Obstacle arrays

Combustion of a natural gas-air cloud in a highly congested obstacle array leads to flame speeds in excess of 100 m/s (pressure in excess of 200 mbar). 

Interaction of a jet flame and an obstacle array can result in an increase of flame speed and production of pressures in excess of 700 mbar. 

The presence of horizontal or vertical obstacles (Figure 4.4) in the propane cloud hardly influenced flame propagation. On the other hand, flame propagation was influenced significantly when obstacles were covered by steel plates. Within the partially confined obstacle array, flame speeds up to 66 m/s were observed (Table 4.2) they were clearly higher than flame speeds in unconfined areas. However, at points where flames left areas of partial confinement, flame speeds dropped to their original, low, unconflned levels. 

FIgura 4.4. Obstacle array used in large-scale propane explosion tests by Zeeuwen et al. (1983). 

Van Wingerden and Zeeuwen (1983) demonstrated increases in flame speeds of methane, propane, ethylene, and acetylene by deploying an array of cylindrical obstacles between two plates (Table 4.3). They showed that laminar flame speed can be used as a scaling parameter for reactivity. Van Wingerden (1984) further investigated the effect of pipe-rack obstacle arrays between two plates. Ignition of an ethylene-air mixture at one edge of the apparatus resulted in a flame speed of 420 m/s and a maximum pressure of 0.7 bar. 

Propagation of quasi-detonation in obstacle array in stoichiometric H2/O2 mixture (a) initial pressure 140 torr, detonation reinitiation via Mach reflection at the bottom wall (b) initial pressure 120 torr, detonation reinitiation by normal Mach stem reflection from the obstacle with subsequent enhancement via reflection from the top wall 6 ps between frames. (From Teodorczyk, A., Lee, J.H.S., and Knystautas, R., Prog. Astr. Aeron., 138,223,1990. With permission.)... 

UDM (Urban Dispersion Model) (Hall et al., 2002 [248]) is a widely-used model developed by the UK Defence Science and Technology Laboratory (DSTL) based on assumptions of a Gaussian shape and empirical parameterizations developed from special field and laboratory experiments involving obstacle arrays. 

Macdonald, R.W., Carter-Schofield, S.L., and Slawson, RR. (2001) Measurements of mean plume dispersion in simple obstacle arrays at 1 200 scale. Presented at the 5th Annual GMU/DTRA Transport and Dispersion Modelling Workshop, 18-19 July 2001, George Mason University, Fairfax, VA. 

A fourth stage that assumes increasing importance for shorter obstacle arrays may also be dehned ... 

FIGURE 3.4 (See color insert following page 46.) Schematic illustration of the evolution of a turbulent flow upon encountering an obstacle array. 

Hanna, S.R., Tehranian, S., Carissimo, B., Macdonald, R.W., and Lohner, R., 2002. Comparisons of model simulations with observations of mean flow and turbulence within simple obstacle arrays, Atmos. Environ., 36, pp. 5067-5079. 

S. Choi and J.-K. Park, Sheathless hydrophoretic particle focusing in a microchannel with exponentially increasing obstacle arrays. Analytical Chemistry, 80, 3035-3039 (2008). 

Current state-of-the-art in the understanding of these phenomena, as well as progress made in achieving empirical and quantitative descriptions of these combustion processes, are reviewed. The specific topics discussed are i) the maximum attainable turbulent flame speed in an obstacle array, ii) computer simulation of turbulent flame accelerations, iii) correlation between the detonation cell size and the dynamic parameters of fuel-air detonations, and iv) the transition from deflagration to detonation. Future directions in the investigation of these problems are also discussed. 

Figure 3. Peak overpressures for turbulent H -air flames propagating in an orifice obstacle array.

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