A Study On Urea Water Solution Spray Characteristics And Structural Parameters Of Urea-SCR

With the tightening of vehicle emission regulations,the problems of NOx and PM emission are becoming increasingly serious.It is difficult to meet the standards through the internal purification technology alone.Optimizing combustion and Selective catalytic reduction?SCR?can greatly reduce pollutants emission on the basis of maintaining diesel engine power performance.With this advantage,this route has become the mainstream technology route for reducing diesel engine emissions in China.However,the conversion efficiency of NOx is low at low temperature,and Urea water solution?UWS?that has not been completely decomposed tends to form deposition which blocks the exhaust pipe.Furthermore,ammonia leakage is another challenge for the application of the technology.In this work,the particle size distribution of different injection parameters was first analyzed by the laser particle size analyzer.Then the morphological changes of the spray in the process of impingement were captured by the industrial camera.Finally,the Urea-SCR system was modeled by using AVL_FIRE software,and the validation of the model was verified by the bench test.The effects of structure parameters on NOx conversion efficiency,wall film formation and ammonia leakage of Urea-SCR system were analyzed.The main research contents are shown as below:?1?The particle size distributions at different injection pressures,injection rates and different sections were analyzed by laser particle size analyzer.The results show that the droplet size can be decreased by increasing the injection pressure,and the number of droplet with larger diameter can be decreased gradually.The droplet diameter increases gradually with the increase of injection rates.As the droplets leave the nozzle,the droplet diameter decreases gradually under the action of air shear force and evaporation.?2?The morphological development of spray wall-impingement with different injection rates,injection pressures,injection heights,injection angles and wall temperatures was analyzed.AVL_FIRE was used to simulate the spray module and analyze the changes of wall film quality,wall film area and wall film formation position under different injection parameters.The results show that the quality and area of the wall film increase with the increasing of injection flow.With the increase of injection pressure,the quality of wall film decreases and the area of wall film increases gradually.With the increase of wall temperature,the quality and area of the wall film decreased,and the spray height and the spray angle have a greater influence on the formation position of the wall film.?3?Based on the AVL_FIRE spray model and the Urea-SCR system bench test,the Urea-SCR system was numerically simulated.The NOx conversion efficiency,ammonia leakage and the wall film formation of the Urea-SCR system were analyzed at different injection angles,expansion angles of catalyst,mixer types and its locations.The result show that with the increase of the injection angle,the impingement position on the wall moves forward,and the NH₃ volume fraction distribution and uniformity at the inlet of the catalyst shows a trend of decreasing after increasing.With the increase of expansion angle of catalytic,NH₃ uniformity coefficient at the catalyst inlet of the SCR system and NOx conversion efficiency both decreased,and NH₃leakage tends to increase.It is reasonable to install the mixer at 100mm²00mm downstream of the nozzle.After the mixer is installed,the NH₃ uniformity coefficient at the inlet of the catalyst can be increased by 40%,the NOx conversion efficiency can be increased by 20%,and the NH₃volume fraction at the outlet of the SCR system can be reduced.