Void Fraction Values Research Articles

An experimental study on the influence of fluid flow pattern on microbubble generation

Geometrical properties of generated microbubbles induced by different fluid flow patterns were investigated experimentally. Image processing method has been used to find microbubble size distribution and to determine bubbles’ roundness as well. Three types of flow patterns were produced by changing microbubble generator configuration in order to improve bubbles’ size distribution. These different geometrical configurations of designed microbubble generator were used to generate microbubbles in various air volume fractions. Obtained results confirm direct relation between generated microbubbles size and their distribution with fluid flow patterns. More complex geometry creating high turbulent regions are suggested to increase bubble breaking up especially for high values of void fractions.

Void Fraction Variations in a Fractal-Like Branching Microchannel Network

Based on predictions of lower pressure drop penalties in fractal-like branching channels compared to parallel channels, an experimental investigation of two-phase void fraction variations was performed. The flow network, mimicking flow networks found in nature, was designed with a self-similar bifurcating channel configuration and etched 150 μ m into a 38.1 mm diameter silicon disk. A Pyrex® cover was anodically bonded to the silicon disk to allow for flow visualization. The length and width scale ratios between channels on either side of a bifurcation are fixed. The channel widths range in size from 100 μ m to 400 μm over a total channel length of approximately 17 mm. Experimental results of flow boiling are presented for a heater energy input power of 66 W and an inlet water flow rate of 45 g/min at a fixed inlet fluid temperature of 88°C. High-speed, high-resolution imaging was used to visualize the flow and quantify void fraction values in several channels within a branching structure. Both time-averaged and instantaneous two-dimensional void fraction data are presented, showing a correlation between channels at the same bifurcation level and between channels at different bifurcation levels.

Study of two-phase flows in reduced gravity using ground based experiments

Experimental studies have been carried out to support the development of a framework of the two-fluid model along with an interfacial area transport equation applicable to reduced gravity two-phase flows. The experimental study simulates the reduced gravity condition in ground based facilities by using two immiscible liquids of similar density namely, water as the continuous phase and Therminol 59® as the dispersed phase. We have acquired a total of eleven data sets in the bubbly flow and bubbly to slug flow transition regimes. These flow conditions have area-averaged void (volume) fractions ranging from 3 to 30% and channel Reynolds number for the continuous phase between 2,900 and 8,800. Flow visualization has been performed and a flow regime map developed which is compared with relevant bubbly to slug flow regime transition criteria. The comparison shows that the transition boundary is well predicted by the criterion based on critical void fraction. The value of the critical void fraction at transition was experimentally determined to be approximately 25%. In addition, important two-phase flow local parameters, including the void fraction, interfacial area concentration, droplet number frequency and droplet velocity, have been acquired at two axial locations using state-of-the-art multi-sensor conductivity probe. The radial profiles and axial development of the two-phase flow parameters show that the coalescence mechanism is enhanced by either increasing the continuous or dispersed phase Reynolds number. Evidence of turbulence induced particle interaction mechanism is highlighted. The data presented in this paper clearly show the marked differences in terms of bubble (droplet) size, phase distribution and phase interaction in two-phase flow between normal and reduced gravity conditions.

Performance evaluation of a newly developed flow diverted bed system for stream restoration

The flow diverted bed (FDB) system was developed to improve the stream water quality. This system (Run 2) had two wetlands where the flow from the upper layer (aerobic) in the first wetland could be redirected to the lower layer (anaerobic) in the second wetland by the flow diverter, and vice versa. In addition, the yellow-soil media placed at the bottom layer allowed a larger influx in the lower layer and had a sorption capacity of phosphate ions. The removal efficiencies of COD and TN at Run 2 [hydraulic loading rate (HLR) = 177 cm/d] was 1.7–3.6 times higher than that at Run 1 (HLR = 192 cm/d) without the diverter. The values of void fraction, dimensionless variance, dispersion number, Peclet number and velocity for this system were found to be 0.6637, 0.2847, 0.1717, 5.8241, and 53.2 m/d, respectively. The reducing trend of each pollutant was exponentially decreased along the overall length of this system. Also, the transport was dominated by convection (plug flow) rather than dispersion (complete mix). The volumetric removal rate constants for COD, SS, TKN, TN, and TP were found to be 4.854, 7.988, 1.388, 1.625, and 1.767 d −1 via the first-order rate model. Consequently, the flow diverter and yellow-soil media in this system played an important role in enhancing the treatability of pollutants and operating at a higher HLR.

Two-phase flow structure close to orifice contractions during horizontal intermittent flows

Two-phase horizontal intermittent flow through orifice contractions is experimentally investigated by means of a new impedance meter able to collect void fraction data simultaneously at different pipe locations. The investigation focuses on two-phase structure in the region downstream of orifice. Experiments were carried out on horizontal air–water flows in 40 mm inner diameter pipes. Six different orifice plates were considered, as combinations of two restriction ratios and three plate thicknesses. The results demonstrate that the orifices generally induce a marked increase in the downstream void fraction values, and that this is associated to a reduction in film thickness in the stratified regions.

The derivation of thermal relaxation time between two-phase bubbly flow

Thermal relaxation time constant is derived analytically for the relaxed model with unequal phase-temperatures of a vapour bubble at saturation temperature and a non-steady temperature field around the growing vapour bubble. The energy and state equation are solved between two finite boundary conditions. Thermal relaxation time perform a good agreement with Mohammadein (in Doctoral thesis, PAN, Gdansk, 1994) and Moby Dick experiment in terms of non-equilibrium homogeneous model (Bilicki et al. in Proc R Soc Lond A428:379–397, 1990) for lower values of initial void fraction. Thermal relaxation is affected by Jacob number, superheating, initial bubble radius and thermal diffusivity.

Réduction de traînée par injection de microbulles dans la couche limite d'écoulements en canal

An investigation of turbulent structure modification of fully developed channel flow by microbubble injection close to the upper wall was studied. Two-dimensional velocity components at Reynolds number of 5128 based on the half height of the channel and bulk velocity were measured. The particle image Velocimetry technique (PIV) was utilized to obtain the two-dimensional velocity fields of the fluid and the microbubbles. Microbubbles with an average diameter of 30 um were injected into the buffer layer. Various values of void fractions were used to evaluate the effects of microbubble concentrations on the drag reduction. Modifications in the length and time scales were detected by calculating two-point correlation coefficients and Wavelet maps. The energy spectra were calculated. It is interesting to note a shift from large frequency domain to lower low frequency structure.

A problem in the COBRA-EN code related to the void fraction calculation

When a certain void fraction value is reached in the two-phase flow regime, a problem occurs in the COBRA-EN code. This problem was observed in the drift-flux model option and interrupts code execution. Two solutions are proposed to solve the problem.

Temporal correlation modification by microbubbles injection in a channel flow

In an effort to achieve a better understanding of the drag reduction mechanism by microbubbles injection, the structure of flow turbulence in a water channel with microbubbles is studied using particle image velocimetry technique (PIV) at Re h = 5128. Full field of the two-dimensional velocity components (streamwise and normal) within a location close to the wall was measured. The effects of several values of local void fraction upon the drag reduction are investigated.

Measurement of dynamic void fractions in stratified types of flow

A new non-intrusive computerized image analysis and optical observation method for accurately measuring dynamic void fraction has recently been developed by Thome and coworkers [Dynamic void fractions in stratified types of flow, part I: new optical measurement technique, Int. J. Multiphase Flow 31 (2004); Dynamic void fractions in stratified types of flow part II: measurements for R-22 and R-410a, Int. J. Multiphase Flow 31 (2004)]. This technique is applied to circular horizontal sight glass tubes using a monochromatic laser sheet to illuminate the two-phase flow coupled with image processing to measure cross-sectional void fractions and dry angles in stratified types of flow. The refraction effects on the cross-sectional images are overcome by reconstructing the video images by computer. From these, the shape at the vapor–liquid interface is detected and the void fraction is accurately determined (to an estimated accuracy of about ±0.01) over a wide range of void fraction values (from 0.05 to 0.95). In addition, the dry angle around the upper perimeter of the tube is also obtained. The system has been coupled to a flow boiling test facility to obtain dynamic and time-averaged void fractions in a horizontal tube for two refrigerants: R-22 and R-410A. About 227,000 images were analysed so far in this study for a 13.6 mm tube to provide the same number of dynamic void fraction measurements and 238 time-averaged void fraction values. A summary of the technique is described here with additional analysis of the previous results and new results (based on processing 83,000 new images) are presented here for void fractions in an 8.0 mm sightglass tube with R-22 at mass velocities of 100 and 150 kg/(m 2 s). The new method also has the potential to measure interfacial wave contours and other pertinent geometrical characteristics of stratified two-phase flows.

Interfacial measurements in stratified types of flow. Part II: Measurements for R-22 and R-410A

A new optical void fraction measurement system has been coupled to a flow boiling test facility to obtain dynamic and time-averaged void fractions in a horizontal tube. A series of evaporation tests have been run for two refrigerants. R-22 was tested under mass velocity conditions of 70, 100, 150 and 200 kg/m 2 s and R-410A for 70, 150, 200 and 300 kg/m 2 s in a 13.6 mm diameter glass tube. Using our newly developed image processing system, about 227 000 images have been analyzed in this study to provide the same number of dynamic void fraction measurements. From these images, 238 time-averaged void fraction values have been obtained for vapor qualities from 0.01 to 0.95. These experimental points show very good agreement with the horizontal version of the Rouhani–Axelsson drift flux void fraction model.

Pressure drop and void fraction profiles during horizontal flow through thin and thick orifices

Two-phase flow pressure drop through singularities such as thin and thick orifices has been experimentally investigated. The study refers to air–water horizontal flows in 60 and 40 mm pipes. The operating conditions cover the gas and liquid superficial velocity ranges V sg=0.3–4 m/s and V sl=0.6–2 m/s, respectively. Intermittent flows have been observed. The local pressure drop has been obtained by means of extrapolation from upstream and downstream linearized pressure profiles. Single-phase measurements have been carried out for local liquid Reynolds values ranging from 3×10 4 to 2×10 5 to obtain the discharge coefficient and calculate the two-phase local multiplier. The effect of orifice geometry has been considered by selecting six different singularities with two different area contraction ratios ( σ=0.73 and σ=0.54) and different thickness (from 0.025 to 0.59 restriction diameters). The analysis of the results and their comparison with available relationships revealed a lack in the prediction capability of literature correlations for high values of the flow area ratio. Finally the void fraction profiles obtained by means of ring impedance probes showed that generally the effect of the singularity is to increase the void fraction values just downstream the orifice with differences up to 50% with respect to the straight pipe values.

Stability Criteria of the Steady Flow of a Liquid Containing Bubbles Along a Nozzle

The steady cavitating flow through a converging-diverging nozzle is considered. A continuum model is assumed with the Rayleigh-Plesset equation to account for the bubble dynamics. A similar problem has been studied previously by Wang and Brennen, and they found that if the upstream gas volume fraction of the bubbles exceeds a critical value there is flashing flow instability. In the present work, a perturbation analysis is made introducing a small parameter, ε, that is the ratio of the initial bubble diameter to the length scale of the nozzle. As a result of this analysis, the critical value of the upstream void fraction is calculated as a function of the several parameters appearing in the problem, and turns out to be very small and proportional to ε3. A correlation is proposed giving explicitly the functional dependence of this critical value.

The growth of vapour bubbles in superheated water between two finite boundaries

The paper analyses the behaviour of vapour-bubble growth between finite boundaries. The Plesset and Zwick theory is modified by assumptions that are different than those studied before. The growth problem is solved analytically in terms of initial bubble velocity and initial void fraction. The results are compared with those obtained from previous theories and experiment. Good agreement is obtained for certain values of the initial void fraction and the initial bubble velocity.PACS No.: 47.50Dz

Effect of size distribution on tapping properties of fine powder

As well known, a size distribution affects the packing properties of a fine powder, but no systematic data can be found about the effect of size or size distribution on packing density of fine powder under 50 μm. Fine powders with log-uniform size distribution were prepared by mixing of mono-sized silica or fly ash powder that was classified by Turbo classifier. The tapping property of the powder packed beds was measured using the Powder tester. From our experimental results, void fraction of the tapped powder beds is affected by size distribution in all tapping numbers. For log-uniform size distribution, the value of void fraction decreases with decreasing inclination of the size distribution. In the case of Andreasen (Gaudin–Shuhmann) distribution, the minimum void fraction was obtainable at the Fuller constant of 0.7–0.9. The experimental results were compared with the estimated values by our proposed model for a multi-component particle mixture.

Pressure drop changes during dissolution of pyrite cinder in a fixed bed

The percolation leaching of heavy metals from pyrite cinder in a packed bed is considered in this work. Flow rate and physicochemical parameters have been analysed in a gravity percolation leaching system with a simultaneous dissolution reaction. The structure of the unconsolidated bed of porous cinder changes during operation as a consequence of the leaching phenomenon. The variables that influence the flow rate are: bed length, particle size distribution and leaching liquor characteristics. Head loss changes during operation are mainly explained in terms of the fine particle redistribution, migration along the bed and thickening at the bottom. Experimental values of effective void fraction are compared with values from Ergun's equation modified to be applied to a layered bed.

Gas-Liquid Distribution in the Developing Region of Horizontal Intermittent Flows

Experiments have been carried out to analyze the evolution of the air/water flow structure along a horizontal 60 mm i.d. straight pipe. Plug and slug flow regimes were observed. In order to investigate the local structure of the flow, dynamic void fraction measurements have been performed by means of ring impedance probes. From dynamic void fraction measurements the mean void fraction values, the probability distributions of void, the liquid film height in the gas cavities and the slug frequencies have been evaluated. The analysis of the results showed that the flow structure deeply modifies along the pipe and that minor effects have to be ascribed to gas injection mode. The examination of the probability distribution function enabled the identification of the operating conditions at which the transition occurs, thus making it possible to propose a new objective criterion of identification.

Skill-based interpretation of noisy probe signals enhanced with a genetic algorithm

The paper presents the design and evaluation of an adaptive signal processing procedure based on human skill. The focus is on interpreting probe signals detected in gas–liquid flow in the presence of noise where existing signal interpretation techniques may encounter difficulties. Interpretation of a probe signal requires construction of a corresponding two-state signal that denotes the presence of the phases, i.e. gas and liquid, at the probe tip. To develop a computer procedure that would imitate a skilled operator in probe signal interpretation, manual knowledge acquisition and evolutionary optimization were employed. First, a prototype signal interpretation procedure based on operator skill was designed, and the procedure parameters were then optimized with a genetic algorithm. In the optimization process, a two-state signal reconstructed from the probe signal by an operator was used as a reference. The robustness of the approach was tested in a series of numerical experiments. They included local evaluation on training and test signals, calculation of global void fraction values, and an assessment of variability among different experts. The results showed that the developed technique is highly consistent with the operator way of signal interpretation and represents a reliable prerequisite for gas–liquid flow measurements.

Homogeneous hydraulic and pneumatic conveying of solid particles

A discussion is provided of the difficulties and limitations involved in the application of the one-dimensional two-phase flow model to estimate pressure gradients associated with the transport of particles. Two particular cases are investigated, namely, hydraulic conveying and pneumatic transport. The validity of the model is checked by comparing experimental data with predicted values of pressure gradients and void fractions. New approaches for estimating the interaction forces in hydraulic and pneumatic transport are proposed. It is demonstrated that, despite its limitations, the two-phase flow model may provide good predictions for the pressure gradients.

Local measurements of void fraction and liquid holdup in packed columns using X-ray computed tomography

A ‘tailor made’ computed X-ray tomographic scanner has been developed as a tool for the analysis of the distribution of gas, liquid and solid phases in packed columns. The very good spatial resolution of the scanner has first been assessed by the imaging of objects of known shape and size, called ‘physical’ phantoms. Images have then been realized on a 0.6 m diameter and 2 m height column packed with Cascade Mini-Ring 1A packing elements, which is a random polypropylene packing designed to be used in absorption columns. The solid phase distribution, leading to the void fraction distribution, has been analyzed on cross-section images of the dry packed column (without any liquid flowrate). The measured value of the bed void fraction is equal to the value provided by the manufacturer. The analysis of the axial profile of void fraction shows that the ‘end effect’ may be neglected, whereas the analysis of the radial profile evidences the existence of a non negligible ‘wall effect’. Images have then been carried out on the packed column irrigated by a liquid flowrate ranging between 0 and 6000 l h −1 (0–6.10 −3 m s −1). Hold-up values have been measured in different cross-sections of the column and averaged in order to obtain the total hold-up value in the bed. The computed values are in very good agreement with those reported in the literature for similar packing. The dependence of the liquid hold-up on the liquid superficial velocity can be expressed in terms of a power law. The fitted value of the exponent, equal to 0.65, is in the range of exponent values found in correlations of the literature.