For a given pair of fluid phases, liquid-liquid flows are generally described in terms of regimes (e.g. stratified, wavy or dispersed), which are a function of the Reynolds numbers of the individual phases, the geometry of the flow, as well as the inlet conditions and the distance from the inlet. Typically, injecting the heavier phase at the bottom of the channel and the lighter phase at the top is the common inlet configuration when establishing a liquid-liquid flow for study in a laboratory environment. This configuration corresponds to that expected in a naturally separated flow orientation, on the assumption that at long lengths the density difference between the two phases will lead to this arrangement of the two phases. In this study, a series of experiments were designed to investigate the influence of injecting the heavier phase at the top of the pipe rather than at the bottom. This modification introduces the possibility of phase breakup near the inlet by an additional instability mechanism (due to the density difference between the two liquids), which would not appear had the phases been introduced in the conventional inlet flow arrangement. We perform detailed flow measurements and observe that this flow arrangement gives rise to altered flow structures downstream. Moreover, our results suggest that the effects of this instability near the inlet may persist along the pipe and influence the observed flow behaviour even at long lengths.
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