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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
Influence of increase of mixture temperature on flammability limit
widens flammability limit (range)
26
Influence of increase of pressure on flammability limit of hydrogen
narrows lean & rich flammability limit
27
Influence of increase of pressure on flammability limit of hydrocarbons
narrows lean FL | widens rich FL
28
How does the flame temeprature change when flammability limit is approached?
It decreases
29
Advantages of Flammability limits
Useful concept to know what could cause a failure of flame propagation
30
Disadvatnages of flammability limits
Empirical observation without an established fundamental understanding
31
Mechanisms contributing to flammability limits
Chemical kinetics - flammability limit is characterized by THERMAL EXTINCTION + - CHAIN TERMINATION overwhelming chain branching
32
What's extinction?
When leakage and the reduction in flame temperature becomes relatively severe, extinction occurs.
33
How to achieve extinction?
1 Remove a certain amount of chain-carrying or chain branching RADICALS - introduce chemical INHIBITANTS 2. Remove certain amount of HEAT - reduce reaction rate (by cooling, decreasing reactant concentration, decreasing system pressure)
34
Does the disappearance of the flame imply extinction?
No
35
When is extinction not possible?
For D(ac) > D(ac,I)
36
Causes of extinction for premixed flames
- aerodynamic stretch - insufficient residence time - influence of mixture nonequidiffusion
37
Causes of extinction for nonpremixed flames
- reactant leakage : incomplete reaction
38
General causes for extinction
- thermal cooling | - chemical radical depletion
39
Difference between flammability & extinction
- extinction is only part of flammability - flame shape - flow stretch - conductive heat loss to the wall
40
What's the stretch factor?
Lagrangian time derivative of - the logarithm of - an area A with - its boundary surface moving - tangentially to - the surface of - the fluid velocity
41
Equation for stretch
k = 1/A * (dA/dT)
42
Einheit für Stretch
s^-1
43
Causes of stretched flames
- flow induced - flame motion - flame curvature
44
Examples of stretched flames
- flow induced: stagnation flames - flame motion: propagating spherical flame - flame curvature: axisymmetric bunsen flame
45
Turbulence generation
- grid - jet - fan
46
Definition of explosion
- Corresponds to rapid heat release | - Does not necessarily require the presence of a waveform
47
Gemeinsamkeiten DEFLAGRATION & DETONATION
- waveforem - substained by chemical reaction - need an explosive gas
48
Characteristics of DEFLAGRATION
- flame - waves travel subsonic - pressure decreases p(b) < p(u) - velocity increases - combustion wave
49
Characteristics of DETONATION
- waves travel supersonic - pressure increases p(b) > p(u) - velocity decreases - shock wave
50
What's the laminar flame speed?
Wave propagation speed of a - 1D - planar - absent of heat loss - steady - premixed flame
51
Einheiten von Laminar flame speed
cm/s
52
Relation between flame speed and equivalence ration
If the equivalence ratio increases, so does the flame speed
53
Relation between flame speed and LEWIS
S ~ (1/Le - 1) k | k = stretch factor
54
Liftoff mechanism main strain
Siehe heft
55
Liftoff mechanism close to jet exit
- strain rate rises | - > flame extinguished
56
Liftoff mechanism downstream of jet
- strain rate decreases | - > flame exists
57
Jet flame types
- momentum controlled | - buoyancy controlled
58
Steigung log(y)-log(Q) for round jet flames
= 1 both buoyancy & momentum controlled
59
Steigung log(y)-log(Q) for slot jet flames
= 4/3 for buoyancy | = 1 momentum
60
Is the flame height for buoyancy controlled slot jet dependent of the slot width
No, it's not
61
log(tRES)-log(Q)
Siehe heft
62
Tubulent premixed flames regime
Siehe Aufschriebe
63
Advantages of bunsen method for measurements
- simple - top & bottom effect - radial dependence along surface
64
Advantages of spherical bomb method for measurements
- simple | - single run to get laminar flame speed
65
Advantages of counter flow method for measurements
- simple - well defined boundary conditions - 1D modelling around center flow - applicable for (non/partially) premixed flames
66
Disdvantages of counter flow method for measurements
- high pressure | - hard for weak flames
67
Disdvantages of spherical bomb method for measurements
- temperature & pressure variations - variations of flame stretch - only dry gaes - flame instability / instability
68
Disdvantages of bunsen method for measurements
- curvature - wall effect - hydrodynamic dependency
69
Main nitrogen oxides
- NO | - NO2
70
Main mechanisms for NOx formation
- thermal - fuel - prompt
71
Coal-fired engines
- N2O: Coal fired boiler | - NOx: Coal fired burner
72
Turbulence combustion modelling - RANS
Widely used for industrial scale modeling
73
Turbulence combustion modelling - DNS
Limited to small scale and low RE
74
Types of Turbulence combustion modelling
- RANS - DNS - LES
75
Turbulence combustion modelling - LES
can be used in scales in meters under rapid development
76
At 1 atm, near the lean flammability limit, the laminar flame speed of methane/air premixed flame is closest to
1 cm/s
77
The mechanisms for the flammability limits of a premixed flame are
- raditive heat loss | - chemical kinetics
78
To premixed flame, if the reaction rate doubles, the laminar flame speed will increase how much?
Wurzel(2)
79
At 1 atm and room temperature, for methane/air premixed mixture at an equivalence ratio of 1, the laminar flame speed is closes to
40 cm/s
80
At 1 atm, the flame thickness of methane air flames at equivalence ration = 1 is
0,1 mm
81
When pressure increases, the thickness of hydrogen/air premixed flames will
decrease
82
At 1 atm the lean flammability limit of methane/air mixture is closes to
0,5
83
Main chain branching reaction
H + O2 = OH + O
84
Main chain termination
H + O2 + M = HO2 + M
85
Semonov's criterion
Heat loss curve is tangent to the heat generation curve

Flames and Gas Combustion (10 decks)

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