Total pressure fluctuations and two-phase flow turbulence in self-aerated stepped chute flows

Current knowledge in high-velocity self-aerated flows continues to rely upon physical modelling. Herein a miniature total pressure probe was successfully used in both clear-water and air-water flow regions of high-velocity open channel flows on a steep stepped channel. The measurements were conducted in a large size facility (θ=45°, h=0.1 m, W=0.985 m) and they were complemented by detailed clear-water and air-water flow measurements using a Prandtl-Pitot tube and dual-tip phase-detection probe respectively in both developing and fully-developed flow regions for Reynolds numbers within 3.3×10⁵ to 8.7×10⁵. Upstream of the inception point of free-surface aeration, the clear-water developing flow was characterised by a developing turbulent boundary layer and an ideal-flow region above. The boundary layer flow presented large total pressure fluctuations and turbulence intensities, with distributions of turbulence intensity close to intermediate roughness flow data sets: i.e., intermediate between d-type and k-type. The total pressure measurements were validated in the highly-aerated turbulent shear region, since the total pressure predictions based upon simultaneously-measured void fraction and velocity data agreed well with experimental results recorded by the total pressure probe. The results demonstrated the suitability of miniature total pressure probe in both monophase and two-phase flows. Both interfacial and water phase turbulence intensities were recorded. Present findings indicated that the turbulence intensity in the water phase was smaller than the interfacial turbulence intensity.     

Analysis of two-phase flow regimes and parameter distribution in 5 x 5 rod bundle using image processing techniques

With the recent developments in image processing and analysis, this paper presents bubble characteristics distribution in adiabatic air-water two-phase flow through a 5 × 5 rod bundle. The experiment covered water superficial velocities (J_l = 0.012 m/s – 0.421 m/s) and air superficial velocities (J_g = 0.042 m/s – 0.987 m/s) in which three distinct flow regimes were identified. The flow regime map was compared with existing flow regime transition criteria for vertical rod bundles. Distinct features from the two-phase flow images were extracted to train a classifier model to distinguish between regimes from a separate experiment. The model distinguishes between the bubbly flow regime and others accurately. The void fraction and velocity distributions were also extracted from the R–CCN masked images. Bubble-induced turbulence that was dominant in the subchannel at (J_l = 0.28 m/s) shifted to the outer subchannels and gaps when the flow rate increased (J_l = 0.42 m/s). These methods over-predicted the void faction around the surfaces of the inner rods.     

Experimental PIV and LIF characterization of a bubble column flow

Experimental 2D Particle Image Velocimetry (PIV) measurements, with uniform background lighting and Laser Induced Fluorescence (LIF) of the tracking particles, were performed in order to characterize the air-water biphasic flow and the 2D bubble column rising velocity in static water. Some applications require knowledge of the simultaneity of two-phase flow characteristics. The two phase flow air/water are common application in industry as chemical, hydraulic and nuclear industry, water treatment by aeration, and measurements are implemented to characterize the behaviour of the air bubbles column flow. The bubble flow studied in this paper is related to the optimization of the aeration in hydraulic turbines with micro-bubbles. The first step of this study, presented in the paper, is a complete characterization of a bubble column issued from a metallic sparger with holes of 0.5 mm diameter. For its complete characterization is determined simultaneously, via image processing technics, the flow velocity field induced by the column of bubbles in water, and the bubbles features: the bubble ascension velocity, diameter variation, interfacial area and shape factor. The results are compared with bibliographical data.     

Statistical characterization of the interfacial behavior of the sub-regimes in gas-liquid stratified two-phase flow in a horizontal pipe

The aim of the present study was to identify the interfacial behavior of the sub-regime of gas-liquid stratified two phase flow by using the pressure differential signal data. Here, the probability distribution function (PDF), power spectral density (PSD), kolmogorov entropy and discrete wavelet transform (DWT) were used to analyze the differential pressure signals. The indicators of each flow sub regime were analyzed on the basis of the quantitative values of the statistical curves, which were also validated by visual observation of the video images. The results indicated that there are six identified sub-regimes of stratified flow namely stratified smooth (S), two-dimensional (2-D) wave, three-dimensional (3-D) wave, roll wave (RW), entrained droplet + disturbance wave (ED + DW) and pseudo slug (PS). Next, the increase of liquid viscosity will shift the transition line from the RW to ED + DW to a lower both of J_L and J_G. The increase of the liquid viscosity provides a stabilizing effect to reduce the chaos of the pressure gradient fluctuation. For the RW and the ED + DW sub-regime, the increase of the liquid viscosity shifts the wavelet energy to a larger scale and lower frequency. For the PS sub-regime, the increase of liquid viscosity shifts the wavelet energy to a smaller scale with a higher frequency. For the RW sub-regime, the increase of J_G will increase the wavelet energy at the small-scale and high-frequency decomposition levels.     

Application of PIV measurement techniques to study the characteristics of flame–acoustic wave interactions

The present paper introduce the development of the experimental setup and measurement methodologies to study the structure of acoustic wave and the interaction between the acoustic fields and flame flow inside a square tube by using a time-resolved Particle Image Velocimetry (PIV). The design of signal synchronisation system, the selection of seeding particles, the test of seeding methods and the statistical analysis of the PIV data were introduced. The titanium dioxide (TiO₂) was selected as the seeding particle for the PIV. The accuracy of synchronisation system was checked by a simple experiment. A time-resolved PIV results showed that the acoustic velocity difference was less than 2.8% at specific phase angle over 1000 excitation cycles at an excitation frequency of 385 Hz. For the case of hot flame gas, the largest difference is 4.4% over 100 excitation cycles at an excitation frequency of 10 Hz. Results proves that the present experimental system has high reliability to measure and analyse the characteristics of flame–acoustic interactions.     

Application of shallow-water acoustic tomography to measure flow direction and river discharge

High-frequency Fluvial Acoustic Tomography System (FATS) was initially used to measure flow velocity and river discharge in a mountainous river. The results showed the high-frequency FATS, not only improves the velocity resolution, but also reduces the minimum operational range from 76 m to 43 m in compare with the previous type of FATS. The analysis of sound wave propagation (Ray tracing) showed the bottom topography can be the reason of multi-ray paths of sound wave in the shallow freshwater rivers. A new formula based on the continuity equation introduced to estimate the variations of the angle between the flow direction and the FATS transmission line. The flow direction was measured using two crossed FATS transmission lines of 53 kHz and 30 kHz. The results compared to the up-looking ADCP (Acoustic Doppler Current Profiler) deployed near the intersection of the two lines which measured the changes in flow direction. The results affirmed the efficiency of the proposed method. Finally, the river discharge was estimated by both FAT systems and compared to the Rating Curve method and moving-boat ADCP estimates. The relative error of the FATS discharge measurements was less than 10%.     

Evaluating phase-detection-based approaches for interfacial velocity and turbulence intensity estimation in a highly-aerated hydraulic jump

Estimation of turbulence intensity within a highly-aerated turbulent flow is challenging. A possible way is to record bubble arrival information using intrusive phase-detection probes, but the derivation of velocity variation is subject to the correlation and de-correlation of the signals, especially in highly-turbulent and rapidly-varied flows with complex bubble transport such as in hydraulic jumps. Although attempts have been devoted to the improvement of data processing, it is difficult to assess the existing approaches in strong hydraulic jumps due to the lack of alternative measurement techniques applicable to the internal air-water flow region. In this letter, a substantial amount of work is devoted to manual analysis of instantaneous interfacial velocity in strong hydraulic jumps, and the velocity variation results are compared with the results of full-signal cross-correlation and adaptive-window cross-correlation approaches, to evaluate their performance in approximating the velocity turbulence. The manual results are validated in terms of the time-averaged velocity. The interfacial turbulence intensity is suggested to be greater than the water-phase velocity turbulence, typically between 20% and 40% in the unidirectional jet-shear region. Overall the manual results agree better with the calculation of adaptive-window cross-correlation technique. The relevance of the signal decomposition processing is also discussed, and it is emphasized that the phase-detection-based approaches are subject to the limitations of one-dimensional measurements in a complex three-dimensional flow.     

Unsteady velocity profiling in bores and positive surges

In an open channel, steady flow conditions may be achieved when the discharge and boundary conditions remain constant for a reasonable period of time. The operation of any regulation device (e.g. gate) is associated with some unsteady surge motion. In the present study, new velocity profiling measurements were performed systematically under controlled flow conditions. Both steady and unsteady measurements were conducted in a relatively large laboratory facility. An ensemble-averaged technique was applied in unsteady flows to investigate positive surges. The experiments were repeated 25 (or 50) times for each controlled flow condition and the results were ensemble-averaged. The quality and accuracy of the Profiler data set were validated against data collected with an acoustic Doppler velocimeter, in both steady and unsteady rapidly-varied flows. A careful sensitivity analysis was conducted to test the appropriate number of runs. The results indicated that the selection of 25 runs was suitable for ensemble-averaging in rapidly-varied unsteady flows. Some instrumental error was observed however with the velocity profiler. Outside the boundary layer, the Profiler tended to produce errors in terms time-averaged velocity data and velocity fluctuations for a number of points in a profile. Overall, the study demonstrated that the propagation of positive surges is a highly unsteady turbulent process, and the performance of ADV Vectrino II Profiler in such an unsteady turbulent flow was satisfactory, provided that a careful validation was undertaken for all Profiler outputs.     

On air entrapment onset and surface velocity in high-speed turbulent prototype flows

In a free-surface spillway, the upstream flow is non-aerated and the flow becomes a strong air-water mix downstream of the onset location of air entrapment. Field observations were conducted over a steep spillway chute, and detailed quantitative measurements were undertaken in real-world high-speed flows with strong turbulence and very high Reynolds numbers within the range 10⁷ to 10⁸. The data showed that the onset of air entrapment is a complicated transient three-dimensional process in high-speed strongly-turbulent flows. A robust optical flow (OF) technique was applied and provided physically-meaningful surface velocities in the non-aerated flow region. The streamwise velocities were reasonably close to ideal fluid flow calculations, with large streamwise surface velocity fluctuations, in the non-aerated flow region. Overall, the study demonstrated the application of optical techniques to prototype spillway flows, provided that some careful validation was undertaken.     

Experimental study of bubbly swirling flow in a vertical tube using ultrasonic velocity profiler (UVP) and wire mesh sensor (WMS)

Two-phase air-water bubbly swirling flow through a pipe is a complex turbulent flow and its prediction is still challenging. The present paper describes the experimental investigation of the air-water bubbly swirling flow in vertical co-current flow. Swirling flow is induced by a twisted tape in a 20 mm inner diameter pipe. The flow is investigated using Ultrasonic velocity profiler (UVP), which allows the measurement of liquid and gas velocities simultaneously. Furthermore, simultaneous measurement of void fraction is performed using Wire mesh sensor (WMS). The experimental results reveal that swirling flow has significant impact on bubbles’ distribution. In low liquid flow rate, the average bubble velocity is fairly uniform along the radial position and void fraction increases in the near wall region. However, increasing liquid flow rate at constant gas flow rate leads to increase in void fraction in the core region, this is mainly due to drift velocity which is affected by centrifugal force. Experimental findings and parametric trends based on the effects of swirling flow are summarized and discussed.     

The effect of air velocity on slugs in a confined channel

The present work focuses on the study of slugs occurring in a two-phase flow of a confined rectangular channel: conditions of appearance and effect on the flow behavior. Three-dimensional numerical simulations have been carried out to examine the effect of superficial air velocity on flow behavior. The Volume Of Fluid model (VOF) is used to track the air-water interface. Validation of the numerical model is obtained by comparing the results of the simulated axial velocity with experimental data determined using the Laser Doppler Anemometry (LDA) technique. The numerical results revealed that for a fixed water level and superficial water velocity, higher superficial air velocities generate a slug flow that causes channel blockage. The position of these slugs and the timing of their occurrence were correlated in terms of air and water superficial Reynolds numbers.     

Structure and dynamics of the turbulent flow through a central baffle

Central baffle flume (CBF) can be utilized as a control structure to measure flow discharge in irrigation channels under free and submerged flow conditions. Stage-discharge relationship has been extensively studied for various geometrical parameters and flow conditions, whereas internal structure of the flow around a baffle has not been investigated in the literature. To address this need, the present work investigates the turbulent flow around a central baffle through high-resolution numerical simulations using an open source computational model. Velocity measurements were conducted in a laboratory flume to setup and validate the numerical model. Comparison of the numerical results with the experimental measurements proves that the present numerical model can predict water depth and velocity field. Longitudinal distance from the apex to the intersection point of water and critical depths can be estimated as L_xc = 2L_e, where L_e is the longitudinal length of the guide walls. A horseshoe vortex system identified in front of the baffle produces a significant bump on the free-surface and rib vortices generated from the baffle extend up to the sidewalls of the channel. The vertical separation layer observed downstream of the baffle results in a reverse flow and a vortex pair is formed by the impingement of the resulting reverse flow on the back of the baffle. Reverse flow, plunging flow structure, splash and rebounding wave events observed at the downstream produce substantial hydrodynamic effects on the baffle. Geometry of the central baffle was modified to suppress recirculation effects based on the insights into the complete flow structure around the baffle. Eventually, vortex structures were suppressed and the length of the recirculation zone was reduced by 76%.     

Characterization of gas-liquid two phase flows using dielectric Sensors

Two phase flow regime identification and void fraction measurement is an area of considerable interest because of its wide applications in process industries. The principle involved in dielectric measurement is that the two phase flow regime is characterized by the changes in effective permittivity of the two phase fluid mixture. In the present work, a pair of parallel copper electrodes on the two sides of a glass tube acts as a dielectric sensor. As the void fraction in the glass tube changes, the effective permittivity of the medium changes. This causes a variation in the capacitance value across the electrodes. A standard IC, Oscillator 555 is employed as a tool to generate a rectangular wave. The variation in dielectric constant is analyzed based on the change in time period of the trough (T₀) of the rectangular wave that is recorded online by a data acquisition system. Experiments were performed in a 4.7 mm diameter tube with air-water, air-palmolein oil two phase fluids to study the variation in dielectric constant which is indicated as a change in time period of trough. The effect of conductivity of water on the capacitance variation is examined with water having Total dissolved solids (TDS) which is a measure of movable ions in the range 10-4000 ppm (16 µS/cm–6.3 mS/cm). The novelty in the present work is the determination of changes in capacitance value based on the change in time of trough of the rectangular wave. The technique does not require amplification or a filtering circuit, thereby leading to a precise identification of two phase flow regime.     

Miniaturized liquid film sensor (MLFS) for two phase flow measurements in square microchannels with high spatial resolution

Two miniaturized liquid film sensors (MLFS) based on electrical conductance measurement have been developed and tested. The sensors are non-intrusive and produced with materials and technologies fully compatible and integrable with standard microfluidics. They consist of a line of 20 electrodes with a purpose-designed shape, flush against the wall, covering a total length of 5.00 and 6.68 mm. The governing electronics achieve 10 kHz of time resolution. The electrode spacing of the two sensors is 230 μm and 330 μm, which allows measurements of liquid films up to 150 μm and 400 μm for sensors MLFS_A and MLFS_B, respectively. The sensor characteristics were obtained by imposing static liquid films of known thickness on top of the actual sensor. Further dynamic measurements of concurrent air-water flow in a horizontal microchannel were performed. The line of electrodes is placed across the flow direction with an angle of 3.53° from the direction of flow, allowing for a spatial resolution perpendicular to the flow of 14.2 μm for sensor MLFS_A and 20.5 μm for sensor MLFS_B. The high time and spatial resolution allows for fast and accurate detection of the presence of bubbles, and even measurement of film thickness and bubble velocity. Further information, such as the bubble shape, can be gathered based on the shape of the liquid layer underneath the bubble, which is particularly important for heat transfer studies in microchannels.     

Study on a new electromagnetic flowmeter based on three-value trapezoidal wave excitation

Excitation technology plays an important part in the field of electromagnetic flowmeter (EMF). The form of the magnetic field directly determines the characteristics of the flow signal. No breakthrough has been made on the excitation technology since the invention of sinusoidal wave excitation, rectangular wave excitation, and double-frequency excitation. In this paper, firstly a new three-value trapezoidal wave excitation with transient responses is proposed. Then the signal model is established, and the verification experiments are carried out. Finally, flow calibration experiments and comparative experiments on the trapezoidal wave excitation are conducted on the experimental platform. The experiment results show that the electromotive force output by the EMF based on three-value trapezoidal wave excitation is linearly related to the flow velocity. When the flow velocity is 0.257 m/s, the relative error is only 1.635%. When the flow velocity reaches 2.133 m/s, the relative error is reduced to 0.432%. The three-value trapezoidal wave excitation with the transient analysis satisfies the requirements of the EMF with high accuracy. The research also shows that the excitation frequency has a great influence on the measurement accuracy of the EMF based on three-value trapezoidal wave excitation.     

3D and 2D experimental views on the flow field of gas-evolving electrode cold model for electrolysis magnesium

In the magnesium electrolysis process, chlorine gas bubbles release at the surfaces of anode and affect electrolyte flow patterns. This paper presents an experimental apparatus to simulate the flow field induced by chlorine gas evolution at the gas-evolving electrodes of magnesium electrolysis cell. The three-dimensional flow structures were determined by using volumetric three-component velocimetry (V3V) technique, which has the ability to capture the out-of-plane velocity component. The three-dimensional flow structures in the region with a depth about 120 mm can be obtained. To achieve this, approximately 15,000 three-dimensional velocity vectors were detected in the flow measurements and constituted the three-dimensional flow field, which eliminated the perspective error caused by the out-of-plane motion in Particle Image Velocimetry (PIV) method. In experiments, comparisons are made between the V3V and PIV results. The in-of-plane velocities data obtained by V3V technique have the same trend with the PIV results, and V3V provides more details in the third direction for the flow field accurately.     

新型旋转射流多孔喷嘴流场的分析

设计了一种适用径向水平井技术的旋转射流多孔喷嘴。其前端面均布3个不同切向倾角15°、30°、45°的孔眼。利用数值模拟方法,采用RNG k-ε湍流模型对该喷嘴流场特征进行了研究,并与试验破岩结果进行了对比。结果表明,15°孔眼轴向速度较大,扩散性差,衰减缓慢,类似直射流,破岩效果适中;30°孔眼轴向、径向、切向速度均较大,扩散性适中,衰减较慢,其破岩效果最佳;45°孔眼虽具有较高切向、径向速度,但扩散性较强,衰减较严重,可形成较大破岩面积,但破岩深度有限;3个孔眼破岩面积在径向存在重叠,当喷嘴旋转时,可轨道扫描式完成联合破岩;不同倾角破岩效果不同,本试验中30°倾角的孔眼破岩效果最佳。流场数值模拟结果与实际破岩效果基本一致。     

Measurements of the flow of supercritical pressure carbon dioxide through Venturi flow meter

The Venturi flow meter exhibits relatively low pressure loss, simple design, and low manufacturing costs. This study describes flow rates measurements for supercritical pressure CO₂ using the Venturi flow meter with pressure ranging 7.379–7.836 MPa and 5.84–7.272 MPa for in supercritical and gaseous regions, respectively. The flow rates of supercritical pressure CO₂ were accurately measured using a Venturi flow meter with a diameter ratio of 0.6468, having large and small diameters of 87.32 mm and 56.48 mm, respectively. The convergent and divergent angles were 21 $°$ ± 1 $°$ and 15 $°$ ± 1 $°$, respectively. The averaged discharge coefficient of 0.9975 was obtained, which was independent of the pressure ratio. Additionally, the expansion factors were also calculated using the experimental results, which ranged from 0.99976 to 0.99987 and 0.99945–0.99995 for the supercritical and gaseous regions, respectively. The experimental results showed that the Venturi flow meter had uncertainties ranging from 0.1 to 2.8%.