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subsequent reduction of NOx gases by urea–generated ammonia. To make things more complicated, automobile engines do not run under constant-load conditions, and because of this, the quantity of urea needed for the complete conversion of NOx becomes time-varying. Furthermore, urea needs to be injected in precise quantities to achieve complete conversion of NOx and avoid excess ammonia slip (ammonia leaving the exhaust).
Performance optimization of mobile SCR systems under such challenging conditions demand, among other things, a good understanding of the evaporation behaviour and the thermal decomposition process of urea-water-solution (UWS) droplets. The optimization of complex engineering systems, such as an SCR system, is often done by conducting extensive simulations on computers as it is almost impossible to experimentally investigate the effect of every single parameter on the overall
system performance. Although,
such computational studies
offer a faster and cheaper
method of optimization, the
results from a computational
study are only as good as the
experimental data used to
validate the base computational
model.
For any experimental data
to be useful for the validation
of computational models, the
conditions under which the
experiments are conducted
should be very well defined.
For droplet evaporation,
these conditions include the
temperature of gas surrounding the droplet, its velocity, ambient pressure and relative humidity (a measure of water content in the air). But having searched through existing research findings on UWS droplets, I
Mr. Mikhil Surendran || 415
realized that the experimental data on the evaporation behaviour of UWS droplets was inadequate, and the process of evaporation/ dissociation was not completely understood. This realization motivated me, and as a part of my research, I decided to perform controlled experiments on droplets of UWS under conditions typical of a diesel engine’s exhaust, which would allow us to further understand their evaporation behaviour, and to generate reliable experimental data that can form the basis of future optimization studies.
The study started by building a suitable experimental setup. I knew that the pressure inside the exhaust pipe of a diesel engine was close to atmospheric and that the temperature of the exhaust gas could be anywhere between 100oC and 700oC (depending on the load on the engine). We successfully built a setup that could achieve similar conditions and additionally decided to use dry air
instead of room air so as to have a well-defined condition of relative humidity being zero. Since it is almost impossible to keep the air in a motionless state at high temperatures, we decided to have a controlled and measured flow around the droplet. Through the measures we took, we were eventually able to have control over four of the most important factors influencing the evaporation behaviour of the droplets.
Over the past few months, we have been able to generate a decent amount of experimental data and have now reached a
state wherein we can start using this data to validate computational models for UWS droplet evaporation. A better reliable computational model will someday help researchers improve the performance of SCR systems, hopefully
   In diesel engines, nitrogen oxides, or NOx as they
are commonly called, are predominantly produced by the reaction of nitrogen with oxygen at high temperatures created by the burning of diesel inside the combustion chamber (thermal NOx).
  






































































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