Modeling the Kinetic and Thermal Interaction of UWS Droplets Impinging on a Flat Plate at Different Exhaust Gas Conditions

Abstract

T he selective catalytic reduction has seen widespread adoption as the best technology to reduce the NO x emissions from internal combustion engines, particularly for Diesels. This technology uses ammonia as a reducing agent, which is obtained injecting an ammonia carrier into the exhaust gas stream. The dosing of the ammonia carrier, usually AdBlue, is the major concern during the design and engine calibration phases, since the interaction between the injected liquid and the components of the exhaust system can lead to the undesired formation of solid deposits. To avoid this, the thermal and kinematic interaction between the spray and the components of the after treatment system (ATS) must be modeled accurately. In this work, the authors developed a Conjugate Heat Transfer (CHT) framework to model the kinetic and thermal interaction among the spray, the eventual liquid layer and the pipe walls. The Nukiyama curve has been embedded in the calculation of the heat flux between the droplet and the walls to limit the heat transfer in the proximity of the Leidenfrost point and in the transition region. To validate this model, an experimental data set was provided by EMPA (CH) and used for comparison with calculated values. The measurement of the thermal footprint of the spray have been performed on the back of a thin plate where the spray impinges. Several injections have been considered with the intent of showing the transition to the different interaction regimes. The simulations performed show that after the initial cooling of the wall, due to impingement, a liquid film is formed, which is then dragged along the plate. As the number of injection progresses, the effect of the transition between the different evaporation regimes translates into high temperature gradients on the back of the plate. The comparison with the experimental data both in terms of temperature and temperature gradient shows a good agreement with the experiments, showing the capabilities of the model developed to predict the temperature drop.

Numerical and Experimental Investigation of Ammonia Uniformity in a Compact Selective Catalytic Reduction System

“…Although most of the spray and wallfilm submodels used in this work are based on the state of the art, the wallfilm boiling model is based on the Nukiyama curve for water. A recent study by Nappi et al 40 shows that although the shape of the Nukiyama curve is the same for water and UWS, there is a significant difference in the Nukiyama and critical heat flux (CHF) temperatures. Utilizing the Nukiyama curve of UWS in the wallfilm boiling model should improve the prediction accuracy of wall temperature and wallfilm formation.…”

A preliminary computational analysis towards the use of Electrically Heated Mixing Catalyst for innovative SCR after-treatment systems

“…As the atomization of the lowpressure injectors used to provide UWS is low, the interaction between the spray and the walls of the SCR system is inevitable. This interaction can lead to localized cooling [3] and the formation of a permanent film, which is the main source of solid deposits. The formation of these deposits is not only detrimental to the operation of the SCR system (as they can lead to clogging) but also alters the dosing strategy of ammonia, causing a reduction of the conversion efficiency.…”