Browse Publications Technical Papers 2019-01-0982
2019-04-09

Accelerating Accurate Urea/SCR Film Temperature Simulations to Timescales Needed for Urea Deposit Predictions 2019-01-0982

In a NOx-reducing aftertreatment system utilizing urea water solution as the ammonia precursor, solid deposits are a side effect that reduce system performance by blocking flow passages, the injector, and causing maldistribution of ammonia at the catalyst. These solid urea decomposition by-products can take many minutes or hours to form. Since the urea-containing liquid film must lie in a specific temperature range for solid deposit formation, knowledge of the quasi-steady location, temperature, and composition of the film are an essential first step to deposit chemistry modeling. Of equal importance is developing a technique to accelerate CFD simulations such that minutes to hours of simulation time can be achieved in a reasonable wall-clock time. Determining the quasi-steady film locations and temperatures accurately with CFD is an area of active development. In this study, multiple methods of solution acceleration are compared separately and in combination, to determine the best methods for CFD. Methods examined include reducing the specific heat of solids, using the steady-state solver, utilizing super-cycling for solid temperatures, using intermittent frozen flow where the flow solution is suspended, and representing pulsed sprays as continuous sprays. The experiments of Birkhold et al. [1] are simulated with CONVERGE and used to evaluate the efficacy and accuracy of simulation acceleration approaches. For dry walls above the deposition limit temperature, the Wruck [2] model estimates the heat flux removed from the walls, while also accounting for the Leidenfrost effect. For determination of spray-wall interaction outcomes, the Kuhnke [3] and Bai-Gosman (as extended for urea-water sprays by Smith et al. [4]) models are examined. [1] Birkhold et al., “Analysis of the Injection of Urea-Water-Solution for Automotive SCR DeNOx-Systems: Modeling of the Two-Phase Flow and Spray/Wall-Interaction,” SAE 2016-01-0643 [2] Wruck, N.M. and Renz, U., “Transient Phase-Change of Droplets Impacting on a Hot Wall,” Wiley-VCH Verlag GmbH, ISMN 978-3-527-27149-8. [3] Kunke, D., “Spray/Wall-interaction Modelling by Dimensionless Data Analysis,” Ph.D. Thesis, Shaker Verlag, 2004, SIBN 3-8322-3539. [4] Smith, H., Zochbauer, M., and Lauer, T. “Advanced Spray Impingement Modelling for an Improved Prediction Accuracy of the Ammonia Homogenisation in SCR Systems,” SAE 2015-01-1054

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