选择性催化还原技术是控制柴油机NOx排放的最有效方法之一。然而,固体尿素和副产品的产生,会导致尿素分解不完全,降低NOx的还原效率。沉积物的积累也会影响背压,甚至堵塞催化剂,造成恶性循环。在本文中,建立了尿素分解机理的数值模型。基于这个模型,计算了薄膜中的四种传热方法和化学反应。结果表明,薄膜与平板之间的传热方式主要有传导和蒸发,两者的最大值都高于107W/m2。薄膜的质量随热通量而变化,可分为四个阶段。在第一阶段,电影迅速形成,和强烈的热交换发生。在第二阶段,薄膜缓慢增加,尿素的分解质量增加。膜的主要成分是固体尿素。第三阶段,固体尿素质量开始减少。第四阶段,薄膜质量下降,大量的尿素被分解。尿素分解的主要副产物是前三个阶段的缩二脲和第四阶段的三聚氰酸。固体尿素及其副产物主要分布在低热通量区域。随着初始板温度的升高,热通量增加,薄膜和固体尿素质量降低,副产物出现得更早,质量增加得更快。当温度为523K时,528K,533K,缩二脲质量的最大值出现在3.1s左右,1.4s和0.5s,分别。本工作为理解传热特性与尿素分解副产物形成之间的关系提供了指导。
Selective catalytic reduction technology is one of the most effective methods to control NOx emissions from diesel engines. However, the generation of solid urea and by-products, which will lead to the incomplete decomposition of urea and reduce NOx reduction efficiency. The accumulation of deposits will also affect the back pressure and even block the catalyst, causing a vicious cycle. In this paper, a numerical model coupled with a detailed decomposition mechanism of urea was established. Based on this model, four heat transfer methods and chemical reactions in the film were calculated. The results show that the main heat transfer methods between the film and the plate are conduction and evaporation, both of which have maximum values higher than 107 W/m2. The mass of the film varies with heat flux can be divided into four stages. In the first stage, the film forms rapidly, and intense heat exchange occurs. In the second stage, the film increases slowly and the decomposition mass of urea increases. The main component of the film is solid urea. In the third stage, the solid urea mass begins to decrease. In the fourth stage, film mass decreases, and a large amount of urea is decomposed. The main byproduct of urea decomposition is biuret in the first three stages and cyanuric acid in the fourth stage. The solid urea and byproducts are mainly distributed in regions with low heat flux. With an increase in the initial plate temperature, the heat flux increases, the film and solid urea masses decrease, the byproducts appear earlier and their mass increases faster. When the temperature is 523 K, 528 K, and 533 K, the maximums of biuret mass occur at about 3.1s, 1.4s and 0.5s, respectively. The present work provides guidance understanding the relationship between heat transfer characteristics and byproducts formation of urea decomposition.