Low Pressure-Driven Injection Characterization for SCR Applications

Abstract

A queous Urea is a non-toxic and stable ammonia carrier and its injection and mixing represent the basis for the most common de-NOx technology for mobile applications. The reactant feed preparation process is defined by evaporation, thermolysis and hydrolysis of the liquid mixture upstream the Selective Catalytic Reduction reactor, and it is strongly dependent on the interaction between spray and gaseous flow. Low-pressure driven injectors are the common industrial standard for these applications, and their behavior in almost-ambient pressure cross flows is significantly different from any in-cylinder application. For this reason, two substantially different injectors in terms of geometry and design are experimentally studied, characterizing drop sizes and velocities through Phase Doppler Anemometry (PDA) and liquid mass spatial distribution through Shadow Imaging (SI). The measurements involve the analysis of the spray in quiescent air conditions, gathering information at the closest to the nozzle reliable location. Distilled water and Urea Water Solution are characterized, clearly showing that the spray evolution is only slightly affected by the mixture composition. These experimental findings act as the basis for the construction of a numerical description of liquid injection within the Lagrangian tracking framework inside 3D finite volume CFD simulations. A robust injection model is defined, pointing out the singularities of the system and identifying the most critical aspects. The proposed model is built on the direct definition of a droplet diameter distribution, putting particular attention in the droplet-to-parcel ratio definition. One of the two injectors is taken as the input and the other one, which shows a considerable different design and spray pattern is used as a test for the modeling approach. The simulation setup is then assessed in cross-flow conditions and validated on data gathered in a synthetic exhaust gas test bench, where the injectors are installed in an optically accessible chamber, allowing PDA and SI measurements over a wide range of thermal and kinematic cross flow conditions, representative of typical Diesel after-treatment systems. Good agreement with the experimental data is obtained, highlighting how the definition of the initial spray droplet kinematic properties is the key feature in the correct description of a spray for SCR applications, regardless of the Urea mass fraction in the mixture. The evolution of the liquid plume inside a realistic after-treatment channel size geometry shows that most of the momentum of the spray is carried by the largest droplets, which are the ones to be correctly described to address important issues in the Urea Water Solution dosing; these are responsible for the excessive wall impingement and subsequent formation of solid deposits. Specific care is applied in the definition of a reasonable CFD framework, determining mesh sizes and sub-model setups able to fulfill affordable computational costs, according to the current industrial standards.

Numerical analysis of the flow rig for UWS spray examination in exhaust system-relevant conditions

“…The flash-boiling injection is particularly advantageous as it provides a high level of urea decomposition and uniform ammonia distribution [4,5]. However, those sprays are characterised by significantly smaller droplets, lower velocity and increased angles, and therefore are highly exposed to the drift [3,7,10,11,13,14,[17][18][19].…”

“…A substantial double-vortex formation was observed, but surprisingly, an increase in the gas mass flow rate from 100 to 400 kg/h led to its extinction. Nocivelli et al [11] observed that the spray core composed of larger droplets was slightly affected by the crossflow (contrary to the smaller droplets), and pointed out the importance of the definition of droplet parcels in the case of the Lagrangian spray modelling approach. Another factor required for proper spray measurements is a stable flow.…”