EXPERIMENTS WITH RELEASE AND IGNITION OF HYDROGEN GAS IN A 3 M LONG CHANNEL

1. EXPERIMENTS WITH RELEASE AND IGNITION OF HYDROGEN GAS IN A 3 M LONG CHANNEL Ole Kr. Sommersel, Dag Bjerketvedt,Knut Vaagsaether and Torstein K. Fanneløp Telemark University College, Porsgrunn, Norway

2. Outline • Introduction and objective • Experimental setup • Froude scaling • Experimental dispersion results • Numerical dispersion simulations with FLACS • Flame propagation • Conclusions

3. Introduction • The hazard, when hydrogen is leaking, is strongly linked to the dispersion of hydrogen. D. Bjerketvedt and A. MjaavattenICHS Conference, Pisa, Sept., 2005

4. Objective • Get a better understanding of the phenomena and to develop tools that can analyse hydrogen dispersions and explosions in buildings, channels and tunnels D. Bjerketvedt and A. MjaavattenICHS Conference, Pisa, Sept., 2005

5. Experimental setup Test #20, Q =31.8 l/min, L = 2 m

6. Experimental setup • Hydrogen gas supply • Hydrogen (99.9%) was injected into the channel through a vertical 4 mm ID steel tube. • The release was directed vertically upwards and the flow velocities ranged from 2.4 m/s to 99.2 m/s • Ignition • Siemens ZM 20/10 high voltage igniter. • 5 mm from the upper wall • switched on and off in a series of short pulses ten times per second • Pressure recordings • Three Kistler 7001 pressure transducers • High-speed video • Photron Ultima APX-RS high-speed digital video camera. Frame rate was typical 2000 fps.

7. Flow rates and ignition positions

8. Froude scaling and Gravity Currents • William Froude (1810-1879) • In fluid dynamics, a gravity current is a flow in a gravitational field driven by a density difference • The frontal velocity of gravity currents can typically be expressed by the dimensionless Froude number. • The Froude number is the ratio between momentum and gravity forces acting in a fluid flow Ref. 3 Ref. 4

9. Froude scaling • uF is the average frontal velocity • H is the height of the the channel • h is the height of the hydrogen-air layer in the channel • hH is the height of a 100 % hydrogen layer in the channel.

10. Froude scaling

11. Froude versus volume flow

12. “Light-fluid intrusion" for gravity currents • We have extended the model for "light-fluid intrusion" given by Gröbelbauer et. al. [5] when the Froude number is based on the length scale hH • Φ = 0.5 and hH << H we get Fr = 0.68 Φ = h/H

13. Time of ignition Fr = 0.68

14. Hydrogen concentration Fr = 0.68

15. FLACS results

16. Flame propagation • Triple flames (rich layer) • Phillips [8] 1965 • LPF, lean premixed flame • RPF, rich premixed flame • DF, diffusion flame Test #20, Q =31.8 l/min, L = 2 m Ref. Chung [9]

17. Maximum explosion pressures

18. Conclusions • The time of arrival of the gas cloud at the continuous ignition source appear to be well described by Froude scaling with a length scale corresponding to the height of a layer of 100 % hydrogen in the channel • We believe that this Froude scaling can be useful as a tool to analyse the consequences of hydrogen release in buildings, channels and tunnels • Further work is needed in order to establish the validity of this scaling for other conditions than those of the present small scale tests