Helium-air Mixture Research Articles

0110 2成分気体に対応した濃度計の研究(OS1-2 噴流,後流および剥離流れの流動解析と応用,オーガナイズドセッション)

This experimental study aims at measuring concentration of jet diffusion. Hot-wire concentration sensor to measure the concentration of the components in a binary mixture of gases is described. The concentration sensor consists of a concentration probe, a vacuum pump, a hot-wire amplifier. And the concentration probe consists of a brass sonic nozzle, a brass tube, a tungsten hot-wire to measure gas thermal conductivity of gas mixture. There are two main parameter in King's equation which are velocity and thermal conductivity. The sonic nozzle decreases the effect of velocity. The new probe calibrated with helium jet. The calibration curve was made with helium-air mixture. And the time constant was 0.9 ms. Contour of velocity and concentration of helium jet diffusion was measured.

1904 低密度気体噴流に生じるサイドジェットの出現条件(S21-1 噴流,後流及びはく離流れの流動解析と応用(1),21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

A natural development process of a low-density jet, such as a helium gas, is different from that of a constant-density jet. Side jets, which are radial ejections, are formed at vicinity of the nozzle exit and mixing of fluids between the jet and the ambient is enhanced. Main parameter to distinguish formation of the side jet is density ratio of jet fluid to ambient fluids. The other parameters are the Reynolds number and the shear layer thickness at the nozzle exit. Helium gas, helium-air mixture, and air are discharged from a round nozzle upward and visualized by planar laser Mie scattering (PLMS). The region for formation of the side jets in the density ratio-Reynolds number plane is illustrated. The velocity profile at the nozzle exit is measured by a hot-wire concentration probe and a hot-wire probe, and the momentum thickness at the exit is given. The region of the side jets in the density ratio-momentum thickness domain is shown.

A Convection Heat Transfer Correlation for a Binary Air-Helium Mixture at Low Reynolds Number

The results of experiments investigating heat transfer from a hot wire in a binary mixture of air and helium are reported. The measurements were made with a constant temperature anemometer at low Reynolds numbers (0.25<Re<1.2) and correlated by treating the data in terms of a suitably defined Reynolds and Nusselt numbers based on the wire diameter. The correlation was obtained by taking into account the temperature dependency of gas properties, properties of binary gas mixtures, and the fluid slip at the probe surfaces as well as gas accommodation effects. The correlation has been used to measure velocity and velocity-density statistics across a buoyancy driven Rayleigh–Taylor mixing layer with a hot wire. The measured values obtained with the correlation agree well with measurements obtained with a more rigorous and extensive calibration technique (at two different overheat ratios). The reported correlation technique can be used as a faster and less expensive method for calibrating hot wires in binary gas mixtures.

Acoustic Simulation of Coaxial Hot Air Jets Using Cold Helium-Air Mixture Jets

This work examines the ability of small-scale helium-air mixture coaxial jets to simulate the acoustics of large-scale hot air jets representing the exhaust of separate-flow turbofan engines. Experiments employed a one-eighth-scale model of a separate-flow nozzle used in hot tests at NASA John H. Glenn Research Center. Comparisons were conducted for two set points using the following methods: matching velocity and density, and matching velocity and Mach number. For both methods, the helium-air data compare well with the hot data in all measures of noise: spectral shapes, spectral levels, and overall sound pressure levels. The method of matching velocity and Mach number gives slightly better agreement in the spectral shapes at angles close to the jet axis and in the overall sound pressure levels. The overall agreement between the hot air and helium-air mixture data is within 1.2 dB.

Flywheel energy storage—An upswing technology for energy sustainability

Flywheel energy storage (FES) can have energy fed in the rotational mass of a flywheel, store it as kinetic energy, and release out upon demand. It is a significant and attractive manner for energy futures ‘sustainable’. The key factors of FES technology, such as flywheel material, geometry, length and its support system were described, which directly influence the amount of energy storage and flywheel specific energy. It is very suitable to such applications that involve many charge–discharge cycles and little in the way of long-term storage applications including International Space Station (ISS), Low Earth Orbits (LEO), overall efficiency improvement and pulse power transfer for Hybrid Electric Vehicles (HEVs), Power Quality (PQ) events, and many stationary applications. Design margins, fault protection and containment were considered as three good approaches to solve safety issue. Vacuum enclosures or helium–air mixture gas condition were discussed for solution of windage energy loss. In short, with the aid of new technologies the cost of FES can be lowered and the FES will play a significant role in securing global energy sustainability.

Modeling the chemical kinetics of high-pressure glow discharges in mixtures of helium with real air

Atmospheric and near-atmospheric pressure glow discharges generated in both pure helium and helium-air mixtures have been studied using a plasma chemistry code originally developed for simulations of electron-beam-produced air plasmas. Comparisons are made with experimental data obtained from high-pressure glow discharges in helium-air mixtures developed by applying sinusoidal voltage wave forms between two parallel planar metallic electrodes covered by glass plates, with frequencies ranging from 10to50kHz and electric field strengths up to 5kV∕cm. The code simulates the plasma chemistry following periodic pulsations of ionization in prescribed E∕N environments. Many of the rate constants depend on gas temperature, electron temperature, and E∕N. In helium plasmas with small amounts (∼850ppm) of air added, rapid conversion of atomic helium ions to molecular helium ions dominate the positive ion kinetics and these species are strongly modulated while the radical species are not. The charged and neutral species concentrations at atmospheric pressure with air impurity levels up to 10000ppm are predicted. The negative ion densities are very small but increase as the air impurity level is raised, which indicates that in helium-based systems operated in open air the concentration of negative ions would be significant. If water vapor at typical humidity levels is present as one of the impurities, hydrated cluster ions eventually comprise a significant fraction of the charged species.

Statistically steady measurements of Rayleigh-Taylor mixing in a gas channel

A novel gas channel experiment is described to study the development of high Atwood number Rayleigh-Taylor mixing. Two gas streams, one containing air and the other containing a helium-air mixture, flow parallel to each other separated by a thin splitter plate. The streams meet at the end of a splitter plate leading to the formation of an unstable interface and of buoyancy driven mixing. This buoyancy driven mixing experiment allows for long data collection times, has short transients, and is statistically steady. The facility was designed to be capable of large Atwood number studies (At∼0.75). We describe initial validation work to measure the self similar evolution of mixing at density differences (0.035<At<0.1). The purpose of this paper is to describe the new high Atwood number gas channel facility and present validation results for experimental runs at Atwood numbers up to 0.1. Diagnostics include a constant temperature hot wire anemometer, and high resolution digital image analysis. The hot-wire probe gives velocity statistics of the mixing layer. A multiposition single-wire technique was used to measure the velocity fluctuations in three mutually perpendicular directions. Analysis of the measured data was used to explain the mixing as it develops to a self-similar regime in this flow. A digital image analysis procedure was used to characterize various properties of the flow and also to validate the hot wire measurements.

Safety oriented analysis of cold helium–air mixture formation and stratification

Cryogenic installations of superconducting accelerators contain significant amount of high density helium, mostly filling the cryostats and headers located in underground tunnels and caverns. In the case of emergency relief of the helium, oxygen deficiency hazard arises. The paper proposes a description of helium–air mixture formation based on turbulent jet physics. A condition of stratified flow onset is given resulting from dimensional analysis. A performed experiment of cold helium discharge to an instrumented test tunnel enabled identification of different stratified flow types together with corresponding measurements of oxygen concentration and temperature drop. The results can be used for safety analysis of oxygen deficiency hazard in confined areas, especially in the tunnels.

Novel applications of the flywheel energy storage system

Flywheel energy storage system is focused as an uninterruptible power supplies (UPS) from the view point of a clean ecological energy storage system. However, in high speed rotating machines, e.g. motor, generator and flywheel, the windage loss amounts to a large ratio of the total losses. The reason is that windage loss is proportional to the cube of its angular velocity; a windage loss may lead to the reduction of total system efficiency. To cope with this problem, Ajisman et al. proposed the use of helium–air mixture gas into the housing and indicated that the helium (50 vol%)–air (50 vol%) mixture gas can reduce the windage loss to 42% of that in the air (100 vol%) case. Helium is the second lightest and smallest monatomic molecule gas. Its molecular weight and gas density are about 1/7 those of air, thermal conductivity is 10 times as large as that of air. Then, enclosing helium–air mixture gas into the housing of rotating machine, a large amount of windage loss can be reduced. In our first work, applying this mixture gas to the conventional flywheel UPS, we indicate that idling energy loss of the flywheel UPS which is caused by the rotation can be easily reduced, and thus the energy storage efficiency can be improved. Second, we propose one of the novel utilization of a low speed steel flywheel energy storage system for a momentary power failure called a momentary voltage drop.

Temperature variation correction for aspirating probes in helium–air mixtures

An empirical correction procedure for the temperature sensitivity of hot film, aspirating probes in helium–air mixtures has been established. It has been demonstrated experimentally that, within the full concentration range and a fairly wide temperature range, the concentration and temperature sensitivities are decoupled. The procedure consists of calibrating the probe in mixtures of varying concentration under a constant reference temperature and then applying a correction to the probe voltage output that takes into account the difference between the flow and reference temperatures. This correction was found to be proportional to the temperature difference.

Acoustic and Mean Flow Measurements of High-Speed, Helium-Air Mixture Jets

The effectiveness of using helium-air mixture jets to simulate heated jets at both subsonic and supersonic velocities is investigated. Particular emphasis is placed on replicating the aeroacoustic properties of heated jets. Acoustic directivity and spectral measurements of helium-air mixture jets are compared to existing heated jet data. Results typically indicate agreement within 2 dB with some discrepancy attributable to facility and run condition differences. In addition, mean flowfield measurements indicate a shortening of the potential core and a slight decrease in jet spreading with the addition of helium – the same trends observed for heated jets. The similarities in acoustic and mean flow behavior indicate that helium-air mixture jet experiments provide a viable low-cost alternative to heated jet experiments.

Atmospheric Pressure RF (13.56 MHz) Glow Discharge: Characterization and Application to “In Line” Waste Water Treatment

In this work the results obtained from the experimental study of an Atmospheric Pressure Glow Discharge (APGD) appear, generated in Helium (He) and dry air mixture by using a radio frequency (RF 13.56 MHz) power source. The external conditions of the discharge, its macroscopic parameters (i.e., electronic number density ne and temperature Te) and the results of its application to “in line” industrial and domestic waste water treatment are presented.

Noise Measurements in Supersonic Jets Treated with the Mach Wave Elimination Method

We report noise measurements for perfectly expanded coaxial jets composed of a supersonic primary stream at velocity of 920 m/s and a coflow stream at conditions designed to prevent formation of Mach waves. Both the primary and secondary streams consisted of helium-air mixtures to simulate approximately the conditions of hot flows, The resulting sound field was compared to that emitted by a single jet at the conditions of the primary stream. Overall sound pressure levels (OASPL) and noise spectra were obtained at many radial and azimuthal positions around the jet exit. Equal-thrust comparisons were made by using geometric scaling. At equal thrust, Mach wave elimination reduced the near-field OASPL by 11 dB and the far-field OASPL by 5 dB. The mid-to-high-frequency region of the spectrum, which is most pertinent to aircraft noise, was reduced by 20 dB in the near field and by 9 dB in the far field. It is shown that Mach waves account for at least 85 % of the sound field most relevant to aircraft noise.

Hearing threshold of divers in hyperbaric helium-air gas mixture

Hearing threshold of divers in hyperbaric helium-air gas mixture

Behavior of naturally unstable and periodically forced axisymmetric buoyant plumes of helium and helium–air mixtures

Effects of forcing on the behavior of naturally unstable buoyant plumes of helium and helium/air mixtures are investigated experimentally. Axisymmetric buoyant plumes originating from a 10 cm diam nozzle are perturbed in a periodic manner by an upstream loudspeaker introducing a sinusoidal streamwise velocity oscillation with a peak-to-peak velocity magnitude of about 3% of the mean velocity at the nozzle exit. The experimental conditions in this study corresponded to Richardson number, Ri=[(ρ∞−ρp)gd]/ρ∞V02, of 42. Video images of the naturally unstable plumes as well as the forced plumes were analyzed to study the features of the resulting flow oscillations and the vortical structures. It was found that the plume responds readily to the imposed oscillations with the toroidal vortices forming at the forcing frequency. Plume images indicate a more complicated and turbulent state of the flow within the large-scale toroidal vortices as they convect downstream. These vortical structures have lateral dimensions of the order of the nozzle diameter at low oscillation frequencies, but the vortex length scales become smaller at higher forcing frequencies. Additionally, mushroom-shaped smaller-scale vortex pairs appear in the very early part of the forced plumes that are absent in the unforced case. Frequency spectrum of the plume centerline velocity fluctuations, detected by two total pressure probes located at one-half and two diameter heights above the nozzle exit, show that the plume predominantly responds to the imposed flow excitation. However, as the flow evolves downstream, frequency spectra exhibit a broader range of frequencies some of which do not coincide with the imposed one and its harmonics. The streamwise convection velocities of the large-scale vortical structures are not influenced by the imposed oscillation frequency. As the excitation frequency approaches that of the natural frequency of the flow, a more chaotic behavior of the large-scale vortices is observed in terms of their convection velocity. This is in contrast to the behavior of momentum dominated shear flows (jets and mixing layers), where a large degree of spatial and temporal coherence of the flow is attained when the flow is excited at its natural frequency. To complement these experiments and probe into the nature of the axisymmetric plume instabilities, we also visualized the transitional behavior of these buoyant plumes from their initially non-oscillatory state to the final periodic state. This is important as far as identifying the nature of the instability as either absolute or convective. Our findings show that the experiments exhibit features that point toward a convective-type instability.

Helium-Based Simulation of Smoke Spread Due to Fire in Enclosed Spaces

This study concerns helium smoke simulators which have been used for simulating smoke spread in aircraft cabins during inflight tests. The smoke from cabin fires is buoyant due to higher temperatures, compared to the surroundings. However, the smoke from commercially available smoke generators is nonbuoyant and an addition of helium is employed to make it buoyant so that the actual circumstance is closely represented. The dependence of buoyancy on the concentration of helium and Ihe validity of this approach to represent thermal buoyancy effects are important questions that need to be answered. In this technique, helium is added to smoke from commercially available smoke generators in order to provide buoyancy in the flow due to the lower density of helium. The mixture is then used to simulate the flow of smoke and toxic gases in the cabin of an aircraft, or in other enclosed spaces, without employing heated air to obtain the buoyancy. Since buoyancy plays a very important role in the flow and spread of smoke and hot gases, it is important that the simulation include this effect in order to closely represent the actual flow An experimental investigation is undertaken to explore the validity of simulating thermally buoyant gases, from a fire, wilh a helium and air mixture. The experimental flow consisted of a ceiling jet with a corner flow. This same configuration was used in previous studies which were directed at thermally buoyant flows. Thus, a basis for comparison between the two buoyant flows can be established. Parameters, such as the volume flow rate of the air and helium and the initial helium concentration, were varied over wide ranges. Temperature profiles from the thermally buoyant case were compared to helium concentration profiles. In addition, velocity profiles from the two studies were compared to determine the flow spread. From these results, the characteristics of the buoyant helium-air mixture were obtained. It is found that, though this mixture can be used to qualitatively model thermally buoyant flows, large quantitative differences exist between the two and must be taken into account for an accurate simulation.

Measurements of Instantaneous Velocity Field of a Non-Reacting Pulsating Buoyant Plume by Particle Image Velocimetry

abstract Digital particle image velocimetry (DPIVj was employed to measure velocity field of a naturally pulsating plume of helium-air mixture. Velocity measurements were made across a diametric plane of an axisymmetric plume originating from a lOcm. diameter circular nozzle at a Richardson number of 283. The plume oscillated at a frequency between 3.5 and 4.0 Hz. Measurements were performed in a phase resolved manner to characterize the different phases of the plume velocity field during its oscillation period. Because of the large field of view, velocity field was assembled from DPIV data in sub regions at the same phase of the oscillation. For the periodic flow studied, the repeatability and matching of velocities at sub region boundaries were found to be very good. The obtained velocity fields show a strong acceleration along the plume axis within one diameter height above the nozzle. The strong upward acceleration is accompanied by a radial inflow which fixes the entrainment rate of ambient fluid into the plume. Formation of a toroidal vortex structure occurs at around a height of one half nozzle diameter which coincides with the region where the streamwise acceleration goes through zero. Subsequently, the toroidal vortex convects downstream significantly influencing the flow field in its vicinity. Entrainment is predominantly a result of periodic engulfment of ambient air into the plume.

Density and velocity measurements in turbulent He-air boundary layers

A turbulent boundary layer with large density variations has been generated by tangential injection of air or helium into a boundary layer of air-helium mixtures. Instrumentation based on thermoanemometry has been successfully developed for the investigation of this flow. Analysis of the mean and fluctuating density fields shows that the flow is mainly governed by the ratio of the injection to the external velocity and that the density ratio plays a secondary role in the mixing process. A slight enhancement of turbulent activity is, however, observed when helium is injected.

An improved method for measuring velocity and concentration by thermo-anemometry in turbulent helium-air mixtures

A hot wire/film probe is used with new operating conditions for measurements of velocity and concentration in helium/air mixtures. With the upstream wire much more overheated than the downstream film, the sensing elements behave almost independently and the calibration map is composed of clearly distinct iso-velocity and iso-concentration curves. This behaviour is largely explained by the difference in the heated elements’ diameters. Tests of the probe in a turbulent boundary layer with large density variations indicate very satisfactory measurements. The domain of use for the probe is considerably extended with these new operating conditions.

Experiments on the oscillatory behavior of buoyant plumes of helium and helium-air mixtures

Experiments on the oscillatory behavior of axisymmetric buoyant plumes of helium and helium-air mixtures are reported for a range of nozzle diameters (3.6 cm<d<20 cm), source velocities, and plume densities. Measurements include pulsation frequencies as determined from total pressure fluctuations along the plume centerline in addition to the phase resolved laser Doppler velocity measurements. These nonreacting buoyant plumes are found to exhibit periodic oscillations of plume boundaries which subsequently evolve into toroidal vortices within one-half diameter above the nozzle exit. These oscillations and vortices are similar to those observed in pool fires, although their frequency scaling is somewhat different. The frequency relationship is well represented by the expression S=0.8Ri0.381, where the Strouhal number is S=fd/V0 and the Richardson number is defined as Ri1=[(ρ∞−ρp)gd]/ρ∞V20. Parameters f, V0, ρ are frequency, source velocity, and density and subscripts p and ∞ refer to the plume fluid and ambient, respectively. Between Ri1=100 and 500, a transition in the frequency scaling is observed as evidenced by more turbulent and vigorously mixing plumes beyond this transition. In this region, S=2.1Ri0.281. This change in scaling can be explained by the effect of turbulent mixing on local plume density and the resulting modification of the convection speed of the toroidal vortices. These results provide a consistent basis for the mechanism of the observed instability which is quite different than other types of flow instabilities. Additionally, the phase resolved velocity field of a pulsating buoyant plume reveals a strong buoyant acceleration along the plume centerline followed by a deceleration in the region of the toroidal vortex formation. The strong upward acceleration is also accompanied by significant radial inflow toward the plume centerline determining the entrainment characteristics of these pulsating buoyant plumes.