Helium Jet Research Articles

Color schlieren deflectometry study of jet mixing: effect of buoyancy and perforation

Mixing of jets is crucial for optimal performance of many industrial applications and there is a need to optimize both nozzle geometry and flow conditions. The present study reports the influence of buoyancy and perforation on mixing between a jet and its environment. Optical techniques are ideal for the study of jet mixing due to their non-intrusive and inertia free properties. The present study gives an account of mixing between helium jet and the ambient fluid using a combination of color schlieren deflectometry and radial tomographic mathematics. Four different perforation sizes have been used and the experiments are performed for Reynolds numbers 21–676 and Richardson numbers 3.27–0.0015. Color schlieren images show distinct influence of perforation and flow conditions (Richardson number). Oxygen concentration and jet width quantify effectiveness of jet mixing. Buoyancy plays an important role in mixing at high Richardson number. Perforation improves jet mixing i.e. there is about 120% increase in jet width and the size of perforation plays an important role.

Measurements of multiple mole fraction fields in a turbulent jet by simultaneous planar laser-induced fluorescence and planar Rayleigh scattering

This paper presents two-dimensional measurements of all individual mole fractions in a three-species, non-reacting turbulent flow, using planar laser-induced fluorescence (PLIF) and planar laser Rayleigh scattering. The flow is an axisymmetric jet of acetone and helium in an air coflow. PLIF measures the acetone mole fraction, while Rayleigh scattering measures a linear combination of the acetone and helium mole fractions. The simultaneous implementation of these techniques allows for the calculation of the helium and air mole fraction fields. The results of this diagnostic method are being used for the study of multicomponent molecular transport effects in turbulent fluid mixing.

Microstructure of fine-grained Nd–Fe–B sintered magnets with high coercivity

The microstructure of high-coercivity Nd–Fe–B sintered magnets with a grain size of about 1 μm, which was processed by helium jet milling and the pressless sintering process, was studied by scanning electron microscopy, transmission electron microscopy and atom probe tomography. The high coercivity can be attributed to a similar microstructure that is five times smaller than that of conventional sintered magnets. Initial magnetization curves showed a two-step magnetization process, suggesting that the grain boundary phase causes a pinning force.

Modelling of plasma bullet propagation along a helium jet in ambient air

The results of simulation of positive streamer propagation along a helium jet in ambient air are presented. A two-dimensional axially symmetric streamer model, accounting for variation of helium–air mixture composition in the jet, is used. The obtained distributions of plasma parameters have a ring-shaped structure, typical for plasma bullets. The calculated radial profiles of emitting nitrogen molecules agree with experimental data.

Millimeter-wave spectroscopy of titanium dioxide, TiO 2

The millimeter-wave rotational spectrum of TiO 2 in its ground vibrational state has been recorded in the frequency range from 248 to 345 GHz using the Cologne Supersonic Jet Spectrometer for Terahertz Applications (SuJeSTA). Forty-two b-type rotational transitions of the main isotopologue 48TiO 2 and five transitions of 46TiO 2 in natural abundance have been measured up to J = 22 and K a = 8 which corresponds to excitation temperatures of 170 K. TiO 2 was formed by laser ablation and adiabatically cooled in a supersonic jet of helium to rotational temperatures of 20 K. The new transitions have been analyzed together with previously reported data obtained from Fourier-transform microwave spectroscopy in the frequency range from 7 to 42 GHz. The improved and extended set of spectroscopic parameters provides accurate transition frequencies for future astronomical searches in the millimeter-wave region.

Simulation of streamers propagating along helium jets in ambient air: Polarity-induced effects

Results of modeling of streamer propagation along helium jets for both positive and negative polarities of applied voltage are presented. Obtained patterns of streamer dynamics and structure in these two cases are similar to those observed in experiments with plasma jets.

Current state-of-the-art manufacturing technology for He-cooled divertor finger

A divertor concept for DEMO has been investigated at Karlsruhe Institute of Technology (KIT) which has to withstand a heat flux of 10MW/m2. The design utilizes small finger module composed of a small tungsten tile brazed on a thimble made from tungsten alloy. The divertor finger is cooled by helium jet impingement at 10MPa and 600°C. The subject of this paper is technological studies on machining and braze joining the divertor components. Goal of this task, which is considered an important R&D issue, is to find out appropriate manufacturing methods to ensure high functionality and high reliability of the divertor as well as to meet the economic aspect. One of the major requirements for manufacturing is micro-crack-free surface of tungsten parts, since crack propagations in tungsten were observed in the previous high-heat-flux tests at Efremov. Different manufacturing methods and the corresponding results are discussed in the following report.

Stochastic dynamical model of intermittency in fully developed turbulence

A model of intermittency is presented in which the dynamics of the rates of energy transfer between successive steps in the energy cascade is described by a hierarchy of stochastic differential equations. The probability distribution of velocity increments is calculated explicitly and expressed in terms of generalized hypergeometric functions of the type (n)F(0), which exhibit power-law tails. The model predictions are found to be in good agreement with experiments on a low temperature gaseous helium jet. It is argued that distributions based on the functions (n)F(0) might be relevant also for other physical systems with multiscale dynamics.

Submillimeter-resolution radiography of shielded structures with laser-accelerated electron beams

We investigate the use of energetic electron beams for high-resolution radiography of flaws embedded in thick solid objects. A bright, monoenergetic electron beam (with energy >100 MeV) was generated by the process of laser-wakefield acceleration through the interaction of 50-TW, 30-fs laser pulses with a supersonic helium jet. The high energy, low divergence, and small source size of these beams make them ideal for high-resolution radiographic studies of cracks or voids embedded in dense materials that are placed at a large distance from the source. We report radiographic imaging of steel with submillimeter resolution.

Comparative Studies of Atmospheric Pressure Plasma Characteristics Between He and Ar Working Gases for Sterilization

Helium (He) and Argon (Ar) atmospheric pressure plasma jets operated with low-frequency power source are designed and studied. The current and voltage waveforms, formation of plasma jets, estimated rotational and vibrational temperatures, optical emission spectra, and numerical simulations for He and Ar gases are investigated to analyze the plasma characteristics. Ar plasma shows higher discharge current and many instantaneous current peaks compared with He plasma. For gas flow between 1 and 7 L/min and applied voltage between 3 and 10 kV, no significant changes in Ar plasma are observed. He plasma is found to be sensitive as far as gas flow rate and applied voltage are concerned. This sensitivity is associated with a transition from laminar to turbulent mode of gas flow. The estimated gas temperatures show higher values for Ar plasma than those of He plasma. Ar plasma jet emits extremely high intensity of OH (305 nm ~312 nm) and O (777 nm) compared with that emitted from He plasma jet. High concentration of OH and O in Ar plasma is related with high density of electrons with 4-5 eV, which is in the range of the dissociation energy of H-H, O-H, and O=O bonds. As a result, wider sterilization area and higher sterilization efficacy in indirect treatment are observed for Ar plasma than He plasma.

ChemInform Abstract: Spectroscopic Signatures of Structural Aufbau in (Benzene)n; n = 7-19.

Optical spectra of larger (benzene)n clusters, n=7–19, have been investigated in detail using the molecular B2u←A1g000 and 610 transitions. The clusters are formed in a helium jet and are detected mass selectively by two‐color resonant two‐photon ionization spectroscopy at moderate resolution. The association of spectral line shifts with distinct molecular sites leads to a simple picture of the building up of clusters of nonpolar molecules, as proposed earlier [Easter et al. Chem. Phys. Lett. 157, 277 (1989)]. Coarse band shapes and fingerprintlike fine structure are discussed in terms of the numbers and multiplicities of sites predicted by the icosahedral aufbau sequence, and yield a particularly clear correspondence around B13, whose quasiicosahedral structure (as determined by minimum‐energy simulations) forms the sequence’s core. Distinctive multiplet structure observed in the center of the 610 band for n=12–15 is attributed to exciton interactions among equivalent sites.

Applications of the dynamic mode decomposition

The decomposition of experimental data into dynamic modes using a data-based algorithm is applied to Schlieren snapshots of a helium jet and to time-resolved PIV-measurements of an unforced and harmonically forced jet. The algorithm relies on the reconstruction of a low-dimensional inter-snapshot map from the available flow field data. The spectral decomposition of this map results in an eigenvalue and eigenvector representation (referred to as dynamic modes) of the underlying fluid behavior contained in the processed flow fields. This dynamic mode decomposition allows the breakdown of a fluid process into dynamically revelant and coherent structures and thus aids in the characterization and quantification of physical mechanisms in fluid flow.

Exhaust of Underexpanded Jets from Finite Reservoirs

We examine the response of an underexpanded jet to a depleting, nite reservoir with experiments and simulations. An open-ended shock tube facility with variable reservoir length is used to obtain images of nitrogen and helium jet structures at successive instances during the blowdown from initial pressure ratios of up to 250. The reservoir and ambient pressures are simultaneously measured to obtain the instantaneous pressure ratio. We estimate the time-scales for jet formation and reservoir depletion as a function of the specic heat ratio of the gas and the initial pressure ratio. The jet structure formation time-scale is found to become approximately independent of pressure ratio for ratios greater than 50. In the present work, no evidence of time-dependence in the Mach disk shock location is observed for rates of pressure decrease associated with isentropic blowdown of a nite reservoir while the pressure ratio is greater than 15. The shock location in the nitereservoir jet can be calculated from an existing empirical t to innite-reserv oir jet data evaluated at the instantaneous reservoir pressure. For pressure ratios below 15, however, the present data deviate from a compilation of data for innite-reserv oir jets. A new t is obtained to data in the lower pressure regime. The self-similarity of the jet structure is quantied and departure from similarity is noted to begin at pressure ratios lower than about 15, approximately the same ratio which limits existing empirical ts.

Numerical and experimental investigation of buoyant gas release: Application to hydrogen jets

The physics of hydrogen release from compressed storage vessels is investigated for flow conditions corresponding to a subsonic turbulent jet venting into atmosphere at a Mach number ∼0.3 using large eddy simulations (LES) and particle image velocimetry (PIV) measurements. A major focus of the work is investigation of the dynamic features of the flow and mixing processes by analyzing the detailed instantaneous data available from LES. Simulations are first conducted for a helium jet with flow parameters corresponding closely to hydrogen jet release at low Mach number in order to allow direct comparison with PIV measurements. The simulations and measurements are consistent, and show that though exact self-similarity of the turbulent jet is not achieved due to buoyancy effects, the normalized averaged velocity and concentration profiles collapse reasonably well and can be approximated by a Gaussian distribution. Analysis of the instantaneous concentration fields shows that, due to the complex dynamics of the jet, which includes flapping and vortex shedding, concentration levels above the flammability threshold occur intermittently in regions where average concentrations are low. The spatial extent of regions where flammability thresholds are exceeded is nearly twice as large when considering transient effects compared to the mean (time-averaged) flow; the possible implications for safety guidelines are important. LES results are also presented for a hydrogen jet during the transient phase following the onset of release. Compared to the stationary jet flow, the radial extent of concentration above the flammability threshold is found to increase by ∼30% during transients as a result of the strong vortex ring that forms when the hydrogen jet penetrates stationary ambient air.

Generation of sound by flow instabilities in a low‐speed helium jet.

Schlieren images of low‐speed helium flow from a nozzle of circular cross section reveal instability in the jet occurring approximately 1 cm from the nozzle orifice. Accompanying this instability is a pronounced whistle. In this presentation, both qualitative and quantitative analyses of the observed phenomenon will be discussed.

Numerical investigation of potential elimination of ‘hot streaking’ and stratification in the VHTR lower plenum using helicoid inserts

The helium-cooled, high temperature Next Generation Nuclear Plant (NGNP) and Very High Temperature Reactor (VHTR) with prismatic type cores are being designed to operate at reactor exit temperatures ranging from 873 to 923 K and 1123 to 1223 K, respectively. The helium flow velocity in the coolant channels of the core is ∼70 m/s. The high-temperature helium jets exiting the coolant channels impinge onto the bottom plate in the lower plenum (LP), causing “hot spots” (“hot streaking”) and stratification due to the absence of proper mixing and the obstruction caused by the graphite support columns. In order to minimize or eliminate hot streaking and enhance helium mixing in the LP, this work investigates using static, quadruple helicoid inserts at the exit of the coolant channels. These inserts introduce radial and azimuthal momentum flow components, which result in extensive entrainment and mixing of the surrounding gas in the LP, significantly reducing the impingement onto the bottom plate, thereby minimizing hot streaking and stratification. Results of parametric analyses and a comparison of the flow fields of helium free conventional and swirling jets, and of a convectional jet in cross flow are presented and discussed. The analyses with helium at 1273 K and the dynamic Smagorinsky turbulence model are conducted using Fuego, a 3D, finite element, incompressible, reactive flow, massively parallel code with state-of-the-art turbulence models developed at Sandia National Laboratories. The calculations are benchmarked successfully by comparing the numerical results with experimental data and semi-empirical analytical expressions.

Quantitative planar imaging of turbulent buoyant jet mixing

Planar Rayleigh scattering provides quantitative mixing measurements in the developing region of axisymmetric turbulent helium jets issuing into air. The measurements focus on the relatively near field, in which the jets are primarily momentum driven. The imaging parameters are specified to ensure high spatial resolution. The mean jet fluid concentration fields attain self-similarity within the measurement region, though the forms of the mole fraction profiles indicate a reduction in turbulent transport at the jet outer boundary, arising from the reduced jet fluid density. Nevertheless, jet-like scaling pertains for the concentration fields. Mass fraction fluctuations on the jet centreline attain the expected asymptotic value of ≈23% of the centreline mass fraction values. The scalar dissipation rates, however, show an axial decay rate that is slower than theoretical predictions. The two-dimensional extent of the measurements also allows spatial filtering similar to that inherent in large-eddy simulations (LESs). The results confirm that fluctuation levels and scalar dissipation rates determined for the filtered fields are reduced as the effective resolution is reduced, but while the fluctuation profiles for the filtered fields are similar for the different filter sizes, the forms of the scalar dissipation profiles are highly dependent on filter size. These latter results in particular are of a form that will be useful for grid-dependent assessments of LES results.

Effect of nozzle sizes on jet impingement heat transfer in He-cooled divertor

The use of impinging jets for divertor cooling in the conceptual fusion power plant is attracting much attention due to its very high heat removal capability and moderate pumping power requirement. The latest and the most advanced divertor concept is based on modular design cooled by helium impinging jets. To reduce the thermal stresses, the plasma-facing side of the divertor is build up of numerous small cooling fingers cooled by an array of helium jets. In this study the influence of nozzle sizes on the heat transfer and flow characteristics of such cooling finger is investigated numerically. The main objective is to find an optimal size and distribution of nozzle diameters in the jet array in which the heat transfer would be the highest possible at an acceptable pressure drop through the cooling finger. Prior to nozzle diameters modification, the simulation results for the reference finger geometry were validated against high heat flux experiments. A good agreement was obtained. The nozzle diameters were then modified at two different mass flow rates (13.5 g/s and 6.8 g/s per cooling finger). The most critical design parameter of interest was the maximum thimble temperature, which is limited by the melting temperature of the filler material in the brazed finger joint. It has been found that an optimal jet arrangement should have equal nozzle diameters to reach the highest thimble temperature decrease, while keeping the pressure drop within reasonable limits.

A novel spectral analysis algorithm to obtain local scalar field statistics from line-of-sight measurements in turbulent flows

Statistical tomography to obtain local field variables from non-intrusive line-of-sight measurements in turbulent flows has been an intriguing subject for some time. In this study, a novel algorithm is presented to obtain statistical information on the local scalar field in axisymmetric turbulent flows. The algorithm uses line-of-sight transverse deflection angle measurements in only one view direction to greatly simplify the optical configuration. The validity of the algorithm is examined using noise-free synthetically generated scalar data that simulate the concentration field of a turbulent helium jet. Results show that the proposed algorithm provides excellent reconstruction of integral length scale and variance of refractive index difference, which can be related to scalar physical properties such as density, temperature and/or species concentrations. Good reconstruction accuracy and the need for a simple optical configuration make the proposed algorithm a promising method to characterize the scalar field in turbulent flows using path-integrated measurements.

Density and compressibility effects in turbulent subsonic jets part 1: mean velocity field

The behavior of compressible jets originated from initially turbulent pipe flows issuing in still air has been investigated at three different subsonic Mach numbers, 0.3, 0.6 and 0.9. Helium, nitrogen and krypton gases were used to generate the jet flows and investigate the additional effects of density on the flow structure. Particle image velocimetry, high-frequency response pressure transducers and thermocouples were used to obtain velocity, Mach number and total temperature measurements inside the flow field. The jets were formed at the exit of an adiabatic compressible frictional turbulent pipe flow, which was developing toward its corresponding sonic conditions inside the pipe, and continued to expand within the first four diameters distance after it exited the pipe. Theoretical considerations based on flow self-similarity were used to obtain the decay of Mach number along the centerline of the jets for the first time. It was found that this decay depends on two contributions, one from the velocity field which is inversely proportional to the distance from the exit and one from the thermal field which is proportional to this distance. As a result, a small non-linearity in the variation of the inverse Mach number with downstream distance was found. The decay of the Mach number at the centerline of the axisymmetric jets increases by increasing the initial Mach number at the exit of the flow for all jets. The decay of mean velocity at the centerline of the jets is also higher at higher exit Mach numbers. However, the velocity non-dimensionalized by the exit velocity seems to decrease faster at low exit Mach numbers, suggesting a reduced mixing with increasing exit flow Mach numbers. Helium jets were found to have the largest spreading rate among the three different gas jets used in the present investigation, while krypton jets had the lowest spreading rate. The spreading rate of each gas decreases with increasing its kinetic energy relatively to its internal energy.