Impact of Surface Roughness on the Impingement of Urea–Water Solution Droplets

Abstract

The understanding of impingement processes is crucial for optimizing automotive selective catalytic reduction (SCR) systems. An accurate description of this behavior helps design exhaust systems and increases the validity of modeling approaches. A component test bench was set up, featuring a droplet chain generator for producing droplet sizes typically found in the urea–water solution sprays of SCR systems. A heatable impingement plate with an interchangeable surface enabled investigation of the influence of surface roughness. Data were acquired using a high-speed camera and image postprocessing. The droplet–wall interaction could be described using different regimes. An approach to characterizing impingement behavior based on weighted-regime superposition enabled gradual transitions between regimes, instead of step-like changes. It was observed that the surface roughness increased the droplet–solid contact area and generated thermal-induced secondary droplets at lower temperatures. A region of enhanced mechanical disintegration of the droplet was found, caused by peaks of the surface shearing off parts of the droplet. The probability of a droplet rebounding from the wall was reduced on a rough surface, due to the interference of the surface spikes with the droplet’s spreading and contracting motion. Additionally, the influence of surface topography was investigated using a shot-peened surface. Caused by this surface’s reduced root mean square slope, the aforementioned enhancement of mechanical disintegration was not observed.

CFD Simulation of SCR Systems Using a Mass-Fraction-Based Impingement Model

“…The necessity for further detailed analysis of this phenomenon (e.g., secondary droplets, temperature boundaries) could be derived from these results. Corresponding experimental investigations were carried out and reported [11,19]. Apart from the further analysis of TBU, experiments indicated a gradual transition between the various impingement regimes instead of step-like changes [11,17,19].…”

“…Corresponding experimental investigations were carried out and reported [11,19]. Apart from the further analysis of TBU, experiments indicated a gradual transition between the various impingement regimes instead of step-like changes [11,17,19].…”

“…The main parameters are the surface temperature and the droplet impact kinetics [3,[6][7][8]. Additionally, the influence of surface parameters (e.g., roughness, porosity) or fluid parameters (e.g., viscosity, surface tension) have been reported [9][10][11][12].…”

“…The influence of the droplet's impact conditions and its stability is mostly described based on dimensionless numbers, such as Weber-number (We) or K-number [3,4,[13][14][15]. The impingement model presented in the current paper is based on an experimental characterization using the We-number [11]. It is defined in Equation ( 1) with the wallnormal component of the velocity v n , droplet diameter d, density ρ, and surface tension σ.…”

“…The directions of secondary droplets are different due to the different driving factors, i.e., kinetic energy, on the one hand, and intensive boiling on the other hand. It was observed that, in temperature ranges where mechanical breakup (MBU) and thermal-induced breakup (TBU) coexist, the MBU droplets have very flat ejection angles, and the TBU droplets yield very steep angles [11]. This is in contrast to the described gradual transition of droplet properties for all secondary droplets only with the impact of We-number [4].…”