Flow patterns in gas-assisted injection molding process in a channel

The flow field of a long bubble steadily expelling a viscous fluid confined by two closely located parallel plates is examined. In order to investigate the influence of bubble size on the flow field, a theoretical bubble profile is used to replace the complicated procedure for computing simultaneously the interface between the gas surface and fluid flows. The present study showed the two typical flow patterns and also a third flow pattern of the stagnation point moving in the region of the bubble tip front during transformation of the two typical flow patterns. The vorticity patterns are also drawn for various bubble profiles and are examined for their effect on the flow. The velocity field is also presented from two different viewpoints and the phenomena is examined. The stagnation point located on the center line between the bubble tip to the upstream is only found in the small range of in a channel, where λ is ratio of the bubble width to the distance between two parallel plates.     

Linear computed tomography of two-phase distribution in a rectangular channel

Two-phase flow is closely linked with the efficiency and safety of industrial processes in many fields, and there are various instruments for measuring the two-phase distribution. Among them, γ-ray and ultrafast X-ray tomography systems are most promising to break the technical barrier of gas-liquid measurement in the flow channel of high-temperature and high-pressure (up to 15 MPa/342 °C) nuclear reactors. Hence, A CT measurement method has been developed for imaging a two-phase distribution of a central plane oriented in axial direction in a rectangular duct, which was tested theoretically using a numerical phantom and experimentally on a preliminary tomographic hardware with a mechanically traversed gamma source and a detector unit, as well as a static phantom simulating gas bubbles in the pipe. After completing experimental and numerical imaging of a multi-bubbles phantom, the two-phase contrast and locations of bubbles in the experimental and simulated reconstruction images showed a good agreement and supported the feasibility of applying the linear scanning technique to realize two-phase detection in rectangular channels. The sensitivity analyses of scanning range, photon-registering time and scanning step length conveyed the optimal experimental strategy for this system. Morphological operation has also been imposed on image processing achieving elimination of severe ringing artefacts.     

A numerical model for simulating separated gas-liquid two-phase flow with low GVF in a V-cone flowmeter

One of the relatively new types of differential pressure flowmeters is the V-Cone (conical or cone) meter. In addition to having many advantages over other types, this flowmeter can also be used in multiphase flows. In the last few decades, many numerical works have been presented for single-phase flows. But in the discussion of two-phase flow, most of the available works are related to experimental research. Therefore, in this paper, the separated two-phase flow with a low gas volume fraction (GVF) has been numerically investigated. For this purpose, an unstructured grid and finite volume numerical method were used. In order to model the two-phase flow and turbulence, the existing approaches were compared. According to the results obtained, the volume of fluid (VOF) method for simulating two-phase flow and the Realizable k-ε model for turbulence modeling lead to better results than other investigated models. Also, by stimulating the flow with the aforementioned methods, it was found that the accuracy of the pressure calculation decreases with the reduction of the superficial velocity and volume fraction of the gas. Furthermore, for a more detailed analysis, the superiority of the Realizable k-ε turbulence model compared to other investigated models was proved quantitatively.     

A numerical study of impingement heat transfer in a confined circular jet

Heat transfer characteristics of a submerged circular jet impingement with a confined plate was studied numerically. The continuity, momentum and energy equations were solved simultaneously. FIDAP, a finite element code, was used to formulate and solve the matrix equations for fluid elements. The effects of channel height and Reynolds number on the local Nusselt number were considered in the range of H=0.5–1.5 and Re=100–900, respectively. It was found that the channel height influenced strongly on the surface temperature, shear stress and pressure drop. The peak temperature was observed and gradually moved outward to the rim of the heated circular plate with increasing the Reynolds number, which may be related to flow recirculation region in the channel. It is also noted that the pressure drop increased more than the average heat transfer coefficient as the Reynolds number increased. For Pr=7, the Nusselt number was much more dependent on the Reynolds number than the channel height, and the magnitude of the second peak in the Nusselt number distribution increased as the Reynolds number increased. The local Nusselt number calculated based on a mixing-cup temperature was considerably different from that using the inlet nozzle temperature for H=0.5 and Re=100. The present study showed that the local Nusselt number of a confined submerged jet was significantly larger than that of the unconfined free jet which was available in the literature.     

Analytical investigation of an inductive flow sensor with arc-shaped electrodes for water velocity measurement in two-phase flows

An inductive flow sensor with spot-shaped electrodes (IFS-SE) is sensitive to the shape of the flow profile and is restricted to be used to measure the flow rate of axisymmetric single-phase flows in a circular pipe. In many cases of application, it is not possible to provide a fully developed flow profile. Therefore, the inductive flow sensor has to cope with flow profiles that are not fully developed. To improve the accuracy, an inductive flow sensor with a pair of arc-shaped electrodes flush-mounted on the internal surface of an insulating section of a pipe is proposed in this article to investigate the characteristics of vertical gas-water two-phase flows. The effect of the flow profile on the inductive flow sensor is analyzed. A key contribution of the present work is to estimate the relationship between the induced voltage and the velocity of the conductive phase in two-phase flows. The estimation is achieved by the analytical calculation of magnetic-inductive equations through the method of variables separation. The analytical solution is compared with the results from an ideal model and from numerical simulation. Experiments are conducted to calibrate the inductive flow sensor with arc-shaped electrodes (IFS-AE). It is noted that the proposed IFS-AE can be adopted to obtain the velocity of the conductive phase in two-phase flows by measuring the voltage induced on the arc-shaped electrodes.     

Comprehensive experimental study of liquid-slug length and Taylor-bubble velocity in slug flow

Slug flow is an intermittent two-phase flow pattern that provokes undesirable pressure variations in pipes. Mathematical models are commonly used to study these variations; so that it is necessary to know the experimental liquid-slug length, Taylor-bubble length, and pressure drop to validate such mathematical models.In this work, we experimentally studied the water-air slug flow through an acrylic pipe loop 6 m long and 0.01905 m internal diameter. We assembled infrared sensors on the acrylic pipe to get voltage signals accordingly to the presence of liquid-slugs or Taylor-bubbles. We applied Fourier transform on the voltage signals to obtain dominant frequencies to determining the liquid-slug length.Moreover, we obtained the cross-correlation function to get the delay time between two groups of the voltage signals to determine the velocity of Taylor-bubbles. Additionally, we measured the liquid-slug length by video technique and pressure drop with a digital manometer. The liquid-slug lengths obtained by using dominant frequencies are in agreement with the ones measured by video technique.On the other hand, Taylor-bubbles could touch or not the wall pipe at different inclination pipe angles; this affects pressure drop. Then, we observed the inclination angle when the Taylor-bubble detaches from the wall of the pipe, under different flow conditions. We found that the Taylor-bubble detaching angle is 45°, and as the inclination angle is higher, the slug-liquid and Taylor-bubble lengths are smaller. The detaching angle can be used as a criterion to neglect the gas shear-stress into mathematical models to improve predictions of the hydrodynamic behavior of slug flow.     

A neural network approach for prediction of discharge in straight compound open channel flow

Most natural rivers and streams consist of two stage channels known as main channel and flood plains. Accurate prediction of discharge in compound open channels is extremely important from river engineering point of view. It helps the practitioners to provide essential information regarding flood mitigation, construction of hydraulic structures and prediction of sediment load so as to plan for effective preventive measures. Discharge determination models such as the single channel method (SCM), the divided channel method (DCM), the coherence method (COHM) and the exchange discharge method (EDM) are widely used; however, they are insufficient to predict discharge accurately. Therefore, an attempt has been made in this work to predict the total discharge in compound channels with an artificial neural network (ANN) and compare with the above models. The mean absolute percentage error with artificial neural networks is found to be consistently low as compared to other models.     

The development of a multiphase flow meter without separation based on sloped open channel dynamics

The online continuous measurement of multiphase flow is one of the most key technologies which influences the development of oil industry in future. A new type of multiphase meter system is developed based on the open channel flow. The test pipe of the meter is slightly slopped to make the flow pattern mainly stratified flow. Based on the study of oil and gas flow dynamics in the open channel test pipe, the liquid metering model and gas metering model are deduced to calculate the gas and the liquid flow rate, the water cut is measured online by the principle of differential pressure. This device can work online without the separation of the production fluid. By the lab test and field application test, the results of the metering system show that the liquid flow rate errors are within ±5%, the gas flow rate errors can be within ±5%, and the water cut absolute error is within ±2%, which can meet the demands of the field flow rate measurement.     

The humidity effect on air flow rates in a critical flow venturi nozzle

A Critical Flow Venturi Nozzle (CFVN) is usually calibrated using dry air. Yet CFVNs in industrial and calibration service centers are often used to measure flow rates of humid air. Therefore, ISO 9300 provides the calculation method for the humidity effect on discharge coefficients of CFVN. However, since this method is only due to a theoretical analysis, it is important to confirm and check the ISO calculation method for the humidity effect on CFVN with its isentropic analysis by means of an experimental method.In this experiment, three CFVNs with diameters of 0.4, 0.8 and 1.6 mm were calibrated with dry air (with the dew point −40 °C), in a primary air flow standard system with a mercury sealed piston prover, installed at the Korea Research Institute of Standards and Science (KRISS). Another piston prover, a portable dry piston prover, was used as a reference meter and was also calibrated in the primary standard system using dry air. The repeatability of this dry piston prover was confirmed with the deviation being less than 0.05%. The CFVNs were tested with this dry piston prover, using humid air. For air types with high humidity, the humidity effect on flow rates through the CFVNs showed quite significant difference between the experimental results and those from the ISO method with isentropic analysis. But for air types with low humidity, its effect was relatively insignificant.     

Experimental analysis of combined side weir-gate located on a straight channel

Flow measurement and control in open channel system for lateral flow is important to support the system management. The flow characteristics of combined side weir-gate are complicated due to changes in flow conditions along the side weir-gate section. Experimental investigation of the flow characteristics of both weir and sluice gates is crucial for predicting the flow through a combined side weir-gate structure. Although weir-gate structures are widely used for frontal flow in hydraulic structures, the same is not true for lateral flows. In this study, 650 laboratory tests were conducted to determine the flow characteristics of combined side weir-gate for subcritical flow, and the experimental results were analyzed to determine the effect of these characteristics and weir-gate geometry on discharge capacity. Interaction factor of combined side weir-gate is a function of the upstream Froude number, the ratio of gate opening to upstream flow depth, the ratio of distance between the top of the sluice gate and the weir crest to gate opening, the ratio of weir and gate length to upstream flow depth, and the ratio of weir and gate length to main channel width. Consequently, more discharge is passed through combined side weir-gate compared to side weir and sluice gates. The empirically derived equations for the discharge of combined side weir-gate show good compatibility with the experimental data.     

Use of gas bubbles for ultrasound Doppler flow velocity profile measurement

Ultrasonic velocimetry based on the Doppler shift effect accurately provides quasi-instantaneous flow fields for fluids with a sufficiently high acoustic scattering level. However, ultrasonic velocity instruments are known to perform poorly in clear water with low acoustic scattering level, which are frequent conditions in laboratory applications. This work confirms a technique to solve the problem by seeding the flow with micro hydrogen bubbles, generated by means of electrolysis.This paper investigates the influence of gas bubbles density on the quality of the ultrasound Doppler based velocity profiles in an open channel flow. The bubbles are generated by electrolysis of water using different magnitudes of electrical current. The estimation of the number of bubbles in the measurement volume confirms that the bubble diameter is similar to that of the wire used for electrolysis. This enables to determine the minimum density of gas bubbles needed to obtain a reasonably good echo and therefore an accurate velocity profile.     

Identification of gas-liquid two-phase flow patterns in a horizontal pipe based on ultrasonic echoes and RBF neural network

This paper proposes a novel flow pattern identification method using ultrasonic echo signals within the pipe wall. A two-dimensional acoustic pressure numerical model is established to investigate the ultrasonic pulse transmission behavior between the wall-gas and wall-liquid interface. Experiments were also carried out at a horizontal air-water two-phase flow loop to measure the ultrasonic echo pulse signals of stratified flow, slug flow, and annular flow. It is interesting to find that the attenuation of the ultrasonic pulse at the wall-liquid interface is faster than the attenuation at the wall-gas interface. An RBF neural network is constructed for online flow pattern identification. The normalized envelop area and the area ratios of the echo spectrum are selected as the input parameters. The results show that the stratified flow, slug flow, and annular flow can be identified with an accuracy of 94.0%.     

Direct measurement of the effect of non-submerged rigid vegetation-induced flow cross-section area variations on flow force in compound channel

In this experimental work, flow force in compound channel in presence of non-submerged vegetation was obtained via direct measurement method. The direct measurement method was used to address the inefficiency of the conventional energy-based methods for measuring the force within the body of highly rough flows. On this basis, the study was performed using a specially designed flume called knife-edge flume with a length of 14 m, a width of 1.07 m, and a height of 1.05 m at a stream bed grade of 10^-3. The flow force measurements were performed directly using a load cell in absence of the common errors that were otherwise associated with velocity measurement by inserting an external body into the flow. In this study, we examined non-submerged rigid vegetation layers with five different diameters (i.e., 20, 25, 30, 40, and 50 mm), three values of crossline spacing (i.e., 6, 8, and 10 cm), five values of crossline spacing (i.e., 8, 10, 12, 15, and 20 cm), and two different arrangements (i.e., ordered and non-ordered or cruciform) at three different positions (i.e., in the floodplain, in the main channel, and simultaneously in the floodplain and the main channel) by performing a total of 451 tests. The flow force exhibited the highest sensitivity to the increase in vegetation volume under the non-ordered arrangement. In this respect, the improvement in the flow force with increasing the percent volume of vegetation was much larger when the vegetation was arranged in a non-ordered rather than ordered fashion, with the effect of the vegetation been even more pronounced in the floodplain rather than the main channel. In addition, at a certain percent change of the flow force, the trough-vegetation flow velocity was higher under the ordered arrangement rather than the non-ordered arrangement, reflecting the more difficulty encountered by the flow as it passed through the non-ordered vegetation. Based on the analysis results, it was found that the flow force would change by 060% when the vegetation had grown in the floodplain with λ* values in the range of 0–1. In cases where the vegetation had grown in the main channel and simultaneously in the main channel and the floodplain, the flow force variations increased to 10–70% and 20–80% as the λ* changed from 0 to 10, respectively. Analyzing the results of the experiments and the vegetation-modified Reynolds number, a relationship was further presented for calculating the flow force in presence of non-submerged rigid vegetation in a compound channel.     

Free overall in circular channels with flat base: a method of open channel flow measurement

A free overfall at the end of an open channel offers a simple means of measuring flow discharge. In this paper, two methods are presented for the computation of end-depth and discharge of a free overfall from smooth circular channels with flat base. Firstly, applying the momentum equation based on the Boussinesq approximation, the flow upstream of a free overfall is theoretically analyzed to calculate the end-depth-ratio (EDR). This approach eliminates the need of an experimentally determined pressure coefficient. In subcritical flows, the EDR is related to the critical-depth that occurs far upstream. In supercritical flows, the Manning equation is used to express the end-depth as a function of streamwise slope of the channel. Methods to estimate discharge from the end-depth in subcritical and supercritical flows are presented. The upstream flow profiles of a free overfall are computed using the streamline curvature at free surface. Secondly, an alternate method for analyzing free overfall from circular channels with flat base is also presented, where a free overfall in a circular channel with flat base is simulated by the flow over a sharp-crested weir to calculate the EDR. The comparisons of the computed results with the experimental data are satisfactory for subcritical flow and acceptable for supercritical flow.     

Flow regime recognition in a long pipeline-riser system based on signals at the top of the riser

To identify conveniently multiphase flow regimes in subsea pipeline-risers, we study in this paper experimentally two-phase flows in a 1657 m long pipeline with an S-shaped riser to simulate field experiment, within a wide range of gas and liquid velocities. Three flow regimes, namely severe slugging, transitional flows, and stable flows, are analyzed based on three differential pressure and one pressure signals at the top of the riser; comparatively speaking, the positions of these signals in the experimental system are similar to those of the sea level signals in industrial fields, which are easy and less expensive to obtain. The obtained signals are decomposed into six scales via a multi-scale wavelet analysis, and further four statistical parameters on each scale are extracted, including mean values, standard deviations, ranges, and mean values of absolute. We compared the effects of six SVM classifiers with different kernel functions on the recognition rate of flow regimes, and it is found the recognition rates of SVM classifier with quadratic and cubic kernel functions are the highest. Further, the principal component analysis is employed to reduce the dimension of statistical parameters and it indicates that the recognition rate tends to increase with the rising number of principal components from 1 to 6, and it remains constant if the principal component number is further increased. Moreover, The results suggest that the recognition rate obtained from the pressure difference between the top of the riser and the separator peaks, and then it comes that from the pressure signal at the top of the riser, and that for the pressure difference signal at the top of the riser is the least satisfying one. As for the optimal differential pressure signals between the top of the riser and the separator, the results show that the recognition rate increases rapidly from 70.2% to 90.4% when the sample duration rising from 2.3 s to 18.6 s, and when the sample duration exceeds 74.4 s, the recognition rate exceeds 92.9% and remains unchanged.     

Effects of sample size and concentration of seeding in LDA measurements on the velocity bias in open channel flow

A Laser Doppler Anemometer was used to measure the mean flow and turbulence in fluid experiments despite it having a problem with the mean velocity bias when a LDA is used to measure turbulent flows. It is generally considered that given a sufficiently large sample size, LDA will produce measurements free of bias, even with high turbulence intensity, but there is no relative experimental validation to demonstrate how the sample size affects the mean velocity bias. This paper first tries to find the reasonable sample size that ensures a mean velocity calculation free of bias. Furthermore, the effects of particle seeding concentration on the measurements of velocity were also considered. Throughout the experimental process the particle velocities were measured using a Dantec 2-D LDA system. To describe the effects of sample size and particle seeding concentration, this paper will also address the reasonable sample size and range of concentration in the design of water flow prior to any experimental application.     

Machine learning identification of multiphase flow regimes in a long pipeline-riser system

Multiphase flow regime identification is a promising technology for ensuring flow safety in marine gathering pipelines. An experimental study of air-water flow regime was conducted on a 1687 m long S-shaped pipeline-riser system. The flow regimes were quantitatively classified as severe slugging, oscillating flow and stable flow. By combining of two cheap and easily accessible differential pressure signals on the top of the riser as a sample, the flow regime classifier is established based on the support vector machine, and methods to improve the computational efficiency of the classifier are investigated. First, on the premise that the recognition rate is over 90%, a reasonable sample-size reduction strategy based on the K-means clustering method was designed. When the signal duration was 18.6 s, the number of samples was reduced from 7632 to 2658, and the hyperparameter iteration time of the classifier was shortened by 99.3% from 10681 s to 80 s. Second, with the combination of the single-feature recognition rate and the correlation between features, the number of features was reduced significantly. The recognition rate of the three preferred features was 96.3% with a sample duration of 18.6 s. Finally, by using the samples with a signal duration of 18.6 s as the training set, the flow regime classifier built with the three preferred features had a better generalization ability, and the average recognition rate of the testing set was higher than 90%. When the signal duration of the training set is in the range of 9.3 s–55.8 s, the maximum difference of the average recognition rate of the testing set is only 0.7%.     

Improved volume-of-fluid (VOF) model for predictions of velocity fields and droplet lengths in microchannels

Accurate measurement of flow in microchannels is imperative to better understand their flow behaviour, which aids in the design of microfluidic devices. In this work, we present an improved VOF model based on smoothing functions that can effectively minimise the issue of spurious velocities, which causes numerical simulations in microchannels to be less accurate. We use the smoothed VOF to simulate the velocity fields and droplet lengths in microchannels and compare the results with experimental data. The results show that the smoothed VOF is able to simulate flow in microchannels more accurately than the standard VOF model. Microchannel simulations using the standard VOF model are less accurate because the spurious velocities produces artificially higher velocity regions in the flow field results. The spurious velocities also induce a higher but non-physical shear stress during the droplet formation process, resulting in droplets forming prematurely with shorter lengths. Hence the smoothed VOF which resolves the issue of spurious velocities is shown to be a more viable tool in predicting the flow in microchannels by means of numerical simulations.     

无线信道测量设备系统响应消除问题研究

信道测量是获取信道特性最有效最直接的方式之一。随着测量带宽的增加,测量精度的提升以及测量指标的丰富,在无线信道测量过程中消除测量设备系统响应成为不可忽视的重要步骤。研究了测量设备系统响应对客观物理信道的影响,并提出了一种消除测量设备系统响应的理论方法。通过实验室模拟测试与实地场景测量的实测数据,验证了所提出方法的可行性,同时分析了影响测量设备系统响应的客观因素以及采样色散对信道冲击响应的影响。