Innovative Techniques For Two-Phase Flow Measurements

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    Innovative Techniques For Two-Phase Flow Measurements

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    Recent Patents on Electrical Engineering 2008, 1, 1-13 1
    1874-4761/08 $100.00+.00 © 2008 Bentham Science Publishers Ltd.
    Innovative Techniques For Two-Phase Flow Measurements
    Wael H. Ahmed* and Basel I. Ismail**
    *Component Life Technology, Atomic Energy of Canada Ltd., Canada, **Department of Mechanical Engineering,
    Lakehead University, Canada
    Received: November 5, 2007; Accepted: November 27, 2007; Revised: November 30, 2007
    Abstract: Two-phase flow is important in an increasing number of applications and industries. For example, the safety
    analysis codes for the nuclear energy industry require closure relations for the vapor-liquid interfacial transfer terms,
    while accurate two-phase pressure drop models are necessary to design the piping systems in the oil and gas industry.
    Also, two-phase flow occurs in heat exchangers, steam generators, chemical reactors, oil transportation and many other
    process equipments. In addition, development of accurate and suitable instrumentations for on-line monitoring and
    measurement of the solids concentration and velocity in gas-solid two-phase flows has proven to be a challenging problem
    with many scientists and engineers worldwide developing novel techniques for this application. This paper presents a
    review on the electrical-based measurement techniques for gas-solid and gas-liquid two-phase flows along with the most
    recent patents developed for two-phase flow measurements. Also, development of a novel method for the design of
    capacitance sensors for void fraction measurement and flow pattern identification was presented in detail.
    Keywords: Two-phase flow, gas-solid flow, gas-liquid flow, novel design method, capacitance sensor, recent patents, void
    fraction.
    1. INTRODUCTION
    Online, continuous, two-phase flow measurement is often
    necessary, particularly in the oil and gas, nuclear energy and
    chemical processing industries. Reliable measurements of
    the void fraction and flow pattern identification are important for accurate modeling of two-phase systems. Void
    fraction can be measured using a number of techniques,
    including radiation attenuation (X or -ray or neutron beams)
    for line or area averaged values, optical or electrical contact
    probes for local void fraction, impedance technique using
    capacitance sensors and direct volume measurement using
    quick-closing valves. The use of the different techniques
    depends on the applications, and whether a volumetric
    average or a local void fraction measurement is desired. The
    radiation attenuation method can be expensive and from a
    safety aspect difficult to implement, while intrusive probes
    disturb the flow field. On the other hand, the impedance
    measurement technique is practical and cost-effective
    method for void fraction measurement. The technique is
    non-intrusive and relatively simple to design and implement.
    Impedance or capacitance sensors have been used successfully to measure time and volume averaged void fraction,
    and its instantaneous output signal has been used to identify
    the flow pattern [1]. In this paper, electrical-based techniques
    for gas-liquid and gas-solid flows measurements are
    reviewed. The paper is divided into two parts: In the first
    part, gas-solid flow measurements with their important and
    relevant application to pneumatic conveying systems are
    discussed. However, the detailed description of different
    techniques cannot be covered exhaustively here. Therefore,
    detailed consideration is focused on the development of a
    novel method for the design of a capacitance sensor used in
    *Address correspondence to this author at the Component Life Technology,
    Atomic Energy of Canada Ltd., Canada; Tel: +1(613) 584-8811;
    Fax: +1(613) 584-8031; E-mail: ahmedw@aecl.ca
    gas-liquid two-phase flow measurements in the second part
    of the paper.
    2. RECENT DEVELOPMENTS IN TWO-PHASE
    FLOW MEASUREMENTS
    2.1. Gas-Solid Two-Phase Flows
    Gas-solid Two-phase flows, particularly those solids in
    the forms of powder and grain transported in gases (typically
    air) through a pipeline, are important in pneumatic conveying which has widespread applications across many industries including chemical, food processing, cement, mining,
    petrochemical, pharmaceutical, semiconductor, and in transporting pulverized coal in fuel lines of thermal power plants.
    Moving fluidized-beds may also be regarded as a form of
    gas-solid flows. This wide application has led to extensive
    research on gas-solid two-phase flow systems.
    Gas-solid flows (also known as particulate flows), such
    as those in pneumatic conveying systems, are usually
    classified as dilute- or dense-phase flow. The dilute-phase
    flow can be normally achieved with low concentrations of
    solids (typically below 10%) and the simultaneous condition
    of high gas velocities in the gas-solid flow mixture [2]. In the
    dilute-phase conveying, the solid particles are fully dispersed
    or suspended in the flow and no deposition occurs. The
    velocity at which solids are traveling in a dilute-phase
    conveying is important. If they are traveling too slowly,
    solids can drop out from the flowing suspension and pipe
    blockage can occur. If they are traveling too quickly this can
    lead to unnecessarily high pipeline wear and power
    consumption due to system pressure drop [3]. Flow patterns
    in a dense pneumatic conveying system, however, show
    many interesting features and analogies with gas-liquid
    flows, such as slug, plug, and stratified flow regimes [4].
    Dense-phase flow in pneumatic conveying systems have the
    relative benefits of a low air requirement and hence energy
    demand, low pipeline erosion and low product degradation 2 Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 Ahmed and Ismail
    [5]. However, dense-phase pneumatic conveying may lead to
    unstable flows, caused by insufficient air velocity, and
    deposition. These unstable flows and deposition often cause
    pipe blockage and vibration that can be extremely difficult to
    remedy.
    The fundamental system components of a conventional
    pneumatic conveying system are: an air mover, a feeder for
    the solids material to be conveyed, the conveying pipeline,
    and a receiver to disengage the conveyed solids and carrier
    gas (typically air). Local solids concentration and velocity
    are two of the most important parameters characterizing gassolid flows. Development of accurate and suitable instrumentations for on-line monitoring and measurement of the
    solids concentration and velocity in gas-solid two-phase
    flows has proven to be a challenging task with many scientists and engineers worldwide developing novel techniques
    for this application. This is particularly due to spatial and
    temporal fluctuations of both the solids concentration and
    velocity during the pneumatic transportation [6]. The results
    of these measurements can be used to ensure efficient and
    economic pneumatic conveying operation, prevent blockage
    and hazardous damages and maintain safety standards. This
    paper presents a review on a range of existing and recent
    techniques used to measure solids concentration and velocity
    in gas-solid flows, such as those widely used in pneumatic
    conveying systems.
    2.2. Measurement Techniques for Gas-Solid Flow
    Systems
    The measurements and visualization of gas-solid flows,
    such as those involved in pneumatic conveying systems,
    could be performed using a range of noninvasive and nonintrusive electrical impedance, optical transmission [7],
    radiography and radiometric sensing [8], and other measurement techniques. The application of capacitance sensors for
    measuring local solids concentrations in a gas-solid flow was
    established decades ago [9, 10]. Chang et al. [11] tested
    strip- or ring-type capacitance transducers that could be used
    in non-electrically conductive (dielectric) pipes to nondestructively detect powder flow or measure powder
    thickness using the dielectric constant difference of the gas
    and powder. Capacitance impedance rings are noninvasive,
    simple and relatively inexpensive for implementation and
    provide high temporal resolution. However, those devices
    are difficult to calibrate and care must be taken when using
    correlation models for different gas-solid flow patterns to
    convert electrical impedance to solid concentrations [12]. In
    a dense-phase pneumatic conveying where density gradient
    exists in the gas-solid flow caused by a non-uniform
    distribution of solids in a pipe cross section, electrical
    capacitance tomography (ECT) can be effectively used [4; 5;
    13]. In an electrical process tomography, the capacitancesensing electrodes are installed at equal intervals around the
    periphery of the pipe, in order to obtain the threedimensional distribution of solids along the direction of the
    gas-solid flow. The electrodes are usually installed in a
    noninvasive way, i.e. outside the pipe made of dielectric
    material [14]. The process tomography or tomographic
    imaging technique has the potential of producing crosssectional images of the distribution of flow components in a
    conveying pipeline from which solids velocity and
    concentration profiles of a section of a pipeline can be
    obtained. Brown et al. [15] developed a tomographic
    technique for imaging gas-solid flow distributions in
    pneumatic conveying pipelines. Their technique utilized
    ultrasonic transmission-mode measurements constrained to
    the megahertz region. They performed image reconstruction
    using a back-projection method implemented with standard
    graphics algorithms. Brown et al. [15] applied their technique to simulate reconstructions of dense and dilute distributions and obtained results demonstrating the capabilities
    and limitations of their technique. They also addressed
    aspects of transducer array design. Recently, Steiner et al.
    [16] proposed a dual-mode ultrasound and electrical capacitance process tomography sensor for measurements of gassolid flow. In their method, the ultrasound image is used as a
    priori information for the finite element method-based
    capacitance reconstruction algorithm. They [16] reported that
    a significant improvement in image quality can be achieved
    with the proposed dual-mode tomography process sensor.
    Jiang and Xiong [17] introduced a novel noninvasive
    electrostatic method for measurement of the solids velocity
    and mass flow rate in gas-solid flow. In their system, the
    measurement of the velocity and mass flow rate were
    realized by an electrostatic sensor capable of detecting the
    electrostatic signals of solid powder in the gas-solid flow.
    The transit time of powder was measured by cross-correlating electrostatic signals, and the velocity was calculated.
    By correlating the voltage on electrostatic sensor and the
    fixed mass flow rate, the voltage-mass flow rate curve of the
    sensing system was established. Jiang and Xiong [17]
    concluded that electrostatic method can be used to trace and
    measure the velocity of the solid phase, and the method is
    capable of performing on-line measurements of the mass
    flow rate of the gas-solid flow.
    3. CAPACITANCE SENSORS FOR GAS-LIQUID
    TWO-PHASE FLOWS
    The principle of the capacitance method is based on the
    differences in the dielectric constants of the two phases in
    the flow, and the capacitance measured across the sensors is
    dependent on the volume ratio of the two phases. There are,
    however, several disadvantages of the impedance technique,
    which are sometimes difficult to resolve. For example, the
    capacitance measurement is sensitive to the void fraction
    distribution or flow regimes due to the non-uniformity of the
    electrical field inside the measuring volume. This, however,
    can be compensated by first identifying the flow pattern. The
    measurement is also sensitive to the changes in electrical
    properties of the two phases due to temperature. The noise
    due to the electromagnetic field around the sensor and
    connecting wires can significantly affect the signal and needs
    to be minimized through proper design of the sensor shield.
    The sensitivity of the capacitance meter is also improved by
    Andrade [18]. He used a full power supply potential swing
    on the shield electrodes to allow for the use of simpler shield
    electrode design. Using the full swing improves the
    capacitance background and balances such noises as the wire
    capacitances. Furthermore, it is difficult to resolve the
    change in phase distribution within the sensor measurement
    volume. The sensitivity to the flow pattern can be increased
    through better design of the sensors. For example, Hung et Innovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 3
    al. [19] used eight electrodes along the circumference of the
    tube to obtain a tomographic image of the two-phases. On
    the other hand, homogenizing the electric field in the axial
    direction minimizes the error due to the void distribution in
    the measuring volume. This could be done by using rotating
    type sensors as suggested by Merilo et al. [20] and Lucas
    and Simonian [21] or by using helical electrodes as discussed by Geraets and Borst [22] and Kenneth and Rezkallah
    [23]. The electrical capacitance tomography technique was
    also implemented recently by Gamio et al. [24] to image
    various two-phase gas-oil horizontal flows in a pressurized
    pipeline. They emphasized the potential of this technique for
    real-time flow visualization and flow regime identification in
    practical industrial application at high pressure operating
    conditions. Electrical Impedance Tomography (EIT) is also
    introduced in a recent patent by Wang [25] as a new signal
    processing method. The method is used on-line to obtain
    accurate estimates of the local disperse phase volumetric
    flow rate, the mean disperse and continuous phase volume
    fractions and the distributions of the local axial, radial and
    angular velocity components of the disperse phase.
    Several different configurations of capacitance sensors,
    including flat plate, concave, ring, helical and multiple
    helical wound in contact or isolated from the fluid, have
    been widely investigated [23, 26-29]. However, there are
    fewer studies on the optimization of the design to obtain
    good signal to noise ratio and high sensitivity to the different
    flow patterns. Elkow and Rezkallah [23] compared the
    performance of concave and helical type sensors and
    determined that the problems associated with helical type
    sensors, including the nonlinear response, poor sensitivity
    and poor shielding, can be eliminated by using the concave
    type sensors. The accuracy of the concave parallel sensors
    can be improved by having both electrodes of equal length to
    decrease the non-uniformity of the electric field between the
    two electrodes and eliminate the non-linear response. Based
    on several tests, they also recommended that the distance
    between the electrodes and the shield should be large relative
    to the separation distance between the two electrodes in
    order to improve the immunity to stray capacitance.
    In this paper, a systematic method for the design of
    capacitance sensors for void fraction measurement and flow
    pattern identification is presented. Two different configurations of the sensors are considered: concave and ring type
    Fig. (1). For the ring types sensor each electrode covers the
    entire circumference, except for a small gap to facilitate the
    installation of the sensor around the tubes, and are separated
    in the axial direction of the tube Fig. (1a), while in the concave sensor, two brass strips are mounted on the tube circumference opposite to each other Fig. (1b). The difference
    in the electrode geometry results in different electric fields
    within the measurement volume and hence in the sensitivity
    and response of the sensors. The two geometries are
    analyzed for the signal to noise ratio and the sensitivity to
    the void fraction and flow pattern. Experiments were
    performed to validate the design theory and to evaluate the
    sensor characteristics in actual operating conditions using
    air-oil two-phase flow in a horizontal pipe. To more
    objectively identify the flow pattern, the probability density
    function (PDF) and power spectral density (PSD) of the time
    trace of the void fraction signal is analyzed in a similar
    manner to that done by Jones and Zuber [30].
    4. CAPACITANCE SENSOR DESIGN
    The capacitance sensors need to be designed for accurate
    measurement of the void fraction, and have sufficient time
    response to detect the variations for the different flow
    patterns. The value of the capacitance across the sensors is
    related to the phase distribution and dielectric properties of
    the two-phases. The relationship between the capacitance
    and the void fraction is dependent on the dielectric values of
    the two phases, cross-sectional area of the sensors, and the
    separation distance between the two electrodes. A dielectric
    material placed between two conductor plates acts as an
    insulator to increase the charge storage capabilities because
    the dielectric contains charged molecules that are randomly
    oriented. When an external field is applied across the two
    plates, the charged molecules align themselves with the
    electric field and produces dipoles, where the positive
    charges of each molecule are in the direction of the applied
    field and the negative charges oppose the field. An internal
    Fig. (1). Schematic of electrodes configurations.
    Capacitance
    meter
    Elec (1)
    Elec (2)
    Elec (1)
    Capacitance
    meter
    Elec (2)
    a) Ring type b) Concave type
    Capacitance
    meter
    Elec (1)
    Elec (2)
    Elec (1)
    Capacitance
    meter
    Elec (2)
    a) Ring type b) Concave type 4 Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 Ahmed and Ismail
    electric field, opposite in direction of the external electric
    field, will result with a consequent reduction of the overall
    electric field and the overall potential. It is important to
    understand the theory of the capacitance sensor technique to
    properly optimize the sensor design. For simplicity, this can
    be illustrated using a simple parallel plate capacitor to show
    the effect of a dielectric material on the capacitance.
    Neglecting edge effects, the uniform electric field between
    the two plates of the capacitor in vacuum space without the
    dielectric medium, with charge density f
    and the permittivity of free space, o = 8.854x10
    -12
    C
    2
    N
    -1
    m
    -2
    , can be written
    as:
    E =
     f
    o
    (1)
    When a dielectric material is inserted to fill the entire
    space between the plates, the dielectric is polarized, and a
    polarization charge of density p appears on the two surfaces.
    For a uniform electric field out from a plane, the electric
    field can be written as:
    E =
    ( )  f
    +  p
    o
    (2)
    The polarized charge density is:
     p
    = 
    o
    E
    (3)
    where  is the electric susceptibility of the material. Finally
    we can write the capacitance as:
    C =
    
    o
    ( ) 1  A
    d (4)
    and the dielectric constant  is defined as:
     = ( ) 1  (5)
    The permittivity  is sometimes used to characterize the
    dielectric behavior of the matter and is related to the
    permittivity of the free space as
     = 
    o
     (6)
    In order to mathematically represent the output signal as
    a function of the void fraction, the capacitance measured by
    a transducer for two-phase flow can be treated as an approximation of a parallel or series combination of capacitors
    with different dielectric constants. In this work, the performance of two different types of sensor configurations is
    investigated, and a design method to optimize the sensor
    performance is developed. The sensor output is sensitive to
    the flow pattern and to incorporate the flow pattern in the
    theoretical analysis, it is important to clearly classify the
    flow patterns. To minimize the number of flow patterns, only
    the basic flow patterns are considered here. For horizontal
    pipes: bubbly, stratified, slug, and annular flow are used and
    for vertical pipes: bubbly, slug, and annular are considered.
    Churn flow is difficult to simulate in our analysis.
    4.1. Ring Type Sensors
    If we consider only stratified, annular and long slug flow
    patterns among the basic flow patterns listed before, the
    electric field between two ring electrodes in free space can
    be schematically presented in Fig. (2a). Assuming the electrical field is shielded outside the tube and neglecting the
    radial electric field, the distribution can be approximated to
    that shown in Fig. (2b). Therefore the electric field can be
    assumed approximately constant in the axial direction and
    the two rings equivalent to parallel flat disks. The equivalent
    capacitance circuit method can be used to analyze this
    problem [11]. In this method, the two phases are modelled as
    series or parallel capacitors between the electrodes. This
    equivalent circuit is based on the distribution of the two
    phases inside the channel and is illustrated in Fig. (3). By
    considering the two electrodes as two imaginary disks in
    parallel with a separation distance (d), the two phases distributed horizontally will be equivalent to two capacitances in
    series Fig. (3a), while the two phases distributed in the
    vertical direction will be equivalent to two parallel capacitances Fig. (3b). The equivalent circuits for different twophase distributions are shown Fig. (4a) through d as
    discussed by Chang et al. [11]. The theoretical output
    Fig. (2). Schematic of electrical field profile generated by two ring electrodes.
    2
    Electrode 2 Electrode 2
    d
    Electrode 1 Electrode 1
    Imaginary planes
    a) Actual b) Assumed Innovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 5
    capacitance as a function of void fraction for annular,
    stratified and long slug flow can be obtained by considering
    the equivalent circuit method represented in Fig. (4a & b),
    and the total equivalent capacitance for both circuits can be
    calculated as:
    CT
    = Cg
    +CL
    +Cw
    (7)
    For annular or stratified flow
    CT
    =
    Kg
    
    0
    Ag
    d
    +
    KL
    
    0
    AL
    d
    +
    Kw
    
    0
    Aw
    d (8)
    CT
    =
    Kg
    
    0
    Ag
    d
    +
    KL
    
    0 ( ) 1 Ag
    d
    +
    Kw
    
    0
    
    
    4
    ( ) D + t
    2
     D
    2
    ( )
    d (9)
    The void fraction is defined as
     =
    VG
    VT
    =
    AG
     d
    AT
     d
    =
    AG
    AT
    (10)
    where VG and VT are the volume of gas and total volume
    respectively.
    By dividing equation (9) by the cross-sectional area of
    the pipe results in
    CT
    =
    D
    2
    
    o
    4d
    Kg
     + KL ( ) 1 + Kw
    1+
    t
    D



    


    
    2
    1





    
    
    
    
    (11)
    where, t is the pipe wall thickness and w is the dielectric
    constant of the pipe material.
    4.2. Concave Type Sensors
    A similar analogy is used for the concave type sensors,
    however, an equivalent geometrical shape is introduced (Fig.
    5a through c), where the change of the electrical field due to
    the curvature of the electrode is neglected [26]. Then, the
    total capacitance for stratified or annular flow pattern can be
    written as:
    Fig. (3). Capacitance circuit equivalent to two-phase flow distribution.
    Fig. (4). Equivalent capacitance circuits for typical flow regimes (Adapted from Chang et al. [11]).
    Cg CL
    CL
    Cg Liquid
    Gas
    d
    x
    d
    Ag AL
    a) Series circuit b) Parallel circuit
    Cg CL
    CL
    Cg Liquid
    Gas
    d
    x
    d
    Ag AL
    a) Series circuit b) Parallel circuit
    3
    c) Bubbly flow d) Slug Flow
    a) Annular flow b) Stratified flow
    CG6 Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 Ahmed and Ismail
    CT
    =
    1
    CW
    +
    1
    CL2
    +
    1
    Cg
    +CL1
    +CL1
    +
    1
    CW
    +
    1
    CL2
    
    
    
    
    (12)
    The equivalent geometrical parameters can be defined as
    d =  ( ) D  2  t e = d =  ( ) D  2  t
    f =
    1
    2
    D   ( ) D  2  t a =
    1
    2
    ( ) D  2 z
    (13)
    Based on the two phases distribution shown in Fig. (5),
    and using the capacitance circuit analogy, the capacitances in
    equation (12) can be written as:
    Cw
    = w
    
    o
    a  L
    t
    Cg
    =  g
    
    o
    h  L
    e
    CL1
    =  L
    
    o
    a  L
    e
    CL2
    =  L
    
    o
    a  L
    f
    (14)
    The above analysis may be used to estimate the
    capacitance of such a sensor by substituting the permittivity
    of the free space and the dielectric constant of the liquid as
    well as the geometrical parameters listed in equation (13) as
    a function of the void fraction. The set of equations (12) to
    (14) relate the capacitance to the void fraction and can be
    used to investigate the sensitivity and/or to compare different
    designs.
    4.3. Design Method Applied to Different Flow Patterns
    Although the theoretical model discussed above considered only stratified and annular flow, the analysis could be
    extended to other flow patterns in order to design and
    optimize the capacitance sensor. As an example, plug flow is
    considered, and in particular the case where an elongated
    bubble with a length lG in a horizontal pipe is passing
    through a pair of ring type sensors as shown in Fig. (6a). The
    bubble can be assumed as a horizontal cylinder with cross
    sectional area AG and occupying a length l between the two
    electrodes. Assuming a constant electric field between the
    electrodes, the equivalent capacitance circuit using the same
    analogy as before is shown in Fig. (6). In this case
    CT
    =
    Cg
    +CL1
    Cg
    CL1
    +CL2
    +Cw
    (15)
    and the volumetric void fraction is different from the cross
    sectional void fraction and can be calculated as
     =
    VG
    VT
    =
    AG
     lG
    AT
     d
    (16)
    The capacitances in equation (15) can be written as
    Cw
    =
    w
    o
    
    4
    ( ) D + t
    2
     D
    2
    ( )
    d
    CL1
    =
     L
    
    o
    Ag
    d  l
    CL2
    =
     L
    
    o ( ) At
     Ag
    d
    Cg
    =
     g
    o
    Ag
    l
    (17)
    The corresponding relation between the void fraction and the
    output signal of the capacitance sensor in the case of
    elongated bubble can be written as
    Fig. (5). (a),(b) Geometrical simplification of the concave type capacitance sensor, (c) Equivalent capacitance circuits for annular and core
    flow regimes.
    Fig. (6). a) Geometrical simulation of elongated bubble in ring type sensor.
    b) Equivalent capacitance circuit.
    z
    D
    Elect(2)
    Elect(1)
    63.5 mm
    h
    e
    D
    f
    a
    t
    Cw
    Cg CL1
    CL2
    CL2
    Cw
    CL,2
    (a) (b) (c)
    z
    D
    Elect(2)
    Elect(1)
    63.5 mm
    h
    e
    D
    f
    a
    t
    Cw
    Cg CL1
    CL2
    CL2
    Cw
    CL,2
    (a) (b) (c)
    CW
    CG CL1
    CL2
    D
    l
    t
    AG
    AL
    Liquid
    Gas
    Elect2 Elect1
    d
    (a) Elongated bubble simulation b) Equivalent circuit
    CW
    CG CL1
    CL2
    D
    l
    t
    AG
    AL
    Liquid
    Gas
    Elect2 Elect1
    d
    (a) Elongated bubble simulation b) Equivalent circuitInnovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 7
    CT
    =
    o
    d
    D
    2
    4
    w
    1+
    d
    t



    


     1



    


    + L
    1
    d
    l



    


    
    
    
    
    
    
    
    +
    l
    2
     g
    +
    l
    2
    d
    l
    1



    


    
     L
    
    
    
    
    ,
    where
    > 1
    l
    d
    .
    The above analysis can be extended to different flow
    regimes, where the level of complexity of the analysis
    depends on the complexity of the flow regime and the
    required accuracy of the modelling.
    5. DESIGN AND CHARACTERIZATION OF THE
    SENSORS
    Based on the theoretical analysis, ring and concave type
    sensors were designed and constructed for a tube having
    inside and outside diameter of 12.7 and 15.8 mm, respectively. The schematic diagrams with the basic dimensions of
    the sensors are shown in Figs. (7 & 8). The ring type sensor
    consists of two pairs of active electrodes made of 5 mm
    brass strips spaced 2 mm apart to increase the signal to noise
    ratio by amplifying the absolute value of the capacitance
    circuit. This geometry provides sufficient volumetric
    resolution to resolve the characteristics of the slug flow
    regimes. Two separate half hollow cylinders made of acrylic
    are used for housing the electrodes as show in Fig. (7a & b),
    and the unit is shielded using a 0.5 mm thick grounded brass
    electrode to eliminate stray capacitance between any of the
    electrodes, circuit and the wires. The housing is sandwiched
    together over the test section pipe and fastened using acrylic
    screws to make sure the electrodes are firmly in contact with
    the tube surface. The noise on the signal due to the
    surroundings and orientation of the connection wires was
    eliminated in the present design by increasing the separation
    distance between the electrodes and the shield relative to the
    separation distance between electrodes as suggested by
    Elkow and Rezkallah [23]. The length of the electrode for
    the concave type sensor is taken as 10 mm, and both
    electrodes are equal in length to decrease the nonlinearity in
    the sensor response as recommended by Elkow and
    Rezkallah [23]. Two pairs of electrodes were used with
    spacing of 6 mm to have the same spatial resolu-tion as the
    ring type for comparison. In the experimental work reported
    here, the distance between the electrodes and the shielding
    was experimentally optimized to reduce the noise due to
    ground current and electric field leakage. The output
    capacitance from the sensors is measured using a Boonton
    72B capacitance meter operated at an excitation frequency of
    1 MHz. The accuracy of the meter is + 4% in the 10 pF
    range with meter resolution of at least 0.001 of the range.
    The analog output voltage signal from the capacitance meter
    is sampled using a 16-bit A/D converter at a sampling rate
    up to 2 KHz.
    The sensors were calibrated over the full range of void
    fractions to verify the theoretical design approach. A static
    calibration was performed using simulated stratified and
    annular flow patterns, and a dynamic in situ calibration was
    performed to investigate the effect of the noise from the
    surroundings such as from other operating devices on the
    output signal of the sensors. It should be noted that the effect
    a) Schematic of the ring type capacitance sensor.
    b) Solid model.
    Fig. (7). Ring type capacitance sensor.
    Fig. (8). Geometrical simplification of the concave type capacitance
    sensor.
    of the operating temperature was taken into consideration in
    the calibration tests. The calibration tests were used to
    compare the performance of the two types of sensors based
    on better sensitivity for the same spatial resolution as
    described in details by Ahmed [31].
    5.1. Sensor Calibration
    Canvas based Phenolic rods with the same dielectric
    constant as oil (=5.45) are used to represent the liquid in the
    static calibration at ambient temperature. The solid rods with
    Elct(2)
    A
    Elct(1)
    A
    Sec A-A
    d= 5 mm
    50.8 mm
    63.5 mm
    D= 15.8 mm
    z
    D
    Elect(2)
    Elect(1)
    63.5 mm8 Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 Ahmed and Ismail
    a diameter equal to the inside pipe diameter of the test
    section (12.7 mm) are machined to mimic different flow
    regimes. The lengths of these pieces are the same and equal
    to 13 cm. Calibration was carried out for two different flow
    patterns, namely annular and stratified flow. To represent
    annular flow, holes of different diameter were drilled
    through the center of each piece. Nine pieces were generated
    to simulate different void fractions. For stratified flow,
    eleven separate sections were milled off the rod. The value
    of void fraction for each piece can be calculated using the
    geometry.
    A comparison between the results from the theoretical
    models and the experimental data for the off-line calibration
    is presented in Figs. (9a & b) for both ring and concave
    sensors. The experimental results show that the relation
    between the capacitance and void fraction for both types of
    sensors is approximately linear. The deviation between the
    model and experimental results is due to the assumption of
    a) Static calibration for ring type capacitance sensor.
    (b) Static calibration for concave type capacitance sensor.
    Fig. (9). Static calibration for capacitance sensors.
    perfect shielding and also due to a non-uniform electrical
    field between the electrodes, with the discrepancy being
    higher for the concave type sensor. This is because the
    assumption of a constant electric field between the electrodes
    is more applicable for the ring type sensor than for the
    concave type. The non-linearity of the model results for the
    concave type sensor is due to the electrical circuit analogy
    which gives rise to a series capacitance circuit, where the
    total capacitance is a non-linear function of the individual
    capacitances. For the ring type sensors, the total capacitance
    between the electrodes is a summation of capacitances in
    parallel. The calibration data showed a correlation coefficient of 0.928 and a precision error of 0.04 with 95% confidence level for the ring type sensor, while the agreement
    between the experiments and the theory for the concave
    sensor was within 18%.
    Measurements were performed with the sensors installed
    on the air-oil two phase flow loop which discussed in details
    by Ahmed [32]. The density and viscosity of the oil used are
    973.05 kg/m
    3
    and 0.026 Pa.s, respectively. The air and oil
    are then mixed through an annular mixer. The oil flows on
    the outside of an inner perforated pipe, while the air flows in
    the inner pipe and enters the oil stream through 380 0.79-mm
    diameter perforations. The air-oil mixture passes through the
    horizontal test section, which has a total length of 3.5 m.
    Tests were performed with single phase oil (a=0) and
    single phase air (a=1) to check the range of the output signal
    of the sensors. A number of tests were performed for
    stratified flow by adjusting the depth of the oil in the pipe to
    check the linearity of the capacitance-void fraction relation.
    Results show that the sensitivity is the same for both the in
    situ and static calibrations, however, the absolute values of
    the capacitances at zero and 100% void for the two cases are
    off by 5 to 10%. Therefore, the calibration curves were
    checked against the dynamic results before the experimental
    data were taken. Repeatability tests were also performed and
    the results indicated that the variation in void fraction is
    between 2% to 3.5%.
    The capacitance measurements in the air-oil flow tests
    are in the range 0.1 to 15 pF, and proper shielding against
    stray capacitance is required to obtain a good signal to noise
    ratio (SNR). The range of the noise frequencies due to the
    equipment and electrical devices near the test rig were
    determined using a frequency analyzer and was found to be
    lower than 100Hz in this case. The electronic circuit was
    modified to eliminate the environment noise using a high
    pass filter.
    The void fraction can be related to the normalized
    capacitance (Cn) defined as:
    Cn
    =
    Calloil
    Cmeasured
    alloil
    allair
    (18)
    The correlation between the void fraction and normalized
    capacitance is found by applying a linear regression to the
    calibration data for the ring type sensor and is given by,
     = 0.756 Cn
    (19)
    with a correlation coefficient of 0.95.
    0 0.2 0.4 0.6 0.8 1
    1.5
    2
    2.5
    3
    3.5
    Void fraction (%)
    Capacitance (pF)
    Model
    Annular simulation
    Stratified simulation
    0 0.2 0.4 0.6 0.8 1
    0
    0.5
    1
    1.5
    2
    2.5
    Void fraction (%)
    Capacitance (pF)
    Model
    Annular simulation
    Stratified simulationInnovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 9
    5.2. Time Constant
    The definition of the time constant of the capacitance
    sensor here is referred to the time interval required for the
    sensor-meter to change 70% from one state or condition to
    another. The time constant for both types of capacitance
    sensors was obtained experimentally by applying a unit step
    signal and recording the sensor response Fig. (10). The time
    constant for both types of capacitance sensors using equation
    (20) is found to be approximately 40 μs, corresponding to a
    dynamic response of 25 kHz. Fast Fourier transform analysis
    of the current flow fluctuations in the present study showed
    that the major fluctuations in the void fraction were below 1
    kHz, thus the response of the sensors is more than adequate
    Yt
    Yintial
    Ymax
    Yintial
    = 1 e
    T
    
    (20)
    5.3. Sensitivity
    The sensitivity of the sensor can be defined as:
    Senistivity =
    Callliquid
    Callair
    allliquid
    allair
    = Callliquid
    Callair
    (21)
    and needs to be maximized. The sensitivity depends on the
    geometrical shape and gap between the electrodes, which
    also affects the spatial resolution of the sensor. The effect of
    the design parameters such as the electrode width and
    spacing were studied using equations (11) and (12) for both
    sensors. For the ring type sensor, the main dimension that
    affects the sensitivity is the spacing between the electrodes
    (d). The results show that the sensitivity increases as the
    spacing decreases Fig. (11), which also results in a better
    spatial resolution, with the only limitation being in the
    fabrication of the sensor. For the concave sensor, the
    sensitivity increases as the electrode separation (z) decreases
    Fig. (12a), and the electrode length increases. However,
    increasing the electrode length leads to a poor spatial
    resolution as shown in Fig. (12b). In general, the sensitivity
    of the ring type sensor is found to be higher than the concave
    type for the same spatial resolution. The sensitivity of the
    ring sensor was found to be approximately 0.75 pF, while for
    the concave sensor it was 0.6 pF.
    Fig. (10). Capacitance Sensor response to an input step function.
    Fig. (11). Effect of electrode spacing on the sensitivity of ring type
    sensor.
    (a)Effect of electrode separation.
    (b) Effect of electrode length.
    Fig. (12). The effect of sensor dimensions on the sensitivity for
    concave type sensor.
    -2
    -1
    0
    1
    2
    0.4 0.45 0.5 0.55 0.6
    Time (µs)
    Volt (v)
    0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
    0
    5
    10
    15
    d/D
    Sensit vi ity (pF)
    Elct(2)
    d
    Elct(1
    D
    0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
    0
    0.2
    0.4
    0.6
    0.8
    1
    z/D
    Sensit vi ity (pF)
    z
    D
    Elect(2
    Elect(1
    0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
    0
    0.2
    0.4
    0.6
    0.8
    1
    z/D
    Sensit vi ity (pF)
    z
    D
    Elect(2
    Elect(110 Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 Ahmed and Ismail
    The effect of the flow regime on the sensitivity of the
    capacitance sensor can b estimated. For example, in the case
    of elongated bubbles, the sensitivity can be calculated using
    equation (18) along with equation (21). The sensitivity as a
    function of void fraction for different bubble length to
    electrode spacing length ratios for an electrode spacing of 10
    mm is shown in Fig. (13). It should be noted that the
    sensitivity is also affected by the electrode spacing and for
    the ring type sensor an electrode spacing of less than 2 mm is
    required for a high sensitivity. For this electrode spacing, the
    effect of (l X ) is found to be neglected, which means that
    the sensor gives an approximate volumetric void fraction
    equal to the cross sectional value for bubbly flow.
    Fig. (13). The sensitivity of ring type sensor in case of elongated
    bubble flow.
    5.4. Measurement Uncertainty
    The total flow rate through the test section was maintained within +2% of the average value while the void fraction measurements were taken. The main source of uncertainty in the void fraction measurement was the noise in the
    sensor signal from the surrounding equipment such as the oil
    pump and the biases of the voltage signal, sensor spacing
    and position. This uncertainty was calculated to be in the
    range of +6% over the entire range of void fraction.
    6. FLOW PATTERN IDENTIFICATION
    Flow pattern identification can be performed either by
    visual inspection of the flow in a transparent pipe or by
    measuring and quantifying the fluctuations of the flow
    parameters such as void fraction (Jones and Zuber [30]) or
    dynamic pressure (Keska and Williams [33]), which reflect
    the flow structure. The flow pattern recognition from the
    signal fluctuations can be done by analyzing the probability
    density function (PDF) or power spectral density function
    (PSD) of the time trace signal [30]. In the present study, the
    applicability of the capacitance sensors for flow pattern
    identification was investigated. Only the ring type sensor is
    considered here since it has a better sensitivity than the
    concave type for the same spatial resolution. The time
    averaged void fraction and the probability density function
    (PDF) of the void fraction signal is used in this instance to
    identify the flow pattern. The flow regimes are also obtained
    using a high speed camera for validation. Three main flow
    regimes and their corresponding PDF distributions are
    discussed here for two-phase air-oil through a horizontal
    pipe. These three basic flow regimes are: elongated bubble,
    slug and annular flow. All the data presented was collected
    at 2 kHz over a period of 50 sec.
    Figures (14a,b & c) illustrate the time trace, PDF and
    void fraction signal respectively for a typical slug flow while
    an image of the corresponding flow pattern using the high
    speed video system is presented in Fig. (14d). The time trace
    is characterized by an intermittent void fraction signal that
    fluctuates between a high and low value. The high void
    fraction (a=0.29) correspond to the passage of a long gas
    bubble, while the low void fraction indicates the passage of
    the liquid slug. Small bubbles in the tail of the long bubbles
    gives rise to the low void fraction (a=0.04). Although the
    capacitance sensor measures the volumetric void fraction
    over a tube length of 1.65 D, the sensor seems to be sensitive
    to bubbles that are entrained in the liquid slugs as can be
    seen in the time trace signal. In order to distinguish between
    two signals having double peaks on the PDF diagram, the
    power spectral density (PSD) was used. In this case,
    additional information about the flow structure was obtained
    such as the bubble and liquid slug frequencies. The power
    spectral density (PSD) for the slug flow signal shown in Fig.
    (14c) is characterized by two dominant frequencies, corresponding to the liquid slug and the gas bubble frequencies. In
    Fig. (14c), the slug frequency is approximately 118 Hz,
    while the frequency of the small bubbles is 228 Hz. For
    elongated bubble and slug flows, a double peak PDF is
    obtained, one at low void fraction corresponding to the liquid
    slugs and the other at high void fraction due to the gas slugs.
    The time trace and PSD in this case are used to distinguish
    between the two cases. For elongated bubbly flow, the void
    peaks on the time trace signal occur at a higher frequency
    than for slug flow (Fig. (15a) through d). This is indicated in
    the power spectral density (PSD) of the signal where a
    higher frequency of 394 Hz is obtained for the case of
    elongated bubble flow Fig. (15c), while a frequency of 228
    Hz is obtained for the case of slug flow Fig. (14c).
    The PDF for annular flow is characterized with a single
    peak at high void fraction as shown in Fig. (16b), while the
    time trace shows small oscillations around a high mean value
    of void fraction (a=0.54) indicating the unsteady surface
    waves of the liquid film. It should be noted that, as the peak
    in the PDF becomes narrower, the liquid film thickness
    becomes more nearly constant.
    7. CURRENT & FUTURE DEVELOPMENTS
    Electrical capacitance is an innovative technique for
    measurement of two-phase flows. In this paper, a review
    including most recent patents developed for two-phase flow
    measurements using capacitance-based sensor was presented. Also, in this work, a novel systematic method for the
    design of capacitance sensors for void fraction measurement
    and flow pattern identification was developed and discussed
    in detail. Two types of capacitance sensors, ring and concave
    type, have been analyzed. The phase or void distribution is
    represented by an equivalent capacitance circuit to estimate
    = 16.0
    d
    l
    = 4.0
    d
    l
    = 7.0
    d
    l
    Sensitivity (pF)
    0 0.2 0.4 0.6 0.8 1
    0.1
    0.2
    0.3
    0.4
    Void fraction
    = 16.0
    d
    l
    = 4.0
    d
    l
    = 7.0
    d
    l
    Sensitivity (pF)
    0 0.2 0.4 0.6 0.8 1
    0.1
    0.2
    0.3
    0.4
    Void fractionInnovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 11
    Fig. (14). (a) Time trace signal, (b) PDF. (c) PSD and (d) Flow image for intermittent (slug) flow regime (GL1 = 605 kg/m
    2
    .s and x=0.0077).
    Fig. (15). (a) Time trace signal, (b) PDF, (c) PSD and (d) Flow image for elongated bubble flow regime (GL1 = 790 kg/m
    2
    .s and x=0.001).
    0 2 4 6 8 10
    0
    0.2
    0.4
    0.6
    0.8
    1
    Time (Sec)
    Void Fraction
    0 0.2 0.4 0.6 0.8 1
    0
    5
    10
    15
    20
    25
    30
    Void Fraction
    PDF (%)
    10
    1
    10
    2
    10
    3
    10
    4
    10
    -3
    10
    -2
    10
    -1
    10
    0
    10
    1
    Freqency (Hz)
    PSD
    1 in
    (a) (b)
    (c)
    (d)
    0 1 2 3 4 5
    0
    0.2
    0.4
    0.6
    0.8
    1
    Time (Sec)
    Void Fraction
    0.2 0.4 0.6 0.8 1
    0
    5
    10
    15
    20
    25
    30
    Void Fraction
    PDF (%)
    10
    1
    10
    2
    10
    3
    10
    4
    10
    -3
    10
    -2
    10
    -1
    10
    0
    10
    1
    F (Hz)
    PSD
    1 in
    (a) (b)
    (c) (d) 12 Recent Patents on Electrical Engineering, 2008, Vol. 1, No. 1 Ahmed and Ismail
    Fig. (16). (a) Time trace signal, (b) PDF, (c) PSD and (d) Flow image for annular flow regime (GL1 = 395 kg/m
    2
    .s and x=0.15).
    the sensitivity of both concave and ring type sensors. The
    sensitivity of the ring type sensor increases as the separation
    distance between the electrodes decreases, which also
    increases the spatial resolution of the sensor. The sensitivity
    of the ring type sensor is found to be higher than the concave
    type for the same spatial resolution. Both types of sensors
    were fabricated and tested in an air-oil flow loop. The void
    fraction predictions from the theoretical models for the
    sensor design are within 15% of the experimental data.
    Calibration of the sensors shows that the output capacitance
    is linearly related to the void fraction for both types of
    sensors. The volumetric average void fraction is independent
    of the flow pattern for both ring type and concave type
    sensors, which is useful to obtain accurate measurements of
    average void fraction. The capacitance sensors are very
    sensitive to flow regime change and can be used to identify
    the flow pattern. The mean value and the probability density
    function of the instantaneous void fraction signal is used to
    identify the flow regime. Both the slug and elongated bubble
    flow patterns are characterized by a double peak PDF with
    one peak at a high void fraction and one at a low void
    fraction. However, the time trace signal and the power
    spectral density (PSD) can be used to distinguish between
    the two flow regimes. The time trace signal shows that, for
    elongated bubble the fluctuations occur at a higher frequency
    than for slug flow, which is reflected in the power spectral
    density (PSD). A single sharp peak in the PDF at high void
    fraction characterizes the annular flow pattern.
    NOTATION
    A = Area
    D = Diameter (mm)
    E = Electric field
     = Void fraction
    l = Electrode length (mm)
    t = Pipe wall thickness (mm)
    T = Time (sec)
    V = Potential difference (volt)
     = Time constant (sec)
     = Dielectric constant
    SUBSCRIPT
    o = Refer to free space
    c = Charge
    P = Polarization charge
    L = Liquid
    G = Gas
    T = Total
    w = wall
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    paria - من نمیتونم دانلود کنم کتاب عایقو
    پاسخ: درود گرامی لینک دانلود بازیابی شد - 1394/10/11/t3tekn
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    اگه ميشه برای منم يكم اطلاعات درباره موتور شراگ همراه با عكسش ميخواسنم براي پروژه ممنون ميشم خواهشن بفرستين خيلي ضروريه

     

    - 1394/8/28/t3tekn
    mohsen - عالی میشه اگه محبت کنید برام ایمیل کنید - 1393/10/17/t3tekn
    رضا - سلام جناب مهندس خیلی برام جالب بود . اگر براتون مقدور بود باز هم از این نمونه آزمایش ها رو بزارید در صورت بودن فیلم و عکس واقعا ممنون میشم با تشکر از شما به خاطر وقتی که گذاشتید - 1393/9/23/t3tekn
    رضا - سلام جناب مهندس خیلی برام جابل بود . اگر براتون مقدور بود باز هم از این نمونه آزمایش ها رو بزارید در صورت بودن فیلم و عکس واقعا ممنون میشم با تشکر از شما به خاطر وقتی که گذاشتید - 1393/9/23/t3tekn
    mohamad - hi - 1393/9/12/t3tekn
    reza - ججهت تعمیرات این نوع موتور ما به شما خدمات میدهیم
    - 1393/8/8/t3tekn
    بامداد کریمی - سپاسگزارم موید باشین. - 1393/8/3/t3tekn
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    پاسخ: در ارسال های اینده منبع هم گذارده خواهد .سرافراز باشید. - 1393/5/25/t3tekn
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    سلام اگه ميشه يكم اطلاعات درباره موتور شراگ همراه با عكسش ميخواسنم براي پروژه ممنون ميشم خواهشن بفرستين خيلي ضروريه

    - 1393/1/30/t3tekn
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