Spatial and temporal structures of turbulent bubble-driven flows in a rectangular water tank

The spatial and temporal structures of turbulent water flows driven by air bubbles in a rectangular water tank were investigated. The time-resolved particle image velocimetry (PIV) technique was adopted for quantitative visualization. Flow rates of compressed air were changed from 2 to 4 ℓ/min at 0.5 MPa, and the corresponding range of bubble-based Reynolds number ranged from 6,740 to 13,220. The dynamics of flow structures was further investigated by the time-resolved proper orthogonal decomposition (POD) analysis technique. When the flow rate was increased, the main vortex core moved to the side and bottom wall. Locations of peak turbulent kinetic energy regions depended on the bubble Reynolds number. Both spatial and temporal modes were quite different with respect to the flow rates. The first temporal mode was harmonized with the second temporal mode, with small oscillations in the case of the lowest Reynolds number. However, temporal modes oscillate with higher frequencies when the Reynolds number increases. Based on the result of the FFT analysis of each temporal mode, we conjectured that low-frequency oscillation was attributed to the recirculating flow, whereas a higher dominant frequency was related to the vibration of the free surface that interacts with the rising bubbles.     

A Spectroscopic refractometer for a 375–1150 nm wavelength range

A spectroscopic refractometer is intended for measuring the index of refraction n _λ of liquid and solid media at any wavelength λ in a 375–1150 nm wavelength range with an accuracy of ±5 × 10⁻⁵. It can be used, in particular, for determining n _λ of monomers, compositions, and polymers, which are used for creating integrated-optical waveguides operating in a telecommunication spectrum region near 850 nm.     

Flow patterns in gas-assisted injection molding process in a channel

The flow field of a long bubble steadily expelling a viscous fluid confined by two closely located parallel plates is examined. In order to investigate the influence of bubble size on the flow field, a theoretical bubble profile is used to replace the complicated procedure for computing simultaneously the interface between the gas surface and fluid flows. The present study showed the two typical flow patterns and also a third flow pattern of the stagnation point moving in the region of the bubble tip front during transformation of the two typical flow patterns. The vorticity patterns are also drawn for various bubble profiles and are examined for their effect on the flow. The velocity field is also presented from two different viewpoints and the phenomena is examined. The stagnation point located on the center line between the bubble tip to the upstream is only found in the small range of in a channel, where λ is ratio of the bubble width to the distance between two parallel plates.     

A light-erosion method for high-pressure dust-gas-plasma flows generation

A light-erosion method for generating high-pressure dust-gas-plasma flows during ultraviolet (λ = 213 nm) laser ablation of a polymeric matrix ((C₂F₄) _n ) containing dust particles (thin-wall borosilicate glass microspheres with a ∼15- to 80-μm diameter d) is described. The carrying-out of the dust particles by ionized vapors of the substance of the target matrix, their space-time localization in the gas-plasma flow in a period Δτ ∼ 15–75 μs after the laser exposure up to the further spatial separation of the vaporized substance of the target matrix and the dust particle cloud is recorded by laser interferometry and shadow photography methods. The importance of certain selection of the matrix-dust system for realizing the light-erosion method for generating dust-gas-plasma flows, in particular laser exposure conditions, is shown. When condensed media with a low ionization potential (Al, Ce) are used as a dust component, the proposed method for generating heterogeneous gas-plasma flows can be efficient for their further heating by coherent radiation.     

Numerical study on characteristics of turbulent two-phase gas-particle flow using multi-fluid model

The purpose of this research is to study numerically the turbulent gas-particle two-phase flow characteristics using the Eulerian-Eulerian method. A computer code is developed for the numerical study by using the k-ɛ-k _p two-phase turbulent model. The developed code is applied for particle-laden flows in which the particle volume fraction is between 10⁻⁵ and 10⁻² for the Stokes numbers smaller than unity. The gas and particle velocities and the particle volume fraction obtained by using this code are in good agreement with those obtained by a commercial code for the gas-particle jet flows within a rectangular enclosure. The gas-particle jet injected into a vertical rectangular 3D enclosure is numerically modeled to study the effect of the Stokes number, the particle volume fraction and the particle Reynolds numbers. The numerical results show that the Stokes number and the particle volume fraction are important parameters in turbulent gas-particle flows. A small Stokes number (St ≤ 0.07) implies that the particles are nearly at the velocity equilibrium with the gas phase, while a large Stokes number (St ≥ 0.07) implies that the slip velocity between the gas and particle phase increases and the particle velocity is less affected by the gas phase. A large particle volume fraction (α _p ≥ 0.0001) implies that the effect of the particles on the gas phase momentum increases, while a small particle volume fraction (α _p ≤ 0.0001) implies that the particles would have no or small effect on the gas flow field. For fixed Stokes number and particle volume fraction, an increase of the particle Reynolds number results in a decrease of the slip velocity between the gas and particle velocities.     

Fabrication of diamond nanopowder using microwave plasma torch technique

This article presents a manufacturing process for diamond nanopowder by using a microwave plasma torch technique in a laboratory at near atmospheric pressure. The unique technique utilized in the arrangement is the hybrid plasma torch which was patented in 1997 by Dr. Cheng-Ming Wu in Taiwan. It has the advantage of working at near atmospheric pressure and does not require an extreme vacuum system, which is a necessary condition for fabrication of a large amount of nanoparticles. The applied constituents of gas mixtures for synthesizing diamond nanopowder in the process are CH₄ with AR and CH₄ with N₂, where AR and N₂ serve as catalysts. In processing the reaction chamber, it is first pumped to varied pressures from 40–300 Torr to induce plasma; then, the input reactive gas CH₄ is fixed at a constant flow rate of 0.6 l/min and mixed up with varied input flow rate of the catalysts Ar and N₂ from 0.6–1.2 l/min. The particle size of synthesized diamond nanopowder is within about 25--50 nm diameter, which mainly depends on flow rate of CH₄:AR and CH₄:N₂.     

Flow structure in the downstream of square and circular cylinders

In this experimental work, a technique of digital particle image velocimetry (DPIV) is employed to characterize instantaneous vorticity and time-averaged velocity, vorticity, root mean square (rms) velocities, Reynolds stress correlations and phase-averaged contours in the downstream of circular, sharp-edged square and 45^∘ orientated square cylinders in a uniform flow. Strouhal numbers for 550≤Re≤3400 are calculated from wake flow patterns. Shear layers surrounding the recirculation bubble region behind the cylinder are discussed in terms of flow physics and vortex formation lengths of large-scale Kármán vortices. Enhancement levels of Reynolds stress correlations associated with cross-stream velocity are clarified. Finally, flow structures depending on the cylinder geometry and Reynolds number are interpreted with quantitative representations.     

Characteristics of NOx emission with flue gas dilution in air and fuel sides

Flue gas recirculation (FGR) is a method widely adopted to control NO_x in combustion system. The recirculated flue gas decreases flame temperature and reaction rate, resulting in the decrease in thermal NO production. Recently, it has been demonstrated that the recirculated flue gas in fuel stream, that is, the fuel induced recirculation (FIR), could enhance a much improved reduction in NO_x per unit mass of recirculated gas, as compared to the conventional FGR in air. In the present study, the effect of FGR/FIR methods on NO_x reduction in turbulent swirl flames by using N₂ and CO₂ as diluent gases to simulate flue gases. Results show that CO₂ dilution is more effective in NO reduction because of large temperature drop due to the larger specific heat of CO₂ compared to N₂ and FIR is more effective to reduce NO emission than FGR when the same recirculation ratio of dilution gas is used.     

The choice of a spectral interval within which a heated opaque object radiates as a gray body

The temperature of opaque objects with the emissivity ɛ(λ) depending on the wavelength can be rather precisely determined with an error of ≤1–5% from their heat emission spectrum using the model of a gray body ɛ = const(λ). This method is based on the much more strong spectral dependence of the heat emission intensity I(λ) within the short-wavelength region in comparison with that of the emissivity ɛ(λ). At relatively low temperatures T ≤ 3000–4000 K, any opaque object radiates as a gray body in the short-wavelength spectrum region (λ ≤ 350–400 nm), even if it appreciably differs from a gray body in its optical properties. An experimental heat emission spectrum of tungsten (T ≈ 1970 K), for which the influence of the emissivity manifests itself in the long-wavelength region (λ > 580 nm), is given.     

A grazing incidence spectrograph for recording spectra in the range of 5–50 nm

A wide-range grazing-incidence spectrograph based on a spherical grating has been developed for recording integrated (over time) line spectra in the region of 5–50 nm. Resolution of the spectrograph λ/Δλ has been determined; at a wavelength of 5 nm, it exceeds 500 units. The spectrograph has been used to analyze line spectra of multiply charged ions for strong resonance lines in the range of 15–20 nm and record the line of X-ray lasing on the 3p-3s transition of a Ne-like Ar ion with wavelength λ = 46.9 nm.     

An automatic system for controlling the UV-radiation dose

A photodetector based on an ⁺-GaAs/n-ClInPc heterojunction, photosensitive in the 200- to 1000-nm wavelength range, is described. The ClInPc absorbtance is high:K _λ=2×10⁷ m⁻¹ for λ=220 and 380 nm. The photosensitivity of the photodetector is 2250 V/W (S=1 cm²). An automatic system for controlling the UV-radiation dose was developed on the basis of this photoreceiver.     

Experimental investigation on dilute gas-solid multiphase jet in crossflow

In the current study, an experimental setup was built to investigate the gas flow and particle distribution in a normal jet crossflow to a main flow in a confined test section. The experiments were conducted under two test conditions: with Re_c/Re_jet of 7.9×10⁴/3.1×10⁴ and 7.0×10⁴/1.8×10⁴. Four classes of particles were used in both tests. The planar gas flow field and particle distribution on the symmetric cross-section were measured by a DPIV system. Mean fluid velocity results and transient flow visualization images were used to analyze the jet influence on the gas flow field. The analysis of the time-average particle concentration reveal that the jet control method may set a gas barrier in the flow field, which the tiny particles are able travel around, large particles are able travel through, and only 10-micronscale particles could be successfully blocked. The results show that the wall jet control method can be applied in inertia particle separator.     

Non-intrusive laser-based, full-field quantitative flow measurements aided by digital image processing. Part 1: Eccentric cylinders

A technique based on computer-aided image processing was combined with a full flow field particle tracking procedure (FFFT) to yield a non-intrusive method for quantification of qualitative images. The method yields time dependent trajectories, velocities and accelerations throughout the entire flow field. The method has been applied to a geometry of eccentric cylinders with the inner cylinder in rotation. The observed flow patterns are similar to the ones postulated by the three-dimensional flow simulations in narrow gaps. In this application the flow is characterized by a low bearing Reynolds number (Re_h = Rωc/ν). The qualitative and quantitative part of the study concentrates on the flow characteristics in and around the cylinders' minimum clearance. The minimum radial clearance can be varied from 0.0254 to 0.889 mm (0.001–0.035 in). The study of this geometry yields knowledge with immediate application to flow in hydrodynamic journal bearings.     

Pumping effect of sliders in hard disk systems

The accumulation of contaminants on the slider surface is of paramount importance in hard disk drives because only an ultra small amount of contaminants on the slider surface will cause catastrophic failures for hard disk drives with a spacing between the slider and the hard disk as small as 10 nm, which will be reduced further in the near future to about 5–6 nm in order to attain a recording density of 100 Gbit/in². In this paper the pumping effect of the slider is proposed as one mechanism of the contaminant accumulation on the slider. Analysis of the pumping effect is conducted by considering the adsorption process and the shear flow process on the slider surface in terms of the continuum. It is found that the pumping effect can be divided into two different classifications depending on the value of the parameter λ which is the ratio of the maximum shear flow of the adsorbed film to the maximum adsorption amount: the shear flow rate-controlling pumping effect for λ < 0.1 and the adsorption rate-controlling pumping effect for λ > 0.4. For the shear flow rate-controlling effect, the accumulation rate of the contaminant is directly proportional to the disk surface velocity, while inversely proportional to the flying height of the slider. An erratum to this article can be found at .     

On optical flow techniques applied to breaking surges

Surface wave breaking is a challenging two-phase flow process which plays an important role in numerous physical processes. A highly-turbulent unsteady breaking surge was investigated experimentally in a large facility, and substantial aeration occurred in the roller. The application of three optical flow techniques (Lucas-Kanade, Horn-Schunck and Farnback) to the air-water region was tested. The results indicated that the Farnback technique provided most accurate results, although some misleading results could be obtained near the air-water boundaries of the roller. The bore generation by a rapid gate closure showed a highly-unsteady complicated velocity field, with substantial free-surface deformations, wave breaking and formation of large coherent structures before the surge detached from the gate. Further upstream, the surge propagated as a hydraulic jump in translation and the data showed a marked shear region with a recirculation zone above, showing air-water flow features comparable to stationary hydraulic jumps. The upper and lower bounds of air-water flow region yielded data implying an air-to-water velocity ratio about 4–5 for a Froude number Fr₁ = 2.1.     

The parameters measurement of air–water two phase flow using the electrical resistance tomography (ERT) technique in a bubble column

The air–water two-phase flow is investigated in a bubble column with a height of 2 m and a diameter of 0.282 m by using the Electrical Resistance Tomography (ERT) technique. The flow characterization are measured by applying ERT sensors of three vertical sections with superficial gas velocities in the range 0.027–0.156 m/s. Based on the cross-correlation technique and dynamic gas disengagement (DGD) theory, the bubble Saunter diameters are obtained and the local axial velocity about two phases flow can be calculated. The results show that with increased gas superficial velocity the distribution of bubble size is gradually widespread. Moreover, the local velocity of gas bubble swarm has a center peak distribution with increased gas superficial velocity.     

Intermittent airblast atomization and spray particle mixing in pulsating air flow

A new twin-fluid injector applicable to a single-point injector for gasoline engine was developed. Using this injector's transient spray characteristics, the mixing mechanism of spray particles and the deposition mechanism on the inner wall of an intake manifold were investigated. The measurements of spray particle sizes, velocities and deposition rates were experimentally conducted in a pulsating air flow. The particle deposition takes place due to the particle inertia at high air flow rates. At low air flow rates it comes due to the recirculation appearing on the inner wall at the entrance of the intake manifold. On the other hand, the deposition rate of spray particles is strongly influenced by air pulsation. The behavior of spray particle is mainly influenced by air pulsation when the velocity of the atomizing air is low and when the velocity of the air flow around the injector is high. Single small particles follow the air flow more easily than large particles, and this causes the spatial particle size distribution in the spray clump. As the spray particles approach to the tip of a spray clump, the size of particles.     

Numerical study on no emission with flue gas dilution in air and fuel sides

Flue gas recirculation (FGR) is widely adopted to control NO emission in combustion systems. Recirculated flue gas decreases flame temperature and reaction rate, resulting in the decrease in thermal NO production. Recently, it has been demonstrated that the recirculated flue gas in fuel stream, that is, the fuel induced recirculation (FIR), could enhance much improved reduction in NO per unit mass of recirculated gas, as compared to conventional FGR in air. In the present study, the effect of dilution methods in air and fuel sides on NO reduction has been investigated numerically by using N3 and CO2 as diluent gases to simulate flue gases. Counterflow diffusion flames were studied in conjunction with the laminar flamelet model of turbulent flames. Results showed that CO2 dilution was more effective in NO reduction because of large temperature drop due to the larger specific heat of CO2 compared to N₂. Fuel dilution was more effective in reducing NO emission than air dilution when the same recirculation ratio of dilution gas was used by the increase in the nozzle exit velocity, thereby the stretch rate, with dilution gas added to fuel side.     

Creep characterization of type 316LN and HT-9 stainless steels by the K-R creep damage model

The Kachanov and Rabotnov (K-R) creep damage model was interpreted and applied to type 316LN and HT-9 stainless steels. Seven creep constants of the model,A, B. k, m, λ, γ, andq were determined for type 316LN stainless steel. In order to quantify a damage parameter, the cavity was interruptedly traced during creep for measuring cavity area to be reflected into the damage equation. For type 316LN stainless steel, λ=ε _R /ε* and λ _f =ε/ε _R were 3.1 and increased with creep strain. The creep curve with λ=3.1 depicted well the experimental data to the full lifetime and its damage curve showed a good agreement whenr=24. However for the HT-9 stainless steel, the values of A and A/ were different as λ=6.2 and λ _f =8.5, and their K-R creep curves did not agree with the experimental data. This mismatch in the HT-9 steel was due to the ductile fracture by softening of materials rather than the brittle fracture by cavity growth. The differences of the values in the above steels were attributed to creep ductilities at the secondary and the tertiary creep stages.     

Measurements of nonstationary temperatures by the spectral pyrometry method

Recording of a sequence of thermal-radiation spectra allows determination of a nonstationary temperature T(t) without using the data on the emissivity of an object. For a КЭФ-4.5 silicon single crystal heated with radiation from a continuous-wave Nd:YAG laser (λ = 1.064 μm), sequences of hundreds of emission spectra in wavelength ranges of λ = 350–760 nm and λ = 650–1000 nm were recorded at a signal storage time of a CCD array of τ = 15–35 ms and a frequency of recording spectra of f ≈ 30–66 Hz. The spectra were automatically processed, and the dependences of the crystal temperature on the time after the irradiation onset were obtained in the range T ≈ 1100–1450 K.