A Novel Approach for the Impingement of AdBlue-Droplets based on Smooth Regime Transitions

Further development of exhaust aftertreatment systems based on selective catalytic reduction (SCR) requires detailed knowledge of all involved physical and chemical processes. One major influence is the impingement of the injected urea water solution (UWS) droplets on the hot walls of the exhaust system. Due to the numerous influencing factors of this complex phenomenon, it is described by empirical impingement maps based on experimental investigations. Frequently, the impacting droplet is assigned to a single impingement regime (e.g. splash) according to surface temperature and a kinetic parameter (e.g. Bai-Gosman, Bai-ONERA, Kuhnke). A transitional range between regimes has been reported experimentally before, but was abandoned in most cases for model simplification reasons. Only rarely a smooth transition for a selected regime boundary was implemented. However, the still widespread use of hard limits is questionable for CFD, as an arbitrarily small temperature difference around a defined threshold value can yield completely different results. This is special interest for SCR applications with their wide range of surface temperatures and possible temperature changes during injection. To understand the physical behaviour in more detail and to address the mentioned problem, the impingement of UWS on stainless steel was studied using high-speed imaging. The regime transitions were investigated and a new possibility to describe them is now proposed. Instead of one diagram with several regimes separated by strict threshold values, a set of four maps - one for each main impingement scenario - is used. Each map describes the mass fraction of the impacting droplet that is assigned to the respective behaviour depending on wall temperature and Weber-number. Together, the four maps define the outcome of the impinging droplet. The experimental results also describe the thermal induced breakup in more detail. It gradually increases in intensity with rising temperature before being progressively replaced by the high temperature Leidenfrost-regimes. No hard limits remain, as the mass fraction of the droplet that is assigned to a given regime in- and decreases incrementally. A derived impingement model consequently results in smooth transitions which describe the observed behaviour more accurately.