垂直向上气液两相流中两相斯托拉赫数的研究

试验研究了三角形、T形两种形状4种规格的物体,在垂直上升气液两相流中,发生气液两相涡街时,气液两相斯托拉赫数的变化规律,在测得大量数据的基础上,得出了发生气液两相涡街时,气液两相斯托拉赫数的通用关系式,研究表明,气液两相斯托拉赫数在两相工况下为一变数,其值与来流截面含气率、涡街发生体形状和特征尺寸、来流方向等因素有关,应用此关系式,根据测得的两相涡街频率可将涡街发生体作为测量两相流流量与组分的测量元件。     

Fiber-optic probe measurements of void fraction and bubble size distributions beneath breaking waves

Experiments were performed to determine the accuracy of single-tip fiber-optic probes for making simultaneous measurements of the void fraction and bubble size distributions under breaking waves. Tests in a vertical bubble column showed that the normalized RMS error in the void fraction measurements was ∼10%. The relationship between bubble rise time and bubble velocity was investigated in a unidirectional flow cell. Similar to previous studies the rise time and bubble velocity were found to be related by a power law equation. The probes can provide mean bubble velocities accurate to ±10% when a minimum of ∼15 individual bubble velocities are averaged. The fiber-optic probes were deployed beneath a plunging breaking wave in a laboratory wave channel. The slope and shape of the bubble cord length size distribution measured with the probes was found to agree closely with the size distribution measured from digital video recordings. The probes were then positioned in the splash-up zone of a plunging breaker and the resulting cord length distribution had a shape and slope that was in agreement with previous measurements. These results demonstrate that single-tip fiber optic probes can provide accurate simultaneous measurements of the void fraction and bubble sizes inside the dense bubble clouds entrained by breaking waves.     

Experimental investigation of the flow pattern,pressure drop and void fraction of two-phase flow in the corrugated gap of a plate heat exchanger

The two-phase flow in the corrugated gap created by two adjacent plates of a plate heat exchanger was investigated experimentally. One setup consisting of a transparent corrugated gap was used to visualize the two-phase flow pattern and study the local phenomena of phase distribution, pressure drop and void fraction. Saturated two-phase R365mfc and an air-water mixture were used as working fluids.In a second experimental setup, the heat transfer coefficients and the pressure drop inside an industrial plate heat exchanger during the condensation process of R134a are determined. Both experimental setups use the same type of plates, so the experimental results can be connected and a flow pattern model for the condensation in plate heat exchangers can be derived. In this work the results of the flow pattern visualization, the two-phase pressure drop in the corrugated gap and the void fraction analysis by measurement of the electrical capacity are presented. A new pressure drop correlation is derived, which takes into account different flow patterns, that appear during condensation. The mean deviation of the presented pressure drop model compared to the experimental data and data from other experimental works is 18.9%. 81.7% of the calculated pressure drop lies within ±30% compared to the experimental data.     

Void fraction and pressure drop during external upward two-phase crossflow in tube bundles – part I: Experimental investigation

The present paper is the part I of a broad study concerning void fraction and pressure drop for air-water upward external flow across tube bundles. Experimental results were obtained for liquid and gas superficial velocities ranging from 0.02 to 1.50 m/s and 0.20 to 10.00 m/s, respectively. Void fraction measurements were performed for bubbly flow using a capacitive probe. The test section consisted of a triangular tube bundle counting with 19 mm OD tube and transverse pitch of 24 mm. Initially, the paper describes the test facility and the data regression and experimental procedures. Then, the pressure drop and void fraction measurements are validated based on tests for single-phase flow and quiescent liquid conditions, respectively. Finally, the experimental data are presented and analyzed. In the second part of this study (Part II), a literature review on predictive methods for void fraction and pressure drop is presented. Additionally, these methods are compared with the database presented in Part I and new predictive methods for void fraction and frictional pressure drop are proposed.     

Studies on two-phase cross flow. Part I: Flow characteristics around a cylinder

A two-phase flow around a body has scarcely been studied until now, though the flow is used in many industrial components. The cross flows around a spacer in a fuel assembly of light water reactors (LWR) and tube supports in a steam generator are closely related to the long-term reliability and the safety. The present study has been planned to clarify the two-phase flow and heat transfer characteristics around a body including the unknown complicated flow behavior. In the first report, the flow characteristics near and behind a cylinder which was located in a vertical upward air-water bubbly flow were investigated. From the observation of the flow patterns and the measurements of the distributions of void fraction, liquid velocity and static pressure, it is revealed that the vortex flow and the change of the static pressure and liquid velocity distribution around the cylinder resulted in the large distortion of the void fraction distribution around the cylinder. The most noticeable phenomena in the wake were that the peaks of the local void fraction appeared in the vicinity of the cylinder surface near the separation point and in the wake behind the cylinder.     

Time-resolved two-dimensional X-ray densitometry of a two-phase flow downstream of a ventilated cavity

To measure the void fraction distribution in gas–liquid flows, a two-dimensional X-ray densitometry system was developed. This system is capable of acquiring a two-dimensional projection with a 225 cm² area of measurement through 21 cm of water. The images can be acquired at rates on the order of 1 kHz. Common sources of error in X-ray imaging, such as X-ray scatter, image distortion, veiling glare, and beam hardening, were considered and mitigated. The measured average void fraction was compared successfully to that of a phantom target and found to be within 1 %. To evaluate the performance of the new system, the flow in and downstream of a ventilated nominally two-dimensional partial cavity was investigated and compared to measurements from dual-tip fiber optical probes and high-speed video. The measurements were found to have satisfactory agreement for void fractions above 5 % of the selected void fraction measurement range.     

Bubble and liquid turbulence characteristics of bubbly flow in a large diameter vertical pipe

The bubble and liquid turbulence characteristics of air–water bubbly flow in a 200 mm diameter vertical pipe was experimentally investigated. The bubble characteristics were measured using a dual optical probe, while the liquid-phase turbulence was measured using hot-film anemometry. Measurements were performed at six liquid superficial velocities in the range of 0.2–0.68 m/s and gas superficial velocity from 0.005 to 0.18 m/s, corresponding to an area average void fraction from 1.2% to 15.4%. At low void fraction flow, the radial void fraction distribution showed a wall peak which changed to a core peak profile as the void fraction was increased. The liquid average velocity and the turbulence intensities were less uniform in the core region of the pipe as the void fraction profile changed from a wall to a core peak. In general, there is an increase in the turbulence intensities when the bubbles are introduced into the flow. However, a turbulence suppression was observed close to the wall at high liquid superficial velocities for low void fractions up to about 1.6%. The net radial interfacial force on the bubbles was estimated from the momentum equations using the measured profiles. The radial migration of the bubbles in the core region of the pipe, which determines the shape of the void profile, was related to the balance between the turbulent dispersion and the lift forces. The ratio between these forces was characterized by a dimensionless group that includes the area averaged Eötvös number, slip ratio, and the ratio between the apparent added kinetic energy to the actual kinetic energy of the liquid. A non-dimensional map based on this dimensionless group and the force ratio is proposed to distinguish the conditions under which a wall or core peak void profile occurs in bubbly flows.     

Gas–liquid flows in a microscale fractal-like branching flow network

Two-phase air–water flows in a microscale fractal-like flow network were experimentally studied and results were compared to predictions from existing macroscale void fraction correlations and flow regime maps. Void fraction was assessed using (1) two-dimensional analysis of high-speed images (direct method) and (2) experimentally determined using gas velocities (indirect method). Fixed downstream-to-upstream length and width ratios of 1.4 and 0.71, respectively, characterize the five-level flow network. Channels were fabricated in a 38 mm diameter silicon disk, 250 μm deep disk with a terminal channel width of 100 μm. A Pyrex top allowed for flow visualization. Superficial air and water velocities through the various branch levels were varied from 0.007 m/s to 1.8 m/s and from 0.05 m/s to 0.42 m/s, respectively. Two-phase flow regime maps were generated for each level of the flow network and are well predicted by the Taitel and Dukler model. Void fraction assessed using the indirect method shows very good agreement with the homogeneous void fraction model for all branch levels for the given range of flow conditions. Void fraction determined directly varies considerably from that assessed indirectly, showing better agreement with the void fraction correlation of Zivi.     

Experimental determination of the speed of sound in cavitating flows

This paper presents measurements of the speed of sound in two-phase flows characterized by high void fraction. The main objective of the work is the characterization of wave propagation in cavitating flows. The experimental determination of the speed of sound is derived from measurements performed with three pressure transducers, while the void fraction is obtained from analysis of a signal obtained with an optical probe. Experiments are first conducted in air/water mixtures, for a void fraction varying in the range 0–11%, in order to discuss and validate the methods of measurement and analysis. These results are compared to existing theoretical models, and a nice agreement is obtained. Then, the methods are applied to various cavitating flows. The evolution of the speed of sound according to the void fraction α is determined for α varying in the range 0–55%. In this second configuration, the effect of the Mach number is included in the spectral analysis of the pressure transducers’ signals, in order to take into account the possible high flow compressibility. The experimental data are compared to existing theoretical models, and the results are then discussed.     

A neutron scatterometer for void-fraction measurement in heated rod-bundle channels under CANDU LOCA conditions

The difficult problem of directly measuring the void fraction of rapidly boiling water in rod-bundle channels, such as those simulating a large loss-of-coolant accident (LOCA) in a nuclear reactor, is overcome by using a fast-neutron scattering device (scatterometer). The neutron scatterometer measures the number of neutrons scattered from a test section exposed to fast neutrons, and relates these measurements to the channel void fraction. The concept of the device and its design features are discussed. Such a scatterometer was constructed and installed on a test section (channel) of the RD-14M Thermalhydraulic Test Facility of Atomic Energy of Canada Limited. Experiments (30-mm diameter break in the inlet header) were conducted to evaluate the scatterometer’s performance under transient conditions. Results of these tests show that the scatterometer can continuously measure the channel void fraction to within ±10% void (95% confidence interval) when corrections for non-linearity and time response are applied.     

On the development of an objective flow regime indicator

A high intensity dual beam X-ray system was designed and constructed to make chordal-average void fraction measurements. This X-ray system employed a DC excited tube filament, full wave rectification and high voltage filtering to produce a stable photon source. The large photon flux produced by the X-ray system allowed analog linearization of the signal.A series of chordal-average void fraction measurements were made and used to generate probability density functions (PDF) and power spectral density (PSD) functions. The first four moments associated with these distributions were studied as possible flow regime indicators.The moments of the PDF indicated the various flow regime transitions. The moments of the PSD also show some flow regime transition information, but were sensitive to liquid phase velocity. The PDF variance, or second moment about the mean, was found to be the best indicator of flow regime. A variance of 0.04 appear to adequately discriminate between the bubbly, slug and annular flow regimes for low pressure air/water flow in a 2.54 cm I.D. vertical tube.     

An experimental study of drag on a single tube and on a tube in an array under two-phase cross flow

In this work, the drag coefficient and the void fraction around a tube subjected to two-phase cross flow were studied for a single tube and for a tube placed in an array. The drag coefficients were determined by measuring the pressure distribution around the perimeter of the tube. Single tube drag data were taken when the tube was held both rigidly and flexibly. The test tube was made of acrylic and was 2.2 cm in diameter and 20 cm in length. In the experiments, liquid Reynolds number ranged from 430 to 21,900 for the single tube and liquid gap Reynolds number ranged from 32,900 and 61,600 for the tube placed in a triangular array. Free stream void fraction was varied from 0 to 0.4. At low Reynolds numbers, the ratio of two-phase to single-phase drag coefficient is found to be a strong function of εGr/Re2. However, at high Reynolds numbers only void fraction is the important parameter. Empirical correlations have been developed for the ratio of two-phase drag on a single tube and on a tube placed in an array.     

Numerical modeling and experimental investigation of gas–liquid slug formation in a microchannel T-junction

Gas–liquid two-phase flow in a microfluidic T-junction with nearly square microchannels of 113 μm hydraulic diameter was investigated experimentally and numerically. Air and water superficial velocities were 0.018–0.791 m/s and 0.042–0.757 m/s, respectively. Three-dimensional modeling was performed with computational fluid dynamics (CFD) software FLUENT and the volume of fluid (VOF) model. Slug flow (snapping/breaking/jetting) and stratified flow were observed experimentally. Numerically predicted void fraction followed a linear relationship with the homogeneous void fraction, while experimental values depended on the superficial velocity ratio U_G/U_L. Higher experimental velocity slip caused by gas inlet pressure build-up and oscillation caused deviation from numerical predictions. Velocity slip was found to depend on the cross-sectional area coverage of the gas slug, the formation of a liquid film and the presence of liquid at the channel corners. Numerical modeling was found to require improvement to treat the contact angle and contact line slip, and could benefit from the use of a dynamic boundary condition to simulate the compressible gas phase inlet reservoir.     

Study of periodically excited bubbly jets by PIV and double optical sensors

Interactions between large coherent structures and bubbles in two-phase flow can be systematically observed in a periodically excited bubbly jet. Controlled excitation at fixed frequency causes large eddy structures to develop at regular intervals. Thus, interactions between large vortices and bubbles can be studied with PIV and double optical sensors (DOS) using phase-averaging techniques. A number of results on the time and space dependence of velocities and void fractions are presented revealing physical interactions between the liquid flow field and bubble movement as well as feedbacks from bubble agglomeration on the development of flow structures. A clear indication of bubble trapping inside the vortex ring is the generation of a bubble ring that travels with the same velocity as the vortex ring. The DOS results indicate clustering of the bubbles in coherent vortex structures, with a periodic variation of void fraction during the excitation period.     

Sound emission by a vortex ring passing through a circular hole in a large flat plate

Acoustic emission has been studied experimentally when a vortex ring passes through a circular hole in a large flat plate, along its normal axis. The speed of the vortex ring is made high enough for the sound emission to be detectable, but can be regarded as sufficiently low in comparison to the sound speed. Substantial monopole and quadrupole components are observed in the detected wave profiles. Translational motion of the vortex ring in the presence of the flat plate with a hole has been observed optically, and its relation with the sound emission is determined. In this case, the power law of the acoustic pressure amplitude of monopolar vortex sound versus the translation speed U of the vortex ring is first measured in detail and is found to be U^2.1–U^2.4. This means that experimentally determined powers for the monopole components in the two half-spaces also agree approximately with the corresponding values predicted by the theory of vortex sound.     

Computational study of aeration for wastewater treatment via ventilated pump-turbine

Large eddy simulations were performed on a modular pump-turbine to study oxygen dissolution inside the draft tube. Air injection was applied over the runner cone surface during turbine operation. Data regarding bubble size, void fraction and interfacial area concentration were presented to understand their influence on oxygen dissolution. Transient single phase and multiphase flow simulations were carried out to investigate the influence of air injection and dissolution within the flow field and turbine performance. Multiphase simulations were conducted by using the mixture multiphase model. The mathematical modeling of oxygen dissolution employed was validated by comparing predicted oxygen dissolution against experimental measurements performed by Zhou et al. (2013). The averaged dissolved oxygen concentration in the range of 1.2–1.4 mg/l was obtained; which is sufficient for an active aerobic microorganism activity for wastewater treatment processes. Dissolution efficiency and the amount of averaged dissolved oxygen inside the draft tube were sensitive to the inlet bubble size. The efficiency of the dissolution increases strongly as the inlet bubble size was reduced. The obtained results revealed that vortex suppression was achieved through air admission within multiphase flow simulation. Moreover, the power generation of the turbine was hardly influenced by the aeration through the runner cone.     

Acoustic emission from interaction of a vortex ring with a sphere

Acoustic waves emitted by a vortex ring interacting with a fixed solid sphere are studied experimentally and theoretically. The experiments are carried out for two kindsof vortex-sphere arrangement: (A) a vortex ring passes over the sphere, and (B) a vortex ring passes by the sphere. The vortex motion is examined optically by means of a photosensor system, and the pressure signals of the emitted wave are detected by 1/2-inch microphones in the far field. In case A, the measured diameter of the vortex ring after passing the sphere increases from its initial diameter. The observed acoustic wave is dominated mainly by a dipole emission, and some contribution from a quadrupole radiation is present. In case B, the emitted wave is characterized by a rotating dipole emission in which the dipole axis rotates as the vortex position changes relative to the sphere.     

Mass flow rate measurements in gas–liquid flows by means of a venturi or orifice plate coupled to a void fraction sensor

Two-phase flow measurements were carried out using a resistive void fraction meter coupled to a venturi or orifice plate. The measurement system used to estimate the liquid and gas mass flow rates was evaluated using an air–water experimental facility. Experiments included upward vertical and horizontal flow, annular, bubbly, churn and slug patterns, void fraction ranging from 2% to 85%, water flow rate up to 4000 kg/h, air flow rate up to 50 kg/h, and quality up to almost 10%. The fractional root mean square (RMS) deviation of the two-phase mass flow rate in upward vertical flow through a venturi plate is 6.8% using the correlation of Chisholm (D. Chisholm, Pressure gradients during the flow of incompressible two-phase mixtures through pipes, venturis and orifice plates, British Chemical Engineering 12 (9) (1967) 454–457). For the orifice plate, the RMS deviation of the vertical flow is 5.5% using the correlation of Zhang et al. (H.J. Zhang, W.T. Yue, Z.Y. Huang, Investigation of oil–air two-phase mass flow rate measurement using venturi and void fraction sensor, Journal of Zhejiang University Science 6A (6) (2005) 601–606). The results show that the flow direction has no significant influence on the meters in relation to the pressure drop in the experimental operation range. Quality and slip ratio analyses were also performed. The results show a mean slip ratio lower than 1.1, when bubbly and slug flow patterns are encountered for mean void fractions lower than 70%.     

Heat transfer and fluid dynamics of air–water two-phase flow in micro-channels

Heat transfer, pressure drop, and void fraction were simultaneously measured for upward heated air–water non-boiling two-phase flow in 0.51 mm ID tube to investigate thermo–hydro dynamic characteristics of two-phase flow in micro-channels. At low liquid superficial velocity j_l frictional pressure drop agreed with Mishima–Hibiki’s correlation, whereas agreed with Chisholm–Laird’s correlation at relatively high j_l. Void fraction was lower than the homogeneous model and conventional empirical correlations. To interpret the decrease of void fraction with decrease of tube diameter, a relation among the void fraction, pressure gradient and tube diameter was derived. Heat transfer coefficient fairly agreed with the data for 1.03 and 2.01 mm ID tubes when j_l was relatively high. But it became lower than that for larger diameter tubes when j_l was low. Analogy between heat transfer and frictional pressure drop was proved to hold roughly for the two-phase flow in micro-channel. But satisfactory relation was not obtained under the condition of low liquid superficial velocity.     

The use of Hugoniot analysis for the propagation of vapor explosion waves

Experimental studies have shown that a mixture of molten metal and water can support the propagation of a quasi-steady vapor explosion wave. Analysis of steadily propagating vapor explosion waves has been carried out by applying the one-dimensional conservation laws of mass, momentum and energy and appropriate equations of state to a homogeneous mixture of molten tin, water and steam. The effects of void fraction, melt/water volume fraction and melt temperature on the Hugoniot curves have been considered. For low temperature melts, the Hugoniot curve lies partially inside the saturation dome and a Chapman-Jouguet (CJ) detonation point occurs only for low void fractions. For high melt temperatures, the downstream states lie entirely outside the saturation region. Increasing the volatility of the coolant or the addition of chemical reactions increases the predicted CJ detonation pressure and velocity. CJ deflagration solutions were obtained in all cases. The existence of a CJ detonation or CJ deflagration for a multiphase fuel-coolant mixture has yet to be substantiated experimentally and nonequilibrium effects may play a role in the divergence between theory and experiments.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1990.