Schumacher, Olaf ; Mildenberger, Moritz ; Gambaryan-Roisman, Tatiana ; Stephan, Peter (2021)
Wetting and evaporation of pinned urea–water-droplets on substrates of different wettability.
In: International Journal of Heat and Fluid Flow, 92
doi: 10.1016/j.ijheatfluidflow.2021.108886
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

The formation of solid deposits in exhaust treatment systems operating based on selective catalytic reduction is a major reason for the decline in system efficiency and possible damage. To reduce the amount of deposits, precise information about influencing variables but also about necessary preliminary processes is required. One such process is the film formation of liquid urea water solution after the injection. The presented experiments aim to increase the understanding of film formation and evaporation preceding the deposit formation. In a controlled experimental setup, the wetting, spreading and evaporation behavior of single droplets depending on influencing parameters can be investigated. The focus here lies on the effect of hot wall surface finish as well as on the wall temperature. The experiments are conducted in an open test cell. A heated AlMg2 substrate acts as the hot wall on which single AdBlue® droplets are deposited via a needle. The following wetting and evaporation process is captured by a high-speed camera. The droplet geometry is then evaluated by an image processing script to determine sequences of geometry variables such as the contact angle and the contact radius. The results show a strong influence of both, the substrate temperature as well as the surface finishing on the initial wetting and spreading of the droplet. Especially on rough surfaces, the initial spreading decreases significantly for higher temperatures. Evaporation of the droplets is mainly observed in constant contact radius mode. Above the saturation point of water, the surface wettability strongly affects the boiling behavior of the droplets. Significant nucleate boiling is observed on rough surfaces while hydrophobic surfaces show bubbles merging within the droplet, not moving towards the liquid–gas interface. With rising urea concentration, the boiling subsides due to an increased saturation point. The remaining solution shows much stronger wetting of the rough and polished, heated surfaces compared to the initially applied AdBlue®.

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