Neutrally buoyant miscible jets into viscoplastic ambient fluids

We present the results of an experimental study of the jet flow dynamics where a Newtonian fluid is horizontally injected, through a circular nozzle, into a tank filled with a viscoplastic ambient fluid. Motivated by our industrial application (i.e. the jet cleaning of oil and gas wells for various operations), a perforated wall between the nozzle and the tank wall is considered, dividing the flow domain into two main zones. The jet and ambient fluids are miscible and have the same density (i.e. a neutrally buoyant jet). Employing a combination of various non-intrusive experimental methods, including high-speed imaging, time-resolved tomographic particle image velocimetry, and planar laser induced fluorescence techniques, enables us to comprehensively analyze the jet flow behaviour, in terms of the main dimensionless numbers governing the flow. We succeed in identifying four distinct jet flow patterns: a mixing regime, a mushroom regime, a fingering regime, and a fracturing regime, and present a regime classification. We also compare the flow regimes, in terms of the morphological behaviour, jet radius, velocity profile, self-similarity, kinetic energy, and flow intermittency. In general, our results show that increasing the yield stress of the viscoplastic ambient fluid significantly affects the jet flow behaviour. Furthermore, the ratio of yield stress to jet inertia is identified as the key parameter in determining jet flow regimes.