A series of swirl atomizers of rather small dimensions has been experimentally tested in order to determine the minimum injection pressure characterizing the stable zone of working. In this zone the characteristics of the nozzle, i.e. the discharge coefficient and the cone angle, are independent of the injection pressure and the efficiency of the atomizer is always better as far as the small drop production is concerned. The second purpose of this work was to determine the influence of the liquid viscosity on this minimum injection pressure. It has been found that the value of this pressure is imposed by the cone angle that needs more pressure than the discharge coefficient to reach its asymptotic value. When the viscosity of the fluid is increased, we found that the minimum injection pressure and the discharge coefficient increase and the cone angle decreases. The drop diameter distributions measured here appeared to be mainly functions of the cone angle.
Finally, we observed that when the injection pressure becomes too high, swirl atomizers behave as plain orifice atomizers characterized by a sharp increase of the discharge coefficient and a sharp decrease of the cone angle. Therefore, it is concluded that the best range of working for swirl atomizers is bounded by a lower and an upper injection pressure.