The flow fields of large-scale simplex nozzles were investigated by 2-D back-scattered Laser Doppler Velocimetry (LDV). The internal flow structures of a simplex nozzle at two different density ratios of the working fluid and the ambient medium were obtained. The effects of the density ratio, Reynolds Number and orifice geometry on the flow structure were examined. The results revealed that the density ratio only affects the internal flow field in the region where the radius is smaller than the orifice radius. The density ratio and Reynolds Number have stronger influence on the internal flow structure of a sudden contraction and 45° expansion orifice configuration than on that of a 45° contraction and sudden expansion orifice configuration. When the density ratio is one, the effect of the contraction geometry from swirl chamber to orifice on the internal flow is very small compared to the effect of the expansion geometry.

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