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Roberts, C. A.
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Methodology for Modelling a Combined DPF and SCR Catalyst with the Porous Medium Approach in CFD

SAE International Journal of Engines

Coventry Univ.-S. F. Benjamin, C. A. Roberts
  • Journal Article
  • 2014-01-2819
Published 2014-10-13 by SAE International in United States
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. The model is described in full in this paper and values for all the key parameters are presented. The filter is assumed to collect soot in the inlet…
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Modelling of NOx Conversion in a 1D Diesel Engine Exhaust SCR Catalyst System under Transient Conditions Using Ammonia Gas as the Reductant

Coventry University, UK-S. F. Benjamin, M. Gall, C. A. Roberts
Published 2012-09-10 by SAE International in United States
Use of selective catalytic reduction technology is the most popular strategy for removing NOx from lean diesel exhaust. The reductant is essentially ammonia and this has been supplied as a spray of urea droplets, but more recently alternative technology where ammonia gas is released from a storage medium has become a viable alternative. Experiments have been carried out on an engine test rig run to steady state conditions using NOx composed of either 25% or 50% of NO₂, with ammonia gas as the reductant. This was a 1D study where a long 10 degree diffuser provided uniform temperature and velocity profiles to the SCR catalyst brick. Under the transient conditions that occur during drive cycles, the dosing of the ammonia can deviate from the optimum. In this study, the dosage rate of ammonia was held at a fixed value, while the engine load was varied. The variation was from low load to high load and back down for various time periods, based on the rates of change that are typical of those in engine drive…
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Tuning the Standard SCR Reaction Kinetics to Model NO Conversion in a Diesel Engine Exhaust SCR Catalyst System Under Steady State Conditions in 1D and 3D Geometries Using Ammonia Gas as the Reductant

Coventry University, UK-S. F. Benjamin, M. Gall , C. A. Roberts
Published 2012-09-10 by SAE International in United States
Removal of NOx from lean diesel exhaust can be achieved by the use of selective catalytic reduction technology. The supplied reductant is often ammonia, either as urea or as ammonia gas released from a storage medium. Experiments have been carried out on an engine test rig run to steady state conditions using NOx composed mainly of NO, with ammonia gas as the reductant. This was essentially a 1D study because a long 10 degree diffuser was used to provide uniform temperature and velocity profile to the SCR catalyst brick in the test exhaust system. Tuning of the standard reaction, the NO SCR reaction, in a kinetic scheme from the literature and adjustment of the ammonia adsorption kinetics achieved improved agreement between the measurements and CFD simulations. This was carried out for studies at exhaust gas temperatures between 200 and 300°C. The effect of diffuser geometry upstream of the SCR catalyst on NOx conversion was then investigated experimentally using a 180 degree sudden expansion as a 3D diffuser. These were also steady state studies with the…
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Spatial Conversion Profiles within an SCR in a Test Exhaust System with Injection of Ammonia Gas Modelled in CFD using the Porous Medium Approach

Coventry University-M. P. Sturgess, S. F. Benjamin, C. A. Roberts
Published 2010-10-25 by SAE International in United States
Modeling of SCR in diesel exhaust systems with injection of urea spray is complex and challenging but many models use only the conversion observed at the brick exit as a test of the model. In this study, the case modeled is simplified by injecting ammonia gas in nitrogen in place of urea, but the spatial conversion profiles along the SCR brick length at steady state are investigated. This is a more rigorous way of assessing the ability of the model to simulate observations made on a test exhaust system. The data have been collected by repeated engine tests on eight different brick lengths, all which were shorter than a standard-sized SCR. The tests have been carried out for supplied NH₃ /NOx ratios of a 1.5, excess ammonia, a 1.0, balanced ammonia, and a 0.5, deficient ammonia. Levels of NO, NO₂ and NH₃ have been measured both upstream and downstream of the SCR using a gas analyzer fitted with ammonia scrubbers to give reliable NOx measurements. A CFD model based on the porous medium approach has…
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Experimental Study of SCR in a Light-Duty Diesel Exhaust to Provide Data for Validation of a CFD Model Using the Porous Medium Approach

Coventry Univ.-C. A. Roberts, S. F. Benjamin
Univ. Teknikal Malaysia Melaka-N. Tamaldin
Published 2010-04-12 by SAE International in United States
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. Approximately half to three-quarters of the droplets from the spray remained as droplets at entry to the first brick.…
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A NOX Trap Study Using Fast Response Emission Analysers for Model Validation

Coventry University-A. J. Alimin, C. A. Roberts, S. F. Benjamin
Published 2006-04-03 by SAE International in United States
Lean burn after treatment systems using NOX traps for reducing emissions from diesel exhausts require periodic regeneration after each storage stage. Optimising these events is a challenging problem and a model capable of simulating these processes would be highly desirable. This study describes an experimental investigation, which has been designed for the purpose of validating a NOX trapping and regenerating model. A commercial computational fluid dynamics (CFD) package is used, to model NOX trapping and regeneration, using the porous medium approach. This approach has proved successful for three way catalysis modelling. To validate the model a one-dimensional NOX trap system has been tested on a turbocharged, EGR cooled, direct injection diesel engine controlled with an engine management system via DSPACE. Fast response emission analysers have been used to provide high resolution data across the after-treatment system for model validation. Measurements show CO is the primary reductant. After the trap NO and NO2 spikes (NOX slippage) were observed both at the beginning and end of the regeneration period. The former is believed to be due to…
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Pulsating Flow Maldistribution within an Axisymmetric Catalytic Converter - Flow Rig Experiment and Transient CFD Simulation

Coventry University-Z. Liu, S. F. Benjamin, C. A. Roberts
Published 2003-10-27 by SAE International in United States
This paper investigates the flow maldistribution across the monolith of an axisymmetric catalyst assembly fitted to a pulsating flow test rig. Approximately sinusoidal inlet pulse shapes with relatively low peak/mean ratio were applied to the assembly with different amplitudes and frequencies. The inlet and outlet velocities were measured using Hot Wire Anemometry. Experimental results were compared with a previous study, which used inlet pulse shapes with relatively high peak/mean ratios. It is shown that (i) the flow is more maldistributed with increase in mass flow rate, (ii) the flow is in general more uniformly distributed with increase in pulsation frequency, and (iii) the degree of flow maldistribution is largely influenced by the different inlet velocity pulse shapes. Transient CFD simulations were also performed for the inlet pulse shapes used in both studies and simulations were compared with the experimental data. For inlet pulse shapes with low peak/mean ratio, CFD predictions matched measurements fairly well in the bulk central region of the monolith. For inlet pulse shapes with high peak/mean ratio, agreement was less satisfactory. This…
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A Coupled 1D/3D Simulation for the Flow Behaviour inside a Close-Coupled Catalytic Converter

Coventry University-Z. Liu, S. F. Benjamin, C. A. Roberts, H. Zhao, A. Arias-Garcia
Published 2003-05-19 by SAE International in United States
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.
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A Study of the Effect of Flow Pulsations on the Flow Distribution within Ceramic Contoured Catalyst Substrates

Coventry Univ.-S. F. Benjamin, C. A. Roberts, J. Wollin
Published 2001-05-07 by SAE International in United States
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.
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Simulating Heat Transfer in Catalyst Substrates with Triangular and Sinusoidal Channels and the Effect of Oblique Inlet Flow

Centre for Automotive Engineering Research and Technology, Coventry University-E. G. W. Day, S. F. Benjamin, C. A. Roberts
Published 2000-03-06 by SAE International in United States
Heat transfer in automotive exhaust catalyst systems with metallic substrates is modeled using a commercial Computational Fluid Dynamics (CFD) code. The substrate channels are modeled by approximating their geometry as both triangular and sinusoidal. The effect of the packing arrangement of adjacent channels is investigated. The effect of the angle of the flow entering ceramic substrate monoliths on the localised heat transfer is also studied and the related implications for catalyst aging and light off deduced.
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