Simulation of turbulent burning velocities using aqueous autocatalytic reactions in a near-homogeneous turbulence

S. S. Shy, R. H. Jang, C. Y. Tang

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Abstract

In 1992, Ronney, Shy and coworkers introduced experimentally an aqueous autocatalytic reaction system to simulate premixed turbulent combustion in a well-known Taylor-Couette (TC) flow held. This chemical system can produce self-propagating fronts with characteristics which more closely match those assumed by some current theories of turbulent combustion than do gaseous flames, so that these fronts may be used to assess the viability of models of turbulent combustion. In this work, values of the simulated turbulent burning velocities (S(T)) were measured in a newly designed, fully three-dimensional and nearly homogeneous turbulent flow field that was generated by a pair of concurrently vertically vibrated grids in a chemical tank. Effects of flow velocity spectra were obtained from laser doppler velocimetry. Visualizations of the internal structure of propagating fronts were detected by laser- induced fluorescence of a pH indicator. It was found that there are three transitions of the front propagation rates U(T) (≡ S(T)/S(L)) in terms of a normalized turbulent intensity U (≡ u'/S(L)) and a turbulent Karlovitz number (Ka) and Reynolds number (Re). Markstein numbers are probably close to unity. (1) There is a nearly linear increase of U(T) with U only when Ka < 10 (sharp front), in approximate agreement with Yakhot's prediction. Values of U(T) in this thin flame regime are found to be much higher than those in the TC flow, indicating that the flow velocity spectra have an effaced on front propagation rates. At higher values of Ka, values of U(T) depart from the linearity and the behaviour of these propagating fronts suggests that modes similar to distributed combustion (broadening fronts) observed. Also at a fixed value of Ka, the larger values of Re seem to promote front wrinkling, but inhibit front broadening and U(T) increases as Re increases, at any given values of U. (2) At fixed flow Reynolds number, when Ka is greater than a value of order of a hundred, a further increase in Ka may correspond to a decrease in U(T). The apparent front thickness in this regime can broaden to a value of up to 20 times the flow integral length scale. (3) Under some conditions, global quenching of propagating fronts could be observed as Ka increases above three hundred, well beyond the value at which gaseous flames quench. These results suggest that the wrinkled flamelet is persistent at least for Ka up to a value of ten and it is very difficult to globally quench the nearly adiabatic (slightly exothermic) propagating front by intense turbulence, possibly suggesting the importance of heat loss to flame extinction.

Original languageEnglish
Pages (from-to)54-62
Number of pages9
JournalCombustion and Flame
Volume105
Issue number1-2
DOIs
StatePublished - Apr 1996

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