Modeling the Kinetic and Thermal Interaction of UWS Droplets Impinging on a Flat Plate at Different Exhaust Gas Conditions 2021-24-0079

The 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.