of formation

A

Enthalpy/E required to form compounds from their elements at STP (T = 298K P = 1 atm)

37
Q

Sensible enthalpy

A

Change in heat/E of a substance that we can feel

38
Q

Heat of combustion

A

Max amnt of E released from fuel when reacted with a stoichiometric amount of air (and all H2 and C in fuel is converted to CO2 and H2O)

39
Q

T/F: The heat of combustion stays the same regardless of water being vapour or liquid

A

F: 2 diff values for heat of combustion (H_R) based on if water is vapor or liquid

40
Q

Higher heat of combustion/heating value (HHV)

A

Water in products is liquid (more heat is removed from combustion)

41
Q

Lower heat of combustion/heating value (LHV)

A

Water in products is vapour

42
Q

Difference between HHV and LHV

A

Heat of vaporization

43
Q

What is one of the biggest problems with using H2 as a fuel wrt heating values?

A

A/F ratio must be immensely high. For every gram of H2, 34g of air must be added. Air doesn’t have a high energy content at all

44
Q

Adiabatic Flame Temperature (AFT)

A

Final products temperature (for either cst V or P process)

45
Q

T/F: Hand calculations can be used to solve balanced equations regardless of the conditions

A

F: At high temps, minor species will be present due to dissociation of major species, so computer equilibrium solvers can be used

46
Q

Chain branching

A

Chain branching rxns lead to rapid production of radicals which causes the overall rxn to proceed extremely fast (explosively)

47
Q

Chain termination rxns

A

Radicals come together to form final products

48
Q

Explosion limits

A

Critical initial T above which an explosion occurs (no spark ignition tho)

Critical T is also referred to as autoignition T

49
Q

Rapid compression machine

A

Piston-cylinder assembly to estimate explosion limits by raising P and T of F-A mix very quickly to predetermined P2 and T2

50
Q

A flame

A

A thin region in space where chemical rxns convert F-A mix into combustion productions

51
Q

At a given P, T and phi, flame has 2 basic proprties

A
  1. AFT or T_ad
  2. Laminar burning velocity (S_l)
52
Q

What is burning velocity affected by? (3)

A
  1. Equivalence ratio
  2. Initial gas T and P
  3. AFT
53
Q

T/F: Turbulent flame velocity is not a property of the gas but depends on the details of the flow

A

T

54
Q

Turbulent burning velocity depends on…

A

Turbulent intensity

55
Q

T/F: In a channel of width d, the flame can propagate in a channel as long as the tube diameter is larger than the flame thickness

A

T

56
Q

Flammability limit

A

The energy at which a spark-ignited flammable mixture will ignite

57
Q

What affects flammability limit? (2)

A
  1. Mixture
  2. Initial P and T

MECH 453 (4 decks)

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