Instabilities, entrainment and mixing in buoyant reacting jets

Luo, K.H. (2004) Instabilities, entrainment and mixing in buoyant reacting jets. European Journal of Mechanics - B/Fluids, 23 (3), 443-460. (doi:10.1016/j.euromechflu.2003.10.008).

Abstract

Dynamics of buoyant reacting flows from rectangular, square and round sources were investigated using Direct Numerical Simulation (DNS). The general three-dimensional time-dependent governing equations for compressible flow and finite-rate Arrhenius chemistry were solved by high-order numerical methods. The study focused on the intricate couplings among instabilities, vortex dynamics, mixing, entrainment, turbulence and combustion through buoyancy. The main dynamic features of buoyant reacting flows, including the puffing phenomenon, were well reproduced. The instability behind puffing was identified as an intrinsic buoyancy instability that arose from density inhomogeneity in the presence of gravity. No external disturbances were required to trigger or sustain the puffing motions, indicating that the buoyancy instability was a global, absolute instability. The base configurations at the inlet had a significant influence on the subsequent vortex dynamics, entrainment, mixing and ultimately the combustion processes. Base configurations with corners, such as squares and rectangles, were associated with higher entrainment rates than those associated with circular jets/plumes, due to the Biot–Savart instability. A new mechanism based on an extension of the Biot–Savart instability clearly elucidated why rectangular jets/plumes entrain more than square ones, despite the fact that both types of configurations have corners. The aspect ratio effects arising from the mechanism could explain the higher level of vorticity and consequently higher entrainment rate along the major axis as compared with those along the minor axis. Axis switching, as may be deduced from the mechanism, was observed in the rectangular case of aspect ratio 3. The main terms in the vorticity transport equations were calculated for this case, and the coupling between vortex dynamics and combustion was scrutinized. The base configurations were seen to trigger different dynamic couplings among instabilities, vortex dynamics, large-scale entrainment, small-scale mixing and combustion. Such couplings were truly two-way, as exemplified by the modification of the energy frequency spectra by chemical heat release.

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