BAROCLINIC DISTORTION OF LAMINAR FLAMES

Citation
Ga. Batley et al., BAROCLINIC DISTORTION OF LAMINAR FLAMES, Proceedings - Royal Society. Mathematical, physical and engineering sciences, 452(1945), 1996, pp. 199-221
Citations number
27
Categorie Soggetti
Multidisciplinary Sciences",Physics
ISSN journal
13645021
Volume
452
Issue
1945
Year of publication
1996
Pages
199 - 221
Database
ISI
SICI code
1364-5021(1996)452:1945<199:BDOLF>2.0.ZU;2-M
Abstract
One of the most important divisions in studies of premixed gaseous com bustion is that between the theoretically much favoured laminar flames , and the more commonly observed case of turbulent burning. Laminar pr emixed flames clearly represent much simpler cases for theoretical and numerical study. Conversely experimental investigations are much simp ler in the case of turbulent combustion due to the inherent instabilit y of laminar fluid flows. One mechanism which can effect the transitio n from laminar to turbulent combustion is baroclinicity (i.e. the non- alignment of pressure and density gradients). A laminar deflagration, or slow flame, may be thought of as a reaction front which propagates at a low Mach number and whose associated pressure field is therefore close to uniformity. On the other hand, very steep density gradients a re associated with the rapid temperature increase due to the exothermi c chemical reaction. (Note that a typical deflagration thickness is of the order of 1 mm, and densities magi decrease by factors of between five and ten in going from unburnt to burnt gas.) Externally induced p ressure disturbances, which are almost universally present in practica l combustion systems, can introduce a baroclinic effect whenever a ste ep pressure gradient interacts with a flame front in such a way that t he former is misaligned with the density gradient associated with the latter. The differential acceleration of fluid elements can produce si gnificant rotational motion and, if this field of vorticity is suffici ently strong, a laminar flame front may be broken up and the transitio n to turbulent burning may result. This scenario was clearly demonstra ted in an experiment done by Markstein (1963) that involved the double passage of a large amplitude planar pressure signal across an expandi ng spherical flame bubble in a shock tube. The laminar flame front was completely obliterated, and the evolution to fine grain turbulent com bustion was revealed. In the current paper we report on numerical simu lations of a number of similar experiments. Although we are here restr icted to two space dimensions and cannot therefore investigate fully t urbulent behaviour, these simulations do reveal qualitatively similar behaviour to that found in the early stages of the Markstein experimen t. It has been possible to repeat the simulations for a variety of dif ferent flames, so that the effects of the various processes (in partic ular the chemical reaction and the thermoviscous diffusion) can be ass essed. Attention is also given to the question of the grid dependency of the numerical solutions obtained.