Solid Tracer Research Articles

Sub-micron solid air tracers for quantum vortices and liquid helium flows.

The dynamics of quantized vortices in superfluids has received increased attention recently because of novel techniques developed to visualize them directly. One of these techniques [G. P. Bewley et al., Nature 441, 588 (2006)] visualized quantized vortices and their reconnections in superfluid flows of (4)He by using solid hydrogen tracers of micron-size or larger. The present work improves upon the previous technique by using substantially smaller particles created by injecting atmospheric air diluted in helium gas. These smaller particles are detectable thanks to the higher index of refraction of nitrogen compared to hydrogen and thanks to an improved visualization setup. The optical counting estimate, which agrees with terminal velocity estimates, suggests that the tracer diameter is typically 400 ± 200 nm and could be as small as 200 nm; being smaller, but not so small as to be influenced by thermal motion, the particles get trapped on the vortices faster, perturb the vortices less, possess smaller Stokes drag, and stay trapped on fast-moving vortices, as also on vortices generated closer to the superfluid transition temperature. Unlike the past, the ability to create particles in the superfluid state directly (instead of creating them above the λ-point and cooling the fluid subsequently), ensures greater temperature stability for longer periods, and enables the tracking of long and isolated vortices. These advantages have also led to the direct visualization of Kelvin waves. The use of other seed gases could lead to the visualization of even smaller tracers for quantized vortices. We discuss the visualization setup and provide suggestions for further improvement.

Analysis of motion of solid hydrogen tracer particles in oscillating superfluid flows

We have developed a relatively simple cryostat which allows us to image turbulent flows in superfluid helium at temperatures below 2 K, using frozen H2 particles. We analyze the statistics of the velocities of these solid tracers, which follow the turbulent flow generated by oscillating bodies. We have also studied one of the oscillators working in air at room temperature, and traced the flow with solid talcum particles for comparison. Images were recorded by a digital camera at 240 frames per second, while frequencies of the oscillators are between 20 to 45 Hz. The flow is characterized by a modified Reynolds number Reδ based on the viscous penetration depth δ. Software in a dedicated particle tracking velocimetry code allows us to compute the trajectories and velocities of tens of thousands of particles. We have obtained the number of particles for equally spaced intervals of the velocity modulus. For the oscillators in the superfluid, the probability of finding particles at higher velocities has an exponential decay. Within our resolution the statistics in the superfluid for oscillating objects with sharp borders is largely independent of Reδ, while the logarithmic decay at low velocities seems faster than for high velocities for rounded objects. On the other hand, for data taken in air the result is closer to a classical Gaussian distribution of velocities.

Gas–solid flow behavior and contact efficiency in a circulating-turbulent fluidized bed

Abstract Circulating-turbulent fluidized bed (C-TFB) was characterized by high solid holdup, homogenous axial and radial flow structure, no net downflow of solids and high contact efficiency in this study. These flow dynamic properties were mainly represented by solid holdup profiles, the identification of flow regime and cluster dynamics in a riser, 100–150 mm in diameter and 10.06 m in height. To quantify the effect of novel diameter-expanding structure on flow dynamics, a parameter (contact efficiency) was introduced firstly. The effects of gas–solid interaction on flow performance were investigated by a fiber-optic probe to detect solid holdup. CO2 tracer injection and sampling system were used to characterize the flow structure and define the contact efficiency. The experimental results showed that flow regime in C-TFB is belonging to dense riser upflow (DRU) or dense-suspension upflow (DSU) regimes and transient region disappears in axial position. Flow structure is different from previous studies about traditional circulation fluidized bed (CFB) due to the effect of expanding structure and ring-feeder internal. A flow model based on the profiles of solid holdup and CO2 tracer concentration was proposed to account for the gas–solid contact efficiency in the reactor. The contact efficiency in C-TFB is much higher than that of high-density circulating fluidized bed (HDCFB), which means C-TFB reactor would exhibit better performance to optimize the product distribution under the same operating conditions.

A double-spike method for K–Ar measurement: A technique for high precision in situ dating on Mars and other planetary surfaces

A new method for K–Ar dating using a double isotope dilution technique is proposed and demonstrated. The method is designed to eliminate known difficulties facing in situ dating on planetary surfaces, especially instrument complexity and power availability. It may also have applicability in some terrestrial dating applications. Key to the method is the use of a solid tracer spike enriched in both 39Ar and 41K. When mixed with lithium borate flux in a Knudsen effusion cell, this tracer spike and a sample to be dated can be successfully fused and degassed of Ar at <1000°C. The evolved 40Ar∗/39Ar ratio can be measured to high precision using noble gas mass spectrometry. After argon measurement the sample melt is heated to a slightly higher temperature (∼1030°C) to volatilize potassium, and the evolved 39K/41K ratio measured by Knudsen effusion mass spectrometry. Combined with the known composition of the tracer spike, these two ratios define the K–Ar age using a single sample aliquot and without the need for extreme temperature or a mass determination. In principle the method can be implemented using a single mass spectrometer.Experiments indicate that quantitative extraction of argon from a basalt sample occurs at a sufficiently low temperature that potassium loss in this step is unimportant. Similarly, potassium isotope ratios measured in the Knudsen apparatus indicate good sample-spike equilibration and acceptably small isotopic fractionation. When applied to a flood basalt from the Viluy Traps, Siberia, a K–Ar age of 351±19Ma was obtained, a result within 1% of the independently known age. For practical reasons this measurement was made on two separate mass spectrometers, but a scheme for combining the measurements in a single analytical instrument is described. Because both parent and daughter are determined by isotope dilution, the precision on K–Ar ages obtained by the double isotope dilution method should routinely approach that of a pair of isotope ratio determinations, likely better than ±5%.

C143 界面の光学的特性を用いた固気液三相流の空間速度分布計測(OS-9:沸騰・凝縮伝熱および混相流の最近の展開(2))

Bubbly flows are found in a lot of industrial fields such as power and chemical plants and water purification system. PIV is useful tool to investigate the bubble motion and flow structure in the liquid phase simultaneously. In the present study, we have developed velocity measurement system of solid-gas-liquid three phase flow by phase discrimination using interferometric imaging of scattered light. We applied this system to microbubble flow, measuring area was about 9.6mm×8.0mm and we could discriminate the bubbles from the solid tracers within a range of ±1mm from the focal plane. Moreover, bubbles going up and solid tracers settling out were captured. From these results, it is concluded that this system can be applied successfully to measurement of the velocity distribution of three phase flows.

Counts of high-redshift GRBs as probes of primordial non-Gaussianities

We propose to use high-redshift long $\gamma$-ray bursts (GRBs) as cosmological tools to constrain the amount of primordial non-Gaussianity in the density field. By using numerical, N-body, hydrodynamic, chemistry simulations of different cosmological volumes with various Gaussian and non-Gaussian models, we self-consistently relate the cosmic star formation rate density to the corresponding GRB rate. Assuming that GRBs are fair tracers of cosmic star formation, we find that positive local non-Gaussianities, described in terms of the non-linear parameter, \fnl, might boost significantly the GRB rate at high redshift, $z \gg 6$. Deviations with respect to the Gaussian case account for a few orders of magnitude if \fnl$\sim 1000$, one order of magnitude for \fnl$\sim 100$, and a factor of $\sim 2$ for \fnl$\sim 50$. These differences are found only at large redshift, while at later times the rates tend to converge. Furthermore, a comparison between our predictions and the observed GRB data at $z > 6$ allows to exclude large negative \fnl, consistently with previous works. Future detections of any long GRB at extremely high redshift ($z\sim 15-20$) could favor non-Gaussian scenarios with positive \fnl. More stringent constraints require much larger high-$z$ GRB complete samples, currently not available in literature. By distinguishing the contributions to the GRB rate from the metal-poor population III regime, and the metal-enriched population II-I regime, we conclude that the latter is a more solid tracer of the underlying matter distribution, while the former is strongly dominated by feedback mechanisms from the first, massive, short-lived stars, rather than by possible non-Gaussian fluctuations. This holds quite independently of the assumed population III initial mass function.

Testing the performance of a cryogenic visualization system on thermal counterflow by using hydrogen and deuterium solid tracers

An experimental apparatus has been designed to analyze by visualization cryogenic flows of liquid (4)He and consequently address unresolved problems of quantum turbulence. The newly implemented flow visualization setup is described and its specific features discussed. Thermal counterflow experiments have been performed and the motion of solid hydrogen and deuterium tracers studied by using the particle tracking velocimetry technique in order to probe the system capabilities. It is shown that the obtained results are consistent with the two-fluid model describing the behavior of superfluid (4)He. A number of technical and fundamental issues, such as particles' aggregation, role of rotating particles in counterflow and evidence of non-Gaussian distribution of tracers' velocities, are also discussed. The apparatus appears to be well-suited to the task of analyzing cryogenic flows and potentially capable of obtaining new results stimulating further understanding of the underlying physics.

Analysis of tracer particle characteristics for micro PIV in wall-bounded gas flows

The investigation of the gas flow in a working fuel cell by means of optical measurement techniques like micro Particle-Image Velocimetry (µPIV) requires the generation of suitable tracer particles. In this work the analysis of tracer particle characteristics is described which serves as a means to identify suitable particles for wall-bounded gas flows. Several materials and different types of particle generators were examined to check for obtainable particle size distributions and particle concentrations. A simple experiment was designed to investigate the capability of the generated particles to adequately follow the flow. An optically transparent micro-channel with a 90° elbow was manufactured and the µPIV measurement technique is applied. Firstly, the gas-flow seeded with tracer particles is investigated within this 90° elbow micro-channel. Secondly, to check whether the measured flow structure in this previous case matches with the real flow, the same flow conditions are investigated using water as working fluid with solid tracer particles taking into account Re-ynolds number similarity. Thirdly, CFD-calculations using the same reference parameters as in the experimental investiga-tions were performed to quantify the deviation of the particle traces from the real flow streamlines. The results show ethy-lene glycol to be a suitable tracer material since the obtained tracer particles are optically detectable without severe image post-processing and since this material can be easily transformed into an aerosol with suitable concentrations and particle size distributions. The application of the µPIV technique on such a gaseous particle laden flow provided promising results concerning the intended application of this technique to operating fuel cells.

Simulation of Counterflow Turbulence by Vortex Filaments

By using the vortex filament model with the full Biot-Savart law, we have succeeded for the first time in generating the statistically steady state of counterflow turbulence in superfluid 4He under periodic boundary conditions. This state exhibits the characteristic relation $L=\gamma^{2} v_{\mathit{ns}}^{2}$ between the line-length density L and the counterflow relative velocity v ns and there is quantitative agreement between the coefficient γ and some measured values. Since we obtained the realistic state of quantum turbulence, we will use the numerical data to study the statistical property. We focus on the statistics of vortex reconnections from about 350 events in our steady counterflow turbulence simulation, and characterize the dynamics by the minimum separation distance between two reconnecting vortices, which was used in the visualization experiments using the solid hydrogen tracer particles by Paoletti et al. From our analysis we may conclude that the quantized circulation is still the dominant controlling feature and obtains the statistics of the correction factor.

Particle tracer response across shocks measured by PIV

The experimental approach used for the evaluation of the particle response time across a stationary shock wave is assessed by means of PIV measurements. The study focuses on the experimental requirements for a reliable and unbiased measurement of the particle response time τ p and length ξ p based on a single-exponent decaying law. A numerical simulation of the particle response experiment returns the parameters governing the measurement: namely the normalized spatial and temporal resolution, shock strength, and digital resolution. Representing the velocity decay in logarithmic coordinates it is shown that measurements performed with laser pulse separation time up to τ p and interrogation window up to ξ p still yield unbiased results for the particle response. A set of experiments on the particle response across a planar oblique shock wave was conducted to verify the results from the numerical assessment. Liquid droplets of DEHS and solid tracer particles of silicon and titanium dioxide with different primary crystal size are compared. The resulting temporal response ranges from 2 to 3 μs, corresponding to values commonly reported in literature, to almost 0.3 μs when particles are properly dehydrated and a filter is applied before injection into the wind tunnel. It is the first experimental evidence of particle tracers with a measured response time lower than 0.4 μs. The same procedure is applied to attempt the measurement of individual particle tracers by particle tracking velocimetry to estimate the spread in the distribution of tracer time response. The latter analysis is limited by the particle image tracking precision error, which biases the results introducing a wider broadening of the particle velocity distribution.

Residence time distribution in large industrial flotation cells

The radioactive tracer technique was used to measure the Residence Time Distribution (RTD) of the liquid and solid in a rougher flotation bank consisting of seven cells of a volume of 130 m³. Thus, a pneumatic system of high reliability was used in order to introduce a small amount of radioactive tracer (around 100 mL of liquid or pulp) at the feed pulp entrance. Then, the time response of the radioactive tracer was measured online along the flotation bank using noninvasive sensors located in the discharge pipe of each cell. Activity (cps) was measured by Saphymo Srat scintillating crystal sensors of NaI(Tl) of 1″ × 1.5″, thus allowing the simultaneous data acquisition of up to 12 control points with a minimum period of 50 ms. A solution of Br-82 was used as a liquid tracer, while mineral gangue was used as a solid nonfloatable tracer. The solid tracer was also tested at three size classes. An advantage of using the radioactive tracer technique is the direct testing of the actual solid particles (similar physical and chemical properties, shape, etc.). From a hydrodynamic point of view, the experimental data confirmed that single mechanical flotation cells of large size can deviate significantly from perfect mixing.

Bacteria in the global atmosphere – Part 2: Modeling of emissions and transport between different ecosystems

Abstract. Bacteria are constantly being transported through the atmosphere, which may have implications for human health, agriculture, cloud formation, and the dispersal of bacterial species. We simulate the global transport of bacteria, represented as 1 μm and 3 μm diameter spherical solid particle tracers in a general circulation model. We investigate factors influencing residence time and distribution of the particles, including emission region, cloud condensation nucleus activity and removal by ice-phase precipitation. The global distribution depends strongly on the assumptions made about uptake into cloud droplets and ice. The transport is also affected, to a lesser extent, by the emission region, particulate diameter, and season. We find that the seasonal variation in atmospheric residence time is insufficient to explain by itself the observed seasonal variation in concentrations of particulate airborne culturable bacteria, indicating that this variability is mainly driven by seasonal variations in culturability and/or emission strength. We examine the potential for exchange of bacteria between ecosystems and obtain rough estimates of the flux from each ecosystem by using a maximum likelihood estimation technique, together with a new compilation of available observations described in a companion paper. Globally, we estimate the total emissions of bacteria-containing particles to the atmosphere to be 7.6×1023–3.5×1024 a−1, originating mainly from grasslands, shrubs and crops. We estimate the mass of emitted bacteria- to be 40–1800 Gg a−1, depending on the mass fraction of bacterial cells in the particles. In order to improve understanding of this topic, more measurements of the bacterial content of the air and of the rate of surface-atmosphere exchange of bacteria will be necessary. Future observations in wetlands, hot deserts, tundra, remote glacial and coastal regions and over oceans will be of particular interest.

Ring vortex scenario in engraved champagne glasses

The simple idea this study rests on is that one cannot be concerned by the bubbling and aromatic exhalation events in champagne tasting without being interested in the study of the flow mixing mechanisms inside the glass. Indeed, a key assumption is that a strong link of causality may exist between inherent liquid-phase flow structures due to bubble motion and the flavors exhalation process. This is the reason why, to underscore the impact of glass-shape and glass-engravement conditions on mixing flow phenomena, classical flow visualization techniques were used to capture fluid motion in traditional flutes and coupes poured with champagne. Laser tomography combined with fluorescent dyes and solid tracers have been used to give the quasi-instantaneous velocity field from which streamline patterns are deduced as well as the vorticity convection.

New estimates of the gamma-ray line emission of the Cygnus region from INTEGRAL/SPI observations

The Cygnus region harbours a huge complex of massive stars at a distance of 1.0-2.0kpc from us. About 170 O stars are distributed over several OB associations, among which the Cyg OB2 cluster is by far the largest with about 100-120 O stars. As a consequence of their successive nuclear-burning episodes, these massive stars inject large quantities of radioactive nuclei into the interstellar medium such as 26Al and 60Fe. The gamma-ray line signal from the latter is a solid tracer of ongoing nucleosynthesis. We want to compare the decay emission from the Cygnus region with predictions of recently improved stellar models. As a first step, we establish observational constraints upon the gamma-ray line emission from 26Al and 60Fe, with particular emphasis placed on separating the emission due to the Cygnus complex from the foreground and background mean Galactic contributions. We used the high-resolution gamma-ray spectrometer INTEGRAL/SPI to analyse the 26Al and 60Fe decay signal from the Cygnus region. The weak gamma-ray line emissions at 1809 and 1173/1332keV have been characterised in terms of photometry, spectrometry, and source morphology. The 1809keV emission from Cygnus is centred on the position of the Cyg OB2 cluster and has a typical size of 9 degrees or more. The total 1809keV flux from the Cygnus region is (6.0 +/- 1.0) x10e-5 ph/cm2/s, but the flux really attributable to the Cygnus complex reduces to (3.9 +/- 1.1) x10e-5 ph/cm2/s. The 1809keV line centroid is agrees with expectations considering the direction and distance of the Cygnus complex, and the line width is consistent with typical motions of the interstellar medium. No decay emission from 60Fe has been observed from the Cygnus region and an upper limit of 1.6 x10e-5 ph/cm2/s was derived.

Reconnection dynamics for quantized vortices

By analyzing trajectories of solid hydrogen tracers in superfluid 4He, we identify tens of thousands of individual reconnection events between quantized vortices. We characterize the dynamics by the minimum separation distance δ ( t ) between the two reconnecting vortices both before and after the events. Applying dimensional arguments, this separation has been predicted to behave asymptotically as δ ( t ) ≈ A ( κ | t − t 0 | ) 1 / 2 , where κ = h / m is the quantum of circulation. The major finding of the experiments and their analysis is strong support for this asymptotic form with κ as the dominant controlling feature, although there are significant event to event fluctuations. At the three-parameter level the dynamics may be about equally well-fit by two modified expressions: (a) an arbitrary power-law expression of the form δ ( t ) = B | t − t 0 | α and (b) a correction-factor expression δ ( t ) = A ( κ | t − t 0 | ) 1 / 2 ( 1 + c | t − t 0 | ) . The measured frequency distribution of α is peaked at the predicted value α = 0.5 , although the half-height values are α = 0.35 and 0.80 and there is marked variation in all fitted quantities. Accepting (b) the amplitude A has mean values of 1.24 ± 0.01 and half height values of 0.8 and 1.6 while the c distribution is peaked close to c = 0 with a half-height range of −0.9 s −1 to 1.5 s −1. In light of possible physical interpretations we regard the correction-factor expression (b), which attributes the observed deviations from the predicted asymptotic form to fluctuations in the local environment and in boundary conditions, as best describing our experimental data. The observed dynamics appear statistically time-reversible, which suggests that an effective equilibrium has been established in quantum turbulence on the time scales (≤0.25 s) investigated. We discuss the impact of reconnection on velocity statistics in quantum turbulence and, as regards classical turbulence, we argue that forms analogous to (b) could well provide an alternative interpretation of the observed deviations from Kolmogorov scaling exponents of the longitudinal structure functions.

Visualization of Superfluid Helium Flow

We discuss an experimental technique to visualize the motion in the bulk of superfluid $^4$He by tracking micron-sized solid hydrogen tracers. The behavior of the tracers is complex since they may be trapped by the quantized vortices while also interacting with the normal fluid via Stokes drag. We discuss the mechanism by which tracers may be trapped by quantized vortices as well as the dependencies on hydrogen volume fraction, temperature, and flow properties. We apply this technique to study the dynamics of a thermal counterflow. Our observations serve as a direct confirmation of the two-fluid model as well as a quantitative test of the normal fluid velocity dependence on the applied heat flux.

Velocity Statistics Distinguish Quantum Turbulence from Classical Turbulence

By analyzing trajectories of solid hydrogen tracers, we find that the distributions of velocity in decaying quantum turbulence in superfluid 4He are strongly non-Gaussian with 1/v(3) power-law tails. These features differ from the near-Gaussian statistics of homogenous and isotropic turbulence of classical fluids. We examine the dynamics of many events of reconnection between quantized vortices and show by simple scaling arguments that they produce the observed power-law tails.

Froth mean residence time measurement in industrial flotation cells

Froth plays an important role in flotation processes preventing the pulp transport to the concentrate (short-circuit). Thus, it contributes to increasing the concentrate grade by gravity drainage of entrained particles, back into the pulp. Key parameters affecting the froth performance are the mean residence times of solids, liquid and gas in the froth. The froth mean residence time depends on the froth depth, gas flowrate, gas hold-up, and flow regime. In this work, the froth mean residence times were evaluated from direct measurements of liquid and solid time responses in the froth of self-aerated copper flotation cells of 130 m 3. For this purpose the radioactive tracer technique was applied, using 82Br as liquid tracer, and non-floatable mineral particles in three size classes (+150; −150 + 45;−45 μm) as solid tracers. All tracers were injected at the cell feed entrance, which allowed the tracer to circulate first through the rotor, and become well distributed over the whole cross-sectional area before entering the froth. Each tracer time response was measured on-line below the pulp/froth interface (input signal) and at the concentrate overflow discharge (output signal). The froth mean residence time was then obtained by difference between the average times of the froth input and output tracer signals, previously modelled. For the copper rougher flotation, the froth mean residence time (9–12 s) of non-floatable solids, derived from experimental measurement, was comparable with that obtained by measuring the gas flowrate and estimating the effective gas volume in the froth. While, the froth mean residence times of liquid and floatable solid were significantly larger, 21 and 24 s, respectively.

Cold flow studies of a slurry transport reactor–hydrocyclon system

The fluid dynamics of a new reaction system composed of a slurry transport reactor–hydrocyclon were studied using two column diameters at different gas and liquid linear velocities. The cold models used a gas collector in the top that allowed measurement of the gas disengaged by radial zones and a conductimetric probe that measured the frequency of the bubbles exiting from the top of the reactor. Gas and liquid hold-ups were determined. Liquid and solid tracers were also employed to determine the resident time distribution (RTD), global residence time, and the recycle of slurry near the wall. The results show the effect of sparger and disengaging design, as well as the effect of gas and liquid flow rate on the radial and axial gas hold-up profiles and on the recycle of slurry by the wall. This recycle is similar to those observed with a draft tube. No significant effect of column diameter was observed. A smooth circulation of slurry and solid was achieved through a mechanical optimization of the inlet and outlet of the reactor. It was demonstrated that the RTD of the system can be simulated using a set of continuous stirred tank reactors and plug flow reactor in a recycle (three parameters). Empirical equations are proposed for predicting the hold-up and the three parameters needed by the model. The similarity to a spouted bed reactor is discussed.

Classifying flow regimes in three-phase fluidized beds from CARPT experiments

Flow regimes in gas–liquid–solid fluidized beds encountered at different operating conditions, column diameters and particle sizes are classified using a statistical method [Diks, C., van Zwet, W.R., Takens, F., DeGoede, J., 1996. Detecting differences between delay vector distributions. Physical Review E 53, 2169–2176] that allows comparison of the attractors characterizing their dynamics. Time series of the single solid tracer particle positions, obtained by the computed automated radioactive particle tracking (CARPT) technique, are used to reconstruct the attractors for each examined operating condition. The test constitutes a reliable way of objectively diagnosing the flow regime in a three-phase fluidized bed since experiments within the same flow regime have similar dynamical characteristics, while there is a huge hydrodynamic change between experiments carried out in different flow regimes.