Numerical and Experimental Evaluation of the Optical Connectivity Technique for Measurement of Liquid Breakup Length in Atomizers

Abstract

During the first stages of atomization in an air-blast atomizer, the liquid stream is destabilized under the influence of a coaxial stream of air, until its continuity is broken. A novel technique [Charalampous et al. 2007 (1)] has been proposed to measure the length of the continuous liquid jet, based on the internal illumination of the liquid jet through the spray nozzle. The liquid jet acts as a light guide, which propagates along the length of the jet, in the same way as light travels along the length of an optical fiber. The light excites a fluorescent dye that is dissolved in the liquid jet, making the volume of the liquid jet luminous. Then, the connectivity of the liquid jet is linked to the optical connectivity of the fluorescent jet. However, since the surface of the liquid jet is not smooth as that of an optical fiber due to the development of waves on the jet surface, there are losses of light intensity due to refraction through the jet surface and absorption by the fluorescence dye, as it propagates along the liquid jet. The optical connectivity technique is examined numerically and experimentally. First, numerical simulations of the light propagation within liquid columns of various geometries are presented. The effects of the morphological characteristics of the interfacial waves on the gas-liquid interface (wavelength and amplitude) and the type of perturbation (sinus or varicose) as well as the absorption of laser light within the liquid jet and the characteristics of the laser beam (divergence and intensity profile) were considered. Next, a comparison of the continuous length of the liquid jet of an airblast atomizer is presented as measured with the optical connectivity technique and an electrical connectivity technique. In the latter approach, the continuity of the jet is determined by the electrical conductivity between the spray nozzle and a probe placed in the electrically charged liquid downstream of the liquid nozzle exit. The comparison shows that the optical connectivity technique performs better than the electrical connectivity technique in measuring the mean breakup length of the liquid jet.