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Coherent deflagrations in a system enclosure-atmosphere and the role of external explosions

Apparently, Swedish scientists were the first to undertake an experimental investigation in 1957 that emphasised the importance of the external explosion during a vented deflagration [1]. They found that in some cases the maximum explosion overpressure outside a 203 m3 enclosure exceeded the maxi-mum overpressure inside of the enclosure. In 1980 Solberg et al further highlighted the danger of ex-ternal explosions [2]. They found that during vented deflagrations in a 35-mm3 vessel, the flame front propagation velocity could reach 100 m/s in a lateral direction outside the vent. In their experiments the flame propagated up to 30 m outside the enclosure though the vessel was only 4 m long. Harrison and Eyre [3] came to conclusion that for “large vents, where the internally generated pressures are low, the external explosion can be the dominating influence on the internal pressure” and that this influence “could be very important for large volume, low strength structures such as buildings or off-shore modules”. In 1987 Harrison and Eyre [3], and Swift and Epstein [4], independently suggested that the mechanism of external explosion influence on the internal pressure dynamics is contained in the decrease of the mass flow rate through the vent. Theoretical analysis performed by Molkov in 1997 [5], based on the processing of experimental data by Harrison and Eyre [3] confirmed that the turbulence factor inside the enclosure was practically unaffected by the occurrence of the external explosion. Instead, a substantial decrease of the generalised discharge coefficient, i.e. mass outflow, was found for tests with pronounced external combustion. It was concluded that the decrease of the pressure drop on the vent due to combustion outside the enclosure was the main reason for reduced venting of gas outside the enclosure. In 1991 Catlin [6] studied the scaling of external explosions. He found that the external overpressure grows proportionally to the velocity of the flow and flame emerging from the vent.

The physics of the phenomenon of coherent deflagrations, i.e. coupled internal and external explo-sions, has been recently clarified [7-9] through application of the large eddy simulation (LES) model developed at the University of Ulster to experiments in 547 mm3 SOLVEX facility [10]. The comparison between experimental and LES pressure transients inside and outside the enclosure and external flame shape led to some important conclusions on the nature of coherent deflagrations. The formation of the turbulent starting vortex in the flammable mixture pushed out of the enclosure during the internal deflagration is a prerequisite for a subsequent intense combustion outside the enclosure. The rapid acceleration of combustion outside the enclosure commences not at the moment when the flame front emerges from the vent, but after the flame front “touches” the edges of the vent. At this point the flame reaches the region of strong turbulence generated in the shear layers at the perimeter of the external jet. There is consequently a rapid increase in the rate of combustion and a coherent steep pressure rise is observed both inside and outside the enclosure. The external pressure rise in the atmosphere is a direct consequence of the highly turbulent deflagration there, but there is no increase of the burning rate inside the enclosure. The pressure rise inside the enclosure is caused by the decrease of mass flow rate from the enclosure to the atmosphere due to the high pressure just outside the vent. In such scenarios the mitigation strategy should aim primarily at the suppression of combustion outside the enclosure.

1. Report of Committee for Explosion Testing, Stockholm, 1957, Kommitten for Explosions Forsok, Bromma 1957, Slutrapport, Stockholm, April 1958.
2. Solberg D.M., Pappas J.A., Skramstad E. (1980). Experimental Investigations on Flame Acceleration and Pressure Rise Phenomena in Large Scale Vented Gas Explosions. In Proceedings of 3rd Int. Symposium on Loss Prevention and Safety Promotion in Process Industries. Basel, p.16/1295.
3. Harrison A.J., Eyre J.A. (1987). ‘External Explosions’ as a results of explosion venting. Com-bustion Science and Technology, 52 (1-3), 91-106.
4. Swift I., Epstein M. (1987). Performance of Low-Pressure Explosion Vents. Plant/Operations Progress, 6 (2), 98-105.
5. Molkov V. (1997). Venting of Gaseous Deflagrations. DSc Thesis, Moscow, VNIIPO.
6. Catlin C.A. (1991). Scale Effects on the External Combustion Caused by Venting of a Confined Explosions. Combustion and Flame, V.83, 399-411.
7. V. Molkov, D. Makarov, J. Puttock, The nature of coherent deflagrations, Proceedings of the 5th International Symposium on Hazards, Prevention and Mitigation of Industrial Explosions (10-14 October 2004, Krakow, Poland), pp.35-44, 2004. Accepted for publication in Journal of Loss Pre-vention in the Process Industries (2005).
8. Molkov V.V., Makarov D.V., Rethinking physics of large-scale vented explosion and its mitiga-tion, Proceedings of the IChemE Symposium “Hazards XVIII: Process Safety - Sharing Best Prac-tice”, 22-25 November 2004, UMIST, Manchester, UK. Accepted for publication in journal Proc-ess Safety and Environmental Protection (2005).
9. V. Molkov, D. Makarov, J. Puttock, Dynamics of external explosions in vented deflagrations, Proceedings of the 11th International Symposium on Loss Prevention and Safety Promotion in the Process Industries (31 May – 3 June 2004, Praha, Czech Republic), Vol.C+E, pp.3275-3280, 2004.
10. Puttock, J.S., Cresswell, T.M., Marks, P.R., Samuels, B. and Prothero, A., 1996, Explosion as-sessment in confined vented geometries. SOLVEX large-scale explosion tests and SCOPE model development. project report, Health and Safety Executive, TRCP 2688R2.

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