?Electrohydrodynamic Atomization of n-Heptane in a Needle-to-Plate System?

A study of polarity inversion effects in EHD spraying is developed by characterizing the several atomization regimes that can be observed by varying liquid flow rate and applied voltage. An experimental approach is adopted. N-heptane, doped with an antistatic additive, is atomized in a classical needle-to-plate system. Study of polarity effects is essentially based on the comparison between two-dimensional maps of the stability regimes occurring by varying the controlling parameters. Direct visualization by digital camera has been used for this purpose. Furthermore a quantitative validation of the maps has been obtained by matching spray images with laser PDA system measurements, providing informations on droplet size and velocity. Results show that occurrence of the regimes is not qualitatively influenced by polarity inversion, but when negative polarity is imposed to liquid, any modifications on spraying modes need higher variations in applied voltage, and so the regimes are stable in larger voltage ranges. Availability of a valid atomization in negative polarity, most of all in conditions interesting for combustion applications, such as multi-jet mode, could solve problems due to the loss of atomization quality observed when positively charged sprays interact with flame. Negative polarity widens the spectrum of ways by which an active control on spraying process, in order to prevent combustion instabilities, could be realized.