A Study On Urea Water Solution Spray Characteristics And Spray Impingement Of Urea-SCR System

The increasingly serious environmental problems and the stringent regulations about emission from vehicles entail more research about the after-treatment technology.The selective catalytic reduction?SCR?technology is widely employed to reduce nitrogen oxides?NO_x?.However,the application of the technology posed some challenges for the researchers to deal with urgently such as the improvement of adequate NO_x conversion rate efficiency,atomization and thermolysis of urea aqueous solution,the avoidance of crystallization formation and ammonia?NH₃?leakage.In this work,the spray characteristics of air-less nozzle,internal-mix nozzle and external-mix nozzle is investigated with Phase Doppler Analyzer under different injection conditions firstly.Then,the infrared thermography is developed to capture the wall temperature distribution and evolution under different spray impingement conditions.Finally,the Urea-SCR system model was proposed with the code AVL FIRE to investigate the influence of NH₃/NO_x feed ratio,exhaust temperature,exhaust flow rate and the location of mixer on the NO_x conversion rate efficiency,liquid film formation and NH₃ leakage.In the present thesis,the general research contents and conclusions are as follows:?1?Laser particle size analyzer is used to measure the spray characteristics of air-less nozzle,internal-mix air-assisted nozzle and external-mix nozzle under different injection conditions.The results show that the atomization of air-assisted nozzle is much better than that of air-less nozzle.As the liquid pressure increases,Sauter mean diameter?SMD?of airless nozzle spray decreases and spray uniformity increases.The increasing air pressure reduces the SMD of air-assisted nozzle spray.As the axial distance increases,the SMD of three nozzles spray decreases first and then increases.As the radial distance increases,the SMD of air-less nozzle and internal-mix nozzle spray decreases,while the SMD of external-mix nozzle spray increases.?2?An experiment is established to investigate wall temperature evolution under different SCR spray impingement conditions with the infrared thermography.The wall temperature evolution is analyzed based on the theory of the impingement regime and the heat transfer regime.The research results indicate that the increasing injection rate promotes the spread of the low temperature area and the increase of the average temperature drop rate.Lower injection height contributes to the drop of the wall average temperature.The temperature profiles and the impinging center shift downstream along the direction of the cross-flow.The drop rate of the average temperature presents a descent initially and a slight rise subsequently with the increase of the cross-flow velocity.The temporal evolution and the spatial distribution of the wall temperature present different characteristics under different initial wall temperature due to the different boiling regimes.When the initial wall temperature is higher than 260?,the wall temperature hardly changes due to the Leidenfrost effect.When the initial wall temperature is225?,the heat flux reaches the peak and the temperature drops fast.The wall temperature drop rate is also larger when the initial wall temperature is below 180?because the spray droplets stick on the wall after impingement which makes the wall wet,and form liquid film.?3?Based on the bench test,the Urea-SCR system numerical model was proposed with the code AVL FIRE to investigate the effects of different parameters on SCR system performance.The research results indicate that the increasing NH₃/NO_x feed ratio contributes to reduce the emission of NO_x.However,the excessive supply of urea water solution causes the increase of NH₃ slip and liquid film.As the exhaust temperature rises,NH₃ slip increases and liquid film decreases,while NO_x conversion rate presents a rise initially and a slight descent.The optimum exhaust temperature is 450?.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 which determines the liquid film formation and the interaction between the pipe and the droplets.As the distance between the nozzle and mixer increases,NH₃ slip increases first and then decreases,and the liquid film mass decreases first and then increases.The optimum distance is range from 114mm to 176 mm based on the numerical study.