Resonance of a flexible plate immersed in a von Kármán vortex street

Journal of Mechanical Science and Technology - This work presents a theoretical and experimental study of a flexible plate immersed in a von Kármán vortex street. The wake is generated in...     

Void fraction measurement of gas–liquid two-phase flow from differential pressure

Void fraction is an important process variable for the volume and mass computation required for transportation of gas–liquid mixture in pipelines, storage in tanks, metering and custody transfer. Inaccurate measurement would introduce errors in product measurement with potentials for loss of revenue. Accurate measurement is often constrained by invasive and expensive online measurement techniques. This work focuses on the use of cost effective and non-invasive pressure sensors to calculate the gas void fraction of gas–liquid flow. The differential pressure readings from the vertical upward bubbly and slug air–water flow are substituted into classical mathematical models based on energy conservation to derive the void fraction. Electrical Resistance Tomography (ERT) and Wire-mesh Sensor (WMS) are used as benchmark to validate the void fraction obtained from the differential pressure. Consequently the model is able to produce reasonable agreement with ERT and WMS on the void fraction measurement. The effect of the friction loss on the mathematical models is also investigated and discussed. It is concluded the friction loss cannot be neglected, particularly when gas void fraction is less than 0.2.     

Investigation of using 60Co source and one detector for determining the flow regime and void fraction in gas–liquid two-phase flows

In this study, a simple detection system comprised of one ⁶⁰Co source and just one NaI detector was investigated in order to identify flow regime and measure void fraction in gas–liquid two phase flows. For this purpose, 3 main flow regimes of two-phase flows including stratified, homogenous and annular with void fractions in the range of 5–95% were simulated by Monte-Carlo N Particle (MCNP) code. At first step, 3 features (count under full energy peaks of 1.173 and 1.333 MeV, and count under Compton continuum) were extracted from registered gamma spectrum. These 3 extracted features were used as inputs of artificial neural network (ANNs). A primary network was trained for identifying the flow regimes, but after testing many different structures, it was found that just two regimes of stratified and annular could be completely identified from each other. After identifying the mentioned two flow regimes by the first ANN, two specific ANNs were also implemented for predicting the void fraction. Using the proposed method in this work, void fraction percentages were predicted with a mean relative error (MRE) of less than only 0.42%. Using fewer detectors is of advantage in industrial nuclear gauges, because of reducing economical expenses and also simplicity of working with these systems.     

Theoretical flow instability of the Kármán boundary layer

The hydrodynamic stability of the Kármán boundary-layer flow due to a rotating disk has been numerically investigated for moving disturbance waves. The disturbed flow over a rotating disk can lead to transition at much lowerRe than that of the well-known Type I instability mode. This early transition is due to the excitation of the Type II instability mode of moving disturbances. Presented are the neutral stability results concerning the two instability modes by solving new linear stability equations reformulated not only by considering whole convective terms but by correcting some errors in the previous stability equations. The reformulated stability equations are slightly different with the previous ones. However, the present neutral stability results are considerably different with the previously known ones. It is found that the flow is always stable for a disturbance whose dimensionless wave numberk is greater than 0.75.     

Flow noise characterization of gas–liquid two-phase flow based on acoustic emission

Flow noise of gas–liquid two-phase flow in horizontal pipeline was detected by using the acoustic emission technique (AE); signals were processed by wavelet transform and chaotic analysis. Conclusions were drawn that stratified flow, annular flow and their transition can be divided clearly through multi-scale energy distribution of flow noise, and that dynamic characteristic of flow pattern transition from stratified flow to annular flow, which is described via correlation dimension, acts in accordance with that of annular flow. The dynamic characteristic of the transition condition has already been consistent with that of the annular flow, but due to the low gas flow rate, the energy of the hydrodynamic noise was not enough to reach the complete annular flow pattern. Results were in accordance with experimental facts. Flow noise reflects the complexity of gas–liquid two-phase flow by means of multi-scale energy distribution and chaotic features. Consequently, flow noise based on acoustic emission is a novel and promising point for researching gas–liquid two-phase flow.     

Voidage measurement of gas–liquid two-phase flow based on Capacitively Coupled Contactless Conductivity Detection

Based on Capacitively Coupled Contactless Conductivity Detection (C⁴D) technique, a new method for the voidage measurement of conductive gas–liquid two-phase flow is proposed. 15 Conductance signals, which reflect voidage distribution of gas–liquid two-phase flow, are obtained by a six-electrode C⁴D sensor. With the conductance signals, the flow pattern of gas–liquid two-phase flow is identified by flow pattern classifiers and then the voidage measurement is implemented by a corresponding voidage measurement model (for each typical flow pattern, a corresponding voidage measurement model is developed). The conductance measurement of the six-electrode C⁴D sensor is implemented by phase sensitivity demodulation (PSD) method. The flow pattern classifiers and the voidage measurement models are developed by partial least squares (PLS) technique and least squares support vector machine (LS-SVM) technique. Static voidage measurement experiments and dynamic voidage measurement experiments show that the proposed voidage measurement method is effective, the developed six-electrode C⁴D sensor is successful and the measurement accuracy is satisfactory.     

Gas–liquid two-phase flowrate measurement in pseudo-slug flow with Venturi

The importance of pseudo-slug flow research is becoming increasingly prominent in the petrochemical field. But the gas–liquid two-phase flowrate measurement in the pseudo-slug flow has not been properly understood and modeled. Based on the differential pressure of Venturi, this study proposes a new pseudo-slug flowrate prediction model. By means of Fast Fourier transform (FFT), the representative frequency range (3.125 Hz < f < 6.25 Hz) is determined. Then, the fourth detail component of the differential pressure after wavelet transform is selected as the flag to distinguish the liquid film region and the pseudo-slug body region. Based on the gas–liquid density ratio, a logarithmic model is established to predict the threshold value. In the liquid film region, the gas–liquid two-phase flow is regarded as wet gas and the flowrate is measured through the over-reading model. In the pseudo-slug body region, the volume gas holdup model is established based on the fluctuation information of the differential pressure. Then the gas–liquid two-phase flowrate can be obtained by solving the Bernoulli equation. Compared to the experiment, the confidence probability of ±10% relative deviation band is 97.78% for the gas, and the confidence probability of ±20% relative deviation band is 95% for the liquid.     

On-line measurement of the size distribution of particles in a gas–solid two-phase flow through acoustic sensing and advanced signal analysis

Acoustic emission (AE) technology is a promising approach to non-intrusively measure the size distribution of particles in a pneumatic suspension. This paper presents an experimental study of the AE sensing technology coupled with signal processing algorithms for on-line particle sizing. The frequency characteristics of the AE signals under different experimental conditions are studied and compared. Initially, the characteristics of the background noise and AE signals are compared in the frequency domain for different air velocities and particle feeding rates. Through short-term energy analysis the working features of the suction unit and the vibration feeder are revealed. To find the effective characteristic frequency band of the AE signals, a multiple scanning and accumulation method assisted with a Savitzky–Golay smoothing filter is used to denoise the power spectra of the signals. Wavelet analysis is also deployed to denoise the signals. The denoising performance of different wavelet parameters (wavelet function, decomposition level and thresholding) is compared in terms of signal-to-noise ratio and signal smoothness. Finally, particle size is predicted through a neural network with energy fraction extracted through wavelet analysis. Experimental results demonstrate that the relative error of the particle sizing system is no greater than 23%.     

Refined reconstruction of liquid–gas interface structures for stratified two-phase flow using wire-mesh sensor

Wire-mesh sensors (WMS), developed at HZDR [4], [13], are widely used to visualize two-phase flows and measure flow parameters, such as phase fraction distributions or gas phase velocities quantitatively and with a very high temporal resolution. They have been extensively applied to a wide range of two-phase gas–liquid flow problems with conducting and non-conducting liquids. However, for very low liquid loadings, the state of the art data analysis algorithms for WMS data suffer from the comparably low spatial resolution of measurements and from boundary effects, caused by e.g. flange rings – especially in the case of capacitance type WMS. In the recent past, diverse studies have been performed on two-phase liquid–gas stratified flow with low liquid loading conditions in horizontal pipes at the University of Tulsa. These tests cover oil–air flow in a 6-inch ID pipe and water–air flow in a 3-inch ID pipe employing dual WMS with 32×32 and 16×16 wires, respectively. For oil–air flow experiments, the superficial liquid and gas velocities vary between 9.2 m/s≤ν_SG≤15 m/s and 0.01 m/s≤ν_SL≤0.02 m/s, respectively [2]. In water–air experiments, the superficial liquid and gas velocities vary between 9.1 m/s≤ν_SG≤33.5 m/s and 0.03 m/s≤ν_SL≤0.2 m/s, respectively [17], [18]. In order to understand the stratified wavy structure of the flow, the reconstruction of the liquid–gas interface is essential. Due to the relatively low spatial resolution in the WMS measurements of approximately 5 mm, the liquid–gas interface recognition has always an unknown uncertainty level. In this work, a novel algorithm for refined liquid–gas interface reconstruction is introduced for flow conditions where entrainment is negligible.     

A new method of entrainment fraction measurement in annular gas–liquid flow in a small diameter vertical tube

A new method was introduced to measure liquid entrainment fraction in gas–liquid two-phase upward annular flow in a vertical tube (i.d.=9.525 mm). In this method, a new liquid–gas separator was designed and the chemically-based titration method was used to effectively measure the entrainment fraction in real time. Experiments were conducted at low system pressure (∼1 atm), and relatively low gas and liquid superficial velocities ($V_{\mathrm{sg}}=25.8$–45.5 m/s, and $V_{\mathrm{sl}}=0.15$–0.30 m/s). Data analysis shows that the results are repeatable and occupy the range commonly seen in annular flow. The entrainment fraction was found to be under 7% for all the experimental set points. The repeatability of the test results and comparisons with previous entrainment data indicate that the new technique can perform as well as the film removal technique.     

Void fraction measurement in steam–water two-phase flow using the gamma ray attenuation under high pressure and high temperature evaporating conditions

The void fraction is one of the most important parameters used to characterize gas–liquid two-phase flow, and a myriad of researchers have investigated it under the adiabatic flow conditions. The gamma ray attenuation is a frequently used non-intrusive method for measuring component volume fraction in gas–liquid two-phase flow system. In this paper, firstly, the influence of the various parameters and test conditions on the gamma ray attenuation have been completely examined, such as the calibration of Count Rate for pure gas and liquid phases, the influences of fluid temperature, phase changing point and fluid mass velocity, distance between gamma ray attenuation measuring instrument and experimental section etc. Secondly, the measurement of void fraction was taken in the vertically upward pipes under high pressure and high temperature evaporating conditions. The experimental results of void fraction were compared with the data in reference literature for measurement, the results from the gamma ray attenuation show good agreement with the literature for air–water two-phase flows, but for the evaporating conditions, a small number of compared data beyond the statistical approach for 90% of confidence interval due to some reasons, such as heat flux, the diameter of Taylor-bubbles, longitude of slugs etc. Finally, six predicted correlations from four groups were selected for comparing with the experimental data. The most of compared data were within the statistical approach for 85% of confidence interval. In general, the void fraction was rarely investigated and the available data was limited under high temperature and high pressure evaporating conditions. The investigations of present study are helpful to resolve the difficulties of measuring for gas–liquid two-phase flows concerning to the heated evaporating condition.     

Experimental and numerical studies on metrological characteristics of swirlmeters with different swirler helix angles in a gas–solid two-phase flow

Swirlmeters are widely used to measure natural gas because of their simple structure, absence of internal moving parts and high measurement accuracy. However, fine particles in natural gas can affect swirlmeters’ metering accuracy. Hence, this study evaluated the metrological characteristics of swirlmeters measuring gas–solid two-phase flow with fine particles and different swirler helix angles. Experiments and simulations for swirlmeters with different solid–gas mass ratios and helix angles were performed. The results demonstrated that the swirlmeter meter factors decreased and pressure losses increased as the solid–gas mass ratio increased. With different solid–gas mass ratios, the most effective sensor detection position was at the throat outlet a half radius from the centre of the runner. The swirler with a smaller helix angle could significantly increase the meter factor, while the swirler with a larger helix angle could significantly decrease pressure losses. The swirlmeter internal flow losses were mainly derived from the swirler.     

Volume fraction measurement and flow regime recognition in dynamic gas–liquid two phase flow using gamma ray radiation technique

Gas–liquid two phase f low is probably the most important form of multiphase f lows and is found widely in industrial applications, particularly in the oil and petrochemical industry. In this study, in the first instance a gas–liquid two phase f low test loop with both vertical and horizontal test tube was designed and constructed. Different volume fractions and f low regimes were generated using this test loop. The measuring system consists of a ¹³⁷Cs single energy source which emits photons with 662 keV energy and two 1-inch NaI (Tl) scintillation detectors for recording the scattered and transmitted counts. The registered counts in the scattering detector were applied to the Multi-Layer Perceptron neural network as inputs. The output of the network was gas volume fraction which was predicted with the Mean Relative Error percentage of less than 0.9660%. Finally, the predicted volume fraction via neural network and the total count in transmission detector were chosen as inputs for another neural network with f low regime type as output. The f low regimes were identified with mean relative error percentage of less than 7.5%.     

Measurement of velocity of sand-containing Oil–Water two-phase flow with super high water holdup in horizontal small pipe based on thermal tracers

When oil fields enter the last production period, the water holdup in the well is extremely high. Chemical flooding and horizontal well technology are often used to enhance oil recovery. These techniques result in a high downhole fluid viscosity and serious sand production, which leads to the failure of common velocity measurements because of sticking sand, and yields new logging difficulties. This paper presents a method of the velocity measurement of sand-containing oil–water two-phase flow in a super high water holdup pipe diameter based on thermal tracers. The measurement accuracy of the thermal tracer velocity method is related closely to parameters that affect its performance. Parameter optimization is required to improve the measurement accuracy. ANSYS Fluent was used for a numerical simulation of the heat-source shape and material, thermistor probe installation position and fluid heating power, and the method was verified experimentally. The optimal parameters of the thermal tracer flowmeter were obtained by numerical simulation, the heat source material was aluminum and the shape was rectangular. The thermistor probe was located 160–220 mm from the heat source, and the pulse heating power was 350 W. The experiment results show that the accuracy of the thermal tracer flowmeter was 4%, the repeatability was 2.6%, and the measurement accuracy of the flow velocity was unaffected by water holdup and sand.     

Study on the spatial filtering and sensitivity characteristic of inserted electrostatic sensors for the measurement of gas–solid two-phase flow parameters

A velocity measuring method using an inserted electrostatic sensor with spatial filtering effects is obtained using the point charge mathematical model established in this paper. Employing the established mathematical model helps determine the spatial filtering and spatial sensitivity characteristics of the probe. The spatial sensitivity distribution is obtained by simulating the point charge mathematical model, and when the point charge is near the probe (a>0), the sensitivity of the probe is higher and the spatial sensitivity of the probe has symmetry. The relation between the probe length L inserted into a pipe and the charge induced on the probe can also be obtained using simulation, where the longer the probe length L is, the larger the signal amplitude is. However, the signal amplitude is almost invariant when the probe length L is larger than the radius of the pipe. Experiments prove that the spatial filtering and sensitivity characteristics of the probe are consistent with the simulation results. When the free fall velocity of particles is the same, the probe has a low-pass characteristic for the measured signals. It is proven that the fluid velocity measurement method using spatial filtering effects can completely measure the fluid velocity using the spatial filtering characteristic experiments of the probe. The spatial filtering measurement velocity method is also feasible when measuring continuous objects.     

Numerical simulation of bubble separation length in a gas–liquid separator based on the Euler–Lagrange method

In this study, the bubble separation behavior in a gas–liquid separator is numerically investigated on the basis of the Euler–Lagrange approach, in which the forces acting on bubbles in a swirling flow field are modeled to calculate the trajectories of the bubbles. By adopting this approach, the effects of five parameters, namely, back pressure, Reynolds number, bubble diameter, void fraction, and swirl number, on separation performance in terms of pressure loss, separation efficiency, separation length, and split ratio are computed and analyzed. On the basis of the analysis, correlations of separation length with the two main parameters are established, which can serve as a basis for the optimal design of separator.     

Analysis of signal characteristics of swirlmeter in oscillatory flow based on Hilbert–Huang Transform (HHT)

In the paper, firstly, on an experimental facility, we investigated the measurement characteristics of a diameter 50 mm swirlmeter in uniform flow and oscillatory flow. At the same time, the interference characteristics of oscillatory flow were studied. Then, the signal characteristics of swirlmeter in oscillatory flow were analyzed by Hilbert–Huang Transform (HHT) method. Results show that the response characteristics of swirlmeter in oscillatory flow are addition of that of swirlmeter in uniform flow and the interference characteristics of oscillatory flow. They further prove the conclusions which suppose that the correlation between the velocity pressures of fluid disturbs wave and that of vortex precession in swirlmeter is linear in the literature, and a new method for the oscillatory flow swirlmeter noise removal on HHT was provided.     

Experimental validation of the calculation of phase holdup for an oil–water two-phase vertical flow based on the measurement of pressure drops

The performance of metering the phase holdup of an oil–water two-phase vertical flow has been investigated based on the measurement of the gravity and frictional pressure drops. A U-tube, in which the same flow patterns can be obtained in downward and upward vertical flows, is designed to measure both gravity and fractional pressure drops. During the experiments, the mixture velocities of the oil and water are in the range of 0.28–4.65 m/s and the oil volume fraction from 0 to 1.0. The results show that the oil holdups calculated are satisfactory with the absolute error of ±10%. The method presented in this work can be used to verify the results of tomography due to its simplicity and therefore is sufficient enough to be applied in industry.     

Influence of the electromagnetic gap in gas–magnetic bearings on the output characteristics of high-speed rotor systems

The electromagnetic gap in gas–magnetic bearings has a considerable influence on the output load and rigidity characteristics of high-speed rotor systems, as shown by experiments and simulation.     

Wear of the Counterbody at the Research Friction on Carbon Coating–Orientant in Lubricant Environments

The profile wear scars on steel balls are analyzed upon their friction in lubricant against steel coated with monocrystalline carbon doped by tungsten. Bond of ball wear with the lubricant composition and the range of test loads has been established.