Premixed turbulent combustion: mechanisms, burning rates, extinction
Experiments have shown that turbulence increases flame propagation rates to a point, but sufficiently high turbulence intensities cause propagation rates to decrease and leads to flame extinguishment. This limits the utility of lean mixtures, which thermodynamically promise higher thermal efficiencies and lower emissions, in practical combustion devices. Neither theoretical models nor computational studies have yet been able to explain these observations. In fact, the theoretical models do not agree with the experiments nor with each other.
Predicted effect of turbulence intensity (u'/SL) on turbulent burning velocity (ST/SL) from "thin-flame" theories: Bray (1990) with zero heat release and large (density ratio = 7) heat release; Anand and Pope (1987) with zero and infinite heat release; Yakhot (1988); Sivashinsky (1990); Gouldin (1987) with integral-scale Reynolds number ReT = 1,000; experimental values from Bradley (1992) for ReT = 1,000. Where ReT is not specified, predictions are independent of ReT.
In order to study the behavior of propagating flames at high turbulence levels, without the complications of large density changes or heat losses, we have introduced the use of an aqueous autocatalytic reaction, which produces propagating fronts, as a paradigm for turbulent premixed combustion. These experiments suggest that heat losses or initiation conditions are probably responsible for the observed quenching in gaseous combustion experiments. Comparison of these results to theory suggests that Yakhot's (1988) Renormalization Group (RNG) theory provides the best description of turbulent flame propagation.
Measured effect of turbulence intensity (u'/SL) on turbulent burning velocity (ST/SL) for aqueous autocatalytic chemical reaction fronts in five different flows (rising fronts in cylindrical tubes, rising fronts in Hele-Shaw cells, 2 mm deep capillary-wave flow, Taylor-Couette flow in the "featureless turbulence" regime, and twin opposed vibrating-grid turbulence. Also shown is a comparison with Yakhot's (1988) renormalization-group theory and an empirical power-law fit.
The fractal dimensions of these fronts in capillary-wave flows compare favorably with Kerstein's heuristic model.
Planar laser-induced fluorescence light sheet images (after thresholding) of arsenous-acid/iodate autocatalytic chemical fronts in capillary-wave flow. Black is products, white is reactants. Field of view 14 cm x 14 cm. Left: image of front with u'/SL = 55; right: image of front with u'/SL = 110.
Left: fractal plot of fronts in capillary-wave flow showing measured front perimeter as a function of the measurement scale. u'/SL = 220, d = 1.268. Right: fractal dimension as a function of u'/SL.
We have also developed an extension of Yakhot's model to consider the effect of turbulence scales which are smaller than the flame thickness, and find that this model provides very good predictions of the propagation rates under these conditions.
High rotation rate
Low rotation rate
Planar laser-induced fluorescence light sheet images of downward-propagating arsenous-acid/iodate autocatalytic chemical fronts in Taylor-Couette cells in the Taylor Vortex regime. (front propagating downwards). Inner cylinder diameter: 3.10 cm, outer cylinder diameter: 4.45 cm.
It is not possible to study chemical front propagation at low values of the non-dimensional turbulence intensity (ratio of turbulence intensity u' to the laminar propagation rate SL) without buoyancy effects (see videos below). However, to create a "bridge" between the chemical fronts and premixed gas flames it is precisely these low values of u'/SL that need to be studied. Consequently, a space experiment entitled Front Interaction with Vortex Experiment (FIVE) studying aqueous chemical fronts at microgravity in a Taylor-Couette cell, will be flown on the STS-107 Space Shuttle mission in January 2001.
|Downward propagation||Upward propagation|
Videos showing comparison of downward and upwardpropagating chemical fronts in Taylor-Couette cells in the "Taylor Vortex" flow regime. Actual speed is 4 times slower than these movies.
Ronney, P. D., "Some Open Issues in Premixed Turbulent Combustion," in: Modeling in Combustion Science (J. D. Buckmaster and T. Takeno, Eds.), Lecture Notes In Physics, Vol. 449, Springer-Verlag, Berlin, 1995, pp. 3-22.
Ronney, P. D., "Flame Structure Modification and Quenching By Turbulence," Combustion Science and Technology (Japanese edition), Vol. 6 (Supplement), pp. 53-76 (1999).
Aldredge, R. C., Vaezi, V. and Ronney, P. D., "Premixed-Gas Flame Propagation in Turbulent Taylor-Couette Flow," Combustion and Flame, Vol. 115, pp. 395-405 (1998).
Shy, S. S., Jang, R. H., Ronney, P. D., "Laboratory Simulation of Flamelet and Distributed Models for Premixed Turbulent Combustion Using Aqueous Autocatalytic Reactions", Combustion Science and Technology, Vol. 113-114, pp. 329 - 340 (1996).
Haslam, B. D., Ronney, P. D., "Fractal Properties of Propagating Fronts in a Strongly Stirred Fluid," Physics of Fluids, Vol. 7, pp. 1931-1937 (1995).
Ronney, P. D., Haslam, B. D., Rhys, N. O., "Front Propagation Rates in Randomly Stirred Media," Physical Review Letters, Vol. 74, pp. 3804-3807 (1995).
Zhu, J. Y., Ronney, P. D., "Simulation of Front Propagation at Large Non-dimensional Flow Disturbance Intensities," Combustion Science and Technology, Vol. 100, pp. 183-201 (1994).
Shy, S. S., Ronney, P. D., Buckley S. G., Yakhot, V., "Experimental Simulation of Premixed Turbulent Combustion Using Aqueous Autocatalytic Reactions," Twenty-Fourth International Symposium on Combustion, Combustion Institute, Pittsburgh, 1992, pp. 543-551.
Ronney, P. D., Yakhot, V., "Flame Broadening Effects on Premixed Turbulent Flame Speed," Combustion Science and Technology, Vol. 86, pp. 31-43 (1992).