Experimental study of the droplet characteristics of a SCR injector spray through optical techniques

Abstract

[EN] The requirement of high-fidelity experimental data of microscopic properties of sprays motivates the continuous development of optical tools, since they are determinant in the understanding of atomization and evaporation process in a variety of multiphase flows.This work compares two optical techniques and an interferometry method for the measurement of the droplet size and velocity of a spray produced by a commercial Selective Catalytic Reduction (SCR) dosing unit also known as Urea Water Solution (UWS) injector. The adequate dosing and mixing of the injected fluid is fundamental for the proper functioning of the (SCR) system, therefore studying the spray characteristics with proper techniques is important to comprehend the phenomena occurring during the injection process. The SCR system is employed in the elimination of nitrous oxides present in exhaust gases of an engine. The objective is to compare the techniques in order to obtain accurate and reliable data that can be used to validate Computational Fluid Dynamics (CFD) models and improve current exhaust geometries and mixers. The studied techniques are Phase Doppler Anemometry (PDA), High Resolution Laser Backlight Imaging (HRLBI) and High Speed Microscopic Imaging (HSMI). The PDA determines the properties of the spray, obtaining the diameter and velocity of droplets in the axis parallel to the axis of the injector. With the HRLBI frames of 2040 x 2040 pixels were captured at different instants of the injection event, achieving a spatial resolution of 7 um/pixel, allowing the visualization of the smallest droplets present in the spray. With the HSMI videos of the spray using a fast camera and a microscopic lens were recorded at 150.000 fps to measure the diameter and the velocity of the droplets in 2 directions. The measurements were carried out using water as the injected fluid at 3 different injection pressures and the results are compared to determine the limits and advantages of each technique. The droplet diameter obtained by the PDA tests is similar to the observed results in the HRLBI, whereas the HSMI missed the smaller droplets. On the other hand the droplet velocity is in good concordance in the axis parallel to the axis of the injector, proving the HSMI a suitable technique to quantify the velocity of the droplets.