Fire Storms and Large Scale Modelling Derek Bradley University of Leeds UKELG 50TH ANNIVERSARY DISCUSSION MEETING “Explosion Safety – Assessment and Challenges” 9th to 11th July 2013 Cardiff University
Fire Storms ?
The Buoyant Plume
Conditions for a Fire Storm High column of burned gas Large spillage and favourable topology Turbulence generation at base Rich aerosol mixture topped by lighter fractions Large turbulent length scales (Turbulence, buoyancy and aerosols give positive feed-back)
Atmospheric Turbulence u m/su′ m/sl m z o =.05 mz o = 1 mz o =.05 mz o = 1 m 3 (light breeze) (near gale) (violent storm)
Turbulent Explosion
Turbulent Burning Correlation U = u t /u' K =0.25(u'/u ℓ ) 2 R l -0.5
Cellular Laminar Explosion
Laminar Instability Inner and Outer Cut-offs = (n s /nl) D-2 Flame area ratio = (n s /n l ) D-2 Fractal Dimension, D = 7/3
Spillage Magnitudes Spillage at Explosion (tonnes) Spillage Area (m 2 ) Mean height at lean flammability limit (m) Donnellson (1978) ,00024 Ufa (1989) 4,5002,500,000140
Atmospheric Turbulence u m/su′ m/sl m z o =.05 mz o = 1 mz o =.05 mz o = 1 m 3 (light breeze) K= K= (near gale) K= K= (violent storm) K= K=0.064
Turbulent Burning Correlation U = u t /u' K =0.25(u'/u ℓ ) 2 R l -0.5
Regime of Peak Turbulence- Instability Interaction
Influence of l s /l G on U Masr = -23Masr = 3
Estimated Donnellson Burning Velocity
Ufa X
Ufa Topography
Ufa Ignition Source
The Buoyant Plume
Ufa Topography
Ufa and Donnellson Burning Velocities Compared
23 Congestion:Flame and Shock Wave in a Duct a A Flame Shock wave
The Maximum Turbulent Burning Velocity
Maximum Turbulent Burning Velocity
Influence of Venting Ratio, A/a
Strong, Stable, Detonations require Low (ξε), or (τ i /τ e )
Problems of Large Scale Modelling Uncertain discharge composition, mixing, and circumstances of ignition. Uncertain physico-chemical data (Ma, extinction stretch rates, burning velocities, (τ i /τ e ). Complexity of congestions,venting, shock wave reflection and refraction. Uncertainties in rate of change of heat release rate.
References G.M. Makhviladze, S.E. Yakush, (2002) “Large Scale Unconfined Fires and Explosions,” Proceedings of the Combustion Institute 29: D. Bradley, M. Lawes, K. Liu, M.S. Mansour, (2013) “Measurements and Correlations of Turbulent Burning Velocities over Wide Ranges of Fuels and Elevated Pressures,” Proceedings of the Combustion Institute 34: D. Bradley, M. Lawes, Kexin Liu, (2008) “Turbulent flame speeds in ducts and the deflagration/detonation transition,” Combust. Flame D. Bradley, (2012) “Autoignitions and detonations in engines and ducts,” Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences, 370, no. 1960: 689–714.