Axial Kinetic Energy Research Articles

Numerical Investigation of the Noise Radiated from Hot Subsonic Turbulent Jets

Experimental studies have highlighted the existence of additional sources of sound due to temperature e uctuations in heated jets, in comparison with unheated jets. Whereas cold jets have been the subject of many numerical investigations, little research has been devoted to the topic of hot jets. Thus, a specie c model to investigate the acoustic radiation from these jets based on numerical predictions using a k‐ turbulence closure is attempted. First, a model for the additional source term is suggested, which is a function of mean axial velocity, mean temperature, and turbulent kinetic energy. Next a computation model of acoustic intensity spectrum is developed in which three different contributions appear: The e rst represents the contribution of the velocity e uctuations in the sound emission, the second that of the temperature e uctuations, and the third a mixed term issuing from the two preceding contributions. Then the model is applied to compute the acoustic radiation of hot jets. Results provided quite a full description of the acoustic features of a hot jet: spectrum shape, radiated acoustic intensity levels, and ine uence of jet temperature. Finally, the model seemed capable of predicting the trends in noise radiation of turbulent hot jets correctly.

Performance optimization and fringing field modifications of a 24-mm long RF-only quadrupole mass spectrometer

Modifications to the exit fringing fields of a nominally radio frequency (RF)-only 24-mm long quadrupole mass analyzer result in enhanced signal-to-background levels by increasing the axial kinetic energy of ions on the verge of instability. Specifically, improved signal-to-background levels were observed when the amplitude of the RF voltage applied to opposing poles was unbalanced and a small amount of resolving DC voltage was applied. Only high q-value ions are affected by the changes in the fringing fields allowing more facile energy discrimination between analyte ions and those that give rise to continuum background. Similar performance enhancements were obtained when the exit fringing fields were altered by the application of an appropriately phased auxiliary RF voltage to the exit lens of the quadrupole in the presence of a small resolving DC voltage. These improvements suggest that small, less expensive quadrupole mass analyzers may soon be a reality. Copyright © 1999 John Wiley & Sons, Ltd.

Axial energy distributions of Li + and F − desorbed from LiF surfaces by fast ion impact

Initial axial kinetic energy ( E z ) distributions of Li + and F − secondary ions desorbed from LiF surfaces, bombarded by low current (∼500 particles/s) N 3+ beams in the 0.075–7.5 MeV primary ion energy ( E PI) range, were measured by using the time-of-flight technique. In this energy range, the electronic energy loss, S e, increases from 25 to 160 eV/Å, while the nuclear stopping power, S n, decreases from 8.0 to 0.25 eV/Å. The observed F − initial axial kinetic energy distribution can be described by the linear collision cascade theory and no other contribution was found. The F − total yield decreases proportionally to S n. The Li + distribution presents a remarkable deviation from the cascade prediction, indicating the existence of additional mechanisms related to the electronic energy-loss process. For the range of E PI studied, these mechanisms produce a Li + energy distribution with a Maxwell-Boltzman-like shape, which vanishes above E z ∼ 10 eV and presents a maximum at E z ∼ 1.2 eV. The Li + yield is nonlinear with the electronic energy loss, S e. A simple desorption model, based on the spatial distribution of the energy deposited by the projectile and on the effective energy-loss concept, is presented. This spatial distribution of the deposited energy is due to secondary electron cascades and is connected with a ESD-like mechanism on the surface.

MeV nitrogen bombardment of LiF: From the nuclear to the electronic sputtering regimes

The time-of-flight technique was used to measure the axial kinetic energy (Ez ) distribution of secondary ions emitted from a LiF surface bombarded with N3+ ions in the 0.30–6.0 MeV energy range. Desorption yields were also measured as function of the projectile energy. The analysis of the Li+ and F− secondary ions shows that, at low incident energies (below 0.50 MeV), these ions are sputtered from the surface by a collision cascade (nuclear sputtering). Such indication comes from the E z 2m−2 (where 0 < m< 1) asymptotic behavior of the axial kinetic energy distribution. The observed values for m are m ∼ 0 for Li+ emission and m ∼ 0.4 for F− emission. The mean axial kinetic energies are, respectively, σ 6eV and ∼ 9eV. At higher projectile energy (above 1 MeV), there is a change in the Li+ emission behavior: the high energy tail of the axial energy distribution becomes exponential, the desorption yield increases presenting a maximum near 2.0 MeV and the mean energy of emission decreases to ∼ 3 eV. No such change has been found for F− desorption process. Its yield decreases proportionally to the nuclear stopping power of N projectiles in LiF.

Evaluation of six k-ε turbulence model predictions of flow in a continuous casting billet-mold water model using laser doppler velocimetry measurements

Quantitative fluid velocity and turbulence measurements, which were obtained using laser Doppler velocimetry (LDV) during the course of the physical modeling of the molten steel flow in a continuous casting billet mold, are presented. The experimental findings are compared to the results obtained by the computational simulation of the flow using the high-Reynolds-number and five low-Reynolds-number k-e turbulence models. The predictive capabilities of these turbulence models were assessed in comparison to the experimentally measured axial velocity and turbulence kinetic energy values. The measured turbulence velocity fluctuations in various directions strongly suggest the nonisotropic nature of the flow field, which cannot be predicted with the k-e models. The extent of deviations between the measurements and computations was quantified.

Fourier transform ion cyclotron resonance detection of multiphoton ionization spectroscopy

Fourier transform ion cyclotron resonance (FT-ICR) detection was tested for resonanceenhanced multiphoton ionization (REMPI) spectroscopy. The (2+1) REMPI spectra of acetaldehyde were obtained in the wavelength range 364-354 nm via a two-photon resonant 3s ← n Rydberg transition. The space-charge effects on the REMPI spectra were examined in the vicinity of the 0 0 (0) transition. The trapping efficiency measurement shows that all the ions produced from REMPI dissociation processes are arrested in the ion cyclotron resonance cell even in the presence of space-charge interactions. Axial kinetic energy release distributions of ions were extracted from the trapping efficiency data obtained under a new space-charge-free condition. FT-ICR peak heights were measured as a function of pressure at different laser powers, magnetic field strengths, and ion excitation methods to test for the detection linearity. The FT-ICR detection responds linearly to the number of ions in a low pressure limit. The product branching ratio was measured by using various ion excitation methods and was compared with the previous quadrupole mass spectrometric study. FT-ICR detection yields the mass-selected REMPI spectra and the product branching ratio in the absence of kinetic shifts.

Electron dynamics in a free electron laser with a reversed axial guide field

In order to interpret the experiment of Conde and Bekefi, electron trajectories are studied for a configuration with a reversed axial guide field. In the vicinity of ‘‘antiresonance,’’ electrons are shown to have chaotic trajectories when emitted far enough from the axis. Thus, the interaction efficiency is degraded for most particles. This explains the large dip in radiative power observed and predicted by different simulation codes. The very good efficiency observed in the experiment is explained by a compromise between a high amplitude perpendicular velocity and a regular enough axial momentum for the electrons. It is also explained by the fact that when photons are emitted, the axial kinetic energy is not too much affected. A new situation for which a small peak in efficiency should be observed is predicted.

Stacked-ring electrostatic ion guide

In 1969 Bahr, Gerlich, and Teloy introduced an rf device that consisted of a stack of ring electrodes, with charge sign alternation between neighboring rings, to store or transport ions. Here we propose to operate such a device with electrostatic potentials rather than rf potentials: ions that move axially along the center of the guide are thereby subjected to an oscillating electrical potential similar to the sinusoidal rf potential in familiar rf-only multipole ion guides. The oscillating potential of the stacked-ring static ion guide focuses ions by exerting a field gradient force on the ions so as to push ions toward the central axis where the field is weakest. The stacked-ring ion guide produces an effectively static "pseudopotential" that is much steeper at the edge (potential varies as e (r)) compared to a quadrupole or octupole guide (for which the potential varies as r (2) or r (6), where r is radial position) and that is much flatter near the center of the guide (for potentially higher ion flux). Advantages of the new ion guide include static rather than rf potential, low electrical noise, a large field-free region near the central axis of the guide, and simple mechanical construction. A disadvantage of the stacked-ring ion guide is that high ion axial kinetic energy is required; ions with axial kinetic energy that is too low may be trapped in the shallow pseudopotential well between adjacent ring electrodes.

Three-dimensional motional stabilization in the trapping field of an open-ended trapped-ion cell: application to the remeasurement experiment in Fourier transform ion cyclotron resonance mass spectrometry.

Three-dimensional motional stabilization of radial trajectories of low-mass organic ions in an open cell using only a dc trapping field is applied to the FT-ICR remeasurement experiment. More than 300 remeasurement cycles are observed with 99.59% remeasurement efficiency for benzene (m/z 78) using a high-pressure helium buffer gas. The enhancement in remeasurement efficiency is due to collisional stabilization of the guiding center of ion motion by dynamic motional averaging in the axial position-dependent radial electric field. Such dc-induced radial stabilization is in contrast to the stability produced by application of radio frequency fields characteristic of quadrupolar axialization or rf-only mode operation. The same effect is produced because ions experience a radial "pseudopotential" during axial oscillation as in time-varying fields. Trajectory simulations for ions oscillating in an open cell trapping well above a z-amplitude critical threshold energy of 0.60 eV (in a potential well of 0.84 V) indicate that radially stabilized trapping motion is achieved because the outward-directed radial electric field existing near the cell center line is compensated by an opposing inward-directed radial electric field at extended z-amplitude. Sufficient axial kinetic energy permits ion penetration into the inward-directed radial electric field regions, enabling > 50% residence time of each trapping oscillation period in regions inducing radial stability, thereby inhibiting magnetron radius growth. A high-pressure, low-mass buffer gas such as helium provides the requisite increase in the axial amplitude of the ion cloud, similar to the mechanism observed for axial excitation of low-mass ions observed in collision-induced dissociation. The result is radial stability at high pressure, even after multiple remeasurement cycles. An optimized excitation radius of 12.5% of the cell radius yields maximum remeasurement efficiency with a 500 ms relaxation delay between excitation events. Summed signal intensity decreases with increased trap potential due to the greater radial electric field and reduced axial expansion of the ion cloud and also decreases with buffer gas mass in response to greater radial scattering.

Ion trajectories in an electrostatic ion guide for external ion source fourier transform ion cyclotron resonance mass spectrometry

An electrostatic ion guide (EIG) that consists of concentric cylinder and central wire electrodes can transport ions efficiently from an external ion source to an ion cyclotron resonance (ICR) ion trap for mass analysis, with several advantages over current injection methods. Because the electrostatic force of the EIG captures ions in a stable orbit about the wire electrode, ions with initially divergent trajectories may be redirected toward the ICR ion trap for improved ion transmission efficiency. SIMION trajectory calculations (ion kinetic energy, 1-200 eV; elevation angle, 0.30 °; azimuthal angle, 0.360°) predict that ions of m/z 1000 may be transmitted through a strong (0.01 → 3.0-T) magnetic field gradient. Judicious choice of ion source position and EIG potential minimizes the spread in ion axial kinetic energy at the ICR ion trap. Advantages of the EIG include large acceptance angle, even for ions that have large initial kinetic energy and large radial displacement with respect to the central z-axis, low ion extraction voltage (5-20 V), and efficient trapping because ions need not be accelerated to high velocity to pass through the magnetic field gradient.

Translation Experiments of Field-Reversed Configuration Plasma

Translation dynamics of field-reversed configuration (FRC) plasmas are studied in the FRC Injection Experiment (FIX) machine. FRC plasmas have been formed in, and launched from, a field-reversed theta-pinch source, and subsequently translated into reduced external magnetic fields. When translated into an adjacent confinement region, incident velocity of the formed FRC exceeds the Alfven velocity. Moreover, the translated FRC cools less than the prediction of an adiabatic theory. The plasma reflects from an external mirror and some of its axial kinetic energy is lost during every reflection. In this reflection process, significant plasma heating is observed in the case where the translation velocity exceeds the sound velocity. 11 refs., 5 figs.

Rethermalization of a field-reversed configuration plasma in translation experiments

A translation experiment of field-reversed configuration (FRC) plasma is performed on the FIX machine [Shiokawa and Goto, Phys. Fluids B 5, 534 (1993)]. The translated FRC bounces between magnetic mirror fields at both ends of a confinement region. The plasma loses some of its axial kinetic energy when it is reflected by the magnetic mirror field, and eventually settles down in the confinement region. In this reflection process, the plasma temperature rises significantly. Such plasma rethermalization has been observed in OCT-L1 experiments [Ito et al., Phys. Fluids 30, 168 (1987)], but rarely in FRX-C/T experiments [Rej et al., Phys. Fluids 29, 852 (1986)]. It is found that the rethermalization depends on the relation between the plasma temperature and the translation velocity. The rethermalization occurs only in the case where the translation velocity exceeds the sound velocity. This result implies the rethermalization is caused by a shock wave induced within the FRC when the plasma is reflected by the magnetic mirror field.

Experimental investigation into a turbulent jet with negative buoyancy

An experimental investigation of a steady-state negatively buoyant jet has been performed using a hot water jet projected vertically downwards from a 7.5 mm radius inlet pipe into a cold ambient. The Richardson number was 0.1 based on inlet pipe radius and the Reynolds number was 5000. Mean and fluctuating velocities and temperatures, triple velocity correlations and velocity–temperature correlations were measured for the jet in a constant temperature ambient. Correlations between orthogonal velocity components were also measured. Velocity measurements were performed using laser Doppler anemometry (LDA), and errors associated with beam movement (due to refractive index fluctuations) were experimentally quantified. Temperatures were measured with fast response thermocouples. The two techniques were used in conjunction to provide velocity–temperature correlations. The intermittency factors around the edge of the flow were measured optically, employing refractive index as a tracer. The normal stresses were found to be high in the shear region at the jet-flow/reverse-flow interface and in the region of large-scale fluid reversal. The contribution from the intermittency at the boundary of the flow was evident in the velocity and temperature fluctuations. The triple velocity products were interpreted as fluxes of the Reynolds stresses and in general exhibited net fluxes away from the regions of high stress intensity. Balances of the terms in the axial momentum and turbulent kinetic energy equations showed that the main contribution from buoyancy was in the mean motion, with very little direct input to the turbulence field.

The Flow Inside a Model Gas Turbine Combustor: Calculations

The present study assesses the ability of the k-ε turbulence model to calculate the flow inside gas turbine combustors. Results of calculations using a cylindrical system of coordinates, hybrid differencing, and a mesh with about 40,000 nodes are compared with velocity measurements of the flow inside a perspex model of can-type gas turbine combustor. The larger discrepancies between measurements and predictions were found in the primary region. The complexity of the flow near the primary jet impingement led to underprediction of the maximum negative axial velocity and turbulence kinetic energy by about 35 and 20 percent, respectively. The calculated results exhibited higher levels of momentum diffusion compared to the experiments and did not show the two contrarotating vortices created between the primary jets; no qualitative agreement with the azimuthal velocity downstream of the primary jets could be achieved. Despite these deficiencies, the model gave acceptable results in other regions of the combustor and correct prediction of the main features of the combustor flow was possible.

Mass-selective isolation of ions stored in a quadrupole ion trap. Part 2. a simulation study of consecutive isolation

A novel process of ion isolation, or mass-selective storage, in a pure quadrupole ion trap has been simulated by calculation of ion trajectories. This isolation process for a single ion species entails the ejection of ions of lower and higher mass-to-charge ratio in two ejection events. A simulation programs has been applied to the calculation of ion trajectories during consecutive ion isolation; both a collision-free system and one in which collisons with helium atoms occurred were considered. The simulation program used here was similar to that used previously for the investigation of resonance excitation of stored ions and for concurrent ion isolation. The working points of three ion species ( m/ z 206, 207 and 208), located initially on the q z axis with q z ≈ 0.2, were moved off-axis first to the vicinity of the β z = 1 boundary at which ions of m/ z 306 were ejected, then off-axis to the β z = 0 boundary at which ions of m/ z 208 were ejected. Stable trajectories were maintained only for the ion species of m/ z 207; other ion species were ejected axially. The simulation process extended over a real time of 2.84 ms. Trajectories are displayed as temporal variations with the radial, azimuthal, and axial kinetic energy components which were calculated throughout each trajectory, were found to be substantially modified by collisional cooling prior to isolation.

Collisional focusing effects in radio frequency quadrupoles

The transmission of ions through a conventional two-dimensional radiofrequency-only (rf) quadrupole has been studied for comparatively high operating pressures between 5 × 10 −4 and 1 × 10 −2 torr. Measurements of signals from mass-resolved analyte ions and total ion currents show that, provided the initial injection ion energy is low (1–30 eV), the ion transmission observed through a small aperture at the exit of the rf quadrupole first increases as the gas pressure increases, reaching a maximum at ∼ 8 × 10 −3 torr before decreasing at higher pressures. This is in direct contrast to the expectations of classical scattering. This “collisional focusing” appears to be analogous to effects seen in three-dimensional ion traps. The collisional focusing increases with the mass of the ion (not mass-to-charge ratio) for masses up to at least 16,950 u. The collisional focusing of the ions is found to be accompanied by significant losses of axial kinetic energy. A Monte Carlo simulation of the energy loss process is reported that can provide agreement with the observed losses for reasonable collision cross-sections. The results suggest that operation of rf quadrupoles at relatively high pressure may find practical application in sampling ions from high (e.g., atmospheric) pressure ion sources.

The Prediction and Measurement of Incompressible Flow in a Labyrinth Seal

Predictions and measurements were obtained for incompressible flow in a labyrinth seal for Taylor numbers of 0, 7,600, and 19,000 at an axial Reynolds number near 50,000. A hot-film anemometer with computer data acquisition was used in obtaining the measurements. The computations were made using a turbulent flow Navier-Stokes finite difference code. The QUICK differencing scheme was utilized in order to diminish false diffusion. Comparison between experiments and predictions are given for the axial and swirl velocity components and turbulent kinetic energy. Also, the substantial effect of the Taylor number on the pressure distribution is presented.

Finite element, stream function‐vorticity solution of secondary flow in the channels of a rod bundle

AbstractA finite element method has been applied to predict the overall features of the fully developed turbulent flow in the non‐circular channels of a rod bundle. The finite element discretization is based on the conventional Galerkin method using an isoparametric quadrilateral element with mixed interpolation.The primary axial flow and turbulent kinetic energy distributions have been predicted for fully developed turbulent flow conditions right up to the wall. The secondary velocity is represented by the stream function‐vorticity formulation and the no‐slip boundary conditions are explicitly introduced in the nonlinear equations by a boundary vorticity formula. The Newton‐Raphson method is applied to the stream function‐vorticity equations and solved simultaneously by the frontal solution technique.A one‐equation eddy viscosity model of turbulence and an algebraic stress transport model have been used to predict primary axial velocity, secondary velocities and turbulent kinetic energy. The predictions obtained for a central subchannel of an equilateral‐triangular rod array with p/d= 1.3 are in reasonable agreement with experimental data.

Ion/molecule reaction inside the collision cell of a tandem quadrupole mass spectrometric system. 2. Energy dependence of ammonium ion formation

AbstractThe dependence of the protonation of neutral ammonia on the axial kinetic energy of protonated reactant ions has been studied in the gas phase, using various protonated carbonyl compounds, inside the collision cell of a tandem quadrupole mass spectrometric system. The hypothesis of two different and non‐competitive reaction channels has been proposed. The first is characterized by a very low (peaked at ±0.05 eVcm) and well‐defined axial kinetic energy of the reactant ion, while the second is more energy demanding (estimated threshold at ±0.2 eVcm) and expressed by a collisionally induced dissociation‐like energy curve. Fourier transform mass spectrometric experiments have shown that ammonium ion can be generated by direct proton exchange and fragmentation of the adduct ion obtained.

Experimental studies of field-reversed configuration translation

In the FRX-C/T experiment [Proceedings of the 9th Symposium for Engineering Problems of Fusion Research (IEEE, New York, 1981), p. 1751], field-reversed configuration (FRC) plasmas have been formed in, and launched from, a field-reversed theta-pinch source and subsequently trapped in an adjacent confinement region. No destructive instabilities or enhanced losses of poloidal flux, particles, or thermal energy are observed for FRC total trajectories of up to 16 m. The observed translation dynamics agree with two-dimensional magnetohydrodynamic (MHD) simulations. When translated into reduced external magnetic fields, FRC’s are observed to accelerate, expand, and cool in partial agreement with adiabatic theory. The plasmas reflect from an external mirror and after each reflection, the axial kinetic energy is reduced by approximately 50%. Because of this reduction, FRC’s are readily trapped without the need of pulsed gate magnet coils.