Study On Airflow Uniformity In Marine SCR Backflushing System

As the development of the economy,the economic globalization is accelerating,shipping has gradually become the main tool for world trade.However,the pollution caused by the marine diesel engine exhaust gradually aroused the concern of IMO and all countries.All the countries are all devoted to the research of marine energy-saving and emission reduction devices.Due to their super-denitrification efficiency,selective catalytic reduction devices are being accepted by various countries.However,during the process of the usage of selective catalytic reduction device,the solid particles are adhered to the surface of the ceramic catalystfilter to block the filtration channel.The traditional solution is using the nozzle with equal diameter to blow back,However,this would cause a non-uniform flow of backflush gas at the exit of the nozzle and an uneven flow of gas at the exit of the independent filter unit,which would seriously affect the filtration efficiency and service life of the filter.Therefore,this paper mainly studies how to improve the uniformity of backflushing airflow.First of all,this paper has designed a kind of columnar catalytic filter and a variety of tapered catalytic filters according to the ceramic filter design guidelines.And a nozzle blowing model for changing the nozzle diameter was designed according to the theoretical calculations.Secondly,The blowback tube model with variable diameter and the blowback tube model with equal diameter were used for backflush simulation analysis.In addition,the influence of inlet velocity of the injection tube,the length of the nozzle,and the cross-sectional area of the nozzle on the uniformity of the blowback was studied.Finally,using candle-like and cone-shaped catalyst filters for backflushing simulation analysis,the effect of different taper on backflush uniformity of the individual nano-ceramic catalytic filter unit was studied.it can be obtained that the outlet flow of the nozzle with equal diameter blowback model gradually increases from front to back,While the outlet flow of the nozzle with variable diameter blowback model is uniform.But the nozzles at both ends of the blow tube also have deviation from the average flow.Accroding to the flow correction methods,the nozzle outlet flow could be even more uniform by constantly changing the nozzle size.Different boundary conditions and dimensions also have affects on the uniformity of the backflush:the smaller the inlet velocity of pipe,the larger the nozzle length,the smaller nozzle area,the nozzle outlet flow is more even.By analyzing the velocity vector of the nozzle at different positions,it can be concluded that the streamline of the nozzle close to the injection tube velocity is unevenly distributed and the streamline at the farthest end of the injection tube is even.Finally,cylindrical and tapered catalytic filters were used for backflush simulation analysis to obtain the velocity distribution of the outer surface of the ceramic catalytic filter at different stop diameters,It is found that:the outer surface of the backflush airflow velocity distribution is the most uniform one when the the stop-end diameter is 10mm.By analyzing its velocity profile,it can be concluded that the internal velocity of the filter near the stop-end is small and the static pressure is large,so the air velocity through the filter is large.