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In an attempt to reduce particulate and NOx emissions from Diesel exhaust, the combined DPF and SCR filter is now frequently chosen as the preferred catalyst. When this device functions effectively it saves valuable packaging space in a passenger vehicle. As part of its development, modelling of its emissions performance is essential. Single channel modelling would seem to be the obvious choice for an SCRF because of its complex internal geometry. This, however, can be computationally demanding if modelling the full monolith. For a normal flow-through catalyst monolith the porous medium approach is an attractive alternative as it accounts for non-uniform inlet conditions without the need to model every channel. This paper attempts to model an SCRF by applying the porous medium approach. The model is essentially 1D but as with all porous medium models, can very easily be applied to 3D cases once developed and validated.

Flow maldistribution across automotive exhaust catalysts significantly affects their conversion efficiency. Flow behaviour can be predicted using computational fluid dynamics (CFD). This study investigates the application of CFD to modelling flow in a 2D system consisting of a catalyst monolith downstream of a wide-angled planar diffuser presented with steady flow. Two distinct approaches, porous medium and individual channels, are used to model monoliths of length 27 mm and 100 mm. Flow predictions are compared to particle image velocimetry (PIV) measurements made in the diffuser and hot wire anemometry (HWA) data taken downstream of the monolith. Both simulations compare favourably with PIV measurements, although the models underestimate the degree of mixing in the shear layer at the periphery of the emerging jet. Tangential velocities are predicted well in the central jet region but are overestimated elsewhere, especially at the closest measured distance, 2.5 mm from the monolith.

Removal of NOx from a light-duty diesel automotive exhaust system can be achieved by SCR reactions using aqueous urea spray as the reductant. Measurements of emissions from such a system are necessary to provide data for CFD model validation. A test exhaust system was designed that featured an expansion can, nozzle and diffuser arrangement to give a controlled flow profile to define an inlet boundary for a CFD model and to approximate to one-dimensional flow. Experiments were carried out on the test exhaust using injection of either ammonia gas in nitrogen or aqueous urea spray. Measurements were made of NO, NO₂ and NH₃ at inlet to and exit from the SCR using a CLD analyzer. The NO and NO₂ profiles within the bricks were found by measuring at the exit from different length bricks. The spray and gas measurements were compared, and insights into the behavior of the droplets upstream and within the bricks were obtained.

This study examines the effect of pulsating flow on the flow distribution through contoured substrates. Three ceramic contoured substrates of equal volume were assessed. Two of the substrates were cone shaped with different cone angles and one had a dome shaped front face. The flow distribution was measured for a range of flow rates and pulsation frequencies. Computational Fluid Dynamics (CFD) simulations were also performed. It is shown how a contoured substrate can provide improvements in flow uniformity and that they are less sensitive to changes in flow rate and pulsation frequency when compared to the case of a standard substrate. Improvements in the prediction of flow distribution are reported when substrate “entrance effects” are accounted for.

Performance improvements of automotive catalytic converters can be achieved by improving the flow distribution of exhaust gases within the substrate. The flow distribution is often assumed to be adequately described by measurements obtained from steady flow rigs. An experimental study was carried out to characterise the flow distribution through the substrate of a close-coupled catalytic converter for both steady and pulsating conditions on a flow rig and on a motored engine. Computational fluid dynamic (CFD) simulations were also performed. On the flow rig, the flow from each port was activated separately discharging air to different regions of the substrate. This resulted in a high degree of flow maldistribution. For steady flow maldistribution increased with Reynolds number. Pulsating the flow resulted in a reduction in flow maldistribution. Different flow distributions were observed on the motored engine when compared to composite maps derived from the rig.

With the increasingly widespread use of catalytic converters for meeting exhaust emission regulations, considerable attention is currently being directed towards improving their performance. Experimental analysis is costly and time consuming. A desirable alternative would be a computational model based on established numerical techniques. To this end a transient three-dimensional model has been developed using a commercial CFD code. It simulates the fluid dynamics, chemical kinetics and heat and mass transfer that takes place in catalysts and their associated assembly. As a result the model can be used to predict important performance parameters such as conversion efficiency, incurred pressure drop and the thermal environment.

The purpose of this paper is to characterise the flow field through the inlet valves, and tumble to swirl conversion tube of a steady state flow rig using HWA. LDA and CFD techniques. Three cylinder head configurations were developed which were found to produce three levels of barrel swirl motion. The swirl precesses around the central core of the conversion tube at all degrees of swirl. Varying degrees of swirl produced differing axial velocity profiles, and flow reversal occurred in the central core of the conversion tube for the high swirling case. The results obtained for this study indicate that care must be exercised when deducing the barrel swirl ratio for real engines from steady flow rig analysis.

An experimental and theoretical investigation has been performed on the flow and pressure loss in axisymmetric catalytic converters and isolated monoliths under steady, isothermal flow conditions. Monolith resistance has been measured with a uniform, low turbulence, incident flow field. It has been found that the pressure loss expression for fully developed laminar flow is a good approximation to observations for x+ greater than 0.2. However, for x+ less than 0.2 the additional pressure loss due to developing flow is no longer negligible and a better approach is to use the correlation proposed by Shah (16). From experimental studies on the axisymmetric catalytic converters non-dimensional power law relationships have been derived relating maldistribution and pressure drop to expansion length, Re, and monolith length. These expressions are shown to generally fit the data well within ±5%.

Conversion efficiency, durability and pressure drop of automotive exhaust catalysts are dependent on the flow distribution within the substrate. This study examines the effect on flow distribution using substrates which feature contoured front faces. Three ceramic contoured substrates of equal volume were assessed. Two of the substrates were cone shaped with different cone angles and one had a dome shaped front face. Pressure drop and flow distribution was measured for a range of flow rates and substrate positions. Computational Fluid Dynamics (CFD) simulations were also performed to provide insight into flow behaviour. It is shown how a contoured substrate can provide improvements in flow uniformity and pressure drop when compared to the case of a standard non-contoured substrate.