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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Effects of equivalence ratio transient duration on self-excited combustion instability time scales in a single-nozzle combustor

Fuel staging is an effective way to attenuate combustion instability in lean, premixed combustion systems. Combustion instability occurs due to the coupling between heat release rate oscillations and combustor acoustics. In our experiments, we seek to understand how transient fuel staging affects the structure and dynamics of a single flame to help inform our previous studies of multi-flame systems. In the current experiments, fuel staging is achieved by changing the equivalence ratio in the nozzle. The equivalence ratio in the nozzle is varied over two different timescales, or transient durations, which are 1 ms and 4 s. For each duration, we also vary the percentage change in the equivalence ratio, with levels of ±3.57% and ±7.14% of the mean equivalence ratio, ϕ=0.7. Flame images recorded using CH* chemiluminescence imaging are analyzed to show the variations of the phase difference between pressure and heat release oscillations during transients. Phase difference transition behaves similarly in the flame base region but differently in flame/wall interaction region. More similarities between short and long duration transients are observed in combustion instability onset direction than decay direction.

Chen, Xiaoling↗
DOE OSTI2021

New instrumentation and tools to study dynamic (non-equilibrium) physico-chemical interfacial processes across large length, rates, and time scales (Final Report)

The hydrophobic interaction dictates the behavior of many systems relevant to biological and industrial processes. However, the origin of the hydrophobic interaction has been extensively debated. In this article, we demonstrate the use of nanostructured surfaces to control the range of the hydrophobic interaction. We measure the strength and range of the hydrophobic interaction using bubble probe force microscopy, and interpret our results using the Stokes–Reynolds–Young–Laplace model. Our findings give insights into the mechanisms of the hydrophobic interaction and will lead to better understanding and control of diverse colloidal and interfacial phenomena.

Moskovits, Martin↗
DOE OSTI2012

Joint EM-NE-International Study of Glass Behavior over Geologic Time Scales - 12303

Vitrification has been chosen as the best demonstrated available technology for waste immobilization worldwide. To date, the contributions of physical and chemical processes controlling the long-term glass dissolution rate in geologic disposal remain uncertain, leading to a lack of international consensus on a rate law for glass corrosion. Existing rate laws have overcome uncertainty through conservatism, but a thorough mechanistic understanding of waste form durability in geologic environments would improve public and regulator confidence. If it is possible to take credit for the true durability of the waste form in repository system evaluations, then it is possible to design the repository with less conservatism with concomitant cost savings. To gain a fundamental understanding of the dissolution rate law, six nations have joined together to formulate a joint plan for collaborative research into the mechanisms controlling the long-term corrosion of glass. This report highlights the technical program plan behind the US portion of this effort, with an emphasis on the current understanding (and limitations) of several mechanistic theories for glass corrosion. Some recent results are presented to provide an example of the ongoing research. Atom probe tomography has been used to provide a high-resolution analysis of elemental concentration gradients present at the hydrated glass / pristine glass interface in SON68 after 25.75 years of corrosion in a simulated granitic groundwater at 90 deg. C. The most valuable result of these initial studies is the success of the technique. Characterization by APT had never been previously demonstrated for glass corrosion layers. The resolution of APT is a powerful addition to the tools with which we can investigate the mechanisms dominating glass corrosion. Some other key results of this study include the observation that the elemental interfacial width between the hydrated glass and pristine glass appears to be much sharper (∼2 nm for B, Na and Al) than had been previously measured using nanoSIMS (∼240 nm). It is not clear whether the APT analysis and nanoSIMS characterizations were possibly performed on topographically unique regions, or whether nanoSIMS overestimated the elemental width. However, the APT data seems very convincing that the elemental width can be much sharper than was previously thought. This result calls into question some of the assumptions made for the diffusion-control models of glass dissolution, since such a sharp profile would not match the diffusion coefficients used to date. Other results, such as the observation of apparently layered concentration profiles, show that gel evolution is not as simple as is currently assumed in nearly every model. This task is a good example of the collaborative nature of the I-TEAM effort. Based on experimental needs and differences in expertise, scientists from DOE and CEA worked together to change the level of understanding in the field. These types of interactions are nearly ubiquitous among the tasks in the technical program plan. With the excellence of the team in place and the willingness of the participants to work together for a common understanding, the stated goal of consensus on the mechanistic basis for radionuclide release from glass is well within reach. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
NASA NTRS1986

Impulsively generated fast coronal pulsations

Rapid oscillations in the corona are discussed from a theoretical standpoint, developing some previous work on ducted, fast magnetoacoustic waves in an inhomogeneous medium. In the theory, impulsively (e.g., flare) generated mhd (magnetohydrodynamic) waves are ducted by regions of low Alfven speed (high density) such as coronal loops. Wave propagation in such ducts is strongly dispersive and closely akin to the behavior of Love waves in seismology, Pekeris waves in oceanography and guided waves in fiber optics. Such flare-generated magnetoacoustic waves possess distinctive temporal signatures consisting of periodic, quasi-periodic and decay phases. The quasi-periodic phase possesses the strongest amplitudes and the shortest time scales. Time scales are typically of the order of a second for inhomogeneities (coronal loop width) of 1000 km and Alfven speeds of 1000/kms, and pulse duration times are of tens of seconds. Quasi-periodic signatures have been observed in radio wavelengths for over a decade and more recently by SMM. It is hoped that the theoretical ideas outlined may be successfully related to these observations and thus aid the interpretation of oscillatory signatures recorded by SMM. Such signatures may also provide a diagnostic of coronal conditions. New aspects of the ducted mhd waves, for example their behavior in smoothly varying as opposed to tube-like inhomogeneities, are currently under investigation. The theory is not restricted to loops but applied equally to open field regions.

Edwin, P. M.↗
DOE OSTI2020

Temporal dynamics of large-scale structures for turbulent Rayleigh–Bénard convection in a moderate aspect-ratio cylinder

We investigate the spatial organization and temporal dynamics of large-scale, coherent structures in turbulent Rayleigh–Bénard convection via direct numerical simulation of a 6.3 aspect-ratio cylinder with Rayleigh and Prandtl numbers of 9.6×10 7 and 6.7, respectively. Fourier modal decomposition is performed to investigate the structural organization of the coherent turbulent motions by analysing the length scales, time scales and the underlying dynamical processes that are ultimately responsible for the large-scale structure formation and evolution. We observe a high level of rotational symmetry in the large-scale structure in this study and that the structure is well described by the first four azimuthal Fourier modes. Two different large-scale organizations are observed during the duration of the simulation and these patterns are dominated spatially and energetically by azimuthal Fourier modes with frequencies of 2 and 3. Studies of the transition between these two large-scale patterns, radial and vertical variations in the azimuthal energy spectra, as well as the spatial and modal variations in the system's correlation time are conducted. Rotational dynamics are observed for individual Fourier modes and the global structure with strong similarities to the dynamics that have been reported for unit aspect-ratio domains in prior works. It is shown that the large-scale structures have very long correlation time scales, on the order of hundreds to thousands of free-fall time units, and that they are the primary source for a horizontal inhomogeneity within the system that can be observed during a finite, but a very long-time simulation or experiment.

42 ENGINEERING↗
DOE OSTI2020

Four Theories of the Madden‐Julian Oscillation

Abstract Studies of the Madden‐Julian Oscillation (MJO) have progressed considerably during the past decades in observations, numerical modeling, and theoretical understanding. Many theoretical attempts have been made to identify the most essential processes responsible for the existence of the MJO. Criteria are proposed to separate a hypothesis from a theory (based on the first principles with quantitative and testable assumptions, able to predict quantitatively the fundamental scales and eastward propagation of the MJO). Four MJO theories are selected to be summarized and compared in this article: the skeleton theory, moisture‐mode theory, gravity‐wave theory, and trio‐interaction theory of the MJO. These four MJO theories are distinct from each other in their key assumptions, parameterized processes, and, particularly, selection mechanisms for the zonal spatial scale, time scale, and eastward propagation of the MJO. The comparison of the four theories and more recent development in MJO dynamical approaches lead to a realization that theoretical thinking of the MJO is diverse and understanding of MJO dynamics needs to be further advanced.

58 GEOSCIENCES↗
NASA NTRS1980

Fine-scale characteristics of interplanetary sector

The structure of the interplanetary sector boundaries observed by Helios 1 within sector transition regions was studied. Such regions consist of intermediate (nonspiral) average field orientations in some cases, as well as a number of large angle directional discontinuities (DD's) on the fine scale (time scales 1 hour). Such DD's are found to be more similar to tangential than rotational discontinuities, to be oriented on average more nearly perpendicular than parallel to the ecliptic plane to be accompanied usually by a large dip ( 80%) in B and, with a most probable thickness of 3 x 10 to the 4th power km, significantly thicker previously studied. It is hypothesized that the observed structures represent multiple traversals of the global heliospheric current sheet due to local fluctuations in the position of the sheet. There is evidence that such fluctuations are sometimes produced by wavelike motions or surface corrugations of scale length 0.05 - 0.1 AU superimposed on the large scale structure.

Behannon, K. W.↗