Effects of Lewis number on generation of primary acoustic instability in downward-propagating flames

Abstract

The generation mechanism of primary acoustic instability of downward-propagating flames in a combustion tube is investigated experimentally. The discussion first treats the effects of the coupling constant βM, where β and M represent the Zel'dovich and Mach numbers, respectively, and the effects of the flame surface area variation in equidiffusive flames on primary acoustic instability. A higher coupling constant tends to generate stronger acoustic sound under the same acoustic losses, whereas the variation in the flame surface area does not seriously affect the coupling between pressure and heat release rate fluctuations. We then describe the effect of the Lewis number on primary acoustic instability in non-equidiffusive flames, where the diffusive thermal effects largely determine the internal flame structure. For a Lewis number below (above) unity, where the reaction rate increases (decreases) with reducing (increasing) reaction zone thickness, relatively strong (weak) acoustic sound is produced under the same coupling constant, because the chemical reaction rate becomes very sensitive (insensitive) to gas temperature fluctuations in the acoustic field. Finally, we obtain a linear relationship between the coupling constant and the average acoustic intensity, and results show that relatively large coupling constant is required to generate primary acoustic instability as the Lewis number increases.