| The pollution caused by marine diesel engines has been paid more and more attention by all countries.To meet the Tier III emission standard for NOx emissions from marine diesel engines,the use of SCR(Selective Catalytic Reduction)system is currently a feasible technical routeDuring the operation of SCR system,poor NOx conversion efficiency at low temperature and urea crystallization are the technical problems.These problems are related to SCR urea spray and catalytic reactor.In this paper,urea spray and crystallization risk issues are mainly studied.The exhaust emission performance of 4L20 diesel engine is tested and the basic data of the diesel engine is obtained for the design of SCR reactor.The vanadium-based catalyst is selected and the catalyst size is calculated.A 4L20 diesel engine SCR reactor fluid calculation model was established,and the urea spray flow field was calculated by means of commercial CFD(Computational Fluid Dynamics)calculation software.It is found that the spray atomization angle of the urea solution and the elbow of the exhaust pipe have a great influence on the uniformity of the flow and concentration field of urea spray,which is a key factor affecting the risk of urea crystallization in the exhaust pipe.The urea atomization angles of 30°,40°,50° and 600 was simulated.The results show that the flow field uniformity at 500 spray angle is the best.The non-uniformity of flow and concentration field of urea spray is caused by the centrifugal action of the gas stream in the elbow of the exhaust pipe,which increases the risk of urea crystallizationThe spray flow field was simulated after adding a diversion device at the elbow of the exhaust pipe.The results show that the addition of the mixer device at the elbow of the exhaust pipe has a great effect on the uniformity of the urea spray flow field.At the entrance of reactor,the uniformity index of the velocity and the NH3 concentration reach over 0.95,the average temperature of the urea film and the evaporation rate of the urea solution are increased,and the thickness of the urea liquid film is reduced,so he risk of urea crystallization can be reduced.In addition,the elbow mixer of the exhaust pipe can refine the atomized droplets,make them evenly distributed,and improve the uniformity of NH3 distribution.The research results can be used in the optimization design of SCR system pipeline. |
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