No 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.

This work aims at investigating the possible advantages of substituting the mixer in traditional SCR systems with an Electrically Heated Catalytic structure (EHC). First of all, EHC technology is being widely investigated in literature because it offers a concrete solution for catalyst thermal activation and film formation reduction in engine cold start conditions; however, its adoption as a mixer can also guarantee other important improvements in reducing pollutant emissions. In this work, a low-pressure injection of Ad-Blue impacts an electrically heated structure and a complete analysis of liquid droplets and film evaporation is carried out. A hybrid Eulerian-Lagrangian model has been adopted on a multi-region configuration, accounting for fluid-solid conjugate heat transfer (CHT), which plays a key role in the conversion strategy: it has been demonstrated that the heated structure can be exploited to significantly increase the exhaust gas enthalpy in the cold start, which represents an important improvement for pollutant conversion. Different heating strategies are analysed, with the objective of maximising the spray evaporation and the uniformity of the ammonia distribution downstream of the mixer. The objectives of this work are emphasizing the improvements that an electrically heated mixing catalyst can bring to the traditional SCR configuration, laying the foundation for other following studies.