1. HYDROGEN COMBUSTION EXPERIMENTS IN A VERTICAL SEMI-CONFINED CHANNEL
Friedrich, A.*1, Grune, J.1, Sempert, K.1, Kuznetsov, M.2 and Jordan, T.2 .
1Pro-Science GmbH, Parkstrasse 9, Ettlingen, 76275, Germany, 2Karlsruhe Institute of Technology, IKET, Karlsruhe, Germany.
Funded by:

2. Background/Motivation
In hydrogen safety considerations phenomena of effective flame acceleration (FA) and deflagration-to-detonation transition (DDT) are very important, especially for large semi-confined spaces, such as rooms or tunnels,
Criteria for the spontaneous transition processes of FA and DDT were derived empirically, numerical simulations of these processes on full reactor scale currently not possible.
These criteria that are currently used worldwide in accident simulations were derived for homogenous mixtures and complete inclusion.

3. Background/Motivation
In a previous experimental series in a large-scale horizontal combustion channel (9 x 3 x 0.6 m³) with an open ground face were formulated. These criteria were derived by analyzing numerous experiments with various layer thicknesses, obstacle configurations and mixture properties.
extended criteria for the onset of FA and DDT in semi-confined geometries
The main objective of this work was to evaluate the critical conditions for FA and DDT in a semi-confined obstructed vertical layer and to compare these conditions with results of the previous campaign in the semi-confined horizontal channel.

4. Experimental Set-Up
All experiments were performed in a vertical channel installed to the safety vessel A3 at the Hydrogen Test Site HYKA at KIT:
dimensions 6 x 0.4 x 0.4 m³ (one open side face),
framework structure covered by wooden plates,
welded steel shell seals and protects wooden inner surface against flames,
16 grid obstacles (BR 50%, spacing 25 cm) in first 4 m from ignition in all experiments.
h = 8 m, di = 2.5 m, V = 33 m³, pStat = 60 bar

5. Test-Procedure General
Open channel side face sealed by thin plastic Film,
Test mixture generated by mass flow controllers for H2 and air,
Initial channel atmosphere replaced with test mixture (from top) and pushed out of channel through outlet valve at lower end,
When the desired H2-concentration or H2-gradient is reached inlet and outlet valves are closed,
Film is destroyed prior to ignition.

6. Test-Procedure Homogeneous Mixtures
cH2(in)
Open channel side face sealed by thin plastic Film,
Initial channel atmosphere replaced from top with test mixture and pushed out of channel through outlet valve at lower end,
Procedure completed when cH2(in) = cH2(out).
H5875
H4875
H3875
H1125
H125
H2375
Film
Procedure homogeneous mixtures
Film is destroyed over complete height by falling knife prior to ignition (cut observed by cameras distributed in safety vessel)
Experiment not further evaluated if cut failed or was incomplete.
Time [s]
cH2(out)
homogeneous mixture

7. Test-Procedure Positive Concentration Gradients
cH2(in)
Open channel side face sealed by thin plastic Film,
Channel filled homogeneously with mixture of concentration cH2(0) that corresponds to lowest value in H2-concentration gradient,
H5875
H4875
H3875
H1125
H125
H2375
Film
Then further injection of H2-air mixture with increasing H2-concentration (programmable massflow-controller),
Procedure completed when cH2(in) = cH2(max),
Film destroyed prior to ignition.
Procedure opsitive cH2-gradients
t = 720 s
Ignition
c(max) dc
c(0)
cH2(0)
cH2(out)
Time [s]
positive gradient

8. Test-Procedure Negative Concentration Gradients
Luft
cH2(in)
Open channel side face sealed by thin plastic Film,
Channel filled homogeneously with mixture of concentration cH2(0) that corresponds to highest value in H2-concentration gradient,
H5875
H4875
H3875
H1125
H125
H2375
Film
Then air is inject from top,
Procedure completed when calculated air-volume is injected,
Film destroyed prior to ignition.
Procedure negative cH2-gradients
t = 135 s
Ignition
cH2(0)
cH2(out)
Time [s]
negative gradient

9. Test-Procedure Gradients Available
2 variables in the procedure for positive gradients:
variation of cH2(0) “shifts” gradient along abscissa,
variation of dc (= cH2(max) – cH2(0)) influences slope of gradient,
Gradients are rather stable (several minutes).
In procedure for negative gradients only cH2(0) can be varied,  all gradients have similar slopes,
Gradients not stable  ignition initiated 10 s after end of air-injection in all experiments with negative gradients.

10. Test-Procedure Ignition & Instrumentation
Top Ignition HT NT PT
Ignition via glow wire in perforated tube ( almost planar flame front),
Homogeneous
Positive Gradient
Film
Negative Gradient
Mixture ignited either at top or at bottom end of channel
 6 cases (HT/HB, PT/PB, NT/NB).
Bottom Ignition HB NB PB
Instrumentation:
Bottom
Ignition
6000
4000
3750
3500
3250
3000
2750
2500
2250
2000
125
5875
4375
3625
5125
625
1375
2125
2875
5375
4875
4125
3375
2625
1875
1125
5625
1625
375
Obstacle
Ionization Probe
Thermocouple
Pressure Transducer
Ignition Device
5750
5500
5250
5000
4750
4500
4250
6000
5750
5500
5250
5000
4750
4500
4250
4000
3750
3500
3250
3000
2750
2500
2250
2000
125
5875
4375
3625
5125
625
1375
2125
2875
5375
4875
4125
3375
2625
1875
1125
5625
1625
375
Obstacle
Ionization Probe
Thermocouple
Pressure Transducer
Ignition Device
Top
Ignition
- flame front: 18 modified thermocouples, 9 ionization probes
- shock wave: 8 pressure sensors

11. Test-Matrix Homogeneous mixtures Positive gradients Negative gradients
cH2
[V%]
Case
σ [-]
λ [mm] 6 HB
2.58 8
3.08 9
HT/HB
3.31
9.5 HT
3.43 10
3.54 11
3.77
2319 12
3.99
1289 13
4.21
798 14
4.42
530 15
4.63
361 16
4.83
252 18
5.23
114 20
5.60
44.6 21
5.79
31.9
cH2 max
cH2 min
cH2 av#
Case
σ for
λ [mm] for
[V%]
25.0
24.5
18.6
18.5
22.9
20.5 PT PB
6.45
6.38
6.11
5.70
14.5
15.5
20.6
37.3
23.2
17.1
21.2
6.17
5.82
19.3
30.3
21.9
21.4
15.4
19.8
17.5
5.94
5.86
5.56
5.12
25.3
28.2
48.8
145
14.3
5.72
5.33
35.6
89.5
16.3
10.3
12.3
4.90
4.89
4.48
4.05
158
227
593
1121
15.3
9.2
13.2
4.68
4.26
310
1045
14.0
8.0
12.1
4.43
4.00
521
1257
6.1
10.2
8.1
4.02
3.60
3.11
1123
1206
13.8
6.2
11.2
4.37
3.82
586
1949
6.4
12.4
9.4
4.72
4.71
4.08
3.41
330
312
724
17.2
6.7
13.7
5.08
4.36
224
18.2
12.2
14.2
5.27
4.46
105
493
19.7
19.1
7.2
7.1
11.1
5.55
3.79
4.73
5.44
50.3
67.1
301
2137
19.2
15.2
5.46
4.67
63.4
335
7.9
12.9
4.18
846
24.9
8.4
13.9
6.44
4.40
14.7
552
27.3
9.1
6.77
4.66
11.3
342
31.8
10.7
17.7
7.03
5.17
15.9
130
37.2
21.0
5.79
19.9
31.5
cH2 max
cH2 min
cH2 av#
Case
σ for
λ [mm] for
[V%]
11.1
7.9
10.4 NB
3.80
3.63
2074
3727
12.1
8.6
11.2
4.01
3.82
1243
1965
13.0
9.2
4.21
798
1230
13.9
9.9
11.6 NT
4.41
3.90
4.20
545
1631
814
14.9
10.5
10.6
12.4
13.8
4.60
4.07
4.38
379
1065
569
15.8
13.2
14.7
4.79
4.24
4.56
272
746
405
16.7
11.9
15.5
4.98
4.74
193
297
17.6
12.5
5.16
4.57
133
402
18.1
12.9
15.1
5.25
4.65
109
349
18.6
5.34
4.73
87.4
304
21.4
15.2
17.8
19.9
5.85
5.19
5.58
28.8
125
46.6
22.3
20.7
6.01
5.74
23.1
34.6
25 experiments with homogeneous mixtures (13 HT and 12 HB).
43 experiments with vertical gradients
25 experiments with positive gradients (12 PT and 13 PB)
18 experiments with negative gradients (9 NT and 9 NB)

12. Fast turbulent deflagration
Data Evaluation
x-t-diagrams of ionization probe and pressure sensor signals recorded during the experiments:
14.9
10.6
cH2 [Vol%]
10.3
16.3
cH2 [Vol%]
18.5
cH2 [Vol%]
24.5
Slow deflagration
v < cp
pmax < 3,5 p0
Fast turbulent deflagration
v ≈ cp
5·p0 < pmax < 8·p0
Detonation v ≈ 2·cp
12·p0 < pmax < 17·p0

13. Results and Discussion Flame Acceleration
Transition between regimes not continuous but stepwise, as results of experiments with homogeneous mixtures indicate:
Flame acceleration:
from graph: cH2(krit) ≈ 14,5 Vol% s*
dimensionless: s* ≈ 4.5
equation from earlier work:
With: s*0 = KHC = 0.66 (horiz. channel)
s/h = 0.25 m/0.4 m = 0.625
 s* ≈ 4.16
s* = s*0 (1 + K · s/h)
or, due to differences between facilities: KVC = 0.32 (vert. channel) with s* = 4.5
Criterion derived for horizontal semi-confined channel in earlier work also applicable for homogeneous mixtures in vertical semi-confined channel,
Also applicable on mixtures with concentration gradients?

14. Results and Discussion Flame Acceleration
All experiments were performed in same facility, so representation of σ over s/h not suitable for comparison of different gradient orientations,
so representation of σ over configuration is used: HT HB PT PB NT NB
PB-G s* FA
no FA
PB-G s* FA
no FA
Horizontal channel
No good agreement of data with FA-criterion when maximum H2-concentration is used for determination of σ (earlier work: Concept of Maximum Concentration),
But very good agreement of data with FA-criterion when average H2-concentration in obstructed region (cH2av#) is used,
Strong deviations only for experiments with very steep concentration gradients (series “PB-G” (triangular symbols)).

15. Results and Discussion Detonation-Transition
For evaluation of DDT-criterion representation of h/λ over configuration is used, but again cH2max or cH2av# can be used for calculation of λ:
DDT
no DDT
h/l = 13.5
Homogeneous mixtures show good agreement with DDT-criterion determined for the horizontal channel,
In general better agreement for λ calculated with cH2max, but overestimation for case PT (and very steep concentration gradients (series “PB-G”)),
Use of cH2av# gives better agreement for case PT (and “PB-G”), but underestimates most other cases.

16. Thank you very much for your attention!
Summary & Conclusion
Slow (subsonic) and fast (sonic) deflagrations as well as detonations were observed and the conditions for an FA and DDT were determined and compared with the results of a previous campaign with semi-confined horizontal layers,
σ-criterion for FA in semi-confined horizontal layers was
successfully adapted to vertical layers with homogeneous H2-concentrations,
also applicable to vertical layers with vertical H2-concentration gradients (better results when mean concentration in gradient is used, with maximum concentration in gradient estimation is very conservative)
λ-criterion for DDT in semi-confined horizontal layers was
successfully applied to vertical layers with homogeneous H2-air mixtures,
also applicable to vertical layers with vertical H2-concentration gradients (better results when maximum concentration in gradient is used, with mean concentration in gradient underestimation of many cases).
The previously formulated criteria for FA and DDT in semi-confined horizontal H2-air layers were applied successfully to vertical H2-air layers with and without concentration gradients, which is a proof for the capacity of the methods used.
Thank you very much for your attention!