LIF/Mie Droplet Sizing of Water Sprays from SCR System Injector using Structured Illumination

Abstract

Recent trends in SCR (Selective Catalytic Reduction) systems development increase requirements for UWS (Urea Water Solution) injection. Close-coupled SCR system designs decrease the distance available for water evaporation and urea decomposition. Due to that, much effort is put into static mixing elements design improvement and injection process enhancement. So far, most experimental studies on UWS spray formation were based on Mie scattering visualization using global illumination and shadowgraphy imaging. High speed imaging of Mie signal with global illumination allows to determine global spray parameters such as penetration and angle but does not give information on droplet sizes. Droplet size determination, due to relatively large droplets generated by SCR injectors, can be done with Mie scattering or backlight imaging methods. Then the visualized area becomes narrowed since high magnification is required. Determination of droplet size distribution across whole spray in such arrangement requires number of measurements. LIF/Mie (Laser Induced Fluorescence/Mie scattering) technique provides an attractive alternative for rapid determination of droplet size distribution across the whole spray. This method however suffers from multiple scattering effects which might affect droplet size distribution results even in relatively dilute sprays. In this study, LIF/Mie ratio distribution across sprays from commercial automotive injector for SCR systems was determined by simultaneous LIF and Mie detection using structured illumination. Moreover, the results were compared with conventional LIF/Mie imaging. Nd:YAG pulse laser was used as a light source. Second harmonic beam of 532 nm was used to illuminate the sprays. Instead of UWS pure water doped with Eosin Y was used. The results showed that conventional images exhibited much stronger background signal. Moreover, the conventional imaging was sensitive to reflections from experimental setup elements, specifically reflections from LIF camera filter. These two observations prove the importance of using SLIPI for LIF/Mie droplets sizing in sprays for SCR systems. At the same time the obtained results showed that under certain conditions (no accidental reflections in the background) conventional imaging provides similar LIF/Mie ratio as structured illumination. The results showed that the LIF/Mie ratio remains unchanged over the spray cloud. This suggests that SMD remains unchanged as well. The slight increase of LIF/Mie ratio far from the injector outlet could be caused by absence of small droplets due to lower momentum and thus lower penetration distance. This assumption however should be verified with PIV measurement.

Investigation of the urea-water solution atomization process in engine exhaust-like conditions

“…Injection pressure of 8 bar has a higher droplet probability in the range between 80 μm and 180 μm and for droplets with diameters above 200 μm the tendency reverts, depicting a higher droplet probability for the injection pressure of 4 bar. The overall values obtained in this work were in the same order of magnitude as those found by other authors[8,14,24,25].…”

Characteristics of Water and Urea–Water Solution Sprays

“…These include both spray formation as well as spray-wall interaction. In many studies available in the literature, UWS is replaced with pure water [3][4][5][6][7][8][9]. For preliminary studies, such an approach seems to be justified since the physical properties influencing spray behaviour, specifically surface tension and viscosity, are similar for these two liquids.…”

Quantitative Expression of a Slight Deviation of the Impact Angle in a Collision Atomizer

“…The measurement of atomized droplet particle size is an important support link for the study of nozzle atomization characteristics. At present, there are many methods for measuring atomized droplet particle size, which can be roughly classified as phase Doppler anemometry (PDA) [22,23], the polarization ratio technique [24,25], the digital holography technique [26,27], laser-induced fluorescence/Mie scattering drop sizing (LIF/Mie) [28,29], and laser interference particle measurement technology (IPI) [30][31][32]. Among these methods, IPI technology has the advantages of simple system structure, easy construction, and a large measurement range.…”