Spray technology has a widely application in all kinds of industrial field, it is important to investigate the spray characteristics of different nozzles which have a decisive effect on spray area. In this study, Phase Doppler Analyzer is used to measure and compare the spray characteristics of air-less nozzle, internal-mix nozzle and external-mix nozzle under different injection conditions. The results show that the increase of compressed air pressure reduces the SMD of air-assisted spray. The uniformity of the droplets size distribution of internal-mix nozzle is also improved by higher compressed air pressure, while that of external-mix nozzle deteriorates. As the axial distance increases, the Sauter Mean Diameter and uniformity index of all nozzles decrease first and then increase due to the collisions and coalescence of droplets. As the radial distance increases, the Sauter Mean Diameter of spray for the air-less nozzle and internal-mix nozzle decreases, and the uniformity index increases. While the change of the Sauter Mean Diameter and uniformity index for the external-mix nozzle are opposite because air and droplets interact intensely.

Nitrogen oxides conversion rate, ammonia slip, and urea crystallization are major problems in the application of the selective catalytic reduction systems of diesel engine. Based on the bench test, the selective catalytic reduction system model was proposed with the code AVL Fire to investigate these problems. The research results indicate that the increasing ammonia/nitrogen oxides feed ratio contributes to reduce the emission of nitrogen oxides. However, the excessive supply of urea-water solution causes the increase of ammonia slip and liquid film. Nitrogen oxides conversion rate reaches a higher level when the exhaust temperature is 450 degrees C. The performance of the system goes worse as the exhaust flow rate increases. The location of the mixer has a huge impact on the flow streamline. The formation of the liquid film and the interaction between the pipe and the droplets depend on the flow streamline. The mixer location is optimized to reduce the quantity of ammonia slip and liquid film based on the numerical study.

The impingement behavior is inevitable during the injection process of urea water solution in selective catalytic reduction aftertreatment system of diesel engine. The impingement contributes to the formation of liquid film on the pipe wall and mixer, and it will block the pipe and reduce the urea water solution decomposition efficiency. In this work, the morphological changes of the spray during the impingement process were captured by the CCD camera. The liquid film development under different injection flow rates, compressed air pressures, injection heights, injection angles and metal wall temperatures were simulated and analyzed by AVL_FIRE. A good agreement with the experiment results is achieved. The results indicate that the liquid film area increases with higher injection flow rate, compressed air pressure, injection height and lower injection angle. According to Kuhnke model, the behaviors of impingement droplets are dominated by the sub-velocity of droplets which is affected by the injection flow rate and injection angle. Higher metal wall temperature can promote the evaporation of the liquid film and droplets, which reduces the liquid film area. It is an effective way to prevent the liquid film formation through rising the metal wall temperature.

Wall temperature distribution and evolution is a crucial factor for solid deposit formation in SCR system. An experiment is established to investigate wall temperature evolution under different SCR spray impingement conditions with the infrared thermography. The wall temperature evolution is ananlyzed based on the theory of the impingement and the heat transfer theory. The research results indicate that the increasing injection rate promotes the cooling rate of the wall and the spread of the low temperature area for the higher spray impingement density. Lower injection height leads to the break up of droplets and accerates the evaporation. The cross-flow causes the asymmetric distribution of wall temperature around the impinging center. The temperature profiles and the impinging center shift downstream along the direction of the cross-flow. The average temperature gradient presents a descent initially and a slight rise subsequently with the increase of the cross-flow velocity. When the initial wall temperature is 225 degrees C, the heat flux reaches the peak and the temperature drops fast, when the initial wall temperature is higher than 260 degrees C, the wall temperature hardly changes due to the leidenfrost effect. (C) 2020 Elsevier Ltd. All rights reserved.