CFD Optimization of Exhaust Manifold for Large Diesel Engine Aftertreatment Systems 2011-01-2199
To meet EPA Tier IV large diesel engine emission targets, intensive development efforts are necessary to achieve NOx reduction and Particulate Matter (PM) reduction targets . With respect to NOx reduction, liquid urea is typically used as the reagent to react with NOx via SCR catalyst . Regarding to PM reduction, additional heat is required to raise exhaust temperature to reach DPF active / passive regeneration performance window . Typically the heat can be generated by external diesel burners which allow diesel liquid droplets to react directly with oxygen in the exhaust gas . Alternatively the heat can be generated by catalytic burners which enable diesel vapor to react with oxygen via DOC catalyst mostly through surface reactions . The latest technology trend is to combine both mechanisms together so that (1) small-scale burners will enhance exhaust temperature to DOC activation temperature; (2) DOC with HC dosing will raise exhaust temperature to 650 C to achieve active soot reduction . From the scope of system level design, given the added reagents (HC and urea) and heat to exhaust gases, the need arises to achieve even distributions of exhaust gas velocity, good mixing of burner heat with exhaust gas and good mixing of reagents released from HC or urea injectors with exhaust gas, necessitating optimizations of multiphase heat and mass transport phenomena. To meet these multifaceted heat and mass transport targets, geometrical optimizations of subsystems are deemed critical on top of requirements of subcomponents such as burners and injectors. In this paper, the turbo-out manifold box is selected as the subsystem under study for design optimizations. Located between turbo-out ports and exhaust aftertreatment system, the manifold is subject to burner heat injections and diesel liquid injections at multiple locations. The manifold therefore serves the mechanism to distribute exhaust gas, burner heat and diesel vapor. Downstream, the exhaust aftertreatment system incorporates multiple identical flow paths, each encompassing DOC, DPF, urea injectors, SCR, and ASC with complete PM and NOx reduction functionalities. Downstream of the exhaust aftertreatment system, a common stack outlet is created to be open to the environment. The paper discusses general design considerations, performance metrics and methodology for the development of turbo-out manifold with system targets in scope. CFD modeling has been used as the main tool in performing design iterations. Test validations are forthcoming based on these design optimizations.