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Venting of hydrogen deflagrations through ducts


Figure 1. Explosion vessel with vent duct [8].

The discharge of hot combustion products and blast waves from vent devices is often ducted to safe locations to avoid damage to surrounding equipment and buildings. The presence of a duct, however, is likely to increase the severity of an explosion compared to simply vented enclosures. For instance, with stoichiometric acetone-air mixtures it was observed that connecting a duct of 5 cm diameter and a length of 183 cm to a 5 cm diameter vent opening in a 27 liter sphere, increased the reduced pres-sure from 0.7 atm to 4.7 atm [1]. Various authors [2,3,4,5,6] have identified the phenomena that lead to an enhanced explosion severity when a vent device is equipped with a duct: a secondary explosion in the duct due to ‘burn-up’ of reactants emerging from the enclosure, frictional drag, inertia of the gas column in the duct, acoustic interactions, and, Rayleigh-Taylor instability induced by Helmholtz oscillations. The increased explosion severity due to the presence of a duct may be mitigated by three dif-ferent methods: the application of a diaphragm to the vent opening, increasing the discharge pressure, and suppression of combustion in the duct by injecting an extinguishing agent at the onset of the vent-ing process.


Figure 3. Explosion dynamics for a 17%
hydrogen-air mixture [8], ignited at the rear
location in Figure 1. a) Pressure trace in the
explosion vessel. b) Pressure differenceacross
the duct entrance. c) Pressure in the initial
sections of the duct. d) Rate of pressure rise in
the vessel.

Figure 2. Explosion dynamics for a 4%
propane-air mixture [8], ignited at the
rear location in Figure 1. a) Pressure trace
in the explosion vessel. b) Pressure difference
across the duct entrance. c) Pressure in the
initial sections of the duct. d) Rate of pressure
rise in the vessel.

An interesting study of the venting of hydrocarbon-air (methane and propane) and hydrogen-air defla-grations through ducts is described in ref. [8]. These authors investigated the pressure development in a 200-litre cylindrical vessel (length: 1.0m, diameter: 0.5m, see Figure 1). A vent pipe (length: 1.0m, diameter: 0.162m) was fitted to the explosion vessel. The behaviour of the pressure in the explosion vessel and the vent duct is shown in Figures 2 and 3. It is seen that, while the pressure in the explosion vessel with propane-air mixtures remains in the deflagration mode during the ‘burn-up’ in the vent duct, hydrogen-air mixtures exhibit a sudden detonation-like spike. This happens well after the flame has passed the vessel-duct assembly. This phenomenon has been ascribed to the effect of flow reversal across the vessel-duct assembly due to the secondary explosion, and the consequential enhancement of the burning rate of remaining reactants in the explosion vessel.

1. Nekrasov, V.P., Meshman, L.M., and Molkov, V.V., The Influence of Exhaust Ducts on Reduced Explosion Pressure, Loss Prevention in Industry, 5:38-39, 1983 (in Russian).
2. Molkov, V.V. 4th International Symposium of Fire Safety Science, p. 1245-1254,Ottawa, Canada, 1994.
3. Ponizy B., and Leyer J.C., Combustion and Flame, 116:259-271, 1999.
4. Ponizy B., and Leyer J.C., Combustion and Flame, 116:272-281, 1999.
5. Cubbage, P.A. and Marshall M.R., IChemE Symp. Series, 33:24-31, 1972.
6. Kumar, R.K., Dewit W.A., Greig D.R., Combustion Science and Technology, 158:167-182, 2002.
7. Ponizy, B. and Veyssiere, B. Diaphragm effect in mitigation of explosions in a vented vessel connected to a duct. Proceedings of the Third International Seminar on Fire and Explosion Haz-ards, p. 695-706, 2001.
8. Ferrara, G., Willacy, S.K., Phylaktou, H.N., Andrews, G.E., Di Benedetto, A., Salzano, E. Vent-ing of premixed gas explosions with a relief pipe of the same area as the vent. Proceedings of the European Combustion Meeting, 2005.


Content: Deflagration

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