Visualization Of Two-phase Flow Research Articles

Multielectrode capacitance sensors for the visualization of transient two-phase flows

The use of capacitance tomography for the imaging of transient flows in trickle-bed reactors is described. Preliminary results obtained with a 12-electrode system are displayed. The results show the possibilities available with capacitance tomography and allow a more detailed examination of the fluid dynamic effects resulting in the pulsing of the cocurrent flow of gas and liquid through packed columns. To improve the axial and spatial resolution as well as the signal-to-noise ratio of the 12-electrode sensor, a new 16-electrode sensor was developed. The improvements include the total number of electrode segments, the operation of the electrode segments, the axial shielding of the sensor, and the potential of the free electrodes.

Critical heat flux and flow pattern for water flow in annular geometry

An experimental study on critical heat flux (CHF) and two-phase flow visualization has been performed for water flow in internally-heated, vertical, concentric annuli under near atmospheric pressure. Tests have been done under stable forced-circulation, upward and downward flow conditions with three test sections of relatively large gap widths (heated length = 0.6 m, inner diameter = 19 mm, outer diameter = 29, 35 and 51 mm). The outer wall of the test section was made up of the transparent Pyrex tube to allow the observation of flow patterns near the CHF occurrence. The CHF mechanism was changed in the order of flooding, churn-to-annular flow transition and local dryout under a large bubble in churn flow as the flow rate was increased from zero to higher values. Observed parametric trends are consistent with the previous understanding except that the CHF for downward flow is considerably lower than that for the upward flow. In addition to the experiment, selected CHF correlations for annuli are assessed based on 1156 experimental data from various sources. The Doerffer et al. (1994); Barnett (1966); Jannsen and Kervinen (1963); Levitan and Lantsman (1977) correlations show reasonable predictions for wide parameter ranges, among which the Doerffer et al. (1994) correlation shows the widest parameter ranges and a possibility of further improvement. However, there is no correlation predicting the low-pressure, low-flow CHF satisfactorily.

Two-phase flow visualization and relative permeability measurement in natural rough-walled rock fractures

Two-phase flow visualization and relative permeability measurement in natural rough-walled rock fractures

Visualization and correlation analysis of counter-current two-phase flow in a thermosyphon by neutron radiography

We visualized the working fluid behavior in a bent thermosyphon by a neutron radiography and analyzed it quantitatively with an image processing technique. Working fluid velocity was evaluated by the two-point correlation function and also the spatio-temporal correlation was calculated, which led to a clarification of the pass route of working fluid in the thermosyphon. The probability density function of the void fraction was calculated.

Two-phase flow visualization and relative permeability measurement in natural rough-walled rock fractures: P. Persoff & K. Pruess, Water Resources Research, 31(5), 1995, pp 1175–1186

Two-phase flow visualization and relative permeability measurement in natural rough-walled rock fractures: P. Persoff & K. Pruess, Water Resources Research, 31(5), 1995, pp 1175–1186

103 陽子線ラジオグラフィによるヒートパイプ内の流れの可視化

Experimental results are presented on visualization of two-phase flow in capillary heat pipe by using proton radiography. Proton radiography is non-destructive testing technique making use of the difference in attenuation characteristics of proton beam in materials. 12MeV Proton beam is generated from Tandem accelerator. Beam diameter is 20mm. Proton beam passing through the test section are converted to visible light by fluorescent screen. Heat Pipe is thin-plate type (190mm X 50mm, 1.3mm in thikness), and 0.6×0.7mm channel is snaking in it. Normal speed video camera (recording rate is 60 frame per second) and high speed camera (1000 frame per second) are used. Small bubbles whose diameter are less than 0.6mm can be visualized clearly and recorded with the rate of 1000 frame per second.

Visualization and Measurements of Two-Phase Flow in Metallic Ducts Using Neutrons as Microscopic Probes. 2nd Report. Measurements of Some Flow Characteristics by Image Processing Techniques.

In order to apply the neutron radiography technique to fluid research, quantitative analyses of high-frame-rate neutron radiographs of air-water flows in a narrow rectangular duct were demonstrated by using image processing techniques. From the geometrical information of the images, the flow regime, bubble rising velocity, water-layer thickness, and interfacial area calculated by multiplying the interfacial length by the channel gap and size distribution of bubbles with diameter greater than 1 mm could be measured by the image processing technique. From the gray levels of the images, the spatial and temporal averaged void fractions and the void profile across the duct could also be measured by the image processing technique. It was clarified that the limited value of the measurable liquid-film thickness with the neutron radiography technique was calculated by an error analysis. It was shown quantitatively that the measurement error decreased by integrating the image temporally.

Visualization and Measurements of Two-Phase Flows in Metallic Ducts Using Neutrons as Microscopic Probes. 5th Report. Void Distribution Measurement Method for Two-Phase Flow in a Round Tube.

A method was proposed for measuring the radial void distribution of a two-phase flow in a round tube using neutrons as probes. This method involves (1) assuming an appropriate function which could approximate the radial void distribution, (2) deriving a function to approximate the void distribution projected on a plane perpendicular to the incident neutron beam and along the transverse direction (x-direction) by integrating the function of the radial void distribution, and (3) determining parameters in the function of the radial void distribution by the least squares method. It was shown analytically and experimentally that the effect of scattered neutrons and unparallelness of the incident neutron beam, which might affect the applicability of this method, could be corrected and ignored, respectively, by taking an appropriate distance between the converter and the test section. This method was applied to the measurement of a temporally averaged radial void distribution of an air-water flow in a round tube of 3.90 mm inner diameter. It was revealed that complex void distribution with a saddle-shaped profile could be measured by assuming an appropriate function.

Visualization and Measurements of Two-Phase Flows in Metallic Ducts Using Neutrons as Microscopic Probes. 3rd Report. Quantitative Measurement of Neutron Radiography Image.

The quantitative measurement of an image obtained by neutron radiography technique was proposed in order to accurately measure the void fraction of a two-phase flow in a metallic duct. It was shown experimentally that the spatial distribution of the dark-current component was homogeneous and the temporal variation could not be ignored. Since the scattered neutrons falling on the image converter could be homogenized by setting the test section at a distance from the converter, it was clarified that the corrections for dark-current and scattered neutrons could be represented by an offset value. It was proposed that the offset value could be determined by using the total macroscopic cross section of the material (Σ-scaling method). By comparing the calculated void fractions with the measured ones obtained by simulating the known void profile using a standard test section, the void fraction could be measured by this method within 2% error. The measurement error was estimated to be up to 10% when no corrections for scattered neutrons were made or no arbitrary offset value was used.

Visualization and Measurements of Two-Phase Flows in Metallic Ducts Using Neutrons as Microscopic Probes. 4th Report. Effect of Image Gray Scale and Pixel Number on Image Quantification.

A study on the effects of image gray scale and pixel number on image quantification was performed analytically and experimentally in order to establish a method of measuring multiphase flows using neutron radiography. It was found from a simplified model that the resolution of an object thickness to be measured had logarithmic characteristics for the object thickness, and there is a possibility that the measured void fraction in high- and low-void-fraction regions might include large error. It was shown from the numerical analysis that the measurement error would be within 5% except for the annular flow region with the void fraction greater than 90%, when void fraction of an air-water two-phase flow in a channel with the local gap smaller than 9mm was measured under the condition of the image gray scale=100. It was revealed from a simplified model that the error due to the number of pixels would be within 5% when the number of pixels projecting the object is ten times greater than that projecting the object boundary.

Application of High-Frame-Rate Neutron Radiography with a Steady Thermal Neutron Beam to Two-Phase Flow Measurements in a Metallic Rectangular Duct

To apply the neutron radiography (NRG) technique to fluid research, high-frame-rate NRG with a steady thermal neutron beam was developed by gathering up-to-date technologies for neutron sources, scintillators, high-speed videos, and image intensifiers. This imaging system has many advantages such as a long recording time, high-frame-rate (up to 1,000 frame/s) imaging, and no need for a triggering signal. Visualization of air water two-phase flow in a metallic rectangular duct was achieved at the recording speeds of 250, 500, and 1,000 frame/s. The qualities of those consecutive images were good enough to observe the flow mechanism and to measure the flow characteristics. It was demonstrated that some characteristics of two-phase flow could be measured by using the current imaging system. To quantify geometric information from NRG images, measurements of flow regime, rising velocity of bubbles and wave height, interfacial length, and interfacial area in annular flow were performed by using the image processing technique. To quantify attenuation characteristics of neutrons in materials, measurements of average void fraction and void profile were conducted. It was confirmed that this new technique may have significant advantages in both visualizing and measuring high-speed fluid phenomena when the ordinary methods such as the optical method andmore » X-ray radiography cannot be applied.« less

Visualization and Measurements of Two-Phase Flows in Metallic Ducts Using Neutrons as Microscopic Probes. 1st Report, Time-Resolved Neutron Radiography and Its Limited Time-Resolution.

Experiments on visualization of high-speed fluid phenomena in metallic ducts were conducted by three kinds of time-resolved neutron radiography techniques. In the direct film method with a pulsed neutron beam, frozen neutron radiographs of air-water flows in a rectangular duct were obtained successfully. The high-frame-rate neutron radiography with pulsed and steady neutron beams allowed visualization of boiling water two-phase flow in a tube at the recording speed of 500 frames/s and air-water flows in a rectangular duct at up to 1000 frames/s, respectively. The quality of the images obtained from the time-resolved neutron radiography was sufficient for observation of the fluid behavior. The limit of the time-resolution was clarified based on the sensitivity and light-decay constant of the converter and the measurement error of neutrons calculated by statistical analysis.

Visualization of fluid phenomena using a high frame-rate neutron radiography with a steady thermal neutron beam

Experiments on visualization of fluid phenomena in a metallic vessel were conducted using a high-speed video camera with a steady thermal neutron beam. The Japan Research Reactor 3M, whose flux at the imaging plate is 1.5 × 108 n/cm2 s, of Japan Atomic Energy Research Institute was used as a neutron source. The imaging system for high frame-rate neutron radiography with the steady thermal neutron beam consisted of a high sensitivity scintillator, 6LiF/ZnS:Ag, an image intensifier whose gain was a factor of 100 000 at the maximum, and a high-speed video which could record phenomena at 1000 frames/s for 14 min by using an ordinary VHS tape with the recording time of 120 min at the normal speed. Visualization of air-water two-phase flows in a rectangular duct with 2.4 mm gap and 40 mm width were successfully performed with the steady thermal neutron beam and this imaging system at frame rates up to 1000 frames/s. The shapes and the behavior of bubbles and liquid films were clearly observed. The rising velocities of slug bubbles and the flow regimes could be also measured. It was clarified that this new technique may have significant advantages both in visualizing and measuring high-speed phenomena when the visible light is not applicable.

Application of Neutron Radiography to Visualization and Void Fraction Measurement of Air-Water Two-Phase Flow in a Small Diameter Tube

Abstract The purpose of this study is to investigate the feasibility of visualization and void fraction measurement of air-water two-phase flow in a small diameter tube (I.D.: 4.08mm) by using the real-time neutron radiography and image processing techniques. Video images of two-phase flow were taken by using the real-time neutron radiography system (thermal neutron radiography facility No. 2) installed at the Japan Research Reactor 3 M of the Japan Atomic Energy Research Institute. The shape of bubbles and its moving behavior were clearly observed from the video images. The image corrections for dark current, shading, field intensity fluctuation and electrical system drift were examined in order to measure the void fraction from the video images. Though, generally speaking, the effect of the scattered neutron could not be ignored for quantification of the images taken by the neutron radiography, the scattered neutron could not affect the final results of void fraction in the case of a small diameter tube. The void fraction calculated from the corrected images was correlated well with the drift flux equation, indicating that the existing drift flux equation could be applied to predict the void fraction of two-phase flow in a small diameter tube. It was demonstrated that the real-time neutron radiography technique could be useful for measuring the void fraction of two-phase flow in a small diameter tube.

Flow visualization of two-phase flows using photochromic dye activation method

A non-intrusive flow visualization technique based on light activation of photochromic dye material has been used to obtain velocity profiles in gas-liquid flows including annular, slug and stratified flows. The preliminary results revealed several important two-phase flow mechanisms that have not been clearly seen previously.

Application of Neutron Radiography to Visualization and Void Fraction Measurement of Air-Water Two-Phase Flow in a Small Diameter Tube.

Abstract The purpose of this study is to investigate the feasibility of visualization and void fraction measurement of air-water two-phase flow in a small diameter tube (I.D.: 4.08mm) by using the real-time neutron radiography and image processing techniques. Video images of two-phase flow were taken by using the real-time neutron radiography system (thermal neutron radiography facility No. 2) installed at the Japan Research Reactor 3 M of the Japan Atomic Energy Research Institute. The shape of bubbles and its moving behavior were clearly observed from the video images. The image corrections for dark current, shading, field intensity fluctuation and electrical system drift were examined in order to measure the void fraction from the video images. Though, generally speaking, the effect of the scattered neutron could not be ignored for quantification of the images taken by the neutron radiography, the scattered neutron could not affect the final results of void fraction in the case of a small diameter tube...

An electronic Cranz–Schardin camera

An electronic high-speed multiple-image camera based on the Cranz–Schardin principle has been developed. High-power LEDs are used as a light source. Frames are recorded by a system of eight CCDs and an eight channel image processor enabling a series of eight subsequent images at full-frame (512× 512) resolution or more images (up to 80) at a lower resolution. The maximum framing frequency is 10 MHz. The advantages upon the classical spark illumination and photographic techniques are mainly the precise electronic control, the immediate access to the frames, the on-line image processing and the low cost of the system. Examples of gas- and two-phase flow visualization are presented.

中性子ラディオグラフィーによる金属管内気液二相流の可視化

Visualization of two-phase flow were carried out by using a neutron radiography method, which is suited to visualize gas-water two-phase flow in a metalic tube. Nitrogen gas-water two-phase flow in a stainless steel tube and water boiling two-phase flow with and without a twist tape in a stainless steel tube were visualized.