The interaction of droplets consisting of urea-water solution (UWS) with a wall is of interest for automotive exhaust gas after-treatment of Diesel engines by selective catalytic reduction (SCR). Since the impingement of tiny UWS droplets on the solid substrate is difficult to examine experimentally, little is known about the detailed dynamics of this process. In the present study, the normal impact of single UWS droplets impinging on dry solid substrates of greatly differing wettability is investigated numerically under axisymmetric conditions. Simulations are performed by a diffuse interface phase-field solver developed by the authors where the coupled Cahn–Hilliard Navier–Stokes equations are solved using OpenFOAM. The code is thoroughly validated against a number of experiments from literature considering the rebound of millimetre-sized water droplets from hydrophobic substrates.

The numerical simulations on the impact dynamics of UWS droplets cover wide ranges of sub-millimetre droplet sizes and impact velocities that are relevant in technical SCR systems. A strong influence of substrate wettability on droplet dynamics is identified. Reducing wettability from hydrophilic to superhydrophobic conditions reduces spreading and enables drop rebound with reduced drop-surface contact time. The effects of drop diameter, drop impact velocity and equilibrium contact angle on the maximum spreading ratio are quantified, and regime maps on rebound versus non-rebound (deposition) impact outcomes are provided. The results of the present interface-resolving numerical simulations may be useful for development of more advanced drop-wall interaction models as they are required in CFD codes relying on the Euler–Lagrange approach for large-scale computations of UWS sprays.

由尿素水溶液（UWS）组成的液滴与壁的相互作用对于通过选择性催化还原（SCR）进行的柴油发动机汽车尾气后处理具有重要意义。由于难以通过实验检查微小的UWS液滴在固体基质上的撞击，因此对该过程的详细动力学知之甚少。在本研究中，在轴对称条件下，数值研究了单个UWS液滴撞击在润湿性差异很大的干燥固体基质上的正常影响。由作者开发的扩散界面相场求解器进行仿真，其中使用OpenFOAM求解耦合的Cahn–Hilliard Navier–Stokes方程。

UWS液滴冲击动力学的数值模拟涵盖了技术毫米波SCR系统中相关的亚毫米级液滴大小和冲击速度的广泛范围。确定了基材润湿性对液滴动力学的强烈影响。将润湿性从亲水性条件降低到超疏水性条件可减少扩散，并在减少液滴表面接触时间的情况下实现液滴回弹。量化了液滴直径，液滴撞击速度和平衡接触角对最大扩散比的影响，并提供了反弹与非反弹（沉积）撞击结果的状态图。