| 1. |
- Chanda Nagarajan, Pratheeba, 1978
(författare)
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Flow Maldistribution in Exhaust Aftertreatment Systems - Numerical Simulations
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To comply with the stringent emission norms under every driving condition, including the coldstart conditions and Real Driving Emissions (RDE) tests, accurate models that capture the spatial and temporal variation of flow distribution information in Exhaust AfterTreatment Systems (EATS) are required. This study is aimed at characterizing and quantifying flow distribution in EATS under transient conditions with realistic geometry that illustrates the complex nature of the flow conditions. Catalytic converters are employed to control emissions. Space limitation in the exhaust line creates non-uniform flow in terms of flow through bends and dead volumes. This limits the performance of the EATS. The flow from the engine does not proceed uniformly to the EATS, creating flow maldistribution at the inlet of EATS. Due to the non-uniform flow, the velocity, temperature and concentrations at the exit are functions of the flow distribution at the inlet. Reactor models that predict conversion of emissions require this information for accurate prediction of conversion of emission gases at the tail pipe. To this end, transient Computational Fluid Dynamics (CFD) simulations are performed to understand the evolution of the flow profiles in the catalytic converter, both under non-reactive and reactive conditions. The flow from the engine is turbulent. Hence, Unsteady Reynolds Averaged Navier Stokes (URANS) equation is used solve for the transport equations to obtain the profiles of velocity, temperature, and concentration in the catalytic converter. The turbulence closure is achieved by using k - w model. The catalyst is modeled as porous media. This thesis presents the results of both non-reactive and reactive simulations, to illustrate the importance of flow and temperature non-uniformity and pulsations respectively. Flow uniformity index is used to characterize the extent of variation of the flow parameters in any catalyst plane. In addition to the uniformity index, contours and histograms are employed to demonstrate the non-uniform flow field. The specification of inlet fluctuations is also required to emulate real time flow to a catalytic converter. The contours of velocity and histograms show that there is significant distribution of the flow variables at the catalyst outlet. This illustrates that a single channel model is very ideal, and it requires flow distribution information to predict tail pipe concentrations accurately. The inclusion of flow distribution information in EATS model can make the model more accurate. These models can be then used in control and monitoring of emissions from automobiles. This will take a step closer towards Zero Emissions.
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| 2. |
- Chanda Nagarajan, Pratheeba, 1978, et al.
(författare)
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Numerical Assessment of Flow Pulsation Effects on Reactant Conversion in Automotive Monolithic Reactors
- 2022
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Ingår i: Catalysts. - : MDPI AG. - 2073-4344. ; 12:6
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Tidskriftsartikel (refereegranskat)abstract
- Highly transient engine-out emissions imply significant challenges for the optimization and control of automotive aftertreatment systems, motivating studies of the effects of flow pulsations on the system behavior. In this work, an axisymmetric aftertreatment system with a first-order reaction in the monolith section is chosen to demonstrate the role of pulsations on the time-averaged conversion at the exit. Reactive computational fluid dynamics simulations under transient conditions are performed by applying the SST k-ω turbulence model along with a reactant species balance equation and a porous medium description of the catalyst. Four different types of temporal velocity variations (constant, step-like, sawtooth and sinusoidal) are applied at the inlet. Additionally, the corresponding fluctuations driven by a prescribed inlet pressure are also investigated. It was found that the fluctuations in the incoming flow affect the transient response of the monolith, the time-averaged conversion, the evolution of the flow uniformity index and the dispersion downstream of the catalyst. It is also shown that the retention time distribution is modulated by the pulsations and that the mixed-cup conversion span is different for geometrically identical systems having the same velocity span if the fluctuation characteristics are different. In conclusion, simulations of phenomena that depend on time-resolved boundary conditions from experiments require proper characterization of fluctuations present in the real-world systems; otherwise, the method of recreating the signal at the boundary may influence the obtained results.
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| 3. |
- Chanda Nagarajan, Pratheeba, 1978
(författare)
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Reduced Order Models For Exhaust AfterTreatment Systems - Coupling Modeling, Simulations and Chemometrics
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of hydrocarbon based fuels and the high temperatures generated during combustion processes are major sources of gaseous pollutants that are detrimental to human health and the environment. Emission legislation is increasingly becoming stringent to mitigate the harmful effects of emissions. Exhaust Aftertreatment systems are a group of catalytic devices that convert these harmful emissions into products like carbon dioxide and nitrogen. Space limitation in the exhaust line creates nonuniform flow in terms of flow through bends and dead volumes. This limits the performance of the EATS. The flow from the engine does not proceed uniformly to the EATS, creating a flow maldistribution at the inlet of the EATS. Consequently, velocity, temperature, and concentrations at the exit are influenced by the flow distribution at the inlet. Accurate models that capture the spatial and temporal variations of the flow distribution in EATS, specifically during cold start conditions and real driving emissions (RDE) tests, are essential to comply with stringent emission standards. 1D and 3D-computational fluid dynamics (3D-CFD) models are used to predict the conversion of species at the exit of EATS. While 1D models are robust, they lack accuracy, whereas, 3D-CFD models offer higher accuracy, but require significant computational resources. This study addresses these challenges primarily through Computational Fluid Dynamics (CFD) simulations and proposes a methodology for developing reduced-order models. Firstly, characterizing and quantifying flow distribution in EATS under transient conditions with realistic geometry is performed using non-reactive simulations. Flow uniformity index is used to characterize the extent of variation of the flow parameters in any catalyst plane. In addition to the uniformity index, contours and histograms are employed to demonstrate the non-uniform flow field. The effect of inlet pulsations on the mixed cup conversion at exit of catalyst is studied using transient reactive simulations. Four transient inlet profiles, viz., constant flowrate, sinusoidal, rectangular, and triangular pulse profiles, are chosen to describe the inlet pulsations. The results show that the fluctuations and pulsations in the incoming flow to a monolithic reactor in an aftertreatment system, affect both the transient response of the reactor as well as its time-averaged performance. The method of specifying the inlet boundary conditions also influences the solutions. A methodology for developing a reduced-order model by combining physics-based CFD solutions with multivariate data analysis methods is proposed. This method is demonstrated by combining CFD solutions of transient reactive simulations on a diesel oxidation catalyst with chemometric techniques. Performance evaluations validate the efficacy of the multi-channel model over single-channel models. Computational efforts for creating the multichannel model are comparable to single-channel models, when utilizing available CFD data and coupling chemometrics analysis. This enables rigorous control applications with improved accuracy. The methodology can be extended to real-world emissions aftertreatment systems with complex geometries. Further, predictions of species conversions in systems with flow maldistribution are made by performing steady state reactive 3D-CFD simulations and mapping the same with 1D-SCM. A pseudo-channel is envisaged that provides the same species conversion as the 3D-CFD, by formulating an objective function, which is the difference of species conversions of 3D-CFD and 1D-SCM. The error of the objective function is minimized by iteratively varying the velocity that will provide the same conversion in a 1D-SCM. The pseudo-channel model outputs agree closely with the CFD results in various steady-state and transient test cases. Detached eddy simulations were carried out under nonreactive conditions on the geometry with bends to confirm the validity of RANS simulations, as RANS simulations are computationally more effective than DES. Flow uniformity indices were of the same order in both cases, however, DES showed fluctuations. This thesis aims at developing reduced order models combining CFD simulations and regression and chemometric techniques. It also highlights the limitations of a single channel model in a realistic geometry case. The thesis also attempts to predict species conversion of transient reactive simulations, from the solution of steady state reactive simulations, as the former is computationally more expensive than the latter. The developed pseudo-channel model and multi-channel model can be used for realtime monitoring and control applications. These two methodologies require a computational load comparable to that of 1D models. Validation of these models using EATS experiments under transient conditions is recommended for future research.
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| 4. |
- Chanda Nagarajan, Pratheeba, 1978, et al.
(författare)
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Transient Flow Uniformity Evolution in Realistic Exhaust Gas Aftertreatment Systems using 3D-CFD
- 2022
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Ingår i: Emission Control Science and Technology. - : Springer Science and Business Media LLC. - 2199-3629 .- 2199-3637. ; 8:3-4, s. 154-170
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Tidskriftsartikel (refereegranskat)abstract
- To precisely control a vehicle powertrain to minimize emissions, accurate and detailed models are needed to capture the spatio-temporal variability of the variables of interest. The aim of this work is to analyze flow and temperature fields in a geometrically realistic — and thus complex — exhaust gas aftertreatment system under transient conditions. The spatio-temporal response of these fields to upstream step changes is predicted using three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) κ- ω simulations where the catalytic converter is described as a porous medium. A catalytic converter geometry with a 90∘-bend and a partially dead volume is used to demonstrate the effects of time-resolved flow maldistribution on the profiles of velocity and temperature. Two sets of transient simulations in terms of step changes in velocity and temperature are performed. Uniformity indices are used to characterize the distribution and variability of the different catalyst channels under transient conditions. The evolution of the uniformity indices as functions of time and axial distance into the catalyst are calculated at different cross-sectional planes. The results show that the evolution of the temperature uniformity is rate controlling, continuously modulating the otherwise much faster flow uniformity response via the fluid properties. The temperature uniformity time scale is determined by the balance of flow, thermal inertia, and the heat losses from the system. The interplay between pressure drop and heat losses governs the transition to the new steady state in uniformity. These types of transient simulations and analyses can contribute essential information when developing reduced-order engineering models to represent the spatio-temporal variability in exhaust aftertreatment systems, in particular during rapid events such as cold start.
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| 5. |
- Chanda Nagarajan, Pratheeba, 1978, et al.
(författare)
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Turbulent uniformity fluctuations in automotive catalysts – A RANS vs DES assessment
- 2022
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Ingår i: Results in Engineering. - : Elsevier BV. - 2590-1230. ; 16
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Tidskriftsartikel (refereegranskat)abstract
- Attaining sufficient flow uniformity in catalytic aftertreatment systems is a major challenge for the automotive industry. Computational fluid dynamics (CFD) simulations offer means of analyzing and quantifying this flow uniformity in silico. In this work, predictions from numerical simulations of flow uniformity obtained using a conventional steady-state Reynolds-Averaged Navier-Stokes (RANS) approach are contrasted against comprehensive Detached Eddy Simulations (DES) where the large-scale turbulence is resolved in space and time. It is shown that the DES approach provides access to data on flow uniformity fluctuations that could be significant for the catalyst light-off behavior. However, the computational cost of the DES is approximately three orders of magnitude larger than that of the corresponding RANS simulation.
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