Influence of the wall temperature of exhaust pipe on the atomization effects of urea water solution

Abstract: Diesel engines are widely used in agricultural machinery and engineering vehicles, due to the high performance of thermal efficient power. However, the NOx emission from diesel engines has posed a great threat to the environment at present. Selective Catalytic Reduction (SCR) can be prevalent for NOx emission control in diesel. Nevertheless, the NOx conversion efficiency can be confined to the blockage of the tailpipe, where the low atomization of urea water solution at the low exhaust temperature is easy to form solid urea deposits. This study aims to explore the influence of wall temperature on the atomization of urea water solution, in order to improve the atomization of urea water solution at the low exhaust temperature, and then reduce the solid urea deposits for a better NOx conversion efficiency of the SCR system. A spray/wall impingement test rig was built for the urea water solution. An intelligent temperature controller was used to provide a constant temperature wall. A high-speed camera was used to record the impact of urea water solution on the wall at different temperatures. A laser analyzer was used for the variation trend of droplet size near the wall, where the urea water solution collided with the wall at wall temperatures of 20, 100, 150 and 200 °C. The atomization of urea water solution was compared at different wall temperatures, according to the Sauter Mean Diameter (SMD) and the solid residues. The results show that there were some significant differences in the atomization of urea aqueous solution under wall temperatures. The intensive atomization was achieved with the increase in wall temperature. Specifically, the SMD tended to be stable at the end of the spray process at room temperature (20 °C). The droplet size gradually tended to be stable after 10 ms with the increase in wall temperature. The spray tended to be stable in the middle of the injection process at the wall temperature of 150 °C. The SMD tended to be stable in the spatial range at the wall temperature of 200 °C. At the initial time of the impingement between the urea water solution and the wall, the droplet size SMD values were 120-180 μm and 100-120 μm in the main spray and reflection zone at 20 °C, respectively. Once the wall temperature was 100 °C, the droplet size SMD values were 120-140 μm and 90-110 μm in the main spray and reflection zone, respectively. The droplet size SMD values were 100-120 μm and 80-100 μm in the main spray and reflection zone at the wall temperature of 150 °C, respectively. By contrast, the droplet size SMD values were 50-70 μm and 30-50 μm in the main spray and reflection zone at the wall temperature of 200 °C, respectively. It infers that the droplet size decreased to be stable with the development of the liquid beam. The droplet size SMD values tended to be stable faster in the space, as the wall temperature increased. The solid residues on the wall were represented in four stages: aqueous urea solution, solid urea deposits, molten urea deposits, and a small amount of cyanate polymer. The wall liquid film that was formed by the urea water solution gradually decreased with the increase of wall temperature. No liquid film was observed at the high wall temperature. The findings can provide a theoretical basis for the better atomization of aqueous urea solution and solid urea deposits in the tailpipe at low exhaust temperature.