FULL-SCALE CFD MODELLING OF AFTER-TREATMENT SYSTEMS FOR INTERNAL COMBUSTION ENGINES

The design of an after-treatment system for internal combustion engine is a complex task, which requires the optimization of different aspects to meet the requirements in terms of emission control, fuel saving, durability and engine performances. Moreover, in the last years, as a response to the new more restrictive emission legislation, new devices (SRC, DOC, NOx-trap, DPF/GPF) have been progressively introduced, increasing the complexity and the dimension of the system. As an additional aspect, the trend towards the design of more compact vehicles have determined a progressive reduction of the space available in the engine compartment, making even more difficult the optimization of the after-treatment system.

In this scenario, the aid of CFD simulation tools can be a valuable support for the design of the after-treatment system, in order to describe and predict the complex physical phenomena occurring in the device. In particular, the intrinsic complexity of the problem to be modelled is related to different aspect: a) the flow is strongly unsteady during real drive conditions; b) chemical reactions occur in both homogeneous phase (gas reactions) and heterogenous phase (catalytic surface reactions); c) chemical/diffusive phenomena are strictly related to the evolution of the thermal conditions of the substrate; d) different substrate geometry (e.g. traditional honeycomb or innovative open-cell foams) can be employed.

In this work, a computational framework for the CFD simulation of the after-treatment system has been implemented on the basis of the OpenFOAM code. In particular, it is based on a multi-region approach, where overlapping meshes, describing fluid and solid regions, are employed in order to model the presence of porous substrates. Specific models are implemented in order to couple fluid and solid regions in terms of heat-transfer and mass-transfer. Catalytic reaction model is introduced in order to describe the chemical surface reactions occurring on the washcoat of the porous substrate. Particular treatment is implemented for the solution of the thermal balance of the monolith, distinguishing two different solid zone: the washcoat zone, where catalytic reactions occurs with heat release/adsorption, and the monolith zone, responsible for the conductive heat transfer towards the external canning of the device. The simulation model has been tested on different configurations and validated resorting to experimental data.