Investigation of the flow patterns produced from sudden expansion geometries using pressure difference measurements and flow visualisation techniques

Abstract

The aim of this study was to investigate the flow behaviour produced from inward sudden and outward expansion geometries. A flow apparatus made of Perspex has been used to measure the variations of pressure differences, and to observe the visualised flow streams, downstream of the expansions. For the outward expansion, the pressure measurements showed two types of oscillations: low frequencies at around 1Hz and high frequencies at around 10Hz. The oscillations at low frequencies have been found to be more dominant and related to the precession of the air flow. Further pressure measurements, together with the visualisation results, showed that this precessing behaviour was stronger near the wall of the expansion chamber. In comparison, the pressure measurements in the inward expansion showed higher overall flow stability. Measurements at different radial distances showed that the low-frequency precessing behaviour appeared to be confined to the central area of the chamber (33–50% diameter from the radial centre). This precession has been found to have a small influence on the flow field near the chamber wall, which may contribute to the flow stability. This finding was also supported by the visualisation results. A swirl module has also been used to investigate the effects of inlet swirl angle on the air flow pattern, where the pressure measurements showed no apparent changes in the overall flow behaviour, but a reduction has been observed in the dominance of the low-frequency oscillations and an increase in the frequencies of the high-frequency oscillations. Flow visualisation of the swirler case showed that the precessing eddies started dissipation at a shorter distance from the expansion point. This study showed the close relationship between the flow behaviours and the geometries of the inlet expansions, and possibility of controlling the flow behaviours by design features such as swirler modules. Such findings are valuable to processes such as spray-drying, which require both sufficient mixing and flow stability. The study also illustrated the use of flow visualisation techniques as complimentary tools to assess complex flow behaviour.