Mixed Helium Research Articles

Investigation of kinetics of iso‐octane ignition under scramjet conditions

AbstractA single pulse reflected shock tube was used to investigate iso‐octane ignition over the temperature range of 900–1400 K at a pressure of ∼1 atm. To account for the anticipated long ignition delay times at the lower temperatures, long shock tube dwell times (∼12 ms) at lower temperature and near atmospheric pressure were achieved by using argon–helium mix as a driver gas. Chemical thermometer experiments were conducted to remove any uncertainties in determining postreflected shock temperatures. The ignition delay data obtained in this study are in good agreement (in the overlap region) with the iso‐octane ignition data from a previous shock tube study. However, the activation energy of iso‐octane ignition obtained in this study in the lower temperature region (<1300 K) is significantly smaller (∼15 kcal vs. ∼40 kcal) than that obtained in a previous higher temperature study. The deflagration may be responsible for lowering of activation energy under the conditions of this study. Two detailed iso‐octane kinetic models (Air Force Research Laboratory—AFRL and Curran et al.) were used to model the experimental results of this study. The AFRL mechanism showed a good agreement with the experimental iso‐octane ignition delays and their temperature dependence. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 194–201, 2006

Gas Permeability of Polymers

The recent article, “Applying Chemical Potential and Partial Pressure Concepts To Understand the Spontaneous Mixing of Helium and Air in a Helium-Inflated Balloon”, proposes a model of permeation untenable and in contradiction with correct statements present in the same text and consequently does not consider the material of which the investigated balloons are made.

Observation of Mixed Fermionic-Bosonic Helium Clusters by Transmission Grating Diffraction

Small weakly bound boson-fermion 4He(m) 3He(n) clusters formed in a free jet expansion are identified using nondestructive transmission grating diffraction. The observations confirm the existence of more than 11 very tenuous complexes including the three-body halo molecule 4He2 3He and the pseudo-Borromean complex 4He2 3He2. Effective cluster formation temperatures, extracted from a sudden freeze model for cluster growth using theoretical binding energies, increase smoothly with cluster size, thereby confirming the calculations with the possible exception of 4He2 3He2.

Definition and experimental evaluation of the smoke “confinement velocity” in tunnel fires

An experimental study is carried out on a reduced scale tunnel model (scale reduction is 1:20). The main objective is to evaluate the longitudinal velocity induced into a tunnel when a fire plume continuously released is confined and extracted between two exhaust vents located on both sides of the fire source. For the experimental simulations, fire-induced smoke is simulated by an air and helium mix release. Smoke flow is symmetrical as regards the fire location and experiments are realized for an half tunnel with only one vent activated downstream the source. The vent extraction flow rate is step by step increased and the length of the stratified smoke layer downstream the vent as well as the longitudinal fresh air flow induced, are measured. A confinement velocity is then associated to the minimum value of the longitudinal air flow needed to prevent the smoke layer propagation downstream the vent. This velocity is evaluated for several values of the fire heat release rate and finally compared with the corresponding critical velocity obtained for a longitudinal ventilation system.

Applying Chemical Potential and Partial Pressure Concepts To Understand the Spontaneous Mixing of Helium and Air in a Helium-Inflated Balloon

In developing this laboratory, our initial motivation for the analysis of gases in a balloon was to answer simple and basic questions, such as, Why does a helium-charged balloon left in the air always drop in a few days? Is leakage of helium the only cause of the drop? What is the composition of the gas in the balloon when it falls after deflation? Students were intrigued by these questions, too, as they analyzed the variation over time in the composition in a balloon inflated with helium. Using the concepts of partial pressure and chemical potential, the laboratory experiment described effectively investigates the diffusion process and the behavior of gas molecules for teaching these concepts in general and physical chemistry.

Shell structure in mixed3He−4Hedroplets

Due to the immiscibility of ${}^{3}\mathrm{He}$ into ${}^{4}\mathrm{He}$ at very low temperatures, mixed helium droplets consist of a core of ${}^{4}\mathrm{He}$ atoms coated by a ${}^{3}\mathrm{He}$ layer whose thickness depends on the number of atoms of each isotope. When these numbers are such that the centrifugal kinetic energy of the ${}^{3}\mathrm{He}$ atoms is small and can be considered as a perturbation to the mean-field energy, a novel shell structure arises, with magic numbers different from these of pure ${}^{3}\mathrm{He}$ droplets. If the outermost shell is not completely filled, the valence atoms align their spins up to the maximum value allowed by the Pauli principle.

ESR investigation of hydrogen and deuterium atoms in impurity-helium solids

Impurity-helium solids (Im–He solids) are porous solids created by injecting a beam of mixed helium and impurity gases into superfluid He4. In this work we use electron spin resonance (ESR) techniques to investigate Im–He solids containing atoms and molecules of hydrogen and/or deuterium. We have performed studies of low temperature (T∼1.35 K) tunnelling chemical reactions in which deuterium atoms replace the hydrogen atoms bound in H2 or HD molecules to produce large (up to 7.5×1017 cm−3) and relatively stable concentrations of free hydrogen atoms. The time dependence of H and D atom concentrations has been investigated for Im–He samples with different initial ratios of hydrogen and deuterium ranging from 1:20 to 1:1. The satellite ESR lines associated with the dipolar coupling of electron spins of H and D atoms to the nuclear moments of the hydrogen nuclei found in neighboring molecules have been observed in Im–He solids. The forbidden hyperfine transition of atomic hydrogen involving the mutual spin flips of electrons and protons has also been observed.

Infrared–microwave double resonance spectroscopy of OCS in pure He4 and mixed He4/He3 clusters

We have performed microwave–infrared double resonance measurements on OCS in pure He4 and mixed He4/He3 clusters. In contrast to our previous study on OCS in pure He4 cluster, where an asymmetric inhomogeneous linewidth increasing with rotational quantum number j was observed [S. Grebenev, M. Havenith, F. Madeja, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys. 113, 9060 (2000)], we find the lines in the mixed helium cluster to be much narrower, showing a symmetric, probably homogeneous line width. The spectra indicate a significant difference in the rotational relaxation of OCS in mixed clusters as compared to pure He4 cluster. We have applied the model of Lehmann [K. K. Lehmann, Mol. Phys. 97, 645 (1999)], describing the interaction of a molecule with the helium cluster, and modified it in order to describe particle in a box states of OCS in mixed helium clusters. The calculations indicate while in the mixed cluster the OCS is confined to the inner He4 core, the energy of the particle in the box states is increased as compared to the pure He4 cluster. Relaxation mechanisms via these particle in the box states and surface excitations (ripplons) of the cluster are discussed in the text.

The RELIEF flow tagging technique and its application in engine testingfacilities and for helium-air mixing studies

Raman excitation plus laser-induced electronic fluorescence (RELIEF)images the motion of oxygen molecules in air and other gas mixtures. This isaccomplished by tagging oxygen molecules through vibrational excitation andimaging them after a short period of time by laser-induced electronicfluorescence. The vibrational lifetime of oxygen is sufficiently long and thesignal sufficiently strong to allow this technique to be used over a widerange of flow conditions, from low subsonic to hypersonic, and in a variety ofgas mixtures including high humidity environments. The utilization of amolecular tagging technique such as this is critical for environments in whichseeding is impossible or unreliable and for measurements in which a wide rangeof scales needs to be observed simultaneously. Two experiments which have beenconducted at national laboratories in medium- to large-scale facilities arereported. At the Arnold Engineering Development Center, RELIEF was used toexamine velocity in a 1 m diameter tunnel for applications in the area ofengine testing. At the NASA Langley Research Center, RELIEF is being used toexamine supersonic mixing of helium in air in a coaxial jet in associationwith studies of fuel-air mixing in hypersonic engines. These applications aretwo examples of the wide range of practical uses for this new technology.

Investigations of a thermoacoustic refrigerator prototype

A thermoacoustic refrigerator prototype was built and tested. The system was designed to provide 140-W cooling power with a temperature lift of 40<th>°F, using a 55% mix of helium–argon at 20 bars pressure and operating at 180 Hz. Leaf springs were used to tune the mechanical driver system to be resonant at 154 Hz, such that the sum of mechanical and acoustical reactances is zero near 180 Hz. Initial testing achieved acoustic powers near 50 W, with an electroacoustic transduction efficiency of about 27% (the maximum efficiency is near 45%). Pressure amplitudes were near 3% of the mean in the stack region, and heat rejected to a secondary fluid was measured to be near 60 W. While this acoustic power level is near the target for the desired cooling load, initial tests indicate that the cooling power is well below 140 W. Results of continuing testing and investigations for determination of the cause of discrepancies in pressures, temperatures, and cooling power between measurement and model (Deltae) will be presented. The influence of DC flows, nuisance heat loads, gas mixture, and hardware (stack and heat exchanger) factors will be addressed. Accelerometer, thermocouple, and pressure sensor data will be presented and interpreted.

Globular Cluster Giant Branch Luminosity Functions: “Extra Stars” and “Deep Mixing”

We explore the possibility that is the reason that there are red giants in the Galactic globular clusters M5 and M30. Deep mixing has often been invoked to account for the abundance anomalies observed in many bright Galactic globular cluster giants. Here we assume that it also adds fuel to the hydrogen-burning shell of a typical giant in some clusters and returns helium to the envelope above. We use a simple mixing model to estimate how much fuel has been added to the hydrogen-burning shell of a typical red giant in M5 and M30 from the number of stars in the red giant branch (RGB) luminosity functions for these two clusters. Presumably, red giants are common in Galactic globular clusters. M5 and M30 were chosen solely because the available luminosity functions for both were derived from an exceptionally large sample of stars in two colors: the RGB luminosity functions are especially well defined and well normalized to unevolved main-sequence stars. If deep mixing is the reason that there are too many M5 giants, the number of giants suggests that the envelope of a typical M5 star should be significantly enriched in helium (ΔY ≈ 0.07) by the time it reaches the tip of the giant branch. Hence, deep mixing may be (at least part of) the reason that M5 has a surprisingly blue horizontal branch (HB) for a moderately metal-rich cluster. If deep mixing occurs and ΔY ≈ 0.07, we can account for part, but only a small part, of the very low value of R—and the surprisingly low estimate for Y—that Sandquist et al. found from their B-band luminosity function. Sandquist et al.'s value of the helium indicator delta suggests that the helium abundance is enhanced (ΔY = 0.04 ± 0.02) in HB stars at the blue edge of the instability strip. If deep mixing accounts for the extra giants that have been observed in M30, Y will be about 0.05 higher in the envelope of a typical M30 HB star than it was when that star left the main sequence. In addition to accounting for the M30 giants, deep mixing can give a good account of (1) the lack of a on the RGB or the presence of a small bump in the wrong place, (2) the surprisingly bright horizontal branch, and (3) the anomalous values of both Δ and R. The abundances of the light elements suggest that deep mixing is much more common in some clusters than in others and more extensive in some stars in a cluster than in others. If deep mixing does add fuel to evolving giants in some clusters, there are many interesting ramifications. For example, all horizontal branches may not have been created equal; some might well be brighter than others. Sweigart and Sweigart & Catelan have recently described how mixing helium out into the envelope of a red giant may solve a number of cluster blue star mysteries. We point out a number of additional mysteries that might be solved by looking at the fuel that is mixed down into the hydrogen-burning shell: e.g., the large range in the values that have been observed for R (the ratio of bright giants to HB stars), the observed range in the helium estimator Δ at a constant metallicity, and the variations observed in the RGB bumps among otherwise similar clusters.

Influence of Unbalanced Magnetron and Penning Ionization for RF Reactive Magnetron Sputtering

Thermochromic VO2 films were deposited by reactive rf magnetron sputtering using balanced and unbalanced magnetrons, and during deposition, helium was mixed into the carrier gas in order to activate a reactive gas (oxygen). It was clarified through Vsdc measurements that the plasma parameter was changed by varying the magnet configuration. The electron temperature and Penning ionization increased upon mixing helium into the carrier gas. These effects enabled VO2 films to be deposited under a wide range of sputtering conditions.

Aluminum Abundances on the Red Giant Branch: Testing Helium Mixing as a Second Parameter

We present new data and discuss the relationship between the Al abundances on the red giant branches (RGBs) and the horizontal-branch (HB) morphologies of the second-parameter pair of globular clusters M3 and M13. Since Al is produced during RGB evolution inside the H-burning shell where He is synthesized, its overabundance in stellar atmospheres can indicate He mixing, which has been shown to be a viable second parameter in explaining HB morpohogies.

Review of the research and development for insulation of superconducting fault-current limiters

Research and development of superconducting fault-current limiters (SCFCL) is ongoing in the world. This paper summarizes the fruits of this work, focusing especially on the electrical insulation. Developments of SCFCLs carried out up to now are briefly described. The essential points to be considered in the design of the electrical insulation of SCFCLs are presented. Definite examples of electrical insulation designs of 63 kV–1.25 kA rms–5.3 kA max SCFCL by GEC ALSTHOM and 69 kV–2.0 kA rms–4.0 kA max SCFCL by Los Alamos National Laboratory (LANL) are introduced. A summary of the experimental results for electrical insulation of 6.6 kV/2 kA single phase SCFCL by Toshiba/Tokyo Electric Power Company (TEPCO) is given. That is, a newly developed high Young's modulus insulating bobbin is clarified as being effective for removing the degradation of the superconducting coil of a SCFCL. Investigations of flashover characteristics of ceramic insulators in vacuum at low temperature and the electrical insulation characteristics of the model coil in liquid helium (LHe) and gas mixed liquid helium are described. The experimental results on a `prospective power transmission model system integrated under superconducting environment', abbreviated to PROMISE by Nagoya University are also briefly presented.

Terrigenous helium in deep-sea sediments

We have measured the isotope ratios of helium contained in various terrigenous materials that contribute to deep-sea sediments. These materials include ice-rafted debris from the North Atlantic, Chinese Loess, and sediment collected at or near the mouths of three large rivers: the Amazon, the Ganges, and the Yangtze. We observe terrigenous 3He/ 4He ratios that vary from 1.8 × 10 −9 to 4.6 × 10 −7, i.e., values that are higher than the theoretically-derived range of 10 −9 to 10 −8. Atlantic and Pacific deep-sea sediment 3He/ 4He ratios can be explained by mixing of helium from similar extraterrestrial but different terrigenous sources. Terrigenous sources for North Atlantic and North Pacific sediments are characterized by 3He and 4He contents that are higher, and 3He/ 4He ratios that are lower, than those for central and eastern equatorial Pacific sediments. This is consistent with the supply to the North Atlantic and North Pacific oceans of older cratonic continental material that contains high amounts of in situ-produced nucleogenic and radiogenic helium. Terrigenous material transported to central and eastern equatorial Pacific sediments contains lower amounts of 3He and 4He and higher 3He/ 4He ratios, indicative of supply from a more juvenile Andean source. In the equatorial Atlantic (core V31-135), we have used previously-published 230Th data to determine an extraterrestrial 3He flux of (1.16 ± 0.15) × 10 −12 cm 3STP · cm −2 · ka −1, within error of that previously determined in sediments from the equatorial Pacific Ocean ([0.78 ± 0.29] × 10 −13 cm 3STP · cm −2 · ka −1; Marcantonio et al., 1996).

Radio-frequency-preionized xenon z-pinch source for extreme ultraviolet lithography

Intense amplified spontaneous emission is generated in generally axial directions in a recombining uniform Z pinch. This effect allows the generation of highly efficient soft x-ray beams, including the intense xenon-band emission at 134 ?, of interest for extreme ultraviolet lithography. We discuss the characteristics of this source, including optimization of the xenon-helium mix and measurements of source size, brightness, and positional and amplitude stability. The issues involved in increasing power to the lithography class by an increase in the repetition rate are discussed. The life and operating costs of a lithography source are considered.

Structure and energetics of mixed4He-3He drops

Using a finite-range density functional, we have investigated the energetics and structural features of mixed helium clusters. The possibility of doping the cluster with a molecule of sulfur hexafluoride is also considered. It is seen that the repulsion introduced by the impurity strongly modifies the properties of the smallest drops. Although only a qualitative comparison is possible, the gross features displayed by our calculations are in agreement with recent experimental findings.

Can Helium Mixing Explain the Solar Neutrino Shortages?

Has the time come to abandon hope of an astrophysical—as distinguished from particle‐theoretic—solution of the solar neutrino puzzle? Two recent papers espouse different positions on this important issue. In the 18 November Physical Review Letters Wick Haxton (University of Washington) and Andrew Curnrning (a visiting undergraduate from Cambridge University), argued that there might still be an astrophysical explanation for the observed shortfall of 8B decay neutrinos from the Sun and for the even greater shortfall of solar neutrinos from electron capture by 7Be: namely the slow mixing of 3He down into the solar core. “This is a very speculative departure from the standard model of how the Sun works,” Haxton told us. “But we're all willing to carefully examine some rather exotic particlephysics solutions. So it may be equally important to explore unexpected solarphysics solutions.”

Effects of Helium Mixing on the Evolution of Globular Cluster Stars

New noncanonical sequences for the evolution of globular cluster stars have been computed in order to investigate the effects of mixing helium from the hydrogen shell into the envelope during the red giant branch (RGB) phase. The possible occurrence of such mixing, driven by internal rotation, is suggested by the observed abundance variations involving C, N, O, Na, and Al in globular cluster red giants. We find that helium mixing can substantially increase the envelope helium abundance. By increasing the RGB tip luminosity, helium mixing also leads to enhanced mass loss along the RGB. Both of these effects have a potentially large impact on the subsequent horizontal-branch (HB) evolution. In particular, helium mixing produces a bluer HB morphology, thereby making it easier to explain the hot HB population found in various stellar systems, as well as the difference in HB morphology between some second parameter pairs of globular clusters. Helium-mixed sequences show a larger RR Lyrae period shift and predict a smaller age for the metal-poor globular clusters. In addition, such sequences can reproduce the low gravities observed in blue HB stars.

Formation and Destruction of Silicon Monoxide in SN 1987A

We present a new chemistry model for the silicon monoxide molecules in SN 1987A. It incorporates an enhanced rate of formation by radiative association and an enhanced rate of destruction through charge transfer between SiO and Ar+ and Ne+, which is more efficient than the energetic electron impact ionization and dissociation in a previous model. The conclusion is unchanged that the 10−4–10−3 M☉ of SiO observed in SN 1987A can be produced by chemistry models in which there is no microscopic mixing of helium into the silicon-oxygen region of the ejecta.