Characterization of Electrified Water Sprays

The research is focused on water sprays electrified by induction charging through an external electric field. The aim of this work is to verify and understand how the breakup mechanism of a liquid jet and the droplets’ size population are influenced by the electric field. Two experimental rigs were tested, and they will be named in the text as high-flow rate electrified spray (≈ L/min) and low-flow rate electrified spray (≈ mL/h). The choice to carry experiments on different scaled setup is due to the geometrical simplicity that the low-flow rate water spray system displays, which helped to better understand the physics beyond the electrification of a liquid jet. In both systems, the liquid was water pumped in a grounded nozzle. The liquid jet crossed an electric field generated by a toroidal ring connected to the HV power supply and placed around the jet itself. The experiments were conducted to estimate: i) the electric current (or the specific charge) acquired by the droplets, ii) the droplets’ size population at different electric potentials. For what concerns the high-flow rate electrified spray, hollow cone hydraulic nozzles were used. We found that the droplets current had a non-monotonic trend with the applied electric voltage: it increased linearly until to reach the optimum value and, after that point, it started to decrease fast. At same time, the breakup mechanism was influenced by the electric field. Indeed, for the primary breakup parameters, we observed that the breakup length reduced with the potential of about 20 %compared with the uncharged value until to reach an asymptotic value, while the spray angle enlarged due to the repulsion between equally charged droplets. In the secondary atomization, the droplets’ size distribution shifted toward smaller diameters as the electric potential increased. The percentile diameter, d70, chosen to represent the distribution, was quite constant for all the potentials from 0 kV until the potential at which the electric current was maximum. As soon as the current overcome the optimum point, the d70 started to reduce swiftly. In these conditions, the dimensionless ratio between the electrical stress and the surface tension stress, R, rise and become larger than 1, as confirmation of the effective influence of the electric field on liquid jet dynamics. We envisage that the insurgence of corona discharge on liquid jet surface could explain these phenomena. This hypothesis was confirmed by the experiments made on the low-flow rate electrified sprays or electrospray. In fact, this experimental rig was used to carry studied on the simple jet mode with whipping breakup, where the whipping is an off-axe instability that generates droplets much smaller that the nozzle inner diameter. The tests revealed that as soon as the electric current increased with the potential and the whipping instability took place on the liquid jet surface, the droplets’ size population were composed by droplets of size smaller than 0.5 mm. When it happens, the electrical stress overcomes the surface tension, as observed for the high-flow rate spray, and the corona discharge glow took place. It was confirmed by ad-hoc experiments made in a black-room. The results confirmed that the presence of the electric field modifies significantly the liquid jet atomization dynamic. It could be used to manipulate the droplets’ distribution accordingly to the require application