The Effect of Evaporation Models on Urea Decomposition from Urea-Water-Solution Droplets in SCR Conditions

Stein, Marcus; Bykov, Viatcheslav; Maas, Ulrich

doi:10.1007/s40825-017-0075-1

Cited by 18 publications

(12 citation statements)

References 22 publications

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“…As it has been pointed out earlier, once the water evaporates completely from a droplet, the remaining urea mass transfer is thought to proceed in conjunction with various thermo-chemical processes [5,16]. In addition to the two one-step scenario mechanisms defined as M0 and M1, two two-step scenario mechanisms (M3 and M2) are applied in which gas phase reaction kinetics for decomposition of urea into ammonia (NH 3 ) and isocyanic acid (HNCO) is considered.…”

Section: Effect Of Scenarios For Urea Thermal Decompositionmentioning

confidence: 97%

“…Nu * the modified Nusselt number as a function of Spalding heat transfer number B T is provided in Equation (16). Note that the classical dimensionless Nusselt number Nu is defined as [22]:…”

Section: Droplet Evaporation and Species Mass Fraction At And Around mentioning

confidence: 99%

“…In order to take into account the influence of natural convection on the droplet evaporation, the Sherwood number correlation in Equation (9) and Nusselt number in Equation (16) are modified following [35] as:…”

Section: Droplet Evaporation and Species Mass Fraction At And Around mentioning

confidence: 99%

“…As already mentioned in introduction, two ways to treat urea decomposition can be depicted depending on whether the urea is directly decomposed at the droplet surface or first evaporates into the gaseous phase and then decomposes according to a chemical reaction [5,16]. Relying on the film evaporation model, the second alternative is first adopted in the present paper.…”

Section: Scenarios For Urea Thermal Decompositionmentioning

confidence: 99%

“…To describe the urea decomposition, the ways of heat reaction are provided [16] play a great role in the overall mass transfer dynamics. The first way (W1) consists of an evaporation of molten/solid urea to the gaseous urea, which decomposes in the gas phase in ammonia and isocyanic acid (HNCO).…”

Section: Introductionmentioning

confidence: 99%

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Numerical Investigation of AdBlue Droplet Evaporation and Thermal Decomposition in the Context of NOx-SCR Using a Multi-Component Evaporation Model

2018

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Abstract:To cope with the progressive tightening of the emission regulations, gasoline and diesel engines will continuously require highly improved exhaust after-treatment systems. In the case of diesel engines, the selective catalytic reduction (SCR) appears as one of the widely adopted technologies to reduce NO x (nitrogen oxides) emissions. Thereby, with the help of available heat from exhaust gas, the injected urea-water solution (UWS) turns inside the exhaust port immediately into gaseous ammonia (NH 3 ) by evaporation of mixture and thermal decomposition of urea. The reaction and conversion efficiency mostly depend upon the evaporation and subsequent mixing of the NH 3 into the exhaust gas, which in turn depends upon the engine loading conditions. Up to now, the aggregation of urea after evaporation of water and during the thermal decomposition of urea is not clearly understood. Hence, various scenarios for the urea depletion in the gaseous phase that can be envisaged have to be appraised under SCR operating conditions relying on an appropriate evaporation description. The objective of the present paper is therefore fourfold. First, a reliable multi-component evaporation model that includes a proper binary diffusion coefficient is developed for the first time in the Euler-Lagrangian CFD (computational fluid dynamics) framework to account properly for the distinct evaporation regimes of adBlue droplets under various operating conditions. Second, this model is extended for thermal decomposition of urea in the gaseous phase, where, depending on how the heat of thermal decomposition of urea is provided, different scenarios are considered. Third, since the evaporation model at and around the droplet surface is based on a gas film approach, how the material properties are evaluated in the film influences the process results is reported, also for the first time. Finally, the impact of various ambient temperatures on the adBlue droplet depletion characteristics as well as the effect of gravity is pointed out. The prediction capability of the model variants is assessed by comparing the achieved results to each other and with experimental data. It turns out that satisfactory agreement between experiment and numerical predictions is achieved for a wide range of operating temperatures by using correlations by "Wilke and Lee" for urea and by "Fuller et al." for water. The results are essentially sensitive to gravity. From subsequent comparisons of different ways to account for the thermal decomposition in the gaseous urea, a significant difference is observed. Finally, the 1/3 film rule widely used for evaluating the material properties in the film shows accurate prediction of both evaporation and thermal decomposition regimes of urea.

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Section: Effect Of Scenarios For Urea Thermal Decompositionmentioning

confidence: 97%

Section: Droplet Evaporation and Species Mass Fraction At And Around mentioning

confidence: 99%