Capacitance sensors for measurement of phase volume fraction intwo-phase pipelines

The effect of phase distribution (flow pattern) is considered for determining the volume fraction in two-phase pipelines. Experiments were performed with a variety of capacitance sensor designs. It was found that for a well-agitated mixture, two flow patterns occur for different compositions of the mixture, resulting in a discontinuous calibration curve. Sensitivity loss was found when the continuous phase was electrically conducting. This results in operational difficulties in applications to oil wells. It is concluded that capacitive sensors are practical only when the continuous phase is nonconductive or as a low electrical conductivity     

应用双头电导探针技术测量 气液两相泡状流局部参数

本文研究应用双头电导探针技术测量气泡局部参数,从而揭示了气液两相泡状流的内部流动规律。成功地设计了一种能够快速可靠测量气泡局部统计参数,包括空隙率、气泡速度、气泡尺寸、界面浓度等的电导探针系统。发现探针尖部的导通距离、沿流动方向两探针间的距离和两个探针针尖的间隙是设计电导探针的关键尺寸。     

应用双头电导探针技术测量气液两相泡状流局部参数

本研究应用双头电导探针技术测量气泡局部参数,从而揭示了气液两相泡状流的内部流动规律。     

Visualization and void fraction measurement of decompressed boiling flow in a capillary tube

A capillary tube is often used as a throttle for a refrigerating cycle. Subcooled refrigerant usually flows from a condenser into the capillary tube. Then, the refrigerant is decompressed along the capillary tube. When the static pressure falls below the saturation pressure for the liquid temperature, spontaneous boiling occurs. A vapor-liquid two-phase mixture is discharged from the tube. In designing a capillary tube, it is necessary to calculate the flow rate for given boundary conditions on pressure and temperature at the inlet and exit. Since total pressure loss is dominated by frictional and acceleration losses during two-phase flow, it is first necessary to specify the boiling inception point. However, there will be a delay in boiling inception during decompressed flow. This study aimed to clarify the boiling inception point and two-phase flow characteristics of refrigerant in a capillary tube. Refrigerant flows in a coiled copper capillary tube were visualized by neutron radiography. The one-dimensional distribution of volumetric average void fraction was measured from radiographs through image processing. From the void fraction distribution, the boiling inception point was determined. Moreover, a simplified CT method was successfully applied to a radiograph for cross-sectional measurements. The experimental results show the flow pattern transition from intermittent flow to annular flow that occurred at a void fraction of about 0.45.     

A method for calibrating cryogenic void fraction RF-sensors having a round cross-section and estimating their accuracy

The calibration principle of the cryogenic void fraction RF-sensors of a round cross-section for a wide temperature range is proposed and described in detail. It is shown that the simplest and reliable method of the calibration procedure requires finding a dependence of the resonant frequency, f, of the empty sensor on its temperature and only two f-values when it is filled with saturated liquid and vapor at the given temperature T₀. The calibration test-bench is described. The errors of the calibration are estimated, and the experimental data for helium, hydrogen and nitrogen are presented.     

气液两相低温流体空泡率测量技术及其进展

对目前几种用于测量低温气液两相流空泡率方法的基本原理、传感器结构及国内外研究情况进行了综述,包括辐射法(射线吸收法)、微波法、射频法、电容法,分析了4种方法的优缺点,在比较4种方法的基础上,提出了目前采用电容传感器测量气液两相低温液体空泡率的优势及设计准则.     

Experimental validation of void fraction models for R410A air conditioners

This paper presents the combined void fraction models on the basis of flow pattern to choose suitable models for calculating the two-phase refrigerant mass of R410A air conditioners, and the model is validated experimentally. The existing single void fraction models are also evaluated experimentally. The Taitel–Dukler flow pattern map is recommended to use in the combined void fraction models. The results presented herein show that the calculated refrigerant mass using the combined void fraction models agrees relatively better with the measured refrigerant mass than that of the single void fraction models. T–B model (for intermittent flow) and Premoli model (for annular flow) are recommended for calculating the two-phase refrigerant mass in evaporator, and T–B model (for intermittent flow) and Harms model (for annular flow) are also indicated for calculating the two-phase refrigerant mass in condenser.     

基于同轴线相位法的两相流含气率测量研究

为了准确测量气液两相流含气率,提出一种同轴线相位差测量方法。利用同轴线传感器,通过测量电磁波经过在同轴线内分布状况不同的气液混合介质后相位差的变化,得到混合介质的含气率。完成了同轴线测量电路及测量传感器的设计,建立了一种含气率测量模型。以气液两相做了室内静态实验,并对垂直管状态下传感器响应和实验结果进行了误差分析。结果表明:相位差输出与含气率呈线性关系;同时,在不同频率下,预测结果和实验结果的相对误差在±5%范围内,说明预测模型准确度较好。     

Application of electrical capacitance tomography to the void fraction measurement of two-phase flow

Based on the electrical capacitance tomography technique, a new method for the void fraction measurement of two-phase flow is proposed. A 12-electrode void fraction measurement system is established. A mathematical model of image reconstruction of electrical capacitance tomography is developed. To obtain the quantitative information of two-phase flow, combining the Tikhonov regularization principle and the algebraic reconstruction technique algorithm, a new image reconstruction algorithm is presented. The experimental results show that the accuracy of void fraction measurement is satisfactory. The proposed method is suitable for the void fraction measurement of many kinds of two-phase flow.     

PIV measurements for flow pattern and void fraction in cavitating flows of He II and He I

Cavitation phenomena in both He I and He II flows were investigated through pressure measurements, the optical visualization method, and particle image velocimetry analysis. Quasi two-dimensional cavitation flows of liquid helium under nearly saturated vapor pressure condition were generated downstream of the throat of a Venturi channel and of a converging jet nozzle. At a sufficiently high flow velocity, the throat pressure falls below the saturated vapor pressure, generating vapor bubbles downstream of the throat. The pressure loss of the He II flows was almost independent of temperature and was larger than that for He I. Large differences in the void fraction between He I and He II was concluded from the result of PIV measurements. The void fraction for He II was larger than that for He I, which was found to be in good agreement with the visualization results.     

Application of radio frequency method to measurements in cryogenics

This paper reports the specific features of the radio frequency method used to measure characteristics of cryogenic flows. The techniques for determination of the void fraction and quality of two-phase flows and the mean integral thermodynamic characteristics of single-phase flows, e.g. temperature and density, are described. The design and basic parameters of sensors with channels of round and annular cross-section are discussed. Possible combinations of these sensors with other devices for the measurement of two-phase and single-phase medium flow rates are shown. Information about the measuring system as well as measurement results for helium and nitrogen are presented. The sensor characteristics for hydrogen and methane have been estimated theoretically.     

A Monolithic CMOS Autocompensated Sensor Transducer for Capacitive Measuring Systems

In this paper, a monolithic complimentary metal-oxide-semiconductor (CMOS) autocompensated sensor transducer for capacitive measuring systems is newly presented. The proposed converter is compact and robust to integrate in capacitive measuring systems. The proposed autocompensated sensor transducer is attractive due to the fact that a digitized signal is produced without realizing the analog-to-digital converter. Hence, the hardware cost could be reduced. Furthermore, the output signal of the proposed transducer is a pulse stream; it could be easily sent over a wide range of transmission media, such as package switch networks (PSNs), radios, and optical, infrared (IR), and ultrasonic media. Another innovation is that the proposed automatic compensation circuits enhance and compensate the linear relation between the variable capacitance of the detected sensor and the output digital frequency over a wide dynamic frequency range. Measurement results have successfully verified the functions and the performance of the proposed autocompensated sensor transducer and confirmed that it is possible to apply it to the air pressure sensor. The area of this chip is 940 times 1080 mum², and the power consumption is 6.4 mW. The proposed transducer is not only suitable for capacitive measuring systems but also practical for application in the front-end systems of the wireless sensor network.     

Flexible discretization technique for DEM-CFD simulations including thin walls

The discrete element method (DEM) coupled with computational fluid dynamics (CFD) method is regarded as a standard approach for a simulation of a gas-solid mixture system. In the DEM-CFD method, the local volume average technique is employed, and hence the fluid motion is calculated based on the void fraction. Although the accuracy of the DEM-CFD method has been improved through lots of studies, inflexibility may become a problem due to the local volume average technique. Specifically, calculations of a gas-solid flow involving thin walls is substantially impossible even by the improved DEM-CFD method. This is because the thin wall cannot be represented when its thickness becomes as large as one grid size due to usage of the local volume average technique. In order to solve this problem, a flexible discretization technique is newly proposed, where the signed distance function and the immersed boundary method are introduced into the dual grid model. In this technique, two kinds of grids are used to calculate the void fraction and the fluid flow. Thus, this technique makes it possible to simulate a gas-solid flow involving a thin wall. Verification and validation tests are performed to show the adequacy of this technique. Through this study, the proposed technique is illustrated to reproduce the exact solution and experimental results in the gas-solid flow involving the thin wall. Consequently, the proposed technique is shown to yield reasonable results in gas-solid flows involving the thin walls.     

Experimental measurement of void fraction in cryogenic two phase upward flow

Experimental measurements of the void fraction in two-phase single component upward flow of both helium and nitrogen were carried out using the capacitance probe technique. An electronic circuit was designed and constructed to accurately detect small changes in the dielectric constant resulting from changes in density in the two-phase flow. A coaxial capacitance probe at the exit of the test section was used as the sensing element to pick up changes in void fraction.Tests were carried out at both adiabatic and diabatic conditions on two test section sizes (6.35 and 2.75 mm id and each 1000 mm long) with exit quality ranging from 0 to 100%, liquid flow of 5 to 35 lh^?1 and inlet pressures of 1 to 2 atm.Analysis of the data indicates that the slip ratio (obtained from quality and void fraction measurements) is affected primarily by pressure, mixture quality, superficial velocity and to a lesser degree by the size of the test section.The data of helium and nitrogen is also compared with the predictions of Lockhart-Martinelli, Levy and the homogeneous models.     

Two-phase cryogenic flow meters: Part II – How to realize the two-phase pressure drop method

The pressure drop method to find the mass flow rate of the two-phase helium and hydrogen flows is discussed in this paper. This method is based on a combination of the narrowing device and the RF void fraction sensor described earlier. Advantages of this approach with respect to the calorimetric one are demonstrated and its metrological characteristics for different flow patterns are estimated. The discussed cryogenic technologies and methods may be used for others applications, for example, to solve some tasks in oil-producing industry.     

Experimental characterization of voids in high fibre volume fraction composites processed by high injection pressure RTM

Replacing autoclave processes is a well-known industry drive in the composites community. One of the most recognized candidates for this replacement is high injection pressure resin transfer moulding (HIPRTM), because it is both an out of autoclave process and because the high processing pressures can, hypothetically, reduce the size of voids, thereby reducing void content. In order to clarify this issue, this paper presents our results on the size distribution and total void fraction of composites containing high fibre volume fractions (>60%) composites produced by HIPRTM. To substantiate this work we present a comparative study considering both autoclave and RTM at lower pressure/fibre volume fractions. Results show that HIPRTM is able to produce high fibre volume fraction parts at very low void content (<0.05%) and is comparable to autoclave results. Future work should study the mechanical properties of these laminates in order to clarify further the limits of HIPRTM.     

Piezoelectric Active-Sensor Diagnostics and Validation Using Instantaneous Baseline Data

This paper presents a signal processing tool that efficiently performs piezoelectric (PZT) sensor diagnostic and validation. Validation of the sensor/actuator functionality during structural health monitoring (SHM) operation is a critical component to successfully implement a complete and robust SHM system, especially with an array of PZT active-sensors involved. The basis of this method is to track the capacitive value of PZT transducers, which manifests in the imaginary part of the measured electrical admittance. Both degradation of the mechanical/electrical properties of a PZT transducer and the bonding defects between a PZT patch and a host structure can be identified by the proposed process. However, it is found that the temperature variations in sensor boundary conditions manifest themselves in similar ways in the measured electrical admittances. Therefore, we examine the effects of temperature variation on the sensor diagnostic process and develop an efficient signal processing tool that enables the identification of a sensor validation feature that can be obtained instantaneously without relying on prestored baselines. This paper concludes with experimental results to demonstrate the effectiveness of the proposed technique.     

Predictions of Phase Distribution in Liquid–Liquid Two-Component Flow

Ground-based liquid–liquid two-component flow can be used to study reduced-gravity gas-liquid two-phase flows provided that the two liquids are immiscible with similar densities. In this paper, we present a numerical study of phase distribution in liquid–liquid two-component flows using the Eulerian two-fluid model in FLUENT, together with a one-group interfacial area transport equation (IATE) that takes into account fluid particle interactions, such as coalescence and disintegration. This modeling approach is expected to dynamically capture changes in the interfacial structure. We apply the FLUENT-IATE model to a water-Therminol 59^® two-component vertical flow in a 25-mm inner diameter pipe, where the two liquids are immiscible with similar densities (3% difference at 20°C). This study covers bubbly (drop) flow and bubbly-to-slug flow transition regimes with area-averaged void (drop) fractions from 3 to 30%. Comparisons of the numerical results with the experimental data indicate that for bubbly flows, the predictions of the lateral phase distributions using the FLUENT-IATE model are generally more accurate than those using the model without the IATE. In addition, we demonstrate that the coalescence of fluid particles is dominated by wake entrainment and enhanced by increasing either the continuous or dispersed phase velocity. However, the predictions show disagreement with experimental data in some flow conditions for larger void fraction conditions, which fall into the bubbly-to-slug flow transition regime. We conjecture that additional fluid particle interaction mechanisms due to the change of flow regimes are possibly involved.     

Oil mass fraction measurement of CO2/PAG mixture

In this study, a method of using a capacitance sensor was investigated as a means to measure the mass fraction of a type of PAG oil flowing with CO₂ in a transcritical cycle. The test facility equipped with the capacitance sensor was fabricated to establish and maintain a known oil mass fraction and to measure the capacitance of the CO₂/oil mixture. By using this facility, the relationship among three parameters (reduced CO₂ density (CO₂ density divided by the critical density of CO₂), oil mass fraction, and relative dielectric constant of the CO₂/PAG oil mixture) was developed. For the range of oil mass fraction 0–0.07, the error of new measurement method was within 0.005 for a wide range of pressures and temperatures tested. This study established the method of measuring the oil mass fraction continuously in the transcritical CO₂ cycle without affecting the cycle performance. Through this method, the effect of oil mass fraction on the characteristics of the oil circulation behavior and the performance of the transcritical CO₂ cycle can be investigated.     

Interface for MEMS-based rotational accelerometer for HDD applications with 2.5 rad/s/sup 2/ resolution and digital output

A 0.6 /spl mu/m BiCMOS interface for microelectromechanical systems (MEMS) based rotational accelerometers is presented. It is housed in an inexpensive standard SO-24 plastic package with a capacitive rotational accelerometer sensor produced using MEMS technology. This sensitive interface chip includes the analog-to-digital conversion, filtering, and interface functions. The analog-to-digital conversion is realized through a single-bit electromechanical /spl Sigma/-/spl Delta/ loop able to detect capacitive unbalancing as low as 50 aF (50/spl times/10/sup -18/ F). The produced bitstream is then processed by a digital chain and made available through a standard 3.3 V (5 V tolerant) three-wire serial bus. The signal bandwidth is about 800 Hz, the sensitivity is 2.5 rad/s/sup 2/, with a full-scale sinewave of 200 rad/s/sup 2/ and a signal-to-noise ratio peak of 38 dB over 30-800 Hz. Through the serial bus, it is also possible to program device characteristics including gain, offset, filter performance, and phase delay. The complete sensor is used in a feed-forward compensation scheme to cancel external disturbances acting on computer hard-disk drives so as to steadily keep the read-write heads on track: this allows greater track densities and better speed performance.