DISPERSION TESTS ON CONCENTRATION AND ITS FLUCTUATIONS FOR 40MPa PRESSURIZED HYDROGEN A. Kouchi, K. Okabayashi, K. Takeno, K. Chitose Mitsubishi Heavy Industries, Ltd. 2 nd International Conference on Hydrogen Safety, September 12, 2007, San Sebastian, Spain

Background  Hydrogen 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 X: Distance from the nozzle [m] Lateral Distance [m] Height [m] Time-averaged concentration was measured at each point

Dispersion Experiments for Time-averaged Concentrations  Results(1)  A typical example of concentration contour [ P 0 =40MPa, D=2.0mm] 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 ρ a : Density of ambient air ρ 0 : Density of hydrogen at nozzle throat

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= This formula will enables us to estimate the axial concentration easily in case the release diameter and initial pressure are given.

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 Experiments Nozzle Flame size Hydrogen supply(40MPa) Including 1.5% Methane Spark controller Methane concentration sensor Spark plug Sampling for concentration measurement AnemometerHigh 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 Fluctuations  Experimental 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 : P 0 40MPa Nozzle Diameter : D0.2mm Measurement s Points Distance from the nozzle: X 0.15m 、 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

 Data analysis method (1) Concentration time-history data analysis Concentration time-history of Hydrogen Time-averaged concentration: C m ex) C m = 4.1% Example: X=0.9m Probability density function of concentration: PDF Concentration [%] Probability of occurrence [%] Cm=4.1% Occurrence probability of flammable concentration: P C ex) P C = 57%

300mm Ignition Spark Hydrogen Flame Dispersion and Spark-Ignition Experiments for Concentration Fluctuations  Data analysis method (2) Image data analysis Each image from the high speed video camera Example: X=0.3m -Occurrence probability of hydrogen flame: P F (= number of times flame was generated/ number of times sparks were generated) - Flame size : L F (Maximum size during one spark) - Flame propagation distance : L D Flame size: L F Flame propagation distance: L D Electrode for spark

Dispersion and Spark-Ignition Experiments for Concentration Fluctuations  Results Time-mean concentration Distance from the nozzle [m] Time-mean concentration C m [%] Relationship between C m and P C Time-averaged Concentration C m [%] Occurrence probability of flammable concentration P C [%] (1) Concentration time-history data analysis -C m is larger than about 7% P c is near 100%. -The concentration is always higher than LFL, i.e. 4%. -As C m becomes smaller, P c decreases and almost zero at around C m =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 Fluctuations  Results (2) Image data analysis Time-averaged Concentration C m [%] Flame size L F [mm] Spark (not hydrogen flame) X=0.9m (Cm=4.1%, P C =57%)X=0.6m (Cm=5.4%, P C =88%) X=0.3m (Cm=8.7%, P C =99.8%) X=0.15m Cm=15.2%, P C =100%) Steady jet flame -C m is 4.1% Hydrogen flame cannot be recognized. -As C m larger, L F becomes larger. -X=0.15m, C m is 15.2%, the flame was generated at the first spark and it grew to a steady jet flame.

Dispersion and Spark-Ignition Experiments for Concentration Fluctuations  Results (2) Image data analysis Spark (not hydrogen flame) X=0.9m (Cm=4.1%, P C =57%)X=0.6m (Cm=5.4%, P C =88%) X=0.3m (Cm=8.7%, P C =99.8%) Occurrence probability of hydrogen flame P F [%] -As C m decreases, P F 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.  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.