Joint characteristic timescales and entropy production analyses for model reduction of combustion systems

The reduction of chemical kinetics describing combustion processes remains one of the major topics in the combustion theory and its applications. Problems concerning the estimation of reaction mechanisms real dimension remain unsolved, this being a critical point in the development of reduction models. In this study, we suggest a combination of local timescale and entropy production analyses to cope with this problem. In particular, the framework of skeletal mechanism is in the focus of the study as a practical and most straightforward implementation strategy for reduced mechanisms. Hydrogen and methane/dimethyl ether reaction mechanisms are considered for illustration and validation purposes. Two skeletal mechanism versions were obtained for methane/dimethyl ether combustion system by varying the tolerance used to identify important reactions in the characteristic timescale analysis of the system. Comparisons of ignition delay times and species profiles calculated with the detailed and the reduced models are presented. The results of the application show transparently the potential of the suggested approach to be automatically implemented for the reduction of large chemical kinetic models

Flexible energy conversion and storage via high-temperature gas-phase reactions: The piston engine as a polygeneration reactor

Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quenched by gas expansion. Thus, piston engines could be “polygeneration” reactors that can flexibly change from power generation to chemical synthesis, and even to chemical-energy storage. This may help mitigating one of the main challenges of future energy systems – accommodating fluctuations in electricity supply and demand. Investments in devices for grid stabilization could be more economical if they have a second use. This paper presents a systematic approach to polygeneration in piston engines, combining thermodynamics, kinetics, numerical optimization, engineering, and thermo-economics. A focus is on the fuel-rich conversion of methane as a fuel that is considered important for the foreseeable future. Starting from thermodynamic theory and kinetic modeling, promising systems are selected. Mathematical optimization and an array of experimental kinetic investigations are used for model improvement and development. To evaluate technical feasibility, experiments are then performed in both a single-stroke rapid compression machine and a reciprocating engine. In both cases, chemical conversion is initiated by homogeneous-charge compression-ignition. A thermodynamic and thermo-economic assessment of the results is positive. Examples that illustrate how the piston engine can be used in polygeneration processes to convert methane to higher-value chemicals or to take up carbon dioxide are presented. Open issues for future research are addressed

Predicting Health Impacts of the World Trade Center Disaster: 1. Halogenated hydrocarbons, symptom syndromes, secondary victimization, and the burdens of history

The recent attack on the World Trade Center, in addition to direct injury and psychological trauma, has exposed a vast population to dioxins, dibenzofurans, related endocrine disruptors, and a multitude of other physiologically active chemicals arising from the decomposition of the massive quantities of halogenated hydrocarbons and other plastics within the affected buildings. The impacts of these chemical species have been compounded by exposure to asbestos, fiberglass, crushed glass, concrete, plastic, and other irritating dusts. To address the manifold complexities of this incident we combine recent theoretical perspectives on immune, CNS, and sociocultural cognition with empirical studies on survivors of past large toxic fires, other community-scale chemical exposure incidents, and the aftereffects of war. Our analysis suggests the appearance of complex, but distinct and characteristic, spectra of synergistically linked social, psychosocial, psychological and physical symptoms among the 100,000 or so persons most directly affected by the WTC attack. The different 'eigenpatterns' should become increasingly comorbid as a function of exposure. The expected outcome greatly transcends a simple 'Post Traumatic Stress Disorder' model, and may resemble a particularly acute form of Gulf War Syndrome. We explore the role of external social factors in subsequent exacerbation of the syndrome -- secondary victimization -- and study the path-dependent influence of individual and community-level historical patterns of stress. We suggest that workplace and other organizations can act as ameliorating intermediaries. Those without acess to such buffering structures appear to face a particularly bleak future

Analysis and simplification of chemical kinetics mechanisms with CSP-based techniques

The computational singular perturbation (CSP) method is exploited to build a comprehensive framework for analysis and simplification of chemical kinetic models. The necessity for both smart post-process tools, able to perform rational diagnostics on large numerical simulations of reactive flows, and affordable reduced kinetic mechanisms, to make the simulations feasible, is the driving force behind this work. The ultimate goal is to improve the understanding of the fundamentals of chemically reacting flows. The CSP method is a suitable candidate for extracting physical insights from reactive flows dynamics that can be employed for both the generation of simplified kinetic schemes and the calculation of smart and compact diagnostic observables. Among them, the tangential stretching rate (TSR) is an estimate of the system’s driving chemical timescale that can be profitably employed for characterising the reactive flow dynamics in terms of combustion regimes and role of transport with respect to kinetics. The potentials of TSR are extensively highlighted, starting from prototypical combustion models, such as batch reactor and unsteady laminar flamelet, and getting to real-life usage on 3-dimensional direct numerical simulation datasets. The CSP mathematical foundations are then employed for mechanism simplification purposes, where small and accurate kinetic mechanisms are sought after. An existing CSP-based simplification algorithm is improved, aiming at the minimisation of the required user knowledge, which becomes a critical feature of the algorithm when dealing with new fuels. Practical applications of the revised algorithm are shown and discussed. Finally, the focus is shifted from the quest for tight accuracy in the simplified mechanisms towards a much broader question regarding confidence in detailed kinetic schemes. Uncertainty in the kinetic model parameters, such as Arrhenius coefficients, can jeopardize the efforts spent in the reduction challenge. A new, uncertainty-aware, robust CSP simplification strategy is proposed, discussed and employed, and its robustness demonstrated in a test case involving an uncertain -in its Arrhenius pre-exponential coefficients- kinetic scheme

Development of Improved CFD Tools for the Optimization of a Scramjet Engine

In the present work, a plugin has been developed for use with the DoD HPCMP CREATE-AV Kestrel multi-physics solver that adds volumetric source terms to the energy equation. These source terms model the heat released due to combustion, but are much more computationally efficient than a full chemistry model. A thrust-based optimization study was then carried out under the control of Sandia National Laboratories\u27 Dakota toolkit. Dakota was allowed to control the amount of heat added to three regions of the scramjet combustor. The plugin was then extended to consider ignition delay time. By comparing ignition delay time to dwell time, it is possible to determine whether the fuel in a cell should be combusted. Results from this analysis are compared to results gathered using a 22-species chemistry model. The ignition delay source term is shown to capture relevant flow physics at a reduced computational cost. Additionally, the expression for second-law (exergetic) efficiency for a scramjet engine is derived and optimized using Dakota. Finally, Dakota was extended to control the geometry of the scramjet engine, allowing for the numerical optimization of the scramjet expansion system. The results from these computationally-efficient optimizations can then be used to inform researchers of potentially optimal solutions before higher-fidelity models are used

Research reports: 1990 NASA/ASEE Summer Faculty Fellowship Program

Reports on the research projects performed under the NASA/ASEE Summer Faculty Fellowship Program are presented. The program was conducted by The University of Alabama and MSFC during the period from June 4, 1990 through August 10, 1990. Some of the topics covered include: (1) Space Shuttles; (2) Space Station Freedom; (3) information systems; (4) materials and processes; (4) Space Shuttle main engine; (5) aerospace sciences; (6) mathematical models; (7) mission operations; (8) systems analysis and integration; (9) systems control; (10) structures and dynamics; (11) aerospace safety; and (12) remote sensin

Pressure Gain Combustion: Fuel Spray and Shockwave Interaction

Pressure gain combustion can attain higher thermodynamic cycle efficiency in gas turbine power systems, resulting in the reduction of specific fuel consumption/fuel burn and Carbon dioxide emissions.There are many ways to achieve pressure gain and the present research investigates pressure gain through shock bubble (gas and liquid bubble) interaction (SBI) using computational fluid dynamics (CFD) simulations. The numerical simulations have been performed in 2D and 3D representations of the shock tube to depict the interaction of a planar shock wave with distinct gas and liquid inhomogeneities. The three scenarios considered cover the interaction of a planar shock wave in air with: spherical helium bubble (Mach number, Ma = 1.25); cylindrical helium bubble (Ma = 1.22) and cylindrical water bubble (Ma = 1.47). To perform these simulations, the Unsteady Reynolds-Averaged Navier-Stokes (URANS) mathematical model and the coupled level set and VOF method within the commercial CFD code, ANSYS FLUENT, have been applied. A finite volume method (FVM) is also employed to solve the governing equations. For the spherical and cylindrical gas bubble cases, various quantitative analyses are presented and compared to the experimental work of Haas and Sturtevant (1987). These include: refracted wave, transmitted wave, upstream interface, downstream interface, jet, vortex filament, non-dimensional bubble, and vortex velocities. The predicted non-dimensional bubble and vortex velocities have also been compared with experimental data, a simple model of shock- induced Rayleigh Taylor (RT) instability and other theoretical models. Comparisons are also shown between the predicted bubble length/width and the experimentally measured results to elucidate changes in the shape and size of the 2D and 3D bubbles. Additional quantitative analyses are also presented for the spherical bubble involving the size estimation of the vortex pair as well as their spacing. For the shock cylindrical water bubble interaction case, the quantitative predictions include: displacement/drift, acceleration, distortion in the lateral direction, distortion in flow direction, area variation from bubble distortion, as well as drag coefficient and are compared to the experimental measurements of Igra et al. (2002). It has been demonstrated that 3D simulations compare very well with the experimental data, suggesting that 3D simulations are necessary to capture SBI process accurately. Finally, comprehensive flow visualization has been used to elucidate the shock-bubble interaction (SBI) process from bubble compression to the formation of the vortex filaments (cylindrical helium bubble), vortex rings (spherical helium bubble), vortices (cylindrical water bubble) as well as the production and distribution of vorticity. It is demonstrated for the first time that turbulence is generated at the early phase of the SBI process, with the maximum turbulence intensity reaching about 20% around the vortex filaments/vortex rings regions for the cylindrical/spherical helium bubble cases respectively and about 22% for the cylindrical water bubble case at the later phase of the interaction process

Modelling the impact of fuel in aeronautical gas turbines

The rise of climatic hazards, due to the human contribution, has led some governments and industries of the aeronautical sector to think about solutions to reduce combustion emissions. To create a less environmentally demanding aviation, electrical storage does not fill the power criteria and other promising fuels such as hydrogen require a change of the whole plane engine. Another short-run solution is the use of drop-in alternative fuel, which, despite some drawbacks, would reduce the emissions of the sector in the nearer future. The European H2020-JETSCREEN project, that funded this PhD, falls within this context. Indeed, the development of Large-Eddy Simulations (LES) and Analytically Reduced Chemistries (ARC) coupled with the rise of computer resources has enabled precise kinetics to be used in turbulent combustion chambers. The main topic of this PhD is the development of a methodology to analyse a stabilised turbulent two-phase flow flame with complex chemistry and heat losses for three multi-component fuels : one conventional and two alternative fuels. Before the computation, questions on the chemistry and the evaporation properties of the fuels remain. At first, ARC were developed and validated against the detailed mechanism, testifying the capability of the kinetic reduction code ARCANE to retrieve the chemical fuel sensitivities. Fuels were then analysed on every canonical case concluding that the fuel composition had an influence on the global combustion but little on the pollutants. Furthermore, the simulation of 1D ARC premixed flame explained why such complex kinetics need very few points in the flame front in order to give accurate results and underlined the prominent role of the flame foot and especially the fuel consumption that is monitoring the flame convergence. Second, evaporation properties comparisons led to results close to the experimental work of the DLR and retrieving the two-phase fuel sensitivities. Based on those results, a twophase premixed flame was computed and the flame characteristic variables were found to depend on the degree of pre-evaporation. Furthermore, the spray counter-flow diffusion flame structure was investigated. The polydisperse two-phase flow initiating a change of the flame regime explained the exotic structure observed. Once those canonical analyses studied, the real combustion chamber simulation was tackled. Differences in terms of averaged solutions have then been drawn, showing the capability of the LES code, AVBP, to globally reproduce the experimental behaviour of those fuels whether for the dynamic quantities, the thermal fields or the two-phase flow properties. The comparison between a simple and a complex surrogate for Jet-A1 resulted in a similar stabilisation point, but a different flame structure, assessing the capability of the Takeno sensor to visualise the right flame regime. A Lean Blow-Out(LBO) methodology was suggested on the simple chemistry, starting by the evaluation of the characteristic timescales, key quantities for the transient flame evolution and followed by the right variable choice for the LBO detection. The LBO was detected slightly below the experimental value, following a flame stabilisation by hot gases process. Finally, the flame structure was compared for the three fuels and depicted differences in terms of flame structure mainly due to the evaporation properties that are impacting the thermal field and the local flame regime

An applied mathematical view of meteorological modeling

The earth’s atmosphere is of overwhelming complexity due to a rich interplay between a large number of phenomena interacting on very diverse length and time scales. There are mathematical equation systems which, in principle, provide a comprehensive description of this system. Yet, exact or accurate approximate solutions to these equations covering the full range of complexities they allow for are not available. As a consequence, one of the central themes of theoretical meteorology is the development of simplified model equations that are amenable to analysis and computational approximate solution, while still faithfully representing an important subset of the observed phenomena

Experimental Investigation of Nozzle/Plume Aerodynamics at Hypersonic Speeds

The work performed by D. W. Bogdanoff and J.-L. Cambier during the period of 1 Feb. - 31 Oct. 1992 is presented. The following topics are discussed: (1) improvement in the operation of the facility; (2) the wedge model; (3) calibration of the new test section; (4) combustor model; (5) hydrogen fuel system for combustor model; (6) three inch calibration/development tunnel; (7) shock tunnel unsteady flow; (8) pulse detonation wave engine; (9) DCAF flow simulation; (10) high temperature shock layer simulation; and (11) the one dimensional Godunov CFD code