Catalytic Converters for Automotive Exhaust Applications. Flow Dynamics, Mass Transfer and Optimization
In order to reduce emissions from vehicles powered by an internal combustion engine, an exhaust aftertreatment system is normally mounted on the engine. This aftertreatment system normally consists of one or several catalytic converters. The design of these systems are performed by computerized methods in order to integrate the system in the vehicle in terms of packaging, safety constraints and optimal performance of the systems. In order to evaluate the performance of a given design proposal at an early stage in the development process in a time and cost efficient way, simulations are a preferred tool.
It has been shown that 3D CFD simulations of catalytic converters can provide substantial information about the flow dynamics, which affects catalytic converter performance in terms of pressure drop, emissions and durability. A prerequisite for performing CFD-simulations of monolith catalytic converters is the implementation of a mathematical model of the substrate pressure drop in the CFD-code. Such a model was developed, and it was shown that an additional, length independent term has to be added to the viscous pressure drop. In addition, an increased turbulence in front of the monolith substrate increased the pressure drop in the entrance region of the catalytic converter.
The extension of the CFD-code to also include heterogeneous catalytic reactions, which was done by adding sub-models to the CFD main program, provides the possibility to study how a specific catalytic converter design affects overall reactor performance on a macroscopic level. However, for accurate modelling of the dynamic behaviour of the catalytic converter, knowledge is required about the simultaneous flow, heat and mass transfer and catalytic reactions on a microscopic level. For this purpose a comprehensive 3D CFD-model was developed for a monolith single channel of arbitrary shape. It was shown that the external mass transfer depends on flow, geometry, intra-particle mass transfer and reaction rate, and that the mass transfer rate is varying around the perimeter of the channel, which will give discrepancy between lumped and fully distributed models.
Measurements were performed on an engine bench to study the influence of entrance effects and pulsations on mass transfer. It was found that an increased mass transfer rate was found at the inlet region for a fresh catalyst, which however was diminished with catalyst deactivation. XPS measurements of the surface confirmed this deactivation.
For tuning and development of kinetic models, laboratory reactors are used frequently. It was shown both computationally and experimentally that large temperature gradients can occur in the monolith catalyst sample of such a reactor, and some basic guidelines of how to avoid temperature gradients were established.
Finally, it was shown how an optimisation routine could be used with a CFD-code in order to find the optimal catalytic converter design for any constrained cost function.