Void fraction,bubble velocity and bubble size in two-phase flow

Experiments were performed in atmosphereic vertical air-water flows, for void fractions between 0.25 and 0.75 (cross-sectional averages) and superficial liquid velocities of 1.3, 1.7 and 2.1 m/s. Local values of void fraction and bubble velocity as well as the bubble diameter were measured by means of a resistivity probe technique. Reliable values were obtained for the local void fraction over the entire range $\text{0 ≤ α ≤ 1}$. The void fraction profiles appeared to have a local maximum at the pipe center, local maxima close to the wall were obviously absent. The resistivity probes are shown to measure the velocity of the interface between the conducting and nonconducting phases, which equals the gas velocity only for low void fractions. The measured data for void fraction and bubble velocity were correlated by means of power law distribution functions, with exponents given by a function of the cross-sectionally averaged void fraction. The Sauter mean diameters for the bubble size spectra found, agree reasonably well with diameters predicted by a theoretical model based on the energy dissipation in the flow.     

A simplified method proposal for practical determination of aperiodic two-phase flow instability

A short outline of the instability phenomenon is given. Two methods are explained for the problem solution, i.e. the practical determination of the existence or not of an aperiodic instability and, in the case of existence, what to do to eliminate it. The first method substantially comes from that used by boiler designers. A second, new method is suggested: this is a manual, simple method. The two methods are compared: the new one is reliable, and has the great advantage of the simplicity and the possibility of specifying the diaphragm at the pipe mouth for eliminating the two-phase flow instability.     

Measurement of bubble size, velocity and concentration in flashing flow behind a sudden constriction

In the present paper the results of investigations in flashing flow behind a sudden constriction in vertical upflow are described. Flow visualization, laser-Doppler and phase-Doppler anemometry have been used to measure local bubble and fluid velocities, local bubble sizes and void fractions. The measurements were performed in the midplane of a two-dimensional channel with a 2:1 stepwise constriction.It was found that bubble nucleation takes place in the recirculation zone immediately behind the constriction, which is the location of the lowest static pressure. These bubbles are transported downstream by the mean flow field, while undergoing further growth. No additional nucleation was observed downstream of the recirculation zone. A periodic, cloudwise behaviour of the bubble formation was found which could be explained by the interaction between the bubble growth and the mean flow field. This interaction results in strong disturbances of the mean flow field, which show up as an increase of the fluctuating bubble velocity by a factor of 3 compared to single-phase measurements in a region of 10 step heights behind the constriction. However, these fluctuations appear more like a periodic change in the mean velocity rather than a higher turbulence level. The measured arithmetic mean bubble diameters rise from approx. 50 μm in the recirculation region to about 70–80 μm 50 step heights downstream. Maximum local bubble number density and void fraction were found to be 160001/cm³ and 0.8%, respectively.     

In-line imaging measurements of particle size,velocity and concentration in a particulate two-phase flow

A novel method is developed for in-line measurements of particle size, velocity and concentration in a dilute, particulate two-phase flow based on trajectory image processing. The measurement system consists of a common industrial CCD camera, an inexpensive LED light and a telecentric lens. In this work, the image pre-processing steps include stitching, illumination correction, binarization, denoising, and the elimination of unreal and defocused particles. A top-hat transformation is found to be very effective for the binarization of images with non-uniform background illumination. Particle trajectories measured within a certain exposure time are used to directly obtain particle size and velocity. The particle concentration is calculated by using the statistics of recognized particles within the field of view. We validate our method by analyzing experiments in a gas-droplet cyclone separator. This in-line image processing method can significantly reduce the measurement cost and avoid the data inversion process involved in the light scattering method.     

In-line imaging measurements of particle size,velocity and concentration in a particulate two-phase flow

A novel method is developed for in-line measurements of particle size, velocity and concentration in a dilute, particulate two-phase flow based on trajectory image processing. The measurement system consists of a common industrial CCD camera, an inexpensive LED light and a telecentric lens. In this work, the image pre-processing steps include stitching, illumination correction, binarization, denoising, and the elimination of unreal and defocused particles. A top-hat transformation is found to be very effective for the binarization of images with non-uniform background illumination. Particle trajectories measured within a certain exposure time are used to directly obtain particle size and velocity. The particle concentration is calculated by using the statistics of recognized particles within the field of view. We validate our method by analyzing experiments in a gas-droplet cyclone separator. This in-line image processing method can significantly reduce the measurement cost and avoid the data inversion process involved in the light scattering method.     

Momentum and heat transfer in two-phase bubble flow—II. A comparison between experimental data and theoretical calculations

Momentum and heat transfer process in a two-phase air-water bubble flow is investigated experimentally to confirm the applicability of the theoretical model proposed in Part 1 of this study. Comparisons are made between the measurements and the predictions for both velocity and temperature profiles, and then satisfactory agreement is obtained. Also, the results for bubble flow simulation are presented to clarify the interrelationship of the frictional pressure gradient and the heat transfer coefficient with the void fraction profile.     

Bin size determination for the measurement of mean flow velocity in molecular dynamics simulations

Extracting macroscopic properties from molecular simulation of fluids is required in most of the molecular dynamics problems. However, methods used for this purpose, their accuracy, and their dependence on the sampling and averaging schemes are still ambiguous. Macroscopic properties at any point of a molecular domain can be extracted via sampling and averaging of molecular behavior within a control region around that point, called a bin. The size of this bin has a significant impact on the accuracy of the macroscopic properties' measurement. In this research, we will focus on the measurement of mean flow velocity using ‘binning method’. On the basis of the results of this study, the most appropriate range of the bin size in which mean velocity can be measured accurately is determined. Although this range is determined based on the measurement of mean flow velocity, we believe that it can be used to estimate the proper range for the measurement of other flow quantities as well. Copyright © 2012 John Wiley & Sons, Ltd.     

An LDA technique for in situ simultaneous velocity and size measurement of large spherical particles in a two-phase suspension flow

A relatively simple optical scheme using the reference-mode laser Doppler anemometry for the in situ measurement of flow properties of a dilute particle-fluid two-phase suspension having a predominant flow direction is hereby proposed. It is an extension of the established technique of optical gating for particle sizing which is fully integrated into the established technique of laser Doppler anemometry for velocity measurement. Particles that can be measured by this scheme are limited to those with sizes greater than the smaller dimension of the optical measuring volume. Inherent in the methodology is a procedure for providing information on the local particle number density and velocity distributions for each size range of the particles and the local velocity distribution of the continuous phase. The accompanying electronics and interfaces are also established for data processing and analysis in a mini computer. Validation of the scheme has been accomplished by controlled experiments using stainless steel balls and water droplets of 1 mm and greater in diameter.     

A three-dimensional photographic method for measurement of phase distribution in dilute bubble flow

A three-dimensional photographic method has been developed to measure phase distributions in bubbly flow in a pipe. In this method a mirror was used to reflect a side view of the flow into the front-view direction, and then flow images in both views were taken simultaneously by one camera. After three-dimensional position and size of each bubble in the flow field were determined by matching the two bubble images in the side and front views, the phase distributions were obtained for the bubbly flow. Received: 25 April 2000 / Accepted: 21 June 2001 Published online: 29 November 2001     

A novel data based control method based upon neural network and simultaneous perturbation stochastic approximation

A novel data based control method is proposed by modifying the cost function of a model-free control method based on Simultaneous Perturbation Stochastic Approximation (SPSA). The controller is constructed through use of a Function Approximator (FA), which is fixed as a neural network here. In the novel approach, the ability of the controller has been greatly improved. At last, the proposed modified control method is applied to solve non-linear tracking problems. Simulation comparison tests were done on typical non-linear plants, through which, the effectiveness of the novel data based control method is fully illustrated.     

A novel experimental facility for measuring internal flow of Solid-liquid two-phase flow in a centrifugal pump by PIV

PIV is one of the most effective flow measurement methods, but for solid-liquid two-phase flow in a centrifugal pump with complex internal structure and high speed rotation, many problems exist in the experimental facility design, such as solid particle releasement, synchronization issues among camera, laser and pump impeller, and methods of avoiding introducing bubbles into flow passage, etc. To solve these critical problems, a new experimental facility without agitation was designed for internal solid-liquid two-phase flow test in a centrifugal pump by PIV. The facility was tested by measuring two-phase flow of water-glass beans and water-rape seeds in a centrifugal slurry pump. The new test facility has many obvious advantages, such as the simpler structure without the agitating device, less noise in the test process, less power consumption, less number of particles to get a certain particle volume concentration, and the decrease of the experiment cost. The result shows that the new facility can realize a stable solid-liquid two-phase flow measurement with high reliability in a centrifugal pump.     

A method for assessing nanomaterial dispersion quality based on principal component analysis of particle size distribution data

Seemingly contradictory findings between studies are a major issue in nanoecotoxicological research and have been explained as a result of the lack of comparability between assay methods,with dispersion of nanomaterials being identified as a key factor.Here we show the use of a multivariate method,principal component analysis(PCA),as a tool in protocol development and categorization of dispersion quality.Results show the significance of particle concentration within a protocol,and its effect on repeatability.Our results suggest that future studies should involve the use of PCA as a powerful data exploration tool to facilitate method development,comparability and integration of data across different laboratories.     

A physically based correlation for the effects of power law rheology and inclination on slug bubble rise velocity

A study has been made of the motion of long bubbles in inclined pipes containing viscous Newtonian and non-Newtonian liquids. A semi-theoretical expression for the rise velocity of air bubbles in water is derived on the hypothesis that the dominant factor is the momentum exchange of the bubble underflow, i.e. the bubble nose shape. The correlation calls on empirical inputs from established literature on bubble rise speeds at high Reynolds number. The effects of increasing Newtonian viscosity are analysed with reference to the momentum exchange and it is shown how viscosity reduces the inclination dependence of the bubble Froude number. Results from an experimental survey in seven different non-Newtonian liquids in three different diameter pipes are presented. These data are correlated so as to decouple the effects of surface tension and viscosity. An empirical relation is proposed for the effective shear rate in the fluid travelling around the bubble nose. Our correlation is compared to literature data from a broad range of Reynolds numbers with excellent agreement except at shallow angles.     

A method for assessing nanomaterial dispersion quality based on principal component analysis of particle size distribution data

Seemingly contradictory findings between studies are a major issue in nanoecotoxicological research and have been explained as a result of the lack of comparability between assay methods, with dispersion of nanomaterials being identified as a key factor. Here we show the use of a multivariate method, principal component analysis (PCA), as a tool in protocol development and categorization of dispersion quality. Results show the significance of particle concentration within a protocol, and its effect on repeatability. Our results suggest that future studies should involve the use of PCA as a powerful data exploration tool to facilitate method development, comparability and integration of data across different laboratories.     

Image processing of hydrogen bubble flow visualization for determination of turbulence statistics and bursting characteristics

A technique is described which employs automated image processing of hydrogen-bubble flow visualization pictures to establish local, instantaneous velocity profile information. Hydrogen bubble flow visualization sequences are recorded using a high-speed video system and then digitized, stored, and evaluated by a VAX 11/780 computer. Employing special smoothing and gradient detection algorithms, individual bubble-lines are computer identified, which allows local velocity profiles to be constructed using time-of-flight techniques. It is demonstrated how this techniques may be used to 1) determine local velocity behavior as a function of position and time, 2) evaluate time-averaged turbulence properties, and 3) correlate probe-type turbulent burst detection techniques with the corresponding visualization data.List of symbols Re Reynolds number based on momentum thickness, u / - t ⁺ nondimensional time tu ² / - T VITA variance averaging time period - u shear velocity = - u local instantaneous streamwise velocity,x-direction - u local fluctuating streamwise velocity,x-direction - u ⁺ nondimensional streamwise velocity, /u - local normal velocity,y-direction - w local spanwise velocity,z-direction - x ⁺ nondimensional coordinate in streamwise direction xu /v - y ⁺ nondimensional coordinate normal to wall, yu /v Greek momentum thickness, - kinematic viscosity - _w wall shear stress This paper was presented at the Ninth Symposium on Turbulence, University of Missouri-Rolla, October 1–3, 1984     

A method for flow visualization and measurement of velocity vectors in three-dimensional flow patterns in water models by using colour photography

Summary A method is described which permits the measurement of velocity vectors in a three-dimensional flow field. This method, which is especially useful for the study of water models, uses colour as an indication of the third dimension, thus enabling complete determination of direction and magnitude of the velocity by means of a single colour-photograph.     

A hot-wire probe configuration and data reduction method to minimize velocity gradient errors for simultaneous measurement of three velocity components in turbulent flows

A highly resolved turbulent channel flow direct numerical simulation (DNS) with Re _?? ?=?200 has been used to investigate the influence of the velocity gradients on the measurement accuracy of a hot-wire probe capable of measuring all three velocity components simultaneously. A new proposed sensor arrangement has been tested. First, the effective cooling velocity was determined for each sensor of the idealized probe, where the influence of the velocity component tangential to the sensors and flow blockage by the presence of the probe are neglected. Then, velocity component statistics were calculated, neglecting the velocity gradients over the probe sensing area, and they were compared to the DNS database values. It has been shown that the influence of the velocity gradients on the new proposed arrangement is minimized. Its accuracy was compared to existing three- and four-sensor configurations as well as to two-sensor X- and V-array probes.