Modeling Approach for a Wiremesh Substrate in CFD Simulation

Experimental studies have shown that knitted wiremesh mixers reduce the formation of solid deposits and improve ammonia homogenization in automotive SCR systems. However, their implementation in CFD models remains a major challenge due to the complex WM geometry. It was the aim of the current study to investigate droplet WM interaction. Essential processes, such as secondary droplet generation, wall film formation, and heat exchange, were analyzed in detail and a numerical model was set up. A box with heat resisting glass was used to study urea-water solution spray impingement on a WM under a wide range of operating conditions. High speed videography was used to identify the impingement regimes. Infrared thermography was applied to investigate WM cooling. In order to determine the impact of the WM on the spray characteristics, the droplet spectrum was measured both upstream and downstream of the WM using the laser diffraction method. The probability of interaction between a droplet and the WM was determined based on a high-resolution image of the WM substrate and as a function of the droplet diameters. Impingement was dominated by the three regimes rebound, deposition, as well as deposition and splash. The WM effectively promoted droplet breakup. Wall film evaporation effected distinct cooling spots on the WM. Based on a statistical analysis of the impingement probability an empirical droplet wiremesh interaction model was derived. Therefore the WM geometry was represented as porous boundaries, and the secondary droplet characteristics as well as the droplet wire interaction regimes were adapted from an existing multi-regime impingement model. The resulting model was validated against measurement data. The findings of the current study provide a detailed insight into the mechanism of droplet WM interaction. A new model is now available which can be used for CFD simulations of SCR exhaust systems with WM substrates.