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Flames and Gas Combustion > Final > Flashcards

Final Flashcards

1
Q

What’s the Karlovitz number?

A
  • non-dimensional stretch rate

- measure of the FLAME time in terms of the AERODYNAMIC time

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2
Q

Karlovitz number for laminar flames

A

Ka = l(T) / s(U)

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3
Q

Karlovitz number for premixed flames

A

Ka = turbulent strain rate / chemical reaction rate

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4
Q

How does the Karlovitz number change with the stretch rate?

A

Ka increases with increasing stretch rate

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5
Q

Whats the Lewis number?

A

Thermal diffusivity / mass diffusivity

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6
Q

Equation for the Lewis number

A

Le = Lambda / (DichteDc)

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7
Q

Properties of rich propane/air flame

A

Le <1
S < 0
open tip
dark tip

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8
Q

Properties of lean propane/air flame

A

Le > 1
S > 0
closed tip
bright tip

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9
Q

Properties of lean methane/air flame

A

Le <1
S < 0
open tip
dark tip

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10
Q

Properties of rich methane/air flame

A

Le > 1
S > 0
closed tip
bright tip

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11
Q

What’s the Damköhler number?

A
  • non-dimensional

- relates CHEMICAL REACTION TIMESCALE to TRANSPORT PHENOMENA occuring in a system

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12
Q

Equation for Damköhler number

A

Da = char. Diffusion time / char. collision time

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13
Q

What’s the Mach number?

A

Measures the SPEED in relation to SOUND VELOCITY

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14
Q

Equation for Mach

A

Ma = v/c

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15
Q

What’s the Peclet number?

A

Measures the relative intensities of CONVECTIVE to DIFFUSIVE TRANSPORT

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16
Q

What’s the Reynolds number?

A

Measures the INERTIAL FORCE in relation to the VISCUOUS FORCE

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17
Q

Equation for Peclet number?

A

Pe = Re*Pr

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18
Q

Equation for Reynolds number

A

Re = (DichteuL) / Eta

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19
Q

What’s the Pradtl number

A

Measures the relative influence of VISCOSITY to THERMAL DIFFUSIVITY

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20
Q

Equation for Prandlt

A

Pr = v/Alpha = (Eta*c(P)) / Lambda

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21
Q

What’s FLAMMABILITY?

A

A mixture is said to be flammable if the resulting flame can propagate all the way to the top of the tube

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22
Q

What’s the FLAMMABILITY LIMIT?

A

Concentration limit beyond which flame propagation is not possible

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23
Q

Relations of lean flammability limits & unburned mixture temeprature

A

Increase of initial temperature = Decrease of lean FL

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24
Q

Flammability limit of hydrogen

A 
Lean = 4%
Rich = 75%
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25
Q

Influence of increase of mixture temperature on flammability limit

A

widens flammability limit (range)

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26
Q

Influence of increase of pressure on flammability limit of hydrogen

A

narrows lean & rich flammability limit

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27
Q

Influence of increase of pressure on flammability limit of hydrocarbons

A

narrows lean FL

widens rich FL

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28
Q

How does the flame temeprature change when flammability limit is approached?

A

It decreases

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29
Q

Advantages of Flammability limits

A

Useful concept to know what could cause a failure of flame propagation

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30
Q

Disadvatnages of flammability limits

A

Empirical observation without an established fundamental understanding

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31
Q

Mechanisms contributing to flammability limits

A

Chemical kinetics

  • flammability limit is characterized by THERMAL EXTINCTION +
  • CHAIN TERMINATION overwhelming chain branching
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32
Q

What’s extinction?

A

When leakage and the reduction in flame temperature becomes relatively severe, extinction occurs.

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33
Q

How to achieve extinction?

A

1 Remove a certain amount of chain-carrying or chain branching RADICALS
- introduce chemical INHIBITANTS

  1. Remove certain amount of HEAT
    - reduce reaction rate (by cooling, decreasing reactant concentration, decreasing system pressure)
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34
Q

Does the disappearance of the flame imply extinction?

A

No

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35
Q

When is extinction not possible?

A

For D(ac) > D(ac,I)

36
Q

Causes of extinction for premixed flames

A
  • aerodynamic stretch
  • insufficient residence time
  • influence of mixture nonequidiffusion
37
Q

Causes of extinction for nonpremixed flames

A
  • reactant leakage : incomplete reaction
38
Q

General causes for extinction

A
  • thermal cooling

- chemical radical depletion

39
Q

Difference between flammability & extinction

A
  • extinction is only part of flammability
  • flame shape
  • flow stretch
  • conductive heat loss to the wall
40
Q

What’s the stretch factor?

A

Lagrangian time derivative of

  • the logarithm of
  • an area A with
  • its boundary surface moving
  • tangentially to
  • the surface of
  • the fluid velocity
41
Q

Equation for stretch

A

k = 1/A * (dA/dT)

42
Q

Einheit für Stretch

43
Q

Causes of stretched flames

A
  • flow induced
  • flame motion
  • flame curvature
44
Q

Examples of stretched flames

A
  • flow induced: stagnation flames
  • flame motion: propagating spherical flame
  • flame curvature: axisymmetric bunsen flame
45
Q

Turbulence generation

A
  • grid
  • jet
  • fan
46
Q

Definition of explosion

A
  • Corresponds to rapid heat release

- Does not necessarily require the presence of a waveform

47
Q

Gemeinsamkeiten DEFLAGRATION & DETONATION

A
  • waveforem
  • substained by chemical reaction
  • need an explosive gas
48
Q

Characteristics of DEFLAGRATION

A
  • flame
  • waves travel subsonic
  • pressure decreases p(b) < p(u)
  • velocity increases
  • combustion wave
49
Q

Characteristics of DETONATION

A
  • waves travel supersonic
  • pressure increases p(b) > p(u)
  • velocity decreases
  • shock wave
50
Q

What’s the laminar flame speed?

A

Wave propagation speed of a

  • 1D
  • planar
  • absent of heat loss
  • steady
  • premixed flame
51
Q

Einheiten von Laminar flame speed

52
Q

Relation between flame speed and equivalence ration

A

If the equivalence ratio increases, so does the flame speed

53
Q

Relation between flame speed and LEWIS

A

S ~ (1/Le - 1) k

k = stretch factor

54
Q

Liftoff mechanism main strain

A

Siehe heft

55
Q

Liftoff mechanism close to jet exit

A
  • strain rate rises

- > flame extinguished

56
Q

Liftoff mechanism downstream of jet

A
  • strain rate decreases

- > flame exists

57
Q

Jet flame types

A
  • momentum controlled

- buoyancy controlled

58
Q

Steigung log(y)-log(Q) for round jet flames

A

= 1 both buoyancy & momentum controlled

59
Q

Steigung log(y)-log(Q) for slot jet flames

A

= 4/3 for buoyancy

= 1 momentum

60
Q

Is the flame height for buoyancy controlled slot jet dependent of the slot width

A

No, it’s not

61
Q

log(tRES)-log(Q)

A

Siehe heft

62
Q

Tubulent premixed flames regime

A

Siehe Aufschriebe

63
Q

Advantages of bunsen method for measurements

A
  • simple
  • top & bottom effect
  • radial dependence along surface
64
Q

Advantages of spherical bomb method for measurements

A
  • simple

- single run to get laminar flame speed

65
Q

Advantages of counter flow method for measurements

A
  • simple
  • well defined boundary conditions
  • 1D modelling around center flow
  • applicable for (non/partially) premixed flames
66
Q

Disdvantages of counter flow method for measurements

A
  • high pressure

- hard for weak flames

67
Q

Disdvantages of spherical bomb method for measurements

A
  • temperature & pressure variations
  • variations of flame stretch
  • only dry gaes
  • flame instability / instability
68
Q

Disdvantages of bunsen method for measurements

A
  • curvature
  • wall effect
  • hydrodynamic dependency
69
Q

Main nitrogen oxides

70
Q

Main mechanisms for NOx formation

A
  • thermal
  • fuel
  • prompt
71
Q

Coal-fired engines

A
  • N2O: Coal fired boiler

- NOx: Coal fired burner

72
Q

Turbulence combustion modelling - RANS

A

Widely used for industrial scale modeling

73
Q

Turbulence combustion modelling - DNS

A

Limited to small scale and low RE

74
Q

Types of Turbulence combustion modelling

A
  • RANS
  • DNS
  • LES
75
Q

Turbulence combustion modelling - LES

A

can be used in scales in meters under rapid development

76
Q

At 1 atm, near the lean flammability limit, the laminar flame speed of methane/air premixed flame is closest to

77
Q

The mechanisms for the flammability limits of a premixed flame are

A
  • raditive heat loss

- chemical kinetics

78
Q

To premixed flame, if the reaction rate doubles, the laminar flame speed will increase how much?

79
Q

At 1 atm and room temperature, for methane/air premixed mixture at an equivalence ratio of 1, the laminar flame speed is closes to

80
Q

At 1 atm, the flame thickness of methane air flames at equivalence ration = 1 is

81
Q

When pressure increases, the thickness of hydrogen/air premixed flames will

82
Q

At 1 atm the lean flammability limit of methane/air mixture is closes to

83
Q

Main chain branching reaction

A

H + O2 = OH + O

84
Q

Main chain termination

A

H + O2 + M = HO2 + M

85
Q

Semonov’s criterion

A

Heat loss curve is tangent to the heat generation curve

Flames and Gas Combustion (10 decks)

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