Research and technology

The NASA Lewis Research Center's research and technology accomplishments for fiscal year 1987 are summarized. It comprises approximately 100 short articles submitted by staff members of the technical directorates and is organized into four sections: aeronautics, aerospace technology (which includes space communications), space station systems, and computational support. A table of contents by subject was developed to assist the reader in finding articles of special interest

Corrosion and hydrate formation in natural gas pipelines

Gas industry annually invests millions of dollars on corrosion inhibitors in order to minimize corrosion implications on flow assurance; however, attention has never been focused on possibilities of these chemicals to promote hydrate formation along deepwater pipelines, which would equally result in another flow assurance problem of high magnitude. This study investigated the possibilities of corrosion inhibitors to aid the formation of gas hydrate along offshore (or underwater) pipeline systems; developed a predictive model on corrosion rate for natural gas pipelines with gas hydrates as the corroding agent and finally investigated the ability of pure N2 and H2 gases to inhibit the formation of gas hydrates.All experiments in this thesis were conducted by forming various water-gas systems in a cylindrical cryogenic sapphire cell. The first investigative work on hydrate-corrosion relationship was conducted by allowing contacts between an industrial grade natural gas (with 20% CO2 content) and five different corrosion inhibitors that are commonly used at offshore fields. The equipment, consisting of several fittings could operate at a temperature range of -160oC – 60oC (with accuracy of ± 0.10oC) and pressure range of 1bar to 500bar (with accuracy of ± 0.5bar). Using the ‗Temperature Search‘ method, the hydrate formation temperature point for each inhibitor was located at 500ppm and 100bar and the result compared with that of control experiment. Due to observed significant influence, further investigations were conducted on Dodecylpyridinium Chloride (DPC) at various concentrations and pressures. The corrosion model was developed based on hydrate‘s thermodynamic properties such as the operating temperature, pressure, fluid fugacity, wall shear stress, superficial velocity, enthalpy, entropy and activity coefficient amongst others, and a Matlab computer code was written to simulate the generated solution algorithm. Finally, components interaction study was conducted on various gas mixtures inside the sapphire cell to investigate the ability of pure N2 and H2 gases to inhibit the formation of gas hydrates.The obtained results established that all corrosion inhibitors aid hydrate promotion; this was attributed to their surfactant and hydrogen bonding properties which were essential for hydrate formation. The five investigated inhibitors showed different promotional rates with DPC having the highest promotional ability. The different promotional rate is due to their different sizes and structures, active functional groups and affinity for water molecules which determine the type(s) of hydrogen bonding exhibited by each inhibitor while in solution. The significant performance of DPC compared to other inhibitors was justified by the specific available active functional group which obeys electronegativity trend of periodic table to determine whether the resulting bond type will be polar covalent, ionic or ionic with some covalent characteristic in nature. Also, DPC hydrates revealed strong influence of the chemical‘s surfactant properties at all pressures and concentrations while its Critical Micelle Concentration (CMC) was believed to be 5000ppm due to the various anomaly behaviors exhibited at this particular concentration.The developed mathematical model adequately predicted corrosion rates with gas hydrate as the corroding agent and its effectiveness was confirmed by the level of agreement between its generated results and existing literatures. The resulting corrosion rate from hydrates could be as high as 174mm/yr (0.48mm/day). This is extremely alarming compared to the industry‘s aim to operate below 2mm/yr. At this rate, an underwater pipeline would be subjected to full bore rupture within some days if corrective measures are not quickly taken.Furthermore, the components interaction study revealed that CH4 played key roles on hydrate formation patterns during natural gas transportation through offshore pipeline system; the higher a natural gas CH4 content, the higher the risk of hydrates promotion. It also showed that when alone, CO2 does not form hydrate at low concentrations but showed a remarkable ability to aid hydrate formation when mixed with CH4. This is not surprising since it is also a former with ability to form Type I hydrate due to its very small size. Again, the ability of pure N2 and pure H2 gases to inhibit the formation of gas hydrate was confirmed but with H2 showing more significant effects. This was ascribed to their individual pressure condition to form hydrate. Though, N2 gas with small molecules forms Type II hydrate at a relatively higher pressure above the investigated pressures, it still forms hydrate within higher operating pressures practiced at gas fields during the transportation. However, H2 gas can never form hydrate at any natural gas transportation conditions. H2 gas only forms hydrates at extremely high pressure of about 2000bar because its molecules are too small and usually leaked out of hydrate cage, thus, reducing the amount that could be stored. By extension, these individual properties affect their interactions with natural gas during the hydrate formation process.Conclusively, this study has essentially revealed a new hydrate-corrosion relationship and established the need for comprehensive investigations in this research area. At all the investigated pressures, it was realized that DPC prolonged the complete blockage of the glass orifice at 10000ppm. This special characteristic may suggest the potential in applying the chemical as an additive for natural gas transportation and storage in slurry forms. Finally, the use of pure N2 or H2 as hydrate inhibitor in the offshore pipeline would be very cost effective to the industry. However, extreme care should be taken during the selection process since there are needs to further investigate the safety factors, material availability, cost implication and recovery from the main gas stream in order to choose the better option

Systematic accuracy assessment for alternative aviation fuel evaporation models

Environmental and security of supply concerns cause an increasing demand for alternative fuels in aviation. Different fuel production pathways for alternative aviation fuels have been suggested and approved in recent years. In that respect, changes in fuel production can result in various fuel compositions and properties and thus impose a risk for the use in the aircraft and jet engine; the ASTM D4054 approval process was developed to warrant the safety of flight. Nevertheless, tests are expensive and time-consuming. Particularly for the combustion testing part, numerical simulations can be beneficially used to reduce costs and time. Furthermore, virtual prototyping and robust design methods might be essential in supporting the design of fuel flexible combustion chamber with reduced emissions. The use of simulation in the context of decision making in situations with risks related to humans and the environment raises the questions how reliable and accurate simulations results are. In this work, new methods are applied that have been developed for scientific computing. The focus of these methods is on supporting simulation informed risk-related decision making as the final recipient of validation activities. Hereby, it is of essential importance that metrics describing the accuracy of the models over the domain of application are inferred systematically. Furthermore, by reporting the influence of uncertainties in input quantities on the response quantities, the reliability of the simulation results can be increased substantially. Evaporation is an important sub-process of the fuel preparation in a combustion chamber and depends strongly on the fuel composition and properties. Conventional Jet A-1 and most alternative aviation fuels consist of several hundred of different species. Continuous Thermodynamic Models (CTM) have been successfully used in recent years to describe multicomponent-fuel droplet evaporation of real fuels. CTM capture the details of the fuel evaporation while preserving the information of the fuel composition over the evaporation process with low computational load. Up to the present, validation activities have been performed by comparing numerical simulation results with experimental data from suspended droplets experiments. These tests proved the functionality of the concepts successfully. However, the fuel composition was unknown, and the droplet suspension had a strong intrusive effect. Thus, the validations are limited to qualitative statements. In this work, a validation domain was derived from the character of actual and future alternative aviation fuels to determine quantitative metrics for alternative aviation fuel evaporation models systematically. Experiments with different fuels from the validation domain were performed in a newly designed experiment. The validation experiment enables to study the evaporation of a wide range of fuels under controlled conditions in a non-intrusive way. Global and local metrics for the evaporation models were inferred. The effect of uncertainties in the spray injection conditions on simulation results was determined by using Latin Hypercube Sampling to sample the input domain and to propagate the uncertainties through the governing equations. The resulting uncertainties in the simulation result can be interpreted as the precision of the validation approach. Validation metrics, as well as the precision, give future users (modeler, analyst and decision maker) all information required to assess the model adequacy for the intended use and, if necessary, to determine next actions to improve the model or the validation experiment.Um die langfristige Versorgungssicherheit mit flüssigen Treibstoffen in der Luftfahrt sicherzustellen und die ökologischen Auswirkungen zu minimieren, wurden in den letzten Jahren verschiedene Herstellungspfade für alternative Treibstoffe entwickelt und zugelassen. Jede Änderung im Herstellungspfad hat jedoch einen Einfluss auf die Zusammensetzung des Treibstoffes und birgt somit ein Risiko bei der Nutzung des Treibstoffes in Flugzeug und Triebwerk. Die Zuverlässigkeit der Nutzung neuartiger Treibstoffe wird durch aufwendige und kostenintensive Tests nach dem ASTM D4054 Zulassungsverfahren sichergestellt. Numerische Simulationen haben das Potential, die Zeitdauer von Verbrennungstests beim Zulassungsverfahren maßgeblich zu verkürzen und Kosten einzusparen. Darüber hinaus können der virtuelle Entwurf und Methoden der Entwicklung von robusten Designs eine wesentliche Unterstützung beim Entwurf von neuen brennstoffflexiblen und schadstoffärmeren Brennkammern sein. Hier muss jedoch die Frage gestellt werden, wie zuverlässig und belastbar die aus numerischen Simulationen gewonnenen Informationen sind und inwieweit sie als Basis für Entscheidungen mit Konsequenzen für die Sicherheit von Mensch und Umwelt dienen können. In dieser Arbeit werden neue Methoden für die Validierung von numerischen Modellen angewandt, die im Bereich des wissenschaftlichen Rechnens in den letzten Jahren entwickelt wurden. Das risiko-informierte Entscheiden, basierend auf aus Simulationen gewonnenen Daten, steht hier als Endprodukt im Fokus. Dabei ist es zum einen von wesentlicher Bedeutung, die Genauigkeit der verwendeten Modelle quantitativ und systematisch über den Anwendungsraum der Modelle zu erfassen. Zum anderen wird die Zuverlässigkeit der Modelle maßgeblich erhöht, indem die Auswirkung von Unsicherheiten in den Eingangsgrößen auf relevante Zielgrößen in die Untersuchung einbezogen wird. Die Verdunstung ist ein wichtiger Teilprozess der Treibstoffaufbereitung in der Brennkammer, der unter anderem stark von den Treibstoffeigenschaften und somit der Treibstoffzusammensetzung abhängt. Konventionell hergestelltes Jet A-1 und ebenso die meisten alternativen Luftfahrttreibstoffe bestehen aus hunderten Einzelkomponenten. Um die Mehrkomponenten-Verdunstung realer Treibstoffe abbilden zu können, wurde in den letzten Jahren die Methode der kontinuierlichen Thermodynamik erfolgreich angewandt. Diese Methode ermöglicht es, Veränderungen in der Treibstoffzusammensetzung während der Verdunstung detailliert wiederzugeben und ist dabei sehr rechenzeiteffizient. Bisherige Validierungen wurden durch den Vergleich von Simulationsergebnissen mit Daten durchgeführt, die aus Experimenten an aufgehängten Tropfen gewonnen wurden. Durch diese Tests wurde die Funktionalität der Modelle erfolgreich nachgewiesen, jedoch war die Zusammensetzung der Treibstoffe nicht bekannt und die Tropfenaufhängung hatte einen stark intrusiven Einfluss. Damit sind bisherige Validierungen des Modelles nur auf qualitative Aussagen beschränkt. In dieser Arbeit wurde, basierend auf den Charakteristika aktueller sowie potentieller alternativer Treibstoffe, eine Validierungsdomain für Luftfahrttreibstoffe abgeleitet. Diese ermöglichte es, systematisch quantitative Metriken über die Genauigkeit von Verdunstungsmodellen für alternative Luftfahrttreibstoffe zu bestimmen. In einem neu entwickelten nicht intrusiven Validierungsexperiment wurde die Verdunstung verschiedener Treibstoffe aus der Validierungsdomain unter kontrollierten Bedingungen detailliert untersucht. Anhand der gewonnenen experimentellen Daten und Simulationsergebnisse konnten globale und lokale Validierungsmetriken abgeleitet werden. Der Einfluss von Unsicherheiten in den Spraystartbedingungen auf die Simulationsergebnisse wurde mittels Latin Hypercube Sampling bestimmt. Die resultierenden Unsicherheiten in den Simulationsergebnissen können als Präzision des Validierungsexperimentes interpretiert werden. Die Validierungsmetriken sowie die Information über die Präzision des Validierungsexperimentes ermöglichen dem zukünftigen Nutzer die Adäquatheit des Modells für den Einsatzbereich zu bewerten und gegebenenfalls Maßnahmen für die Verbesserung des Modells oder des Validierungsexperimentes vorzuschlagen