Heat transfer during spray/wall interaction with urea water solution: An experimental parameter study

Understanding the spray/wall interaction of urea water solution in automotive selective catalytic reduction (SCR) of NOx is crucial for robust system performance. The spray impingement of aqueous urea solution on walls at temperatures from 120 to 420 degrees C is investigated experimentally by using high-speed and infrared thermographic recordings. To evaluate wall wetting and spatially resolved heat transfer, the evaporated mass fraction during spray/wall interaction is described quantitatively through an energy balance. Heat transfer at different wall temperatures shows typical non-linear characteristics analogous to those known from pool boiling experiments (Nukiyama boiling curve). The performed extensive parameter study on the influence of injected fluid, injection duration, injection pressure, wall thickness and wall material reveal significant shifts of the boiling regime boundaries and the related characteristic temperatures, e.g. temperature of critical heat flux and Leidenfrost temperature. These basic findings help to understand spray/wall interaction phenomena and give hints for improvement of automotive SCR system design by taking into account the change of the injection system parameters and the choice of advantageous wall material properties.