Laboratory Studies of Fire Whirls (preliminary)

Transcription

1 Laboratory Studies of Fire Whirls (preliminary) Alexander J. Smits, Katie A. Hartl, Stacy Guo and Frederick L. Dryer Princeton University Coupled Atmosphere Bushfire Modelling Workshop May 2012

2 High Reynolds number in the lab: compressed air up to 200 atm as the working fluid Princeton/ONR Hgh Reynolds number Test Facility: boundary layer flow Re θ = 5 x 10 3 to 220 x 10 3 Re = up to 75,000 Princeton/DARPA/ONR Superpipe: Fully-developed pipe flow Re D = 31 x 10 3 to 35 x 10 6 Re τ = up to 10 6 Re λ = up to 2000

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4 Fric & Roshko, 1994; Kelso & Smits, 1995

5 QuickTime and a h264 decompressor are needed to see this picture. Fire tornado Kentucky Bourbon, Josh Grimes

6 Examples of Fire Whirls Peshtigo Fire, WI 1871 (>1000 deaths) Hifukusho-ato, Tokyo 1923 (~38,000 deaths) Great Chicago Fire, USA 1871 Hiroshima, Dresden Hamburg Mann Gulch Fire 1949 (13 deaths) Indians Fire, CA 2008 (4 casualties) (plume shedding, cold fronts, L-shaped fires)

7 Laboratory experiments Rotating screen setup (Emmons and Ying, 1966) Tangential slit setup (Byram and Martin, 1962) Emmons and Ying (1967) Byram and Martin (1962)

8 Previous work Emmons and Ying (1966) rotating frame qualitative Byram and Martin (1962) fixed frame qualitative Saito and Cremers (1995) fixed frame apparatus Satoh and Yang (1996) fixed frame qualitative Hassan (2005) fixed frame quantitative Akhmetov (2007) rotating frame quantitative Lei (2011) fixed frame quantitative

9 Fire Whirl Principles Whirls occur: 1.ambient vorticity (ground BL, nonuniform horizontal density, earth s rotation) 2.concentrating mechanism (rising air in buoyant column encourages turbulent mixing of gas with vorticity bearing air and transports vorticity aloft) Devastation occurs: 1.rotating core decreases turbulence of rising air (centripetal force) 2.ground slows down the rotation of the air and pushes vorticity filled boundary layer towards axis of rotation Implications: 1.buoyancy is not diffused and a large pressure gradient created 2.more air and fuel sucked into vortex core Emmons and Ying (1967)

10 Order in Chaos

11 Order in Chaos Ambient Vorticity Boundary Layers Non uniform density gradients Concentrating Mechanism Centripetal force vertical pressure gradient Ground effects radial pressure gradient

12 Types of Fire Whirls Kuwana et al. (2007 categorized pool fire whirls into three different types: 1) the fire whirl spinning over the downstream-side of the burning area creating a tall fire column 2) the fire whirl periodically spinning off from the burning area and traveling to the downstream unburned area 3) the relatively stable spinning of air initially without fire in the unburned area but then attracting fires into its spinning motion from the burning area.

13 Scaling Type 3 Fire whirls U = wind speed U c = critical wind speed U b = buoyant velocity at the flame tip L = horizontal length scale Γ= circulation H = height of plume m = burn rate (n = 1/4) Kuwana et al. (2007)

14 Known Fuel rich core Rankine vortex model outside core Solid body rotation inside core Order of magnitude decrease in turbulence Increased burning rate Unknown Scaling parameters (air intake velocity, burning rate, flame base size) Velocity profile outside Velocity profile inside whirl

15 Known Fuel rich core Rankine vortex model outside core Solid body rotation inside core Order of magnitude decrease in turbulence Increased burning rate Unknown Scaling parameters (air intake velocity, burning rate, flame base size) Velocity profile outside Velocity profile inside whirl Even with 50 years of research, the combustion dynamics of fire whirls is far from being completely clarified, mainly due to a shortage of quantitative experimental research. (Lei 2011)

16 Experimental Setup Cylindrical entrainment walls (Plexiglas for PIV) Meker burner to generate flame LPG fuel: mixture of propane and butane with tank, regulator, needle valve, toggle valve Diffusion flame

17 Lab Made Whirls

18 Lab Made Whirls QuickTime and a decompressor are needed to see this picture.

19 ORGANIZED FLOW

20 QuickTime and a decompressor are needed to see this picture.

21 1 in 2 in 3 in 4 in 5 in 6 in

22 Qualitative Observations Stable fire whirls were established using gaseous fuel, diffusion flame structure Threshold cylinder size, beyond which it is less important (may be that the outer flow needs some whirl diameters in size to establish) Threshold gap size, beyond which it is less important (may be that the mass flow is more or less constant) Whirl height depends on fuel flow rate but not strongly

23 Going Forward Short Term: Velocity profiles using Particle Image Velocimetry (PIV) outside the flame Velocity profiles using PIV inside the flame Impact of fuel burning rate on velocity profiles using PIV Long Term: Understand scaling of free fire whirls Understand fire whirl influence in propagating the fire line

24 PIV in Fire Particles in combusting flows aluminum oxide, titanium dioxide (Kompenhas (2001)) silica (Hassan (2005)) glass microspheres (Akhmetov (2007)) smoke particles (Hassan (2005), unspecified function) Particle diffusion Cannot recirculate particles Fluidized bed for metal/glass particles (expensive) Difficulties Metal particles in air are hazardous (sealing, cleaning) Expensive metal particle distribution method Light emitted from flame filter to block light from flame and particles (Kompenhas (2001)) Alternatives Oil droplets (not in literature for combusting flames) Smoke particles

25 Thank you, QUESTIONS?

26 Bibliography H. W. Emmons and S.J. Ying, The fire whirl, in Proceedings of the 11 th International Symposium on Combustion, pp , Combustion Institute, Pittsburgh, PA, G. M. Byram and R.E. Martin, Fire whirlwinds in the laboratory, Fire Control Notes, vol. 33, pp , K. Satoh and K.T. Yang, Experimental observations of swirling fires, Proceedings of the ASME Heat Transfer Division, vol. 4, K. Saito and C.J. Cremers, Fire whirl enhanced combustion, ASME Instructional Fluid Mechanics, vol. 220, M.I. Hassan, et al., Flow structure of a fixed frame type fire whirl, Fire Safety Science Proceedings of the 8 th International Symposium, pp , D.G. Akhmetov, N.V. Grecov, V.V. Nikulin, Flow structure in a fire tornado like vortex, Doklady Physics, vol. 52, no. 11, pp , J. Lei, et al. Experimental research on combustion dynamics of medium scale fire whirl. Proceedings of the Combustion Institute 33, pp , J. Kompenhas, et al. Application of particle image velocimetry to combustion flows: design considerations and uncertainty assessment, Experiments in Fluids, vol. 30, pp , 2001.