Helium Jet Research Articles

A quantitative experiment on the fountain effect in superfluid helium

Superfluid helium, a state of matter existing at low temperatures, shows many remarkable properties. One example is the so called fountain effect, where a heater can produce a jet of helium. This converts heat into mechanical motion; a machine with no moving parts, but working only below 2 K. Allen and Jones first demonstrated the effect in 1938, but their work was basically qualitative. We now present data of a quantitative version of the experiment. We have measured the heat supplied, the temperature and the height of the jet produced. We also develop equations, based on the two-fluid model of superfluid helium, that give a satisfactory fit to the data. The experiment has been performed by advanced undergraduate students in our home institution, and illustrates in a vivid way some of the striking properties of the superfluid state.

Overview of Thermal Hydraulics of Helium-Cooled Solid Divertors

Developing ways to effectively remove the extremely high heat fluxes incident on the plasma-facing components is an important challenge for magnetic fusion energy (MFE). In most cases, the target plates of the divertor, which removes helium ash and other impurities from the core plasma, are subject to the most extreme conditions, with steady-state incident heat fluxes of at least 10 MW/m2. Starting from the early 1990s, a variety of divertor designs with target plates of tungsten (W), cooled for the most part by impinging jets of helium (He), have been investigated.This paper reviews and discusses a number of these impinging-jet concepts, including the modular He-cooled finger-type configurations developed by the Karlsruhe Institute of Technology (KIT), as well as the T-tube divertor, the helium-cooled flat-plate (HCFP) divertor, and the combined plate/finger divertor, all evaluated as part of the ARIES studies. Over the last 15 years, a number of studies have shown that the steady-state thermal and structural performance of single units of a number of these divertor designs can be evaluated with reasonable accuracy under prototypical conditions using a combination of numerical simulations and experimental studies. The helium-cooled modular jet (HEMJ) design has been successfully tested at incident heat fluxes as great as 13 MW/m2 at prototypical conditions. Although it remains unclear how much neutron irradiation damage will affect W, or other armor materials, He jet-impingement cooling is a leading candidate for resolving power exhaust heat removal issues in plasma-material interactions.

Electroconductive and magnetic properties of pure carbon soot produced in arc discharge: Regimes of various buffer gas pressure

The electroconductive and magnetic properties of nanomaterials containing carbon nanoforms synthesised for electrocatalytic, electrochromic and ferrofluid applications, etc., are of interest to researchers. This article aims to understand the physical effects influencing the electroconductive and magnetic properties of materials synthesised by arc discharge. The nanomaterial formation processes leading to the formation of magnetic and electroconductive structures are also discussed. Arc discharge between graphitic electrodes results in the emergence of a fan‐shaped jet of helium and carbon. The gasdynamic and temperature parameters of the jet depend on the parameters of the arc discharge and the buffer gas. Variations in the parameters of the carbon vapour flow change the kinetics of carbon condensation, leading to variations in the morphology and structure of the carbon material. The main external parameter of the synthesis (buffer gas pressure) was varied in the study, and correlations between the intensity ratio of the D to G peaks on the Raman spectrum and the electrical conductivity and the magnetic susceptibility values were found. During the synthesis, nanographite structures were formed. However, the formation of an amorphous carbon structure on the free ‘zig‐zag’ edges of the graphite fragments reduced the magnetic susceptibility, the electrical conductivity and the ID/IG ratio.The amorphous carbon layer was deposited on the graphite structure, which reduces magnetic susceptibility, electrical conductivity and the ID/IG ratio.

Numerical modelling of isothermal release and distribution of helium and hydrogen gases inside the AIHMS cylindrical enclosure

Hydrogen is a highly flammable gas and accidental release in confined space can pose serious combustion hazards. Numerical studies are required to assess the formation of flammable hydrogen cloud within confined spaces. In the present study, numerical investigations on the release of helium and hydrogen gases as high-velocity jets and their subsequent distribution inside an unventilated cylindrical enclosure (AIHMS facility) has been carried out as a first step towards numerical studies on hydrogen distribution in confined spaces for safety assessments. Experimental data for jet release of helium at volume Richardson number 0.1 and subsequent distribution has been used as benchmark data. Sensitivity studies on the influence of grid sizes, time-steps and turbulence models are performed. The performance of the validated numerical model is evaluated using statistical performance parameters. Similarity relations are used to determine input parameters for hydrogen jet for corresponding experimental data with helium jets. Finally, the mixing and flammability aspects of hydrogen distribution inside the enclosure are studied using four numerical indices that quantify mixing and deflagration potential of a distribution. It is concluded that the helium experiments can be used for validation of numerical models for hydrogen safety studies and any one of the similarity relationships, viz., equal buoyancy, equal volumetric flow, or equal concentration can be used for assessing the behaviour of hydrogen release and distribution within confined spaces.

PIV Validation of 3D Multicomponent Model for Cold Spray Within Nitrogen and Helium Supersonic Flow Field

This study presents the validation of a developed three-dimensional multicomponent model for cold spray process using two particle image velocimetry (PIV) experiments. The k-e type 3D model developed for spherical titanium particles was validated with the measured titanium particle velocity within a nitrogen and helium supersonic jet. The 3D model predicted lower values of particle velocity than the PIV experimental study that used irregularly shaped titanium particles. The results of the 3D model were consistent with the PIV experiment that used spherical titanium powder. The 3D model simulation of particle velocity within the helium and nitrogen jet was coupled with an estimation of titanium particle temperature. This was achieved with the consideration of the fact that cold spray particle temperature is difficult and expensive to measure due to considerably lower temperature of particles than thermal spray. The model predicted an interesting pattern of particle size distribution with respect to the location of impact with a concentration of finer particles close to the jet center. It is believed that the 3D model outcomes for particle velocity, temperature and location could be a useful tool to optimize system design, deposition process and mechanical properties of the additively manufactured cold spray structures.

Taylor cone and electrospraying at a free surface of superfluid helium charged from below.

Electrically charged metallic micro- and nanoparticles are trapped under a free surface of superfluid He in a vertical static electric field. We observe a static deformation of the charged liquid surface in the form of a Taylor cone and the emission of a charged liquid helium jet (electrospray). Our numeric calculations reproduce the static shape of the cone.

A comprehensive statistical investigation of schlieren image velocimetry (SIV) using high-velocity helium jet

A detailed statistical assessment of seedless velocity measurement using Schlieren Image Velocimetry (SIV) was explored using open source Robust Phase Correlation (RPC) algorithm. A well-known flow field, an axisymmetric turbulent helium jet, was analyzed near and intermediate region $$(0\le x/d\le 20)$$ for two different Reynolds numbers, Re d = 11,000 and Re d = 22,000 using schlieren with horizontal knife-edge, schlieren with vertical knife-edge and shadowgraph technique, and the resulted velocity fields from SIV techniques were compared to traditional Particle Image Velocimetry (PIV) measurements. A novel, inexpensive, easy to setup two-camera SIV technique had been demonstrated to measure high-velocity turbulent jet, with jet exit velocities 304 m/s (Mach = 0.3) and 611 m/s (Mach = 0.6), respectively. Several image restoration and enhancement techniques were tested to improve signal to noise ratio (SNR) in schlieren and shadowgraph images. Processing and post-processing parameters for SIV techniques were examined in detail. A quantitative comparison between self-seeded SIV techniques and traditional PIV had been made using correlation statistics. While the resulted flow field from schlieren with horizontal knife-edge and shadowgraph showed excellent agreement with PIV measurements, schlieren with vertical knife-edge performed poorly. The performance of spatial cross-correlations at different jet locations using SIV techniques and PIV was evaluated. Turbulence quantities like turbulence intensity, mean velocity fields, Reynolds shear stress influenced spatial correlations and correlation plane SNR heavily. Several performance metrics such as primary peak ratio (PPR), peak to correlation energy (PCE), the probability distribution of signal and noise were used to compare capability and potential of different SIV techniques.

Spectroscopic studies of the parameters of plasma jets during their propagation in the background plasma on the PF-3 facility

This paper presents measurement results of neon and helium plasma parameters in axial jets generated in plasma focus discharge. They were obtained in the course of experiments on laboratory modeling of astrophysical jets performed at the PF-3 facility. The plasma concentration was determined according to Stark broadening of spectral lines; the ionization temperature was determined by the average ion charge. The values of the concentration and temperature of jet plasma and background plasma at two distances from the pinch are also presented. In addition, an estimation was made of the heat content losses of the neon and helium jets during their movement through the surrounding medium.

Propulsive jet simulation with air and helium in launcher wake flows

The influence on the turbulent wake of a generic space launcher model due to the presence of an under-expanded jet is investigated experimentally. Wake flow phenomena represent a significant source of uncertainties in the design of a space launcher. Especially critical are dynamic loads on the structure. The wake flow is investigated at supersonic ( $$M=2.9$$ ) and hypersonic ( $$M=5.9$$ ) flow regimes. The jet flow is simulated using air and helium as working gas. Due to the lower molar mass of helium, higher jet velocities are realized, and therefore, velocity ratios similar to space launchers can be simulated. The degree of under-expansion of the jet is moderate for the supersonic case ( $$p_\mathrm{e}/p_\infty \approx 5$$ ) and high for the hypersonic case ( $$p_\mathrm{e}/p_\infty \approx 90$$ ). The flow topology is described by Schlieren visualization and mean-pressure measurements. Unsteady pressure measurements are performed to describe the dynamic wake flow. The influences of the under-expanded jet and different jet velocities are reported. On the base fluctuations at a Strouhal number, around $$\mathrm{St}_D \approx 0.25$$ dominate for supersonic free-stream flows. With air jet, a fluctuation-level increase on the base is observed for Strouhal numbers above $$\mathrm{St}_D \approx 0.75$$ in hypersonic flow regime. With helium jet, distinct peaks at higher frequencies are found. This is attributed to the interactions of wake flow and jet.

A technique to determine the thermal stability of uracil and uracil derivatives in a molecular beam

Uracil derivatives, especially synthesised for use in positron emission tomography studies, are heated to form a gas phase source and introduced into a vacuum system by seeding into a helium jet. Through time of flight mass spectroscopy, these uracil compounds are shown to have resisted any thermal dissociation during the heating phase of introduction into the jet. The successful introduction into a beam clearly demonstrates that dissociative electron attachment, or other scattering or spectroscopic studies, could be performed on these compounds with confidence that the parent molecule remains intact, at least at the temperatures considered in this study.

Numerical simulation of the interaction between helium jet flow and an atmospheric-pressure “plasma jet”

In this work we study the interaction of an atmospheric-pressure “plasma jet” with the hydrodynamic flow of the working gas. The study is based on the comparison between numerical simulation results and experimental data collected from the literature. Plasma reactors of three different configurations are considered, using a simple model, which focuses on the electro-hydrodynamic force importance. The objective is to evaluate the ability of the model to capture the resulting interaction between the “plasma jet” and the working gas for different reactor configurations. It is also aimed to find out possible correlations between the main parameters of the system, which may be useful for theoretical model development and reactor improved designing. In the context of the present model, it is assumed that the local electro-hydrodynamic force can be expressed via the product of a constant-motive part, which depends on the plasma setup and parameters, with the working gas local concentration, which expresses the dependence of the ionization rate on the gas concentration. The simulation results unveil that the constant-motive part is independent of the flow rate and inversely proportional to the diameter of the dielectric tube of the plasma reactor.

How to assess the plasma delivery of RONS into tissue fluid and tissue

The efficacy of helium (He) and argon (Ar) plasma jets are being investigated for different healthcare applications including wound and cancer therapy, sterilisation and surface disinfections. Current research points to a potential link between the generation of reactive oxygen and nitrogen species (RONS) and outcomes in a range of biological and medical applications. As new data accrue, further strengthening this link, it becomes important to understand the controlled delivery of RONS into solutions, tissue fluids and tissues. This paper investigates the use of He and Ar plasma jets to deliver three RONS (hydrogen peroxide—H2O2, nitrite— and nitrate—) and molecular oxygen (O2) directly into deionised (DI) water, or indirectly into DI water through an agarose target. The DI water is used in place of tissue fluid and the agarose target serves as a surrogate of tissue. Direct plasma jet treatments deliver more RONS and O2 than the through-agarose treatments for equivalent treatments times. The former only deliver RONS whilst the plasma jets are ignited; the latter continues to deliver RONS into the DI water long after the plasmas are extinguished. The He plasma jet is more effective at delivering H2O2 and directly into DI water, but the Ar plasma jet is more effective at nitrating the DI water in both direct and through-agarose treatments. DI water directly treated with the plasma jets is deoxygenated, with the He plasma jet purging more O2 than the Ar plasma jet. This effect is known as ‘sparging’. In contrast, for through-agarose treatments both jets oxygenated the DI water. These results indicate that in the context of direct and indirect plasma jet treatments of real tissue fluids and tissue, the choice of process gas (He or Ar) could have a profound effect on the concentrations of RONS and O2. Irrespective of operating gas, sparging of tissue fluid (in an open wound) for long prolonged periods during direct plasma jet treatment, could have implications for healthy tissue function; whilst through-tissue plasma jet treatment may provide a method to reperfuse oxygen-starved tissue. The assays described in this paper can be readily adopted (by others) and may support the future development of plasma sources to deliver specific (metered) doses of RONS.

Comparison of two-layer model for hydrogen and helium jets with notional nozzle model predictions and experimental data for pressures up to 35 MPa

A two-layer, reduced order model of high pressure hydrogen jets was developed which includes partitioning of the flow between the central core jet region leading to the Mach disk and the supersonic slip region around the core. The flow after the Mach disk is subsonic while the flow around the Mach disk is supersonic with a significant amount of entrained air. This flow structure significantly affects the hydrogen concentration profiles downstream. The predictions of this model are compared to previous experimental data for high pressure hydrogen jets up to 20 MPa and to notional nozzle models and CFD models for pressures up to 35 MPa using ideal gas properties. The results show that this reduced order model gives better predictions of the mole fraction distributions than previous models for highly underexpanded jets. The predicted locations of the 4% lower flammability limit also show that the two-layer model much more accurately predicts the measured locations than the notional nozzle models. The comparisons also show that the CFD model always underpredicts the measured mole fraction concentrations.

Visualization of Ambient Mass Spectrometry with the Use of Schlieren Photography.

This manuscript outlines how to visualize mass spectrometry ambient ionization sources using schlieren photography. In order to properly optimize the mass spectrometer, it is necessary to characterize and understand the physical principles of the source. Most commercial ambient ionization sources utilize jets of nitrogen, helium, or atmospheric air to facilitate the ionization of the analyte. As a consequence, schlieren photography can be used to visualize the gas streams by exploiting the differences in refractive index between the streams and ambient air for visualization in real time. The basic setup requires a camera, mirror, flashlight, and razor blade. When properly configured, a real time image of the source is observed by watching its reflection. This allows for insight into the mechanism of action in the source, and pathways to its optimization can be elucidated. Light is shed on an otherwise invisible situation.

Study on helium impingement cooling for a sharp leading edge subject to aerodynamic heating

This paper presents numerical studies on a typical actively-cooled thermal protection system (TPS) which turns out to be a potential candidate for the leading edge thermal management. The leading edge is cooled by the impingement jet of gaseous helium. The external aerodynamic heating and radiation, the heat conduction in solid wall, and the internal impingement cooling are coupled in a unified computational fluid dynamics system by using a quasi-coupling method. The thermal-hydraulic performance of internal impingement cooling is particularly highlighted. Four metal alloys are tested in order to find out the impact of the material property on the TPS’s cooling performance. Results show that the reduction of leading edge temperature is remarkable with the impingement cooling scheme and the maximum temperature can be controlled far below the materials’ melting point. However, the thermal stress does not significant decrease and is still a barrier for the application of the actively-cooled TPS. The operation limits of various materials are demonstrated, which may benefit to the future material selection work. Besides, the coolant flow rate requirements for safely operating the TPS under various flight conditions are ascertained.

Development of thermal mixing enhancement method for lower plenum of the High Temperature Test Facility

The High Temperature Gas-cooled Reactor (HTGR) is one of the most mature Gen IV reactor concepts under development today. The High Temperature Test Facility (HTTF) at Oregon State University is a test facility that supports the R&D needs for HTGRs. This study focuses on the issue of helium mixing after the core section in the HTTF, the results of which are generally applicable in HTGRs. In the HTTF, hot helium jets at different temperatures are supposed to uniformly mix in the lower plenum (LP) chamber. However, the level of mixing is not sufficient to reduce the peak helium temperature before the hot jet impinges the LP structure, which can cause issues with structural materials and operational issues in the heat exchanger downstream.The maximum allowable temperature variation in the outlet duct connected to the lower plenum is defined as 40K (±20K from the average temperature), while the CFD simulations of this study indicate that the reference design suffers temperature variations in the duct as high as 100K. To solve this issue, the installation of mixing-enhancing structures within the outlet duct were proposed and analyzed using CFD modeling. We show that using either an optimized “Kwiat” structure (developed in this study) or a motionless mixer installed in the outlet duct, the temperature variations can be brought dramatically, with acceptable increases in pressure drop. The optimal solution appears to be to install double motionless mixers with long blades in the outlet duct, which brings the temperature variation into the acceptable range (from 100K down to 18K), with a resulting pressure drop increase in the HTTF loop of 0.73kPa (6% of total pressure drop).

Supersonic jets of hydrogen and helium for laser wakefield acceleration

The properties of laser wakefield accelerated electrons in supersonic gas flows of hydrogen and helium are investigated. At identical backing pressure, we find that electron beams emerging from helium show large variations in their spectral and spatial distributions, whereas electron beams accelerated in hydrogen plasmas show a higher degree of reproducibility. In an experimental investigation of the relation between neutral gas density and backing pressure, it is found that the resulting number density for helium is ∼30% higher than for hydrogen at the same backing pressure. The observed differences in electron beam properties between the two gases can thus be explained by differences in plasma electron density. This interpretation is verified by repeating the laser wakefield acceleration experiment using similar plasma electron densities for the two gases, which then yielded electron beams with similar properties.

Effects of metastable species in helium and argon atmospheric pressure plasma jets (APPJs) on inactivation of periodontopathogenic bacteria

The helium and argon have been widely used as discharge gases in atmospheric pressure plasma jets (APPJs) for bacteria inactivation. The APPJs show apparent different in bullet propagation speed and bacteria inactivation rate apparently vary with discharge gas species. This work shows that these two distinctive features of APPJs can be linked through one factor, the metastable energy level. The effects of helium and argon metastable species on APPJ discharge mechanism for reactive oxygen nitrogen species (RONS) generation in APPJs are investigated by experiments and numerical estimation. The discharge mechanism is investigated by using the bullet velocity from the electric field which is obtained with laser induced fluorescence (LIF) measurement. The measured electric field also applied on the estimation of RONS generation, as electron energy source term in numerical particle reaction. The estimated RONS number is verified by comparing NO and OH densities to the inactivation rate of periodontitis bacteria. The characteristic time for bacteria inactivation of the helium-APPJ was found to be 1.63 min., which is significantly less than that of the argon-APPJ, 12.1 min. In argon-APPJ, the argon metastable preserve the energy due to the lack of the Penning ionization. Thus the surface temperature increase is significantly higher than helium-APPJ case. It implies that the metastable energy plays important role in both of APPJ bullet propagation and bacteria inactivation mechanism.

Optical properties of the atmospheric pressure helium plasma jet generated by alternative current (a.c.) power supply

In this work, an atmospheric pressure plasma jet (APPJ) was produced to generate cold flowing post-discharge plasma of pure helium gas. The main aim of this study was to generate cold flowing APPJ of pure helium gas and to determine how their optical emission spectrum change influences varying different flow rates. Lengths of early, middle, and late post-discharge plasma (jet) regions and their fluctuations were determined, respectively. Then, ignition condition dependence of the post-discharge plasma for flow rate was specified at a constant voltage. Spectroscopic studies of an atmospheric pressure plasma jet of helium were presented via analyzing OH, N2, N2+, oxygen, and helium intensities for various flow rates.

Tomographic characterisation of gas-jet targets for laser wakefield acceleration

Laser wakefield acceleration (LWFA) has emerged as a promising concept for the next generation of high energy electron accelerators. The acceleration medium is provided by a target that creates a local well-defined gas-density profile inside a vacuum vessel. Target development and analysis of the resulting gas-density profiles is an important aspect in the further development of LWFA.Gas-jet targets are widely used in regimes where relatively high electron densities over short interaction lengths are required (up to several millimetres interaction length, plasma densities down to ~1018cm−3). In this paper we report a precise characterisation of such gas-jet targets by a laser interferometry technique. We show that phase shifts down to 4mrad can be resolved. Tomographic phase reconstruction enables detection of non-axisymmetrical gas-density profiles which indicates defects in cylindrical nozzles, analysis of slit-nozzles and nozzles with an induced shock-wave density step. In a direct comparison between argon and helium jets we show that it cannot automatically be assumed, as is often done, that a nozzle measured with argon will provide the same gas density with helium.