Light multiple scattering correction of laser-diffraction spray drop-size distribution measurements

This paper reports an experimental investigation on the practical use of a laser-diffraction instrument, the Malvern Spraytec 2007 to characterize sprays produced by a high-pressure GDI injector. The sprays are highly transient, large, composed of very small drops, dense and heterogeneously distributed in space. These characteristics are at the origin of undesirable effects (beam steering, vignetting and light multiple scattering) whose manifestations are experimentally identified. Ignoring the diodes concerned by beam steering erases the effect of this phenomenon but vignetting and light multiple scattering effects combine to bias the measurements and both require to be corrected. A nth order empirical correction procedure, based on the analysis of the light intensity distribution, is developed. It is a generalization of a second order procedure presented in a previous investigation. The increase of the correction order is demonstrated to be necessary when the injection pressure is greater than 11 MPa. The application of this correction procedure reported that light multiple scattering affects laser-diffraction when the transmission is less than 40%. This limit is in agreement with investigations of the literature and gives credit to the empirical correction procedure. Despite the correction procedure is applicable for the present operating conditions only, this work defines an experimental protocol to apprehend laser-diffraction spray characterization in severe operating conditions and that can be reproduced with ease. Furthermore, it is emphasized that the combination of the Spraytec and the correction procedure performances allows cycle-to-cycle spray drop-size distribution variations to be determined. Such information is of paramount importance and the Spraytec is probably the sole instrument able to provide it.