Detonation Tube Experiments

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22 Φεβ 2014 (πριν από 3 χρόνια και 3 μήνες)

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2

Detonation Tube Experiments



8 m long, 280 mm diameter



“cookie
-
cutter” attached to the 150 mm square test section



Initiation with an e
xploding wire and
an acetylene
-
oxygen
-
driv
er

End plate soot foil

3

Cellular Structure

R. Akbar 1997

4

Themes


Some experimental observations


Speculations about combustion
mechanisms


The role of diffusive processes


The classical theory of high
-
speed flames


Transient processes at shear layers


The “turbulent” flame


Future studies

5

Observations


We observe a range of reaction front
features which are strongly correlated with
the sensitivity of the reaction rates to
temperature variations.

6

Shot 1597,

2H
2
+O
2
+ 8 N
2
, 20kPa

5 mm

Shot 1604,

2H
2
+O
2
+ 8 N
2
, 20kPa

2H
2
+O
2
+ 9 N
2
, 20kPa,
Image height 50mm

8 mm

OH fluorescence images
-

irregular
structure

All 150x150mm testsection

Reference: F.

Pintgen, J.

M. Austin, and J.

E. Shepherd, Detonation front structure: Variety and
characterization, Confined Detonations and Pulse Detonation Engines, pages 105
-
116. Torus
Press, Moscow, 2003.

7

Highly Irregular Mixtures:

N
2
O


H
2

mixtures N2
-
diluted

H
2
+ N
2
O +3 N
2
, 20 kPa, Cellsize: 70mm

10 mm

10 mm

10 mm

Shot 1607

Shot 1608

Shot 1609

All 150x150mm testsection

Reference: F.

Pintgen, J.

M. Austin, and J.

E. Shepherd, Detonation front structure: Variety and
characterization, Confined Detonations and Pulse Detonation Engines, pages 105
-
116. Torus
Press, Moscow, 2003.

8

Highly Irregular Mixtures:

N
2
O


H
2

mixtures N2
-
diluted

50 mm

Shot 1643,

H
2
+ N
2
O +2 N
2,

20kPa,

Cell size: 42mm

Shot 1609, H
2
+ N
2
O +3 N
2

50 mm

Shot 1656, H
2
+ N
2
O + 2 N
2

50 mm

20 mm

Shot 1643, close up

9

0
2
4
6
8
10
12
14
16
18
20
0
2
4
6
8
10
M
CJ
Ea/RT
S
Neutral stability
boundary
C3H8-O2-N2
H2-O2-N2
H2-O2-AR
H2-N2O-N2
H2-N2O-O2-N2
H2-O2-CO2
C2H4-O2-N2
Instability driven turbulence

f=1

weakly unstable

(low E
a
/RT
S
)

highly unstable

(high E
a
/RT
S
)


stable

unstable

12

Turbulent detonations?


Reaction zone appears to be dominated by fluctuations
in leading shock velocity


Consequence of highly unstable structure


Leads to large fluctuations in species and energy release
rate


Transverse waves


Consequence or cause of spatial variation of leading
shock?


May lead to explosive events following decoupling


Diffusive processes may operate at very small scales
but:


Smallest scales not resolved in experiments


Suggests that turbulent detonations may be intrinsically
different than turbulent deflagrations

Is this a new or an old regime of turbulent
combustion?


17

Numerical Simulations


Two
-
dimensional inviscid simulations with
simplified reaction mechanism (1
-
step) but
highly refined spatial discretization (A.
Khokhlov’s FTT method)


Ea/RT chosen to match regular and
irregular mixture thermochemical
computations.

18

Regular Instability, Ea/RT = 6.5

Experimental

Numerical

19

Irregular instability, Ea/RT = 10

Experimental

Numerical

20

Conclusions


Two
-
dimensional simulations at CJ
condition feasible even with simplified
reaction mechanism.


Cellular substructure, wrinkling of reaction
front, localized explosions consistent with
experimental observations.


Purely chemical
-
gasdynamic explanation
for inviscid “turbulence”

21

However…


Computations are based on Euler model


no
diffusive processes. Is this what happens in
nature?


What happens at even higher values of Ea/RT?


Most probable situations for diffusive transport:


Thin layers of unreacted material surrounded by
products


Shear layers between warm (shocked) reactants and
hot products.


Could conventional diffusive turbulent
combustion be possible?

22

Regimes for Conventional
Diffusive Turbulent Combustion

from N. Peters (2000) Turbulent Combustion

23

Can we put some points on this
plot?


Are there regimes where laminar flames
can exist in detonation fronts?


What is the characteristic values of
laminar flame speed in post
-
shock
conditions?


What are the range of length scales and
fluctuation velocities in turbulent
detonations?



24

Classical analysis of diffusive
transport in high
-
speed combustion


Numerical study of Clarke’s one
-
dimensional “fast flames”


Re
-
examining conclusions reached with
asymptotics for realistic detailed
chemistry.


Examine key issue not addressed by
Clarke: When do diffusion flames cease
to be well defined?

25

One
-
Dimensional Fast Flame

30

Two Extremes

CJ
-
ZND

Adiabatic flame

31

v=2m/s

v=5.81m/s

v=20m/s

v=100m/s

tconv

tcond

tdiff

treac

tpress

tsum

*

*

Magnitude of the terms in the energy balance equation

33

Existence of flames behind shocks

34

35

Results of flame computations

36

Summary of fast flame study

37

Characterization of fronts

Coastlines

Distribution of lengths

38

Scale
-
dependent fractal?

39

Wrinkled laminar flamelet analysis

M =
r
u A

M =
r
S
L

A
T

A
T
/A = U/S
L

41

Transient ignition of flames at shear layers


Motivated by OH PLIF observations
and numerical simulations.


Analog of classic Marble
-
Adamson
problem for ignition behind flame
holder

Warm reactants

Hot products

42

The Marble
-
Adamson problem


Chemical changes occur by a single reaction step:



Boundary layer scaling: →


Diffusive term is of the same order as convective terms:

→ → →

1
/

l



1
/
~
Re
2




r
l
lu
1
~
Sc
D
r




2
/
~
/

l
D
lu
1
~
Le
/
Sc

Pr




p
c










sp
sp
N
k
k
N
k
k
1
k
1
k



Schvab
-
Zeldovich formulation:


Steady, low speed (isobaric) flow


Binary diffusion coefficients of all species equal:


Unit Lewis number:

k
k
Y
D
ln



V
1
Le


p
Dc
r

43

The Marble
-
Adamson problem

But:


“Warm”


not cold


combustible stream close to reaction.


Hot stream is not inert (role of radicals diffusion vs.
thermal diffusion).

cold combustibles




u
1


hot inerts




u
2


propagating

laminar flame




boundary layer

44

Time scale competition


Two streams:


hot, with temperature
T
h


and adiabatic (c.v.) induction time

t
hot
cv


warm, with temperature
T
w


and adiabatic (c.v.) induction time
t
warm
cv


Time to ignite a homogeneous mixture


Time for a diffusive flame to develop:

where


Diffusive flame at shear layer if

t
m

~


u
m
HMI
m
f
S
t
t
/
~

t


HMI
t
cv
warm
HMI
t
t


→ Look for large separation of
t
hot
cv

and
t
warm
cv

45

Local triple point analysis


Polar analysis for non
-
reactive gas.


Flow is steady in the reference frame of the triple point.


Constant track angle
φ from incident leading shock.



Known shock speed D at the cell centerline.


All waves are straight.

D

φ

(x
-
x
o
)/L

idealized cell

(x
-
x
o
)/L

D

46

Comparison of adiabatic ignition times

2H
2
-
O
2
-
7Ar

P
1
=6.67kPa

Miller & Bowman

C
2
H
4
-
3O
2
-
8N
2

P
1
=20kPa

GRI
-
Mech 3.0

47

Homogeneous mixing ignition at shear layer

4

3

(x
-
x
o
)/L=0.6

Limiting case : Infinitely fast mixing

h,
Y

h,
Y

“air”

“fuel”

Z kg

(1
-
Z) kg

Mixture

fraction Z

t
HMI
(Z)

P

idealized

cell

48

Dependence from chemical mechanism


0
-
dimensional calculations at constant pressure using

the CHEMKIN library [Kee et al., 1989].

300K

0.21 O2
-
0.79 N2


Z kg

(1
-
Z) kg

Mixture

fraction Z

t
HMI
(Z)

1 atm

0.25

H
2
-
0.75

N
2

1100K

49

Time scales competition

C
2
H
4
-
3O
2
-
8N
2

P
1
=20kPa

GRI
-
Mech 3.0

50

Flame development: 1
-
D unsteady code


AMROC provides access to adaptive data management as
well as to detailed reaction mechanisms (via CHEMKIN).


Operator splitting approach: added subroutine to compute
diffusive fluxes







t
L
t
L
t
L
chem
diff
conv





x
t
c


x
t


2

c.v. explosion, implicit

time integration

4

3

x


Species molecular diffusion, mixture thermal
conductivity and thermal diffusion coefficients
from TRANSPORT software package.


Mixture averages using CHEMKIN gas
-
phase
utilities.

53

Weakly and strongly unstable reactive mixtures

2H
2
-
O
2
-
7Ar


2H
2
-
O
2
-
8N
2


Composition at 10% peak temperature in hot
-
stream c.v. explosion.

H
2
-
O
2

mechanism by Miller & Bowman (1989)


54

Example I: Diffusion flame at shear layer

Composition: hot stream at equilibrium

or inert

at 2213 K;


warm stream unreacted at 1020 K


t
warm
cv

= 1470

s



Estimated
t
HMI
(0.95) = 17.

s

2H
2
-
O
2
-
8N
2


55

Composition: hot stream at equilibrium



or inert



at 2213 K;


warm stream: reactants at 1200 K



t
warm
cv

= 19.8

s



Estimated
t
HMI
(0.95) = 8.

s

Example II: Convective explosion at shear layer

T at 23.6

s

X
OH

at 23.6

s

2H
2
-
O
2
-
8N
2


56

Example III: Energy vs. radical diffusion effects

2H
2
-
O
2
-
11N
2


Composition: hot stream at equilibrium

or inert



at 1427 K;


warm stream: reactants at 1000 K


t
warm
cv

= 327


s



Estimated
t
HMI
(0.95) = 162

s