DISPERSION TESTS ON CONCENTRATION AND ITS FLUCTUATIONS FOR 40MPa PRESSURIZED HYDROGEN
A. Kouchi, K. Okabayashi, K. Takeno, K. Chitose
Mitsubishi Heavy Industries, Ltd.
2nd International Conference on Hydrogen Safety, September 12, 2007, San Sebastian, Spain
BackgroundHydrogen refueling stations which are being planned at the present will store high-pressure hydrogen gas at 40 MPa.
The acquisition of basic data on the influence of high-pressure hydrogen gas on the surroundings has become an urgent task.
We focused on a typical leakage scenario, i.e., leakage from a pinhole occurring in equipment, resulting in continuous leakage at a constant mass flow rate (steady leakage)
Purposes of this Study and Basic Methodology To comprehend the basic behavior of the dispersion of high-
pressure hydrogen gas where there is steady leakage from a pinhole.
Field dispersion experiments :Data sets of time-averaged concentrations were obtained.
However, to investigate the safety of flammable gas dispersion, time-averaged concentrations are insufficient.
Because concentration fluctuates due to turbulence.
To study the relationships among the concentration fluctuation, the occurrence probability of flammable concentration and the features of the flame propagation.
Dispersion and Spark-Ignition Experiments:
-Measurements of instantaneous concentration fluctuations
-Spark-ignitions were also applied at the same point simultaneously
-Characteristics of the flame propagation were analyzed
Dispersion Experiments for Time-averaged Concentrations
Experimental Apparatus and Test Field
(in Akita pref.) Tashiro experimental facility
放出口
Concentration Measurement Poles
Release Nozzle-High-pressure Hydrogen gas was released horizontally. -1m above the ground.
Nozzle
Dispersion Experiments for Time-averaged Concentrations
Release conditions
Concentration Measurement
Stagnation Pressure P0= 40 MPa, Nozzle Diameter D = 0.25, 0.5, 1.0 , 2.0 mm
P0= 20 MPa , D = 2.0 mm
10
0 10 20 30 50
-5
0
5
10 20 30 50
X: Distance from the nozzle
[m]
5
Z: H
eigh
t fr
om t
he
grou
nd [m
] L
ater
al d
ista
nce
[m
]
X: Distance from the nozzle [m]Lat
eral
Dis
tan
ce
[m]
Hei
gh
t [
m]
10
10
20 30 50
5030
Time-averaged concentration was measured at each point
Dispersion Experiments for Time-averaged Concentrations
Results(1) A typical example of concentration contour [ P0=40MPa, D=2.0mm] Unit[%]
×
5
30 X[m] 20 0 10
4 2
1
10
Z[m
]
Nozzle
The gas plume: Almost horizontal near the nozzle -The momentum effect is more dominant than the buoyancy effect in a high concentration area
Using an analogy from the turbulent jet characteristics of incompressible jet flow
Time-averaged concentration along the axis of the plume was plotted against X/θ: - θ : Equivalent release diameter
a
D
0 ρa: Density of ambient air
ρ0: Density of hydrogen at nozzle throat
0.1
1
10
100
10 100 1000 10000 100000
Nondimentional distance from nozzle X/ θ
Con
cent
ratio
n C
m (
)%
φ 0.25mm 40MPa、φ 0.5mm 40MPa、φ 1.0mm 40MPa、φ 2.0mm 40MPa、φ 2.0mm 20MPa、Formula(2): a1=6000
Dispersion Experiments for Time-averaged Concentrations
Results(2) Time-averaged concentration along the axis of the plume
Far from the release point : -Scattering is relatively larger due to the fluctuation of meteorological conditions and smaller momentum of hydrogen jet.
At a short distance: -Scattering becomes smaller. -The plotted points are almost in alignment.
- Concentration along the axis of the dispersion plume can be expressed as a simple formula, where a1=6000.
-This formula will enables us to estimate the axial concentration easily in case the release diameter and initial pressure are given.
1
1
X
aCm
Dispersion and Spark-Ignition Experiments for Concentration Fluctuations
-To study the relationships among the concentration fluctuation, the occurrence probability of flammable concentration and the features of the flame propagation.
Experimental Apparatus and the Method of the ExperimentsNozzle Flame size
Hydrogen supply(40MPa)
Including 1.5% Methane
Spark controller
Methane concentration sensor
Spark plugSampling for concentration measurement
Anemometer High speed camera Pressure sensor
-Hydrogen release: horizontally as pinhole leakage.-Methane gas was mixed into hydrogen gas to a concentration of 1.5vol% as a tracer gas, for the purpose of measuring concentration fluctuation using a fast response flame ionization detector.
-Electric spark-ignition also applied simultaneously.-The phenomena of flame generation and propagation were recorded with a high speed video camera.
Sampling tube for concentration measurement
Ignition plug
Dispersion and Spark-Ignition Experiments for Concentration FluctuationsExperimental Apparatus and the Method of the Experiments
- Concentration : Fast response FID (HFR400) ,Sampling rate : 200Hz.- Electric spark-ignition :10 Hz, the electrode gap:1.5 mm, spark energy :120 mJ - High speed video camera : 500 flames/sec. An UV band-pass filter (313 ± 7 nm) was attached for taking images of the hydrogen flame ( a spectrum peak : around 310 nm)
- Pressure sensor : Ambient pressure change due to ignition 2.5m away from the ignition point.
Nozzle
High speed camera Pressure sensor
Release Condition
Stagnation Pressure : P0 40MPa
Nozzle Diameter : D 0.2mm
Measurements Points
Distance from the nozzle: X0.15m 、 0.3m 、 0.35m 、 0.4m 、 0.45m 、 0.7m 、 0.8m 、 0.9m 、 1.0m 、 1.5m 、 2.5m
Experimental Conditions
Dispersion and Spark-Ignition Experiments for Concentration Fluctuations
Dispersion and Spark-Ignition Experiments for Concentration Fluctuations
Data analysis method (1) Concentration time-history data analysis
Concentration time-history of Hydrogen
0
2
4
6
8
10
12
14
30.0 31.0 32.0 33.0 34.0 35.0
Time [sec]
Hyg
roge
n co
ncen
trat
ion
[%]
Time-averaged concentration: Cm
ex) Cm = 4.1%
Example: X=0.9m
Probability density function of concentration: PDF
0.0
1.0
2.0
3.0
4.0
5.0
0 5 10 15 20Concentration [%]
Pro
babi
lity
of o
ccur
renc
e[%]
Concentration [%]
Pro
ba
bili
ty o
f o
ccu
rre
nce
[%
]
Cm=4.1%
Occurrence probability of flammable concentration: PC
ex) PC= 57%
300mm
Ignition Spark
Hydrogen Flame
Dispersion and Spark-Ignition Experiments for Concentration FluctuationsData analysis method (2) Image data analysis
Each image from the high speed video camera
Example: X=0.3m
-Occurrence probability of hydrogen flame: PF
(= number of times flame was generated/ number of times sparks were generated)- Flame size : LF (Maximum size during one spark)- Flame propagation distance : LD
Flame size: LF
Flame propagation distance: LDElectrode for spark
Dispersion and Spark-Ignition Experiments for Concentration FluctuationsResults
Time-mean concentration
0
5
10
15
20
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Distance from the nozzleX[m]
Tim
e-av
erag
ed c
once
ntra
tion
Cm
[%]
Distance from the nozzle [m]
Tim
e-m
ea
n c
on
cen
tra
tion
Cm[%
]
Relationship between Cm and PC
0
20
40
60
80
100
120
0 4 8 12 16Time- averaged concentration Cm[%]
Pc[
%]
Time-averaged Concentration Cm [%]
Occ
urre
nce
pro
babi
lity
of
flam
mab
le c
once
ntra
tion
PC[%
]
(1) Concentration time-history data analysis
-Cm is larger than about 7% Pc is near 100%. -The concentration is always higher than LFL, i.e. 4%.
-As Cm becomes smaller, Pc decreases and almost zero at around Cm =2%. -This implies that there is no possibility of ignition in an area where time-averaged concentration is lower than 2% even if concentration fluctuation is considered.
Dispersion and Spark-Ignition Experiments for Concentration FluctuationsResults (2) Image data analysis
0
100
200
300
400
500
0 2 4 6 8 10 12 14 16Cm(%)
LF [m
m]
Mean valueMaximum value
3系列
Time-averaged Concentration Cm [%]
Fla
me
siz
e L F
[mm
]
100mm100mm
Spark (not hydrogen flame)
X=0.9m (Cm=4.1%, PC=57%)
100mm
X=0.6m (Cm=5.4%, PC=88%) X=0.3m (Cm=8.7%, PC=99.8%)
X=0.15m Cm=15.2%, PC=100%)
100mmSteady jet flame
-Cm is 4.1% Hydrogen flame cannot be recognized. -As Cm larger, LF becomes larger.-X=0.15m, Cm is 15.2%, the flame was generated at the first spark and it grew to a steady jet flame.
0
20
40
60
80
100
120
0 2 4 6 8 10 12 14 16Cm[%]
PF [%]
Dispersion and Spark-Ignition Experiments for Concentration FluctuationsResults (2) Image data analysis
100mm100mm
Spark (not hydrogen flame)
X=0.9m (Cm=4.1%, PC=57%)
100mm
X=0.6m (Cm=5.4%, PC=88%) X=0.3m (Cm=8.7%, PC=99.8%)
Occ
urre
nce
pro
babi
lity
of
hydr
ogen
fla
me
PF [
%]
-As Cm decreases, PF become smaller, approaching zero around at Cm=5%.
-This phenomenon is consistent with the fact that Pc is almost zero when Cm is around 2%, i.e., when there is no possibility of ignition.
(3) Pressure Measurement
-Ambient pressure change caused by flame generation an undetectable level ( i.e. less than 100 Pa)
Conclusions Time-averaged hydrogen concentration can be expressed by a simple
formula. This formula will enable us to estimate axial concentration at each distance easily. 1
1
X
aCm
The occurrence probability of flammable concentration Pc decreases with decrease in the time-averaged concentration and becomes almost zero and no significant flame propagation occurs, where Cm is around 2% or less.
Thus, there is a clear correlation between the time-mean concentration, the occurrence probability of flammable concentration, flame length and occurrence probability of hydrogen flame.
However, these results were derived from experimental data under regulated conditions. Further data acquisition under various conditions such as larger leakage diameter or where there are obstacles is to be expected and this will be our future task.