Abstract Selective catalytic reduction is a key technology for reducing nitrogen oxide ( NO x ) emissions of diesel engines, which requires the injection of urea–water solution (UWS) into the hot exhaust gas. The main objective of this paper is the development of a methodology for predicting the evaporation of urea-water sprays. For this reason a generic experiment was conducted in order to provide validation data under realistic operating conditions. The configuration is based on a twin-fluid atomiser, which is used to inject UWS into a hot pipe flow. Microscopic imaging was applied in conjunction with particle tracking velocimetry to record droplet size and velocity distribution profiles. Using this approach, droplets of a diameter as small as 4 μm were successfully detected at operating conditions of the gaseous phase of up to 773 K and 0.24 MPa. The analysis of droplet data at different positions downstream of the atomiser revealed details of the evaporation characteristics of UWS. In particular, a characteristic droplet size is proposed, which may serve to characterise the overall progress of UWS spray evaporation. Using this criterion, the comparison of experimental results to numerical predictions is straightforward. A numerical model, which is based on one-dimensional droplet kinematics as well as a rapid mixing evaporation model, was developed to describe the most important physical processes of the experimental configuration. A distinct advantage of the experimental methodology is that the velocity of the gas phase can be approximated by the velocity of the smallest detectable droplets with sufficient accuracy. This information was used to calculate the relative velocity between larger droplets and the hot gas, which is an important parameter considering convective heat and mass transfer in the evaporation model. The results obtained via the numerical model were found to agree very well with experimentally obtained evaporation characteristics.

中文翻译：

---

尿素水溶液的喷雾蒸发：实验和建模

摘要 选择性催化还原是减少柴油机氮氧化物（NO x ）排放的关键技术，需要将尿素水溶液（UWS）喷射到热废气中。本文的主要目标是开发一种预测尿素水喷雾蒸发的方法。出于这个原因，进行了一项通用实验，以便在实际操作条件下提供验证数据。该配置基于双流体雾化器，用于将 UWS 注入热管流中。显微成像与粒子跟踪测速结合使用，以记录液滴尺寸和速度分布曲线。使用这种方法，在高达 773 K 和 0 的气相操作条件下成功检测到直径小至 4 μm 的液滴。24兆帕。雾化器下游不同位置的液滴数据分析揭示了 UWS 蒸发特性的细节。特别是，提出了一个特征液滴尺寸，这可能有助于表征 UWS 喷雾蒸发的整体进程。使用此标准，可以直接将实验结果与数值预测进行比较。开发了基于一维液滴运动学和快速混合蒸发模型的数值模型来描述实验配置中最重要的物理过程。实验方法的一个明显优点是气相的速度可以通过足够准确的最小可检测液滴的速度来近似。该信息用于计算较大液滴和热气之间的相对速度，这是考虑蒸发模型中对流传热和传质的重要参数。发现通过数值模型获得的结果与实验获得的蒸发特性非常吻合。