Visualization and two-color DPIV measurements of flows in circular and square coaxial nozzles

High-resolution, reactive Mie scattering laser-sheet visualizations, two-co lor digital particle image velocimetry (DPIV) and thermal anemometry measur ements in flows generated by equivalent coaxial circular and square jets ar e presented. Visualization results were obtained for three square, coaxial configurations, and a reference circular coaxial nozzle, at two Reynolds nu mbers of the outer jet (19,000 and 29,000) and for inner-to-outer jet veloc ity ratios of 0.15, 0.22, and 0.3. These indicated that the internal unmixe d region diminished with decreasing velocity ratio. Strong evidence of unst eady recirculation and back-flow was observed at the end of the core of the inner jet, for the low velocity ratios. Comparisons between circular and s quare jet configurations indicated considerable mixing enhancement when squ are nozzles were used. Low-coherence, organized large-scale structure was e vident from the visualizations and DPIV measurements near the origin of the inner mixing-region shear layers, and more so in the core region of the ne ar field. These observations were confirmed by velocity spectra, which disp layed peaks corresponding to a free shear-layer instability mode in the inn er mixing-region shear layers, and a wake-type mode in the core region wher e the mean flow has a wake-like character. Although some large-scale struct ure was observed in the outer mixing layer during the visualizations, this was found to be incoherent on the basis of the DPIV measurements and the ve locity spectra. It is noted that no axis-switching phenomena were observed in the square nozzle flows examined here. This is attributed to the absence of an organized structure in the outer shear layer, which was initially hi ghly turbulent, and the weakly coherent nature of the organized structure o bserved in the inner mixing-region near field.