Measurement Of Bubble Velocity Research Articles

Experimental study for high Reynolds’ gas-liquid two-phase flow through rod bundle using Wire Mesh Sensor (WMS)

Gas-liquid two-phase flow is a very complex physical system. It is essential to study the physical interactions between gas and liquid phases, towards improving the modeling of the interfacial transport terms between the two phases. Higher resolution experimental data is essential for CFD quality closure model development. The two-phase flow behavior is highly influenced by flow conditions and the interaction between the bubbles of different sizes and different velocities. Currently, the prediction of bubble size distribution, bubble velocity and interfacial area concentration remain the most challenging tasks in analyzing the transport phenomena and simulating the turbulent bubble flows. Accordingly, several experiments have been conducted in tube bundles such as SUBFLOW at PSI. The SUBFLOW test facility is one of the very few facilities in the world that use Wire Mesh Sensors (WMS) to investigate the two-phase flow characteristics in tube bundles. In this work, we have reconstructed and recommissioned the SUBFLOW test facility, and utilize it to generate new data to extend the two-phase flow validation database, which includes void fraction, bubble distribution and bubble velocity measurement in a tube bundle. Separately, this paper also explains the WMS post processing techniques. Several WMS post-processing frameworks are available at PSI. The presented results show the variation between the different post-processing toolkits, as well as a comparison against previous SUBFLOW experimental data was conducted to verify the consistency of the recommissioning of SUBFLOW facility.

Evaluation of the multi-sensor probe methods for the measurement of local bubble velocity and diameter

The evaluation of the multi-sensor probe methodology is required to accurately measure local bubbles or droplet parameters in the two-phase flow conditions. This study evaluated two- and four-sensor probes through Monte Carlo simulation and air-water benchmark experiments. In the simulation, it was found that the lateral distance of sensing elements should be minimized, and their axial distance should be longer than 10% of bubble diameter for the optimal design of the probe. From the benchmark experiments, the Vb,axial uncertainties of two- and four-sensor probes were 6.5% and 4.5% in the velocity range of 0.4–3.7 m/s, respectively. The D32 uncertainty was significantly affected by the bubble shape and data reduction methods for each probe. For the two-sensor probe, the maximum error was 9% when the bubble shape was close to the sphere of which diameter was less than 2 mm, and it increased to 28% when the bubble diameter exceeded 2 mm due to the bubble shape distortion in the experiment. While, the four-sensor probe showed excellent performance with a 14% error regardless of the bubble shape under the distorted bubbly flow.

Bubble dynamics in 2‐D gas–solid fluidized bed with Geldart A or Geldart B particles by image processing method

AbstractGas bubbles in a two‐dimensional gas–solid fluidized bed were identified using an image processing method to characterize the bubble dynamics in a binary fluidization system. The magnetite, sand (two types), and fluid catalytic cracking (FCC) catalyst particles belonging to Geldart groups A and B were used as the particulate materials. Image processing and MATLAB were applied for bubble size and velocity measurements. Bubble properties and bed expansion in fluidized beds of those four types of particles were studied. Bubble size and bubble rise velocity were found to increase with the distance above the gas distributor and excess gas velocity. Geldart A particles have a higher bed expansion than Geldart B particles due to the large number of small bubbles in the bed. A new empirical correlation for estimating bubble diameter along the bed height was proposed, giving good agreement with the experimental data.

Evaluation of the Multi-Sensor Probe Methods for the Measurement of Local Bubble Velocity and Diameter

Evaluation of the Multi-Sensor Probe Methods for the Measurement of Local Bubble Velocity and Diameter

A new, optimized Doppler optical probe for phase detection, bubble velocity and size measurements: Investigation of a bubble column operated in the heterogeneous regime

A new technique dedicated to bubble velocity and size measurements in complex bubbly flows is proposed. This sensor combines the phase detection capability of a conical optical fiber with velocity measurements from the Doppler signal induced by an interface approaching the fiber extremity. It provides the translation velocity of bubbles projected along the probe axis with an uncertainty at most 14%. This sensor has been successfully exploited in a 0.4 m diameter bubble column. In the heterogeneous regime, the mean bubble velocity profiles scaled by the value on the axis are shown to be self-similar in the quasi fully developed region of the column. Besides, the standard deviation of bubble velocity scaled by the mean velocity increases with Vsg in the homogeneous regime, and it remains stable, close to 0.55, in the heterogeneous regime. Such a sensor opens the way to velocity measurements conditioned by the local void fraction.

Error in bubble velocity measurement using 4-sensor void probe due to interface deformation

Multi-sensor void probe is efficient to measure the local parameters of two-phase flow such as bubble interface velocity. In general, meniscus due to surface tension is formed and shape of the gas–liquid interface deforms when an object contacts the gas–liquid interface. The deformation of the interface by penetration of the front sensor affects the penetration time of the rear sensor, and as a result, an error can occur in the measurement of the bubble interface velocity. The characteristics of the meniscus formation around the sensor was investigated and the error in the measurement of the interface velocity was evaluated. It was clarified that the size, shape of the sensor and the contact angle of the sensor surface affect the error in interface velocity measurement as well as the interface velocity itself, and no measurement error would occur in air-to-water penetration by using a sensor with a large surface contact angle. A double sensor void probe was applied to bubbly flow to measure the bubble interface velocity. The measurement error due to the meniscus also occurred in the actual experimental measurement. It was shown, that it is necessary to use only the velocity of the lower surface of the bubble or to make certain appropriate error correction for the velocity of the upper interface of the bubble.

Measurement of the air bubble size and velocity from micro air bubble generation (MBG) in diesel using optical methods

In this paper, we determine the bubble size and velocity from air bubble generation (MBG) in a diesel using optical methods. A KTM Series Pump was used to generate micro air bubbles in diesel. The air bubble radius and velocity measurements can be useful parameters to optimize the bubble generation process. Two optical systems were used for measurement air bubble sizes and their velocities in diesel. First, the optical system without an objective lens was used to determine the velocity of air bubbles in diesel. Another optical system with a 10× objective lens was used to obtain the size distribution of air bubbles generated in diesel. An available optical system with a 10× objective lens can detect a bubble diameter greater than 3.3 µm that air bubble images were processed using the ImageJ program. We measured the size distribution of air bubbles generated using the ImageJ program. The micro air bubble radius measured in diesel was found to be 6.26 µm in the sample after a month from air bubble generation. In addition, the particle image velocimetry (PIV) technique was used to measure the velocity field. Then, we used the OpenPIV program for PIV image processing. The highest velocity distribution was determined to be 90 mm/s for diesel without air bubbles and 20 mm/s for diesel with air bubbles after a month of the bubble generation.

Labelled Object Velocimetry: Simultaneous Measurements of Bubble Size and Velocity

This paper presents labelled-object velocimetry (LOV) – a technique to determine the velocities of labelled objects in multiphase flow. LOV is based on spatial correlations similar to those used in particle-image velocimetry (PIV) but employs grey-level shadowgraph images to determine object velocities. Object detection and labelling are performed using a classical image-processing algorithm. In contrast to PIV, interrogation areas in LOV are not uniformly distributed. Instead, these areas surround objects, and therefore, depend on object positions and sizes. Additionally, the proposed technique recalls a previously developed algorithm (Laupsien et al., 2019) to distinguish between single bubbles and complex situations, such as overlays, breakups and coalescences. This algorithm-based object selection (ABOS) provides a statistically reliable sample of the entire bubble swarm in terms of size and shape. Although both techniques are completely independent, they can be combined to link object velocities to their geometrical characteristics. Thus, the LOV technique can be used to ascertain velocity-object-diameter histograms.

Experimental study of air-water two-phase jet: bubble size distribution and velocity measurements

An experimental investigation on a horizontal two-phase air-water jet has been conducted in order to develop a technique that can provide information on the interaction between the air bubbles and turbulent flow. In order to allow a measurement of the liquid phase flow field and of the bubbles size, the Particle Image Velocimetry (PIV) has been used, with the liquid phase seeded with a fluorescent tracer consisting of Rhodamine WT mixed with a commercial polyester resin. A phase discrimination algorithm has been developed and successfully used for the detection and separation of tracers and bubbles in distinct images. Three data sets have been recorded: one for a one-phase water jet and two for a two-phase water jet. The obtained results allow a valid characterization of the flow field and an estimation of the effect of the gaseous phase in the air-water jet flow. An analysis of the bubbles size and number has been also conducted and here reported.

Ultrasonic Measurement of Velocity Profile on Bubbly Flow Using Fast Fourier Transform (FFT) Technique

In two-phase bubbly flow, measurement of liquid and bubble velocity is a necessity to understand fluid characteristic. The conventional ultrasonic velocity profiler (UVP), which has been known as a nonintrusive measurement technique, can measure velocity profile of liquid and bubble simultaneously by applying a separation technique for both phases (liquid and bubble) and transparent test section is unnecessary. The aim of this study was to develop a new technique for separating liquid and bubble velocity data in UVP method to measure liquid and bubble velocity profiles separately. The technique employs only single resonant frequency transducer and a simple UVP system. An extra equipment is not required. Fast Fourier Transform (FFT) based frequency estimator paralleled with other signal processing techniques, which is called as proposed technique, was proposed to measure liquid and bubble velocity separately. The experimental facility of two-phase bubbly flow in the vertical pipe was constructed. Firstly, the Doppler frequency estimation by using the FFT technique was evaluated in single-phase liquid flow. Results showed that FFT technique showed a good agreement with autocorrelation and maximum likelihood estimator. Then, separation of liquid and bubble velocity was demonstrated experimentally in the two-phase bubbly flow. The proposed technique confirmed that liquid and bubble velocity could be measured efficiently.

Development of a dual optical fiber probe for the hydrodynamic investigation of a horizontal annular drive gas/liquid ejector

A dual-channel optical fiber probe was developed to quantify the bubble characteristics (void fraction, velocity, and bubble size) in a gas–liquid annular ejector system. Water is pumped upstream of the ejector contraction. Since a low pressure region exists downstream in the ejector diffuser, this permits air to be sucked into the flowing liquid by jet pump action and the inlet air volumetric flow rate is measured by a flow meter. Verification of the void fraction (range 0.15–0.5) measured by the optical fiber probe was then possible and deviations were generally around ± 5%. Also, bubble velocity was measured using the optical probe by cross-correlating signals from the two fibers whose tips are separated by a known distance. Alternatively measuring bubble velocity using a particle image velocimetry method provided validation for the optical fiber probe system where a high speed camera was used to capture instantaneous bubble images at time intervals of 0.125ms. Excellent agreement between the velocities using both methods is reported. For bubble size measurements, analyzing the temporal signals from a single probe enabled estimation of the size of a bubble. Bubble sizes measured ranged between 1.5 and 6.0mm and size distributions were constructed for different ejector water volumetric flow rates ranging from 0.0022 to 0.0063m3/s. LabVIEW provided a convenient platform for coding the algorithms for estimating the void fraction, bubble velocity and bubble size. For further comparison, a CFD study of the ejector system was done, and the vertical radial profiles of the void fraction were compared with those obtained by the optical fiber system and these showed good agreement.

A New Contactless Method for Velocity Measurement of Bubble and Slug in Millimeter-Scale Pipelines

Combining C4D technique and cross-correlation velocity measurement technique, a new measurement method, which is suitable for the bubble/slug velocity measurement in millimeter-scale pipelines, is proposed. Based on the series resonance principle and the simulated inductor technique, a new C4D sensor is developed. With two conductance signals obtained by two new C4D sensors (the upstream sensor and the downstream sensor), the bubble/slug velocity measurement is implemented by the cross-correlation velocity measurement technique. Experiments are carried out in three pipelines with different inner diameters of 4.50, 5.46, and 6.44 mm, respectively. The experimental results show that the proposed bubble/slug velocity measurement method is effective, the development of the new C4D sensor is successful, and the velocity measurement accuracy is satisfactory. The relative error of bubble velocity measurement is less than 5.41% and the relative error of slug flow velocity measurement is less than 4.90%.

Sensitivity studies on the multi-sensor conductivity probe measurement technique for two-phase flows

The objective of this study is to advance the local multi-sensor conductivity probe measurement technique through systematic investigation into several practical aspects of a conductivity probe measurement system. Firstly, signal “ghosting” among probe sensors is found to cause artificially high bubble velocity measurements and low interfacial area concentration (ai) measurements that depend on sampling frequency and sensor impedance. A revised electrical circuit is suggested to eliminate this artificial variability. Secondly, the sensitivity of the probe measurements to sampling frequency is investigated in 13 two-phase flow conditions with superficial liquid and gas velocities ranging from 1.00–5.00m/s and 0.17–2.0m/s, respectively. With increasing gas flow rate, higher sampling frequencies, greater than 100kHz in some cases, are required to adequately capture the bubble number frequency and ai measurements. This trend is due to the increase in gas velocity and the transition to the slug flow regime. Thirdly, the sensitivity of the probe measurements to the measurement duration as well as the sample number is investigated for the same flow conditions. Measurements of both group-I (spherical/distorted) and group-II (cap/slug/churn-turbulent) bubbles are found to be relatively insensitive to both the measurement duration and the number of bubbles, as long as the measurements are made for a duration long enough to capture a collection of samples characteristic to a given two-phase flow system (or a statistical ensemble). Fourthly, investigation into the orientation of a double-sensor probe in the pipe indicates that the sensors should be oriented parallel to the pipe wall to ensure symmetric bubble velocity measurements. Lastly, Monte Carlo simulations are performed to study the effects of the axial (s) and lateral (d) probe sensor separation distances. In addition to previous criteria on the ratio of s to the bubble diameter, it is found that s/d should be greater than four to minimize errors in the measured bubble velocity.

Considerations in the Practical Application of the Multisensor Conductivity Probe for Two-Phase Flow

This study investigates two issues in the practical application of the local conductivity probe for two-phase flow measurements. First, the effects of signal “ghosting,” an electrical interference inherent to multiplexing data acquisition systems, on the measured two-phase flow parameters are examined. A revised conductivity probe circuit is proposed to remove the effects of ghosting. The characteristics of signal ghosting are investigated experimentally with a specialized conductivity probe that enables concurrent acquisition of ghosted and unghosted signals within the same flow condition. It is demonstrated that ghosting causes bubble velocity measurements that are artificially high and, consequently, artificially low interfacial area concentration measurements that depend on sampling frequency and sensor impedance. The revised circuit successfully eliminates this variability. Second, the sensitivity of measured two-phase flow parameters to increasing data acquisition sampling frequency is investigated experimentally. Measurements are acquired at incrementally increasing sampling frequencies with a four-sensor conductivity probe in 13 vertical-upward air-water two-phase flow conditions with superficial liquid and gas velocities ranging from 1.00 to 5.00 m/s and 0.17 to 2.0 m/s, respectively. It is found that the void fraction and average bubble velocity are insensitive to the sampling frequency, while the detected number of bubbles and interfacial area concentration can demonstrate a strong dependence. Considerations for selecting appropriate sampling frequencies in different flow conditions are discussed.

Simultaneous measurement of bubble size, velocity and void fraction in two-phase bubbly flows with time-resolved X-ray imaging

Key parameters of two-phase flows, such as void fraction and microscale bubble size, shape and velocity, were simultaneously measured using time-resolved X-ray imaging. X-ray phase-contrast imaging was employed to obtain those parameters on microbubbles. The void fraction was estimated from X-ray absorption. The radii of the measured microbubbles were mostly smaller than 20 µm, and the maximum velocity was 39.442 mm s(-1), much higher than that in previous studies. The spatial variations of the void fraction were consecutively obtained with a small time interval. This technique would be useful in the experimental analysis of bubbly flows in which microbubbles move at high speed.

Monitoring of contamination of coal processing plants and environmental waters using bubble velocity measurements – advantages and limitations

The paper presents fundamentals of a simple physicochemical method (SPMD) and analysis of results obtained when the method was applied for detection of organic contaminations (surface-active substances SAS) in samples of environmental and industrial waters. The method is based on measurements of variations of air bubble local velocities, which can be significantly changed in presence of surface-active contaminants. Lowering of the bubble velocity is a consequence of a motion induced dynamic adsorption layer (DAL) formed over surface of the rising bubble. The DAL formation retards the surface fluidity and the bubble rising velocity can be lowered by over 50% when the bubble surface is completely immobilized. We showed that the SPMD is a very sensitive tool (detection limit even below 1 ppm) for detection of various kinds of surface-active substances (ionic, non-ionic) in water samples. On the basis of results obtained using precise laboratory set-up, an accuracy of the SPMD is discussed. Moreover, effect of inert electrolyte addition on the bubble velocity lowering and value of detection limit of the SPMD is discussed. Simple approach, enabling quantitative analysis of the surface-active contaminants in samples collected, based on equivalent concentrations determination, is proposed. Results obtained for industrial (Jankowice and Knurow coal processing plants, Jaslo Refinery channel) and environmental waters (Wisloka and Ropa river) are used for detailed analysis and critical discussion of advantages and limitations of the SPMD.

Measurement of bubble velocity using spatial filter velocimetry

Spatial filter velocimetry (SFV) based on spatial filtering of time-series particle images proposed by Hosokawa and Tomiyama is applied to a bubble column to examine its applicability to bubble velocity measurement. The edge detection process is added in the SFV processing to reduce the size of image pattern and to obtain accurate velocity data. It is demonstrated that SFV is able to measure the bubble velocity in various spatial scales with the same accuracy as PTV. The measured mean bubble velocity does not depend on the interrogation size in the measurements. To the contrary, the standard deviation of measured bubble velocity becomes higher as the interrogation size decreases due to the superposition of interfacial wave velocity on the measured velocity. Simultaneous measurement of bubble and liquid velocities is also carried out for a planar bubble plume. The results clearly show that SFV can measure bubble and liquid velocities simultaneously.

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 6.68mm. The governing electronics achieve 10kHz of time resolution. The electrode spacing of the two sensors is 230μm and 330μm, which allows measurements of liquid films up to 150μm and 400μm for sensors MLFSA and MLFSB, 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 14.2μm for sensor MLFSA and 20.5μm for sensor MLFSB. 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.

Measurement of bubble velocity using Capacitively Coupled Contactless Conductivity Detection (C4D) technique

The feasibility of applying Capacitively Coupled Contactless Conductivity Detection (C4D) technique to measurement of bubble velocity in gas–liquid two-phase flow in millimeter-scale pipe is investigated. And, a new method, which combines the C4D technique and the principle of cross-correlation velocity measurement, is proposed for the measurement of bubble velocity. This research includes two parts. First, based on the principle of C4D, a new five-electrode C4D sensor is developed. Then, with two conductivity signals obtained by the C4D sensor, the velocity measurement of bubble is implemented according to the principle of cross-correlation. The research results indicate that the C4D technique is highly effective and anticipates a broad potential in the field of two-phase flow. Experimental results show that the five-electrode C4D sensor is suitable for measuring the velocity of single bubbles with a relative error of less than 5%.

An experimental and numerical study on hydrodynamic characteristics of horizontal annular type water-air ejector

A water-driven annular type ejector loop is designed and constructed for air absorption. Fabricated ejector unit is horizontally installed in the loop, and annular water jet at the throat entrained atmospheric air through the circular pipe placed at the center of the ejector. The tested range of water flow rate is 160 L/min to 320 L/min and volumetric flow rate of water and air and local pressure are quantitatively measured using LabVIEW signal express program. For the quantitative measurement of bubble velocity, cinematic PIV technique using a high speed camera is adapted. In post processing, each bubble is used as seeding particles and ensemble averaged bubble velocity field at vertical plane of the ejector system is finally acquired. In the range of experiment, the bubble size distribution at downstream of the ejector seems to be quite uniform so that the flow can be classified as a homogeneous bubbly flow. In case of low range of water flow rate, the transition from bubbly flow to stratified flow occurs at the atmospheric outlet condition. As a comparative study, a numerical simulation on the same ejector shape is performed to understand the more detail hydrodynamic characteristics in the annular type ejector system. Homogeneous bubbly flow regime is used as default two-phase flow regime, and void fraction at the vertical plane of the ejector system is qualitatively compared with that of experiment. In volume flow rate comparison, numerical prediction agrees well with that of experiment where the homogeneous bubbly flow is maintained.