Stephen F. Benjamin

Modelling the flow distribution through automotive catalytic converters

Abstract Conventional computational fluid dynamics (CFD) methods for simulating the flow through automotive exhaust catalysts assume a monolith resistance based on one-dimensional laminar flow. This underpredicts the flow maldistribution in the monolith. Incorporation of an additional pressure loss accounting for entrance effects improves predictions for the maximum flow velocity within the substrate.

Automotive catalyst warm-up to light-off by pulsating engine exhaust:

Abstract This paper presents the results of studies on a range of different catalyst substrates warmed by engine exhaust. Engine speeds were in the range 1200–3000 r/min. One substrate was non-washcoated, four were washcoated but non-reactive and four were washcoated and reactive. The temperature at four locations within the non-reactive substrates was measured. The reactive substrates were warmed to light-off by the pulsating exhaust flow from an engine running fuel rich of stoichiometric. Both substrate temperatures and hydrocarbon conversion were measured. Predicted temperatures and conversion were obtained from a one-dimensional computational fluid dynamics (CFD) model. The model was based on the porous medium approach and incorporated a simple three-way chemical scheme. Comparison was made of measurements with predictions, with particular reference to the time taken to achieve light-off. Pulsing flow CFD predictions were found to be almost identical to steady-flow predictions for the conditions investigated. The CFD predictions were found to be in fair agreement with the engine test results, but using kinetic rate constants higher than previously reported values.

An experimental investigation of oblique entry pressure losses in automotive catalytic converters

Abstract This study investigates oblique entry pressure loss in automotive catalyst monoliths. Experiments have been performed on a specially designed flow rig using different lengths of monolith (17—100 mm) over a range of Reynolds number and angles of incidence (0–75°). Losses were found to be a function of Reynolds number and angle of incidence and a general correlation has been derived. Computational fluid dynamics predictions of the flow distribution across axisymmetric catalyst assemblies have been performed. Incorporating the oblique entry loss provided much better agreement with experimental data with the assumption that such losses were constant above an angle of incidence of 81°.

A Coupled 1D/3D Simulation for the Flow Behaviour inside a Close-Coupled Catalytic Converter

This paper describes the coupling of a 1D engine simulation code (Ricardo WAVE) to a 3D CFD code (STAR-CD) to study the flow behaviour inside a Close-Coupled Catalytic converter (CCC). A SI engine was modelled in WAVE and the CCC modelled in STAR-CD. The predictions of the stand-alone WAVE model were validated against engine bed tests before the coupled 1D/3D simulations were performed at 3000 RPM WOT for both motored and firing conditions. The predicted exhaust velocities downstream of the catalyst monolith in the coupled simulations matched fairly well with Laser Doppler Anemometry (LDA) measurements.

Warm up of an automotive catalyst substrate by pulsating flow: a single channel modelling approach

Abstract Rapid warm up of an automotive catalyst substrate is important for early light off. This work considers the results from a model of warm up in a single channel. The mass flow is pulsating with high amplitude, about 75% of mean flow, but without flow reversal. The flow regime is laminar within the channel. Pulsations occur with frequency in the range 16–100 Hz, and are important in close-coupled systems where the catalyst is located near to the engine and where the rate of rise of gas inlet temperature with time is rapid, about 15 K/s. The use of a single channel model with conjugate heat transfer enables the heat transfer coefficient to be evaluated and compared with results from steady flow simulations. The value of the augmentation factor based on heat flux is found to be less than unity. The value of the augmentation factor based on heat transfer coefficient depends on the method for calculating the mean heat transfer coefficient, but is generally less than unity. The changes caused by pulsations will be small in practical systems. Changes in wall temperature found in the simulations are the result of the cumulative effect of changes in the mass flow rate.

A Study of the Flow Performance of Ceramic Contoured Substrates for Automotive Exhaust Catalyst Systems

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.

A Comparison of Steady, Pulsating Flow Measurements and CFD Simulations in Close Coupled Catalysts

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. For the engine study significantly more flow activity was observed at the periphery of the substrate, each port contributing to the net flow. The results suggest that strong port interactions occur. CFD simulations showed qualitative agreement with measurements but underestimated the flow maldistribution.

Warm up of automotive catalyst substrates: Comparison of measurements with predictions

Abstract An understanding of the warm up of automotive catalysts is important for accurate prediction of light off. This work describes some experimental studies on warm up in the absence of chemical reactions. In parallel with these experiments, the temperatures of the warmed substrate have been predicted. The problem of warm up is a simple one, capable of analytical description, but since the complete problem of catalyst performance with chemical reactions will ultimately require CFD coding, the simple case is dealt with in this way to form the basis of a more complete model. The studies have found measured heat transfer coefficients which are in the range 15 to 20 W/(m2K) for metallic substrates with sinusoidal channels. This is much lower than standard Nu values suggest. The predictions have also illustrated the significance of the heat transfer coefficint in obtaining accurate agreement with measurements in the simple case of warm up.

Lean NOx trap study on a light-duty diesel engine using fast-response emission analysers

Abstract Storage and regeneration events have been studied using fast-response emission analysers (∼10 ms) for a lean NO x trap (LNT) fitted to a light-duty diesel engine. Tests were conducted at both low and high exhaust temperatures for various storage and purging periods. The use of fast-response analysers has provided detailed information during the short regeneration periods and as combustion switched between rich and lean operating modes. It has also enabled quantification of the storage, reduction, and overall conversion efficiencies, as well as the instantaneous trapping efficiency. With exhaust temperatures of 250 °C, storage efficiency was low (∼30 per cent). During purging, two distinct NO spikes (breakthroughs) were measured downstream of the LNT at the beginning and end of regeneration. For this LNT, the primary reducing mechanism is CO reacting with NO, but CO reacting with ceria and/or water, the water—gas shift reaction, is suspected. With exhaust temperatures of 400 °C, storage efficiencies were high (∼80–90 per cent), except for the long-storage/short-purge case when the trap was near saturation. NO x breakthrough during enrichment depended on storage and purge periods and the availability of catalyst sites. NO2 breakthrough was also observed at the end of regeneration as the combustion switched to lean operation. Generally, for the high-temperature case on this LNT, the primary reducing mechanism is CO reacting with NO2.

Warming automotive catalysts with pulsating flows

Abstract Temperatures of an automotive catalyst substrate warmed by convection pre-light-off have been measured. Direct comparison has been made of warm-up by steady and pulsating flow for a one-dimensional flow case. The 32 Hz pulsating mass flow did not feature flow reversal. Pulsations were achieved by interruption of the airflow by a rotating disc. Very small differences between steady and pulsating cases were observed because the effect of mass flow pulsations on heat transfer is minimal. Two different computational fluid dynamics methods were used to predict temperature. A one-dimensional porous medium model, which required input of an assumed heat transfer coefficient, was compared with a single-channel model. Predictions agreed closely and there was also qualitative agreement with measurements. Similar mass flow pulsations in the range 32-100 Hz have been studied for a case with a larger diameter automotive catalyst supplied via a conical diffuser. The radial flow distribution is controlled by pulsation frequency and the effect of frequency on temperature at different depths in the substrate was observed experimentally. Pulsations will affect catalyst warm-up in practical systems because of their effect on flow distribution, rather than on heat transfer.