A Visualized Experimental Study On The Spray Of Selective Catalytic Reduction Unit For Marine Low-speed Engines

In recent years,with the vigorous development of Marine transportation industry,the environmental pollution caused by ships is also increasingly intensified,the governments of various countries and the International Maritime Organization are speeding up the formulation and improvement of the relevant emission standards and policies.It is imminent to carry out research on the control technology of ship’s gaseous pollutant emission.Selective catalytic reduction is one of the most effective methods to reduce NOx emissions.The reduction rate of NO_Xcan reach more than 95%,which can better meet the strict requirements of NOx emissions in some shipping areas.At present,it has been gradually promoted and applied on ships.In order to further explore the selective catalytic reduction technology,this study conducted spray characteristic experiment on the atomization effect of urea aqueous solution.Due to the high volatility and corrosiveness,water with similar physical properties to urea aqueous solution was used to replace it in this experiment.In this study,an air-assisted urea nozzle was used,and a self-built direct photography experiment platform was used to obtain the macroscopic distribution image of the spray longitudinal section,and the obtained spray image was processed through the MATLAB program to obtain the specific value of the spray cone angle,and compared the diffusion effects of three static mixers;used Phase Doppler Anemometry PDA analysis test platform to obtain the diameter of the particles at different measurement points of the spray and speed,explored the influence of different spray water pressure and spray air pressure on spray characteristics.The experimental results show that the initial flow form of the spray is spiral,and the spray form is stable under the combined action of the jet flow and the external environmental resistance.At this time,the center area of the spray is composed of water mist with small particle size,and the wrinkles generated at the outer boundary of the spray are gradually increased,and the mixing effect with the ambient gas is enhanced.With the development of the spray,the cone Angle of the spray will first increase,then decrease,and finally stabilite.With the increase of spray water pressure,the spray cone Angle slightly increases,the spray atomization effect becomes worse under the action of airflow,and the number of large particle size droplets increases.With the increase of injection pressure,the spray cone Angle decreases slightly,and the atomization effect becomes better under the action of airflow.Compared with no mixer,the spray diffusion degree is improved to different degrees after the mixer is added,and the effect of the rotating vane mixer on the spray diffusion is the most obvious,followed by the tapering vane type,and finally the cross vane type.In the phase Doppler particle experiment,the most of the particles are within 30μm-90μm,and about 90%of the particle size is within 100μm,indicating that the overall distribution of spray particle size is relatively uniform.On the same cross section,the velocity of the measuring point located in the center area of each section is about 25m/s.As the distance from the measuring point to the central axis is further,the velocity of the droplet is smaller.The velocity of the measuring point located in the spray boundary area tends to be 0m/s,and the particle size of the droplet becomes lager.Along the direction of the spray axis,the droplet velocity gradually decreases,and the droplet diameter tends to increase.With the increase of spray water pressure,the particle size of each section of spray increases,but the velocity decreases slightly.With the increase of the injection pressure,the particle size of the spray decreases,the speed increases,and the atomization effect is improved.