Abstract The efficacy of NOx reduction in diesel engines is mainly dependent on how uniformly urea-water solutions (UWS) are dispersed onto the catalyst surface of the Selective Catalytic Reduction (SCR) systems. The urea-based SCR systems also suffer drawbacks due to the formation of urea deposits onto the walls of after-treatment devices due to poor atomization characteristics of UWS. In this work, the impact of lowering the surface tension of UWS on the morphology of UWS sprays was explored using high-speed shadowgraph imaging techniques. The surface tension of UWS was lowered by adding surfactants; two surfactants viz., Sodium Dodecyl Sulfate (SDS) and Dodecyl-Dimethyl-Amine-oxide (DDA) were considered in this investigation. The surface tension of UWS was reduced to a maximum from 73.7 to 30.2 mN/m and 39.8 mN/m with the addition of DDA and SDS respectively at 75% of its respective Critical Micelle Concentration (CMC) in UWS. Even at a very low-pressure difference of 500 mbar of co-flowing air, the surfactant-added UWS tends to break-up relatively closer to the nozzle tip due to flapping-induced bag breakup, which improved its drop-size distribution. Under a relatively higher pressure difference of 2000 mbar of co-flow atomizing air, the liquid breakup was mostly due to surface stripping in surfactant-added UWS sprays that generated a large number of fine droplets. The image analyses of sprays were performed at far downstream locations from the nozzle to quantify the variations of their droplet-sizes caused by varying the surface tension of UWS. The surfactants added UWS sprays revealed a considerably narrower drop-size distribution by up to 43% compared to UWS sprays under high-pressure conditions, and this was due to a combination of flapping-induced bag breakup, surface stripping and secondary atomization of big droplets caused by reducing the surface tension of UWS. Reducing the surface tension of UWS has the potential to improve NOx reduction in SCR systems due to the reduction in droplet sizes of UWS sprays and also to reduce the formation of urea deposits.

中文翻译：

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对不同表面张力下尿素水溶液喷雾的空气辅助破碎形态的见解

摘要 柴油发动机中 NOx 还原的效果主要取决于尿素水溶液 (UWS) 在选择性催化还原 (SCR) 系统的催化剂表面上的分散程度。由于 UWS 的雾化特性差，尿素基 SCR 系统也存在缺陷，因为尿素沉积物会沉积在后处理装置的壁上。在这项工作中，使用高速阴影图成像技术探索了降低 UWS 表面张力对 UWS 喷雾形态的影响。加入表面活性剂可降低UWS的表面张力；本次研究中考虑了两种表面活性剂，即十二烷基硫酸钠 (SDS) 和十二烷基二甲基氧化胺 (DDA)。UWS 的表面张力从 73.7 降至 30.2 mN/m 和 39。8 mN/m，分别添加 DDA 和 SDS，分别为其在 UWS 中各自临界胶束浓度 (CMC) 的 75%。即使在 500 毫巴的共流空气压差非常低的情况下，由于扑动引起的袋子破裂，添加了表面活性剂的 UWS 往往会在相对靠近喷嘴尖端的地方破裂，从而改善了其液滴尺寸分布。在 2000 mbar 的同流雾化空气相对较高的压差下，液体破裂主要是由于添加了表面活性剂的 UWS 喷雾的表面剥离，产生了大量的细小液滴。喷雾的图像分析在喷嘴的远下游位置进行，以量化由 UWS 表面张力变化引起的液滴尺寸变化。与高压条件下的 UWS 喷雾相比，添加表面活性剂的 UWS 喷雾显示出的液滴尺寸分布要窄至 43%，这是由于拍打引起的袋子破裂、表面剥离和大液滴二次雾化的综合作用由降低 UWS 的表面张力引起。由于 UWS 喷雾的液滴尺寸减小，因此降低 UWS 的表面张力有可能改善 SCR 系统中的 NOx 还原，并且还可以减少尿素沉积物的形成。