Enhancement of fine-scale turbulence for improving fuel-rich plume combustion

A passive method to enhance fine-scale mixing was developed and studied in cold flows. Its effect on the combustion intensity and flame stability was then studied in reacting flows. Hot-wire anemometry was used to map the mean and turbulent flowfields of the nonreacting flows. Reacting flows were studied in a free flame and ducted gas-generator fuel-rich plume using planar laser-induced fluorescence imaging, a rake of thermocouples , and high-speed photography. A modified circular nozzle having several downstream-facing steps upstream of its exit was used to introduce numerous inflection points in the initial mean velocity profiles, thus producing multiple corresponding sources of fine-scale turbulence generators and reducing the strain rates in the initial region. Cold flow tests showed turbulence increases of up to six times the initial turbulence level relative to a circular nozzle and a substantial decrease of the mean velocity gradient. The flame of this nozzle was more intense with a homogeneous heat release in the free-flame experiments and ducted-plume combustion experiments, even when the gas-generator exhaust velocity was supersonic. Secondary plume ignition was obtained under conditions that prevented sustained afterburning using the circular nozzle.