Abstract
Countercurrent shear flow control is being established as an effective technique for enhancing turbulence and mixing in various turbulent flows. The basic features of the countercurrent shear layer, including the high levels of turbulence and large turbulent structures, suggest that the technique has vast potential for enhancing combustion. With this as motivation, a novel countercurrent dump combustor was designed and tested under cold flow conditions to study the potential benefits of using counterflow for controlling combustion flows. The countercurrent dump combustor is a suction based system which induces a suction counterflow through a variable height gap at the sudden expansion dump plane. The control technique performed well, as peak turbulence levels, cross-stream averaged turbulent kinetic energy, and cross-stream momentum diffusion increased with applied suction. The countercurrent dump combustor operates via two mechanisms: enhanced global recirculation and near field control of the separated shear layer. In general, the enhanced recirculation is a modulation of the natural recirculation bubble which exists with the confined separated flow, and is predominantly sensitive to the suction mass flow rate. The near field dynamics of the separated shear layer is modulated through the development of a countercurrent shear layer near the separation point, and is governed by the velocity ratio near the dump plane. The use of counterflow also produces larger structures, which alleviates the high strain rates caused by the increased turbulent energy. This result will allow for increased burning rates without the detrimental effects of strain-induced flame stretch. This is a significant advantage over the conventional dump combustor. The turbulence is also highly three-dimensional with counterflow, which will allow for control of thermoacoustic instabilities through disruption of spanwise coherence. The observations of the cold flow study of the countercurrent dump combustor suggest that there will be multiple benefits for using countercurrent shear control in reacting flows. Future studies will be conducted on the countercurrent dump combustor under conditions of premixed combustion. Expected benefits include enhanced combustion, increased volumetric heat release, and reduced thermoacoustic instabilities. (Author)