Mixing In Gas Research Articles

Experimental Study of the Solids Velocity Field in Gas−Solid Risers

Solids flow dynamics in gas−solid risers is inherently complex. Model refinement through experimental validation requires the acquisition of detailed nonintrusive measurements. In this study, noninvasive computer-automated radioactive particle tracking (CARPT) is employed to visualize and quantify in a three-dimensional domain the solids dynamics and mixing in gas−solid risers. This technique has the added advantage that, along with the derived Eulerian solids flow field (time-average velocity map and various turbulence parameters such as the Reynolds stresses, turbulent kinetic energy), it also provides directly the Lagrangian description of the solids motion. The solids velocity field data are obtained in two different risers at low and high solids fluxes at varying superficial gas velocity to span both the fast-fluidized (FF) and dilute phase transport (DPT) regimes. The effect of operating conditions on solids flow and mixing is studied. Comparative analysis of the results is presented to provide insi...

Axial mixing in a novel pilot scale gas–liquid reciprocating plate column

Axial mixing in a novel pilot scale landau reciprocating plate column (LRPC) has been investigated for counter-current gas–liquid contacting over a wide range of operating conditions. The experimental results obtained using the dynamic response method were analysed using both the dispersion and compartment models under different boundary conditions and using both the method of moments and the direct time domain parameter estimation techniques. Based on the results, it was identified that axial mixing in this column can be best described using the back flow and the dispersion models solved with “closed–closed” boundary conditions. A general correlation describing the effect of operating parameters on the extent of axial mixing was developed with a mean absolute relative residuals of 6.8%. Similar to other RPC designs, axial mixing in LRPC increases with increasing both phase flows and plates oscillatory velocity. Values of axial dispersion coefficient in this work ranged from ∼10 −4 to over ∼ 3 × 10 −3 m 2/s, which are comparable or less than those in other RPC designs under similar phase velocities and oscillatory conditions, but an almost order of magnitude lower than those measured in bubble columns under similar operating flow rates.

Mixing and segregation in a bidisperse gas–solid fluidised bed: a numerical and experimental study

In many industrial applications of dense gas–solid fluidised beds, mixing and segregation phenomena play a very important role. The extent of mixing and segregation is strongly influenced by the bubble characteristics. Therefore, the extent of mixing and segregation, induced by a single bubble injected in a monodisperse and bidisperse fluidised bed at incipient fluidisation conditions and in freely bubbling fluidised beds has been studied both with well-defined experiments and with a 3D Euler–Lagrangian model. Particle image velocimetry (PIV) was successfully applied to obtain the ensemble averaged particle velocity profile in the vicinity of the bubble in dense gas–solid fluidised systems. The bubble size of a single injected bubble in a fluidised bed at minimum fluidisation conditions calculated with a 3D discrete particle model (DPM) depended strongly on the selected gas-particle drag model. The widely used Ergun equation combined with the Wen and Yu [Powder Technol. 98 (1998) 38; Chem. Eng. Sci. 47 (1992) 1913] relations overpredicted the bubble size due to an overprediction of the drag force. The DPM with the drag model proposed by Koch and Hill [Annu. Rev. Fluid Mech. 33 (2001) 619], based on Lattice–Boltzmann simulations, gave much better agreement with the experimental findings. The segregation rates in a bidisperse freely bubbling fluidised bed predicted by the DPM agreed very well with the experimentally measured segregation rates by Goldschmidt [M.J.V. Goldschmidt, Hydrodynamic modelling of fluidised bed spray granulation, PhD thesis, Twente University, 2001].

Phase conjugation of a quantum-degenerate atomic fermion beam.

We discuss the possibility of phase conjugation of an atomic Fermi field via nonlinear wave mixing in an ultracold gas. It is shown that for a beam of fermions incident on an atomic phase-conjugate mirror, a time-reversed backward propagating fermionic beam is generated similar to the case in nonlinear optics. By adopting an operational definition of the phase, we show that it is possible to infer the presence of the phase-conjugate field by the loss of the interference pattern in an atomic interferometer.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of benzene: Vibrational analysis of C6H6+ and C6D6+ in the X̃ 2E1g state

Vibrational spectra of C6H6+ and C6D6+ in the ground electronic state have been measured by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four wave mixing in Kr gas. The ionization energies of C6H6 and C6D6 determined by one-photon MATI, 74551±5 and 74579±5 cm−1, respectively, are similar to those reported previously. Vibrational spectra are much simpler than the previous zero kinetic energy photoelectron and MATI spectra obtained by two-photon excitation. Almost complete vibrational assignments for the cations have been possible, which will be useful for future theoretical studies of the Jahn-Teller effect in these cations. Implication from the present one-photon spectra agrees with the previous suggestion that the geometry of benzene cation in the ground electronic state belongs to the D6h symmetry.

Four-wave mixing in degenerate Fermi gases: Beyond the undepleted pump approximation

We analyze the full nonlinear dynamics of the four-wave mixing between an incident beam of fermions and a fermionic density grating. We find that when the number of atoms in the beam is comparable to the number of atoms forming the grating, the dephasing of that grating, which normally leads to the decay of its amplitude, is suppressed. Instead, the density grating and the beam density exhibit large nonlinear coupled amplitude oscillations. In this case, four-wave mixing can persist for much longer times compared to the case of negligible back action. We also evaluate the efficiency of the four-wave mixing and show that it can be enhanced by producing an initial density grating with an amplitude that is less than the maximum value. These results indicate that efficient four-wave mixing in fermionic alkali gases should be experimentally observable.

Polarization of multiple rotational Raman sidebands from hydrogen gas by delayed four-wave Raman mixing in the femtosecond regime

We demonstrate delayed four-wave Raman mixing in hydrogen gas and discuss the polarization of multiple rotational Raman radiation in the sub-100-fs regime. The mechanism of sideband generation through the interaction between a probe pulse and coherence of molecular motions induced by a pump pulse in hydrogen is revealed. One can artificially control the ellipticity of the polarization of the rotational Raman sidebands by changing the pump pulse polarization.

Four-wave mixing in degenerate atomic gases

We study the process of four-wave mixing (FWM) in ultracold degenerate atomic gases. In particular, we address the problem of FWM in boson-fermion mixtures. We develop an approximate description of such processes using asymptotic analysis of high-order perturbation theory taking into account quantum statistics. We perform also numerical simulations of FWM in boson-fermion mixtures and obtain an analytic and numerical estimate of the efficiency of the process.

Atomic four-wave mixing: fermions versus bosons.

We compare four-wave mixing in quantum degenerate gases of bosonic and fermionic atoms. We find that matter-wave gratings formed from either bosonic or fermionic atoms can in principle exhibit nearly identical Bragg scattering and four-wave mixing properties. This implies that effects such as coherent matter-wave amplification and superradiance can occur in degenerate Fermi gases. This effect is due to constructive many-particle quantum interferences, which in the boson case are interpreted as "Bose enhancement."

One-photon mass-analyzed threshold ionization spectroscopy of 1- and 2-iodopropanes in vacuum ultraviolet

One-photon mass-analyzed threshold ionization (MATI) spectroscopy of 1- and 2-iodopropanes has been studied using coherent vacuum ultraviolet (VUV) radiation generated by four-wave mixing in Kr gas. Accurate ionization energies to the lower and upper spin–orbit states of the molecular ions have been determined. These are 9.1755±0.0005 and 9.6903±0.0017 eV for the lower and upper spin–orbit states, respectively, of 2-iodopropane ion. For 1-iodopropane ion, gauche and trans peaks were resolved in the MATI spectra. Ionization energies to the lower spin–orbit states are 9.2567±0.0005 and 9.2718±0.0005, respectively, for the gauche and trans conformers. The corresponding values are 9.8332±0.0017 and 9.8466±0.0017 for the upper spin–orbit states. The pure ion beam of the gauche-only or that of trans-only could be selectively generated by tuning the VUV wavelength. Dissociation of 1- and 2-iodopropane ions, C3H7I+→C3H7++I, occurring in the ion core of highly excited Rydberg neutrals has been observed. Fragmentation thresholds for these reactions have been determined. This has led to an accurate potential energy diagram for the dissociation of the C3H7I+ system in the threshold region. The heat of formation at 0 K of 2-C3H7+ has been determined, 821.7±3.8 kJ mol−1, together with the proton affinity at 0 K of C3H6, 741.6±3.9 kJ mol−1. Plausible mechanisms for the production of 2-C3H7++I from 1-C3H7I+ have been proposed.

Local solid mixing in gas–solid fluidized beds

Diffusivity of the solid particles in a 152-mm ID gas–solid fluidized bed was determined at different regimes of fluidization. The gas was air at room temperature and atmospheric pressure and the solids were 385 μm sand or 70 μm FCC particles. The experiments were done at superficial gas velocities from 0.5 to 2.8 m/s for sand and 0.44 to 0.9 m/s for FCC (in both bubbling and turbulent regimes). Movement of a tracer was monitored by radioactive particle tracking (RPT) technique. Once the time-position data became available, local axial and radial diffusivity of solids were calculated from these data. Calculated diffusivities are in the range of 3.3×10 −3 to 5.6×10 −2 m 2/s for axial and 2.6×10 −4 to 1.5×10 −3 m 2/s for radial direction. The results show that the diffusivities, both axial and radial, increase with superficial gas velocity and are linearly correlated to the axial solid velocity gradient. Solid diffusivity in a bed of FCC was found to be higher than that of a bed of sand at the same excess superficial gas velocity.

A broadly tunable extreme ultraviolet laser source with a 0.008 cm−1 bandwidth

A new coherent narrow bandwidth extreme ultraviolet (XUV) laser source has been developed that is broadly tunable in the range 10–17 eV with a bandwidth of 0.008 cm−1 and intensities of 108 photons/pulse at 20 Hz. The source is based on two-photon resonance-enhanced sum-frequency mixing in rare gases. To demonstrate its properties, high-resolution photoionization and photoelectron spectra of argon have been recorded in the vicinity of the second, spin-orbit excited ionization threshold (Ar+2P1/2). From the analysis of the autoionization line shapes of the ns′[1/2](J=1) and nd′[3/2](J=1) resonances, reduced linewidths Γr,l=Γl×n*3 of 499.3(46) cm−1 and 28.76(89)×103 cm−1 have been determined for the s′ and d′ series, respectively. The results for the ns′[1/2](J=1) series confirm the conclusion reached by Klar et al. [Z. Phys. D 23, 101 (1992)] that earlier single-photon XUV investigations have overestimated the width of these sharp resonances. The narrow bandwidth of the source is also used to accurately determine the range of principal quantum number of the high Rydberg states that are probed by selected electric field ionization sequences in pulsed-field-ionization zero-kinetic-energy photoelectron spectroscopy.

Axial mixing in modern packings, gas, and liquid phases: II. Two-phase flow

Axial mixing measurements of air and water under two-phase flow conditions were made in a large-scale packed column (0.43 m diameter) using tracer experiments. Part I of this article dealt with single-phase mixing in the same column, with the same internals. Four packings were studied: 25.4-mm ceramic Raschig rings, 25.4-mm metal Pall rings, Sulzer BX structured packing, and Flexipac 2 structured packing. Air and water flowed countercurrently through the column at atmospheric pressure and at gas rates varying from 0.25 kg/m2·s up to the flooding point, and liquid rates from 3.25 to 8.5 kg/m2 ·s. A diffusion-type model served to reproduce the experimental response curves obtained for both phases. The results confirmed previous observations for first-generation packings: axial mixing in the gas increases with both gas and liquid rates, whereas liquid-phase axial mixing is a decreasing function of liquid rate and is insensitive to gas rate up to the flooding point. It was also found that the BX packing produces the least mixing in both phases. The largest mixing effects in the gas phase are found for the Raschig rings, and the largest mixing effects for the liquid phase are found for Flexipac 2. Correlations were developed to reproduce the results, yielding an average ±22% difference between experimental and correlated data.

Characteristics of four-wave mixing in a Bose-condensed atomic gas

An analysis is made of four-wave mixing of laser radiation interacting with an ultracold degenerate ensemble of boson atoms trapped in a magnetic trap. It is shown that the existence of long-range order in this type of atomic system leads to a new mechanism for the generation of the reversed wave whose experimental manifestation should differ from four-wave mixing in a nondegenerate gas. A similar nonlinear optical effect may be used for diagnostics of an ultracold gas at temperatures not much lower than the phase transition temperature.

Electrical and structural properties of rapid thermal annealed RF sputtered silicon oxide films

The effects of oxygen mixing and rapid thermal annealing (RTA) on the electrical properties of RF sputtered silicon oxide films has been examined. It was found that the insulating property of the sputtered oxide films improved when prepared with oxygen mixing in the sputtering gas and/or after rapid thermal annealing. For films RTA for 50 and 200 s, Schottky emission and space charge limited conduction were suggested as the transport mechanisms. The infrared results show that the film stoichiometry improved with RTA and/or oxygen mixing.

Axial mixing in modern packings, gas and liquid phases: I. Single‐phase flow

AbstractAn experimental study was conducted in a 0.43‐m‐ID packed column to determine the axial mixing properties of air and water under single‐phase flow conditions. The packings used were 25.4‐mm ceramic Raschig rings, one modern random packing, 25.4‐mm metal Pall rings; two structured packings, Sulzer BX and Flexipac 2. The column was operated at ambient conditions with flow rates varying from 0.4 to 4 kg/m2·s for the gas and from 3.25 to 8.5 kg/m2·s for the liquid. Axial mixing was experimentally determined via dynamic response studies based on the pulse injection technique. The diffusion‐type model served to reproduce the experimental response curves satisfactorily and proved to be a suitable means of describing axial mixing in both phases. The results confirm previous observations for first‐generation packings. Axial mixing is much greater in the liquid than in the gas; axial mixing in the gas increases with gas rate, whereas axial mixing in the liquid decreases with liquid rate. It was also found that the two structured packings produce the lowest levels of axial mixing in the gas. Surprisingly, the greatest mixing effects in the liquid were obtained for Flexipac 2 structured packing. Correlations were developed to represent the experimental mixing and liquid holdup data, yielding an average ±16% difference between experimental and correlated data. Mixing measurements in gas or liquid under two‐phase flow conditions will be discussed in Part II.

A peristaltic injection nozzle for collinear laser and molecular beams

A novel gas nozzle design that allows for collinear laser and molecular beams is described. Unlike conventional nozzles that open and close to limit the gas flow into a vacuum chamber, this nozzle opens to release a small volume of gas into the chamber. Gas pulses of 300 μs duration and 65±5 μs opening times have been obtained. When room-temperature H2 is used as the source gas, the rotational temperature was found to be 175±12 K, whereas when room-temperature D2 was used, the rotational temperature was found to be 104±7 K. The collinear optical and molecular beam path provided in this design may be useful in photodissociation experiments, the study of atomic and molecular collisions, Rydberg state lifetime measurements, and vacuum ultraviolet generation by nonlinear mixing in gases.

Rotationally Resolved Vacuum Ultraviolet Laser Spectra of the37Cl211Σ+u←X1Σ+gTransition

Rotationally and isotopically resolved single-photon excitation spectra of jet-cooled Cl2in the wavelength region between 133 and 138 nm were recorded using a tunable vacuum ultraviolet “laser” generated by two-photon resonantly enhanced four-wave difference mixing in Kr gas. The dominant transition (11Σ+u←X1Σ+g) is well known theoretically and experimentally to involve a double-well excited state potential energy curve formed by a strong homogeneous Rydberg-state/ion-pair state avoided crossing. In this work, single isotopomer spectra were obtained by dispersing and detecting ions produced by (1 + 1′) resonance-enhanced multiphoton ionization in a time-of-flight mass spectrometer. In this way, rotational constants were deduced for the first time for many v′ levels of the least abundant molecular isotope,37Cl2, which are both localized in the Rydberg well, and delocalized in the ion-pair portion of the 1-state potential energy curve. Our experimentally derived band origins andB′vvalues test the practical validity of an analytical 11Σ+upotential energy function which is a modified version of the one first proposed by J. Wörmer, T. Möller, J. Stapelfeldt, G. Zimmerer, D. Haaks, S. Kampf, J. Le Calvé, and M. C. Castex (1988. Z. Phys. D,7,383–395).

Generation of self-compressed laser pulses under the condition of two-photon resonant difference-frequency mixing in gases

Two-photon resonant difference-frequency generation is investigated for laser pulses short compared to the medium polarization relaxation time T 2 taking into account coherent propagation effects. On the basis of numerical solution of the Maxwell–Bloch equation we demonstrate that the process can be accompanied by temporal self-compression of the generated field leading to the formation of pulses at the difference-frequency with duration significantly shorter and intensity higher than those of the input pump and injection pulse.

Conduction mechanisms and interface property of silicon oxide films sputtered under different oxygen concentrations

An investigation on the effects of oxygen mixing in the sputtering gas and rapid thermal annealing on the electrical properties of radio frequency sputtered silicon oxide films has been carried out. The conductivity of the oxide film decreases as the concentration of oxygen in the sputtering gas increases, and also for rapid thermal anneal temperatures higher than 900 °C. Transport mechanisms for films annealed for 50 and 200 s were found to be that of Schottky emission and space charge limited conduction, respectively. From the capacitance versus voltage measurements, it was concluded that the improvement in the oxide quality of the annealed films was not related to the reduction of the interface trapped charge density.