Simulation And Structure Optimization Of Diesel Engine Urea SCR Injection System

The nitrogen oxides (NOx), as one of harmful emission materials generated by diesel engines, have aroused worldwide concern since serious environment problems caused by them. The International Maritime Organization (IMO) has stipulated upper limit value for marine diesel engine NOx emission in three tiers in MARPOL 73/78 Annex VI. Urea-SCR is the most promising technology for marine diesel engine NOx emission reduction. Based on the author’s building and testing experience on the testbed of HD 6105AZLD diesel engine, the following study has been carried out in this thesis, so as to get well distribution of velocity and concentration at the entance section of first catalyst layer.Firstly, this thesis elaborates the rationale of the process of urea solution injection and thermolysis process. Rational mathematical models are chosen and lay foundation for three dimensional numerical modeling.Secondly, high-speed camera, together with equipment of the test bed, is used for recording the process of urea solution injection in the atmospheric environment. The records are compared with three dimensional numerical modeling results as illustration for the simulation method and the results.Thirdly, the same simulation method is used for the process of injection, together with thermolysis process of urea and hydrolysis process of thermolysis products. The simulation results shows that the section uniformity index, defined as the evaluation standard for distribution of velocity field and concentration field at the first catalyst layer entrance section, can’t meet expectation as no changes in parameters of structure and load of the engine.Finally, control variable method is used for the study of different structure of injection system. The simulation of injection, together with thermolysis of urea and hydrolysis of thermolysis products are carried out with changing structure parameters, such as distance between injector and first catalyst layer, holes of injector, length of expanded part, as well as adding guide plate and static mixer to the system. The best combination of structure parameters is acquired and showed that the distribution of velocity field and concentration field can meet expectation.