In order to assist in fast design cycle of Diesel engines selective catalytic reduction (SCR) exhaust systems, significant endeavor is currently being made to improve numerical simulation accuracy of urea thermo-hydrolysis. In this article, the achievements of a recently developed urea semi-detailed decomposition chemical scheme are assessed using three available databases from the literature.
First, evaporation and thermo-hydrolysis of urea-water solution (UWS) single-droplets hanged on a thin thermocouple ring (127 μm) as well as on a thick quartz (275 μm), have been simulated at ambient temperature conditions ranging from 473K to 773K. It has been shown that the numerical results, in terms of evaporation rate and urea gasification, as well as droplet temperature history are very close to the experiments if the heat flux coming from the droplet support is properly accounted for. Indeed, an additional conduction flux has proved to be necessary in the evaporation model in order to account for the droplet heating coming from the support (i.e. thermocouple ring or quartz bead). This additional heat conduction flux has shown more critical for droplets suspended on a thick quartz. It is also argued that our detailed kinetic mechanism is able to ensure accurate thermal decompositions as long as the temperature inside the droplet is still nearly uniform. This assumption is shown to be true at low temperature and so, at low evaporation and thermo-hydrolysis rates. However, for high gas temperature, bubble nucleation near the support surface induces non-uniform temperature distribution.. This process makes accurate simulation of thermal decomposition extremely dependent on the local temperature inside such large suspended droplets. These results are also relevant and underline the modelling difficulties that we must tackle when it comes to studying the evaporation, boiling and thermolysis of liquid films and deposits on the exhaust walls.
Next verification of the models has been carried out using UWS sprays injected in 6-m long pipe under typical Diesel engine exhaust manifold conditions. In this case, good agreement with experiments in terms of urea to ammonia (NH3) conversion efficiencies has been obtained under different temperatures and residence times. In addition, it proved that by-products (like solid biuret, Cyanuric acid and even ammelide) can be formed in the spray parcels upon water evaporation is completed during their travel to the exhaust catalyst inlet. These solid by-product particles may clog the catalyst inlet section.