Browsing by Subject "Combustion"

Now showing 1 - 6 of 6
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    Investigation Of Piston Geometry In Rapid Compression Machines And Sampling Methods For Internal Combustion Engines
    (2019-07) Dasrath, Dereck
    There is a growing effort to reduce carbon dioxide (CO2) emissions produced by internal combustion (IC) engines as an effort to curb anthropogenic climate change. The transportation sector accounts for 28% of anthropogenic CO2, motivating fundamental combustion research to understand and develop more efficient advanced combustion modes. Study of ignition delay time, autoignition pressure and temperature, the chemistry of fuel mixtures, and speciation of combustion products provide important insights into phenomena like pre-ignition (knock) and pollutants (CO2, oxides of nitrogen, soot, etc.) from modern-day IC engines. This body of work investigates novel speciation methods for studying combustion products from IC engines and unique piston geometries for rapid compression machines (RCMs). Quantifying combustion products is an important step in creating accurate numerical models for engine combustion. Many groups have used various instruments in conjunction to characterize a range of combustion generated hydrocarbons but few have used instruments in tandem to improve speciation methods during unconventional combustion modes and address the issues associated with off-line speciation. The first part of this thesis presents an investigation that quantified light unburned hydrocarbons (UHC) using a combination of Fourier transform infrared (FT-IR) spectroscopy and gas chromatography-mass spectroscopy (GC-MS). A light-duty diesel engine is used to generate hydrocarbons at various exhaust gas recirculation (EGR) levels and partially premixed low-temperature combustion (LTC) modes. Exhaust samples are extracted with a novel fixed-volume sampling system and sent into a gas chromatograph (GC) while minimizing unknown dilution, light unburned hydrocarbons (LHC) losses, and removing heavy unburned hydrocarbons (HHC). Along with the wide range of LHCs quantified in this study, focus is directed towards the problem of misidentification of propane by the FT-IR during LTC modes. In the region commonly identified as the absorption spectra of propane (2700 and 3100 cm-1), analysis of the FT-IR spectra indicates absorption band interference caused by components found in unburnt diesel fuel. One of the primary findings of this work is that GC-MS can aid in FT-IR spectral analysis to further refine FT-IR methods for real-time measurement of unconventional combustion mode exhaust species. Rapid compression machines (RCMs) and rapid compression and expansion machines (RCEMs) are apparatuses that have the ability to operate at engine-relevant conditions to study fuel autoignition and pollutant formation. These machines are currently limited for use in speciation studies due to thermal and mixture inhomogeneities caused by heat transfer and gas motion during compression. Studies have shown the disadvantages of using common flat and enlarged piston crevice designs for sampling reaction chamber gases during and after combustion. For instance, computer fluid dynamics (CFD) simulations performed by numerous groups, including collaborators on this work, have confirmed that unburnt fuel mixture emerges from the enlarged crevice after compression then subsequently mixes with reaction chamber gases during RCM and RCEM operation. This disadvantage renders whole-cylinder sampling techniques inaccurate for quantifying combustion products and reduces the relevance of RCMs and RCEMs for comparison with IC engines. Complex fast-sampling systems are implemented by a number of research groups to extract small quantities of gas from the center of the chamber before mixing occurs. Drawbacks with this approach include small sample volumes, local composition non-uniformities, and non-uniform progression of chemical kinetics during sampling. Experimental and computational studies emphasize the importance of piston design for the formation of a well-mixed, homogeneous core gas inside RCM and RCEM reaction chambers. In the second part of this thesis, a novel piston containing a bowl-like geometry similar to those used in diesel engines is implemented to overcome thermal and compositional non-uniformities within RCMs/RCEMs. By eliminating the enlarged crevice and introducing squish flow with the bowl piston, CFD studies show increased thermal uniformity for both RCM and RCEM trajectories. Experiments to characterize piston performance includes flat, enlarged crevice, and bowl piston profiles and four fuel mixtures using the University of Minnesota – Twin Cities controlled trajectory RCEM (CT-RCEM). Heat release analysis (HRA) indicates greater combustion efficiencies when using the bowl piston opposed to the standard flat and enlarged creviced pistons. This is indicative of smaller fractions of unburnt fuel left in the combustion chamber after combustion, ideal for dump sampling and the differentiation of unburnt fuel from combustion products during speciation. Ignition analysis for the bowl piston derived stronger ignition characteristics than the enlarged crevice and flat piston designs. As a result of stronger ignition and better uniform burning, the amount of fuel converted to products of combustion is increased.
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    On variable-density subgrid effects in turbulent flows
    (2018-11) GS, Sidharth
    Eulerian mass density variations in a flow relate to compressibility and material inhomogeneities in the fluid. These variations can be caused due to high flow speeds, heat transfer, thermo-chemical reactions and/or phase change. From a local perspective, density gradient in space affects the velocity gradient dynamics due to variable inertia, in the presence of pressure-gradient driven acceleration, and therefore indirectly, the dissipation rate of kinetic energy and enstrophy. In turbulent flows, density variations and their effects on the velocity field influences the interscale interactions. Of particular interest is the turbulent dynamics in the presence of large vorticity generation by baroclinic torque. Although these effects are usually transient (in space or time) as turbulent mixing homogenizes the density field, the deviation from constant-density dynamical evolution can be statistically significant, particularly in instability-dominated flows with high sensitivity to initial/boundary conditions. In unsteady reacting flows, sustained chemi-acoustic interactions result in turbulent vorticity dynamics that is markedly different from the well-studied incompressible constant-density turbulence. Large-eddy simulations of high Reynolds number variable-density flows require adequate representation of unresolved small-scale variable-density effects. The present work is an effort to understand subgrid-scale (SGS) variable-density effects to improve the fidelity and accuracy of our simulations in these regimes. The thesis focuses on Reynolds-filtered governing equations to compute the large-scale vorticity dynamics more precisely. A novel equation set for coarse-grained mass, momentum and energy is derived that employs only second order moment based closures, and allows explicit representation of subgrid-scale compressibility and inertial effects. The new form of the filtered equations has terms that represent the SGS mass flux, pressure-gradient acceleration, and velocity-dilatation correlation. We attempt to quantify the dynamical significance of these terms with direct numerical and large eddy simulations.
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    Simulations of injection, mixing, and combustion in supersonic flow using a hybrid RANS/LES approach.
    (2011-09) Peterson, David Michael
    There is a great need for accurate and reliable numerical simulation of injection, mixing, and combustion in supersonic combustion ramjet engines. This study seeks to improve the accuracy and reliability which these flow can be simulated with by investigating the use of recent improvements in turbulence modeling and numerical methods. The present numerical simulations use implicit time integration and low-dissipation flux evaluation schemes in an unstructured grid framework. A hybrid Reynolds-Averaged Navier-Stokes and large-eddy simulation approach is used to model turbulence. The large-scale turbulent structure of the flow is resolved, while the near-wall structure is fully modeled. The effects of numerics, grid resolution, and boundary conditions are investigated. The simulation approach is thoroughly validated against available experimental data at a variety of flow conditions. The simulations focus on the injection of fuel through circular injector ports that are oriented either normal to the supersonic crossflow, or at a low angle with respect to the crossflow. The instantaneous flow structure resolved by the simulations is qualitatively compared to experimental flowfield visualization. Quantitative comparisons are made to mean wall pressure, mean velocity, turbulence quantities, and mean mixing data. The simulations are found to do very well at predicting the mean flowfield as well as fluctuations in velocity and injectant concentration. The simulation approach is then used to simulate the flow within a model supersonic combustor. The focus is on the non-reacting case. The simulation results are found to agree well with experimental measurements of temperature and species concentrations. The flow is examined to improve understanding of the mixing within the model combustor. Preliminary results for a simulation including hydrogen combustion are also presented.
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    A stochastic particle method for the investigation of turbulence/chemistry interactions in large-eddy simulations of turbulent reacting flows
    (2013-12) Ferrero, Pietro
    The main objective of this work is to investigate the effects of the coupling between the turbulent fluctuations and the highly non-linear chemical source terms in the context of large-eddy simulations of turbulent reacting flows. To this aim we implement the filtered mass density function (FMDF) methodology on an existing finite volume (FV) fluid dynamics solver. The FMDF provides additional statistical sub-grid scale (SGS) information about the thermochemical state of the flow - species mass fractions and enthalpy - which would not be available otherwise. The core of the methodology involves solving a transport equation for the FMDF by means of a stochastic, grid-free, Lagrangian particle procedure.Any moments of the distribution can be obtained by taking ensemble averages of the particles. The main advantage of this strategy is that the chemical source terms appear in closed form so that the effects of turbulent fluctuations on these terms are already accounted for and do not need to be modeled.We first validate and demonstrate the consistency of our implementation by comparing the results of the hybrid FV/FMDF procedure against model-free LES for temporally developing, non-reacting mixing layers. Consistency requires that, for non-reacting cases, the two solvers should yield identical solutions. We investigate the sensitivity of the FMDF solution on the most relevant numerical parameters, such as the number of particles per cell and the size of the ensemble domain. Next, we apply the FMDF modeling strategy to the simulation of chemically reacting, two- and three-dimensional temporally developing mixing layers and compare the results against both DNS and model-free LES. We clearly show that, when the turbulence/chemistry interaction is accounted for with the FMDF methodology, the results are in much better agreement to the DNS data. Finally, we perform two- and three-dimensional simulations of high Reynolds number, spatially developing, chemically reacting mixing layers, with the intent of reproducing a set of experimental results obtained at the California Institute of Technology. The mean temperature rise calculated by the hybrid FV/FMDF solver, which is associated with the amount of product formed, lies very close to the experimental profile. Conversely, when the effects of turbulence/chemistry coupling are ignored, the simulations clearly over predict the amount of product that is formed.
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    Synthesis gas use in internal combustion engines.
    (2010-12) Bika, Anil Singh
    The objective of this dissertation was to investigate the combustion characteristics of a compression ignition, spark ignition, and homogeneous charge compression ignition engine operating on various blends of synthesis gas. To fully investigate the three ICE operating regimes, experimental investigations were carried out to focus on: 1.) A CI engine operating on ethanol and hydrogen fuel 2.) A CI engine operating on diesel fuel with varying blends of synthesis gas 3.) A SI engine operating on varying blends of synthesis gas 4.) An HCCI engine operating on hydrogen fuel 5.) An HCCI engine operating on varying blends of synthesis gas The three operating modes (CI, SI, and HCCI) were selected because it is unlikely that an engine will be able to operate solely in an HCCI regime throughout the complete load range. The more common CI and SI regimes will likely be necessary for high load engine operation. The results from this doctoral work sheds light into the fundamental aspects of syngas combustion and also provides a foundation for future gasification plant designers and synthesis gas producers, regarding the fuel composition needs of a syngas powered internal combustion engine. The first 3 chapters of this dissertation provide an introduction and background for this doctoral work. The remaining chapters present the results and conclusions
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    Thermochemical Recuperation and Catalytic Strategies for Anhydrous Ammonia in Combustion Systems
    (2021-06) Kane, Seamus
    The carbon intensity of combustion engines poses a major challenge to worldwide efforts to minimize climate change. Anthropogenic carbon dioxide (CO2) is greatest source of atmospheric warming and its emission must be curtailed to affect climate forcing in a meaningful way. Carbon-neutral alternatives such as ethanol and biodiesel recycle atmospheric carbon under ideal conditions yet result in net carbon emissions due to process inefficiencies. Fuels decoupled from chemical carbon are necessary to reduce carbon intensity and halt climate change. Anhydrous ammonia is one such fuel as it can be produced entirely by renewable means and contains no carbon. This body of work investigates combustion applications of anhydrous ammonia in compression ignition (CI) engines, methods of catalytically enhancing ammonia for more efficient combustion and use of the endothermic ammonia decomposition reaction for waste-heat recovery.This thesis presents the applications of catalytic ammonia decomposition, specifically pertaining to, but not limited to internal combustion engines. Ammonia has proved a suitable replacement fuel in spark-ignition (SI) engines and as a secondary fumigated fuel for CI engines. Stability of the ammonia molecule results in poor flame propagation and low ignition reactivity. Using ammonia in a dual-fuel arrangement overcomes these issues in existing engines and combustors designed for hydrocarbon fuels. Alternatively, ammonia can be converted to hydrogen using catalysis, which in turn enhances ammonia combustion without the need for a secondary high reactivity fuel. This work explores hydrogen-enhanced ammonia and diesel combustion in CI engines equipped with catalytic waste-heat recovery. Engines were operated over their full range under various levels of ammonia fuel replacement to determine the effects on engine and combustion efficiency as well as emissions and stability. Thermochemical recuperation and thermal recovery were analyzed across the operability range towards identifying optimal system parameters. The primary finding in the first part of this work is that ammonia effectively recovers waste energy using low-temperature high-active catalysts. Activity is demonstrated as low as 200 °C for Ruthenium-based catalysts, and full conversion to hydrogen results in a net lower heating value increase of 15%. Heat transfer to sustain the decomposing ammonia proved difficult however, as the experimental catalyst unit was undersized for engine operation. Despite low conversion, sufficient hydrogen was generated to enhance flame speed, combustion efficiency and engine thermal efficiency as compared to pure ammonia fumigation. Fuel-bound nitrogen in ammonia generated high oxides of nitrogen (NOx) and N2O emissions upon combustion. However, unburned ammonia present in exhaust was measured to be ideal for passive elimination of these species using selective catalytic reduction (SCR). Emissions and efficiencies measured suggest that future implementation of ammonia dual fuel requires higher rates of heat recovery and higher ammonia replacement rates than those demonstrated in this study. Both conditions can be met using a modified catalyst design and higher flow ammonia fueling system, respectively. Ammonia decomposition catalysis is thoroughly described in literature but heat transfer inside a supporting monolith structure is not. This work presents a computationally efficiency quasi-2D modeling procedure for understanding heat and mass transfer in metal monoliths. A finite difference model was developed to simulate thermal behavior of the decomposition catalyst used in experimental studies. The heat transfer model was calibrated against inert gas experiments and showed excellent agreement with convective and conductive values from literature. Argon, air and CO2 were used under identical thermal conditions to demonstrate robustness in simulating generic flows through the reactor. Agreement of the model against the entire experimental dataset demonstrated robustness in predicting metallic support thermal behavior while the simplicity of the approach presented a computationally inexpensive alternative to CFD or physical prototype design screening. Design screening was then conducted using varied input conditions and showed the relative importance of each parameter on heat exchange effectiveness and wall-average heat transfer coefficient. Optimal performance was quantified, and the effects of design parameters on heat exchange was discussed. High catalyst activity and reaction residence time are needed to achieve high hydrogen yield, promoting efficient combustion of ammonia-hydrogen mixtures. To overcome thermal limitations posed by waste-heat driven decomposition, ammonia partial oxidation can be used to create an endogenous heat source and increase yield. Oxidation and decomposition were combined in an autothermal process and were shown to increase both hydrogen fraction and the hydrogen-to-ammonia ratio of the reformate stream. Autothermal ammonia decomposition (ATD) resulted in fuel heating value decrease, which was comparable to heating value losses expected from poor combustion efficiency in engines. A comprehensive reactor model was developed using two global reaction rates and the previous monolith heat transfer model. Rates were determined through non-linear regression and showed excellent fit across thermal and autothermal regimes. Deficiencies in experimental reactor design were identified using the model, and potential design changes were discussed. The model and experiment both suggest that ATD is a promising alternative to waste-heat recovery approaches alone when a high reactivity reformate mixture is needed. The research shows that ammonia ATD reformate is of sufficient reactivity to enable drop-in replacement of hydrocarbon fuels in unmodified engines and combustors.