EXPERIMENTAL STUDY OF THE JET BREAK UP FOR EHDA OF LIQUIDS IN THE CONE-JET MODE

The scaling laws developed by Fernández de la Mora (1994, J. Fluid Mech. 260, 155–184) and by Gañán-Calvo (1994, J. Aerosol Sci. 25S, S309–S310) have been verified since then through various experiments. Chen and Pui (1997, Aerosol Sci. Technol. 27, 367–380) investigated the dependence of the current and droplet diameter with the permittivity of the liquid. Hartman et al. (1997) developed a physical model to describe the spraying of liquids in the cone-jet mode. These two approaches compare remarkably concerning the current produced by the cones. Nonetheless some differences have appeared concerning the jet break up. Namely, the scaling of the jet diameter with the flow rate and the conductivity is different in both approaches.Thus, we present here an original experimental study to investigate these differences. It was conducted using a High-Speed Spray Imaging System (HSSIS) purchased from Oxford Laser. This system consists of a digital camera (KODAK) connected to a computer which is equipped with a frame grabber. A long-distance microscopic lens is fixed to a camera. The illumination of the subject is done by an infrared laser. A control box synchronizes the camera and the laser. Pictures can be made with an illumination time down to 0.5 ms and the separation between two following pictures can be down to 15 ms. The optical system allows us to see objects down to a few micrometers but only objects bigger than 10 mm can be accurately measured. The experiments were done using five different liquids namely, ethanol, butanol, isobutanol, 2-butanone and ethylene glycol. For each liquid the conductivity and the flow rate were varied. For each situation photos were taken to determine the jet size, the break up of the jet, the droplet size and the droplet velocity. Moreover, for every situation the spray current was measured.Our results show that the jet diameter for the different liquids studied exhibits a dependence on the flow rate at a power ∼0.6. This indicates firstly, that the model developed by Hartman is correct in its calculation of the jet diameter and secondly, that the break up of the jet cannot be assimilated to the one of an unchanged jet. Nevertheless our results must be put in perspective with the fact that the jet diameter is measured at a fixed distance from the cone, while the jet length varies with the flow rate. Further, our study brought some other interesting results.First, concerning the jet break-up mechanism. It is a known fact that within the range where IμQ^1/2 the jet can break up due to varicose or kink instabilities. The results showed that for a given liquid the break up was going from varicose to kink with the flow rate going up. Second, they also showed that the number of satellites produced between two main droplets increases with the flow rate.Moreover, we have tried to define, as a function of liquid properties, a limit which separates the two types of jet break up mechanism (varicose or kink). This is of primary importance because it is known that the size distribution of the main droplets is much narrower when the jet breaks up due to varicose instability.