Innovative Techniques For Two-Phase Flow Measurements

Transcription

1 Recent Patents on Electrical Engineering 28,, -3 Innovative Techniques For Two-Phase Flow Measurements Wael H. Ahme* an Basel I. Ismail** *Component Life Technology, Atomic Energy of Canaa Lt., Canaa, **epartment of Mechanical Engineering, Lakehea University, Canaa Receive: November 5, 27; Accepte: November 27, 27; Revise: November 3, 27 Abstract: Two-phase flow is important in an increasing number of applications an inustries. For example, the safety analysis coes for the nuclear energy inustry require closure relations for the vapor-liqui interfacial transfer terms, while accurate two-phase pressure rop moels are necessary to esign the piping systems in the oil an gas inustry. Also, two-phase flow occurs in heat exchangers, steam generators, chemical reactors, oil transportation an many other process equipments. In aition, evelopment of accurate an suitable instrumentations for on-line monitoring an measurement of the solis concentration an velocity in gas-soli two-phase flows has proven to be a challenging problem with many scientists an engineers worlwie eveloping novel techniques for this application. This paper presents a review on the electrical-base measurement techniques for gas-soli an gas-liqui two-phase flows along with the most recent patents evelope for two-phase flow measurements. Also, evelopment of a novel metho for the esign of capacitance sensors for voi fraction measurement an flow pattern ientification was presente in etail. Keywors: Two-phase flow, gas-soli flow, gas-liqui flow, novel esign metho, capacitance sensor, recent patents, voi fraction.. INTROUCTION Online, continuous, two-phase flow measurement is often necessary, particularly in the oil an gas, nuclear energy an chemical processing inustries. Reliable measurements of the voi fraction an flow pattern ientification are important for accurate moeling of two-phase systems. Voi fraction can be measure using a number of techniques, incluing raiation attenuation (X or -ray or neutron beams) for line or area average values, optical or electrical contact probes for local voi fraction, impeance technique using capacitance sensors an irect volume measurement using quick-closing valves. The use of the ifferent techniques epens on the applications, an whether a volumetric average or a local voi fraction measurement is esire. The raiation attenuation metho can be expensive an from a safety aspect ifficult to implement, while intrusive probes isturb the flow fiel. On the other han, the impeance measurement technique is practical an cost-effective metho for voi fraction measurement. The technique is non-intrusive an relatively simple to esign an implement. Impeance or capacitance sensors have been use successfully to measure time an volume average voi fraction, an its instantaneous output signal has been use to ientify the flow pattern []. In this paper, electrical-base techniques for gas-liqui an gas-soli flows measurements are reviewe. The paper is ivie into two parts: In the first part, gas-soli flow measurements with their important an relevant application to pneumatic conveying systems are iscusse. However, the etaile escription of ifferent techniques cannot be covere exhaustively here. Therefore, etaile consieration is focuse on the evelopment of a novel metho for the esign of a capacitance sensor use in *Aress corresponence to this author at the Component Life Technology, Atomic Energy of Canaa Lt., Canaa; Tel: +(63) ; Fax: +(63) ; ahmew@aecl.ca gas-liqui two-phase flow measurements in the secon part of the paper. 2. RECENT EVELOPMENTS IN TWO-PHASE FLOW MEASUREMENTS 2.. Gas-Soli Two-Phase Flows Gas-soli Two-phase flows, particularly those solis in the forms of power an grain transporte in gases (typically air) through a pipeline, are important in pneumatic conveying which has wiesprea applications across many inustries incluing chemical, foo processing, cement, mining, petrochemical, pharmaceutical, semiconuctor, an in transporting pulverize coal in fuel lines of thermal power plants. Moving fluiize-bes may also be regare as a form of gas-soli flows. This wie application has le to extensive research on gas-soli two-phase flow systems. Gas-soli flows (also known as particulate flows), such as those in pneumatic conveying systems, are usually classifie as ilute- or ense-phase flow. The ilute-phase flow can be normally achieve with low concentrations of solis (typically below %) an the simultaneous conition of high gas velocities in the gas-soli flow mixture [2]. In the ilute-phase conveying, the soli particles are fully isperse or suspene in the flow an no eposition occurs. The velocity at which solis are traveling in a ilute-phase conveying is important. If they are traveling too slowly, solis can rop out from the flowing suspension an pipe blockage can occur. If they are traveling too quickly this can lea to unnecessarily high pipeline wear an power consumption ue to system pressure rop [3]. Flow patterns in a ense pneumatic conveying system, however, show many interesting features an analogies with gas-liqui flows, such as slug, plug, an stratifie flow regimes [4]. ense-phase flow in pneumatic conveying systems have the relative benefits of a low air requirement an hence energy eman, low pipeline erosion an low prouct egraation /8 $ Bentham Science Publishers Lt.

2 2 Recent Patents on Electrical Engineering, 28, Vol., No. Ahme an Ismail [5]. However, ense-phase pneumatic conveying may lea to unstable flows, cause by insufficient air velocity, an eposition. These unstable flows an eposition often cause pipe blockage an vibration that can be extremely ifficult to remey. The funamental system components of a conventional pneumatic conveying system are: an air mover, a feeer for the solis material to be conveye, the conveying pipeline, an a receiver to isengage the conveye solis an carrier gas (typically air). Local solis concentration an velocity are two of the most important parameters characterizing gassoli flows. evelopment of accurate an suitable instrumentations for on-line monitoring an measurement of the solis concentration an velocity in gas-soli two-phase flows has proven to be a challenging task with many scientists an engineers worlwie eveloping novel techniques for this application. This is particularly ue to spatial an temporal fluctuations of both the solis concentration an velocity uring the pneumatic transportation [6]. The results of these measurements can be use to ensure efficient an economic pneumatic conveying operation, prevent blockage an hazarous amages an maintain safety stanars. This paper presents a review on a range of existing an recent techniques use to measure solis concentration an velocity in gas-soli flows, such as those wiely use in pneumatic conveying systems Measurement Techniques for Gas-Soli Flow Systems The measurements an visualization of gas-soli flows, such as those involve in pneumatic conveying systems, coul be performe using a range of noninvasive an nonintrusive electrical impeance, optical transmission [7], raiography an raiometric sensing [8], an other measurement techniques. The application of capacitance sensors for measuring local solis concentrations in a gas-soli flow was establishe ecaes ago [9, ]. Chang et al. [] teste strip- or ring-type capacitance transucers that coul be use in non-electrically conuctive (ielectric) pipes to nonestructively etect power flow or measure power thickness using the ielectric constant ifference of the gas an power. Capacitance impeance rings are noninvasive, simple an relatively inexpensive for implementation an provie high temporal resolution. However, those evices are ifficult to calibrate an care must be taken when using correlation moels for ifferent gas-soli flow patterns to convert electrical impeance to soli concentrations [2]. In a ense-phase pneumatic conveying where ensity graient exists in the gas-soli flow cause by a non-uniform istribution of solis in a pipe cross section, electrical capacitance tomography (ECT) can be effectively use [4; 5; 3]. In an electrical process tomography, the capacitancesensing electroes are installe at equal intervals aroun the periphery of the pipe, in orer to obtain the threeimensional istribution of solis along the irection of the gas-soli flow. The electroes are usually installe in a noninvasive way, i.e. outsie the pipe mae of ielectric material [4]. The process tomography or tomographic imaging technique has the potential of proucing crosssectional images of the istribution of flow components in a conveying pipeline from which solis velocity an concentration profiles of a section of a pipeline can be obtaine. Brown et al. [5] evelope a tomographic technique for imaging gas-soli flow istributions in pneumatic conveying pipelines. Their technique utilize ultrasonic transmission-moe measurements constraine to the megahertz region. They performe image reconstruction using a back-projection metho implemente with stanar graphics algorithms. Brown et al. [5] applie their technique to simulate reconstructions of ense an ilute istributions an obtaine results emonstrating the capabilities an limitations of their technique. They also aresse aspects of transucer array esign. Recently, Steiner et al. [6] propose a ual-moe ultrasoun an electrical capacitance process tomography sensor for measurements of gassoli flow. In their metho, the ultrasoun image is use as a priori information for the finite element metho-base capacitance reconstruction algorithm. They [6] reporte that a significant improvement in image quality can be achieve with the propose ual-moe tomography process sensor. Jiang an Xiong [7] introuce a novel noninvasive electrostatic metho for measurement of the solis velocity an mass flow rate in gas-soli flow. In their system, the measurement of the velocity an mass flow rate were realize by an electrostatic sensor capable of etecting the electrostatic signals of soli power in the gas-soli flow. The transit time of power was measure by cross-correlating electrostatic signals, an the velocity was calculate. By correlating the voltage on electrostatic sensor an the fixe mass flow rate, the voltage-mass flow rate curve of the sensing system was establishe. Jiang an Xiong [7] conclue that electrostatic metho can be use to trace an measure the velocity of the soli phase, an the metho is capable of performing on-line measurements of the mass flow rate of the gas-soli flow. 3. CAPACITANCE SENSORS FOR GAS-LIQUI TWO-PHASE FLOWS The principle of the capacitance metho is base on the ifferences in the ielectric constants of the two phases in the flow, an the capacitance measure across the sensors is epenent on the volume ratio of the two phases. There are, however, several isavantages of the impeance technique, which are sometimes ifficult to resolve. For example, the capacitance measurement is sensitive to the voi fraction istribution or flow regimes ue to the non-uniformity of the electrical fiel insie the measuring volume. This, however, can be compensate by first ientifying the flow pattern. The measurement is also sensitive to the changes in electrical properties of the two phases ue to temperature. The noise ue to the electromagnetic fiel aroun the sensor an connecting wires can significantly affect the signal an nees to be minimize through proper esign of the sensor shiel. The sensitivity of the capacitance meter is also improve by Anrae [8]. He use a full power supply potential swing on the shiel electroes to allow for the use of simpler shiel electroe esign. Using the full swing improves the capacitance backgroun an balances such noises as the wire capacitances. Furthermore, it is ifficult to resolve the change in phase istribution within the sensor measurement volume. The sensitivity to the flow pattern can be increase through better esign of the sensors. For example, Hung et

3 Innovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 28, Vol., No. 3 al. [9] use eight electroes along the circumference of the tube to obtain a tomographic image of the two-phases. On the other han, homogenizing the electric fiel in the axial irection minimizes the error ue to the voi istribution in the measuring volume. This coul be one by using rotating type sensors as suggeste by Merilo et al. [2] an Lucas an Simonian [2] or by using helical electroes as iscusse by Geraets an Borst [22] an Kenneth an Rezkallah [23]. The electrical capacitance tomography technique was also implemente recently by Gamio et al. [24] to image various two-phase gas-oil horizontal flows in a pressurize pipeline. They emphasize the potential of this technique for real-time flow visualization an flow regime ientification in practical inustrial application at high pressure operating conitions. Electrical Impeance Tomography (EIT) is also introuce in a recent patent by Wang [25] as a new signal processing metho. The metho is use on-line to obtain accurate estimates of the local isperse phase volumetric flow rate, the mean isperse an continuous phase volume fractions an the istributions of the local axial, raial an angular velocity components of the isperse phase. Several ifferent configurations of capacitance sensors, incluing flat plate, concave, ring, helical an multiple helical woun in contact or isolate from the flui, have been wiely investigate [23, 26-29]. However, there are fewer stuies on the optimization of the esign to obtain goo signal to noise ratio an high sensitivity to the ifferent flow patterns. Elkow an Rezkallah [23] compare the performance of concave an helical type sensors an etermine that the problems associate with helical type sensors, incluing the nonlinear response, poor sensitivity an poor shieling, can be eliminate by using the concave type sensors. The accuracy of the concave parallel sensors can be improve by having both electroes of equal length to ecrease the non-uniformity of the electric fiel between the two electroes an eliminate the non-linear response. Base on several tests, they also recommene that the istance between the electroes an the shiel shoul be large relative to the separation istance between the two electroes in orer to improve the immunity to stray capacitance. In this paper, a systematic metho for the esign of capacitance sensors for voi fraction measurement an flow pattern ientification is presente. Two ifferent configurations of the sensors are consiere: concave an ring type Fig. (). For the ring types sensor each electroe covers the entire circumference, except for a small gap to facilitate the installation of the sensor aroun the tubes, an are separate in the axial irection of the tube Fig. (a), while in the concave sensor, two brass strips are mounte on the tube circumference opposite to each other Fig. (b). The ifference in the electroe geometry results in ifferent electric fiels within the measurement volume an hence in the sensitivity an response of the sensors. The two geometries are analyze for the signal to noise ratio an the sensitivity to the voi fraction an flow pattern. Experiments were performe to valiate the esign theory an to evaluate the sensor characteristics in actual operating conitions using air-oil two-phase flow in a horizontal pipe. To more objectively ientify the flow pattern, the probability ensity function (PF) an power spectral ensity (PS) of the time trace of the voi fraction signal is analyze in a similar manner to that one by Jones an Zuber [3]. 4. CAPACITANCE SENSOR ESIGN The capacitance sensors nee to be esigne for accurate measurement of the voi fraction, an have sufficient time response to etect the variations for the ifferent flow patterns. The value of the capacitance across the sensors is relate to the phase istribution an ielectric properties of the two-phases. The relationship between the capacitance an the voi fraction is epenent on the ielectric values of the two phases, cross-sectional area of the sensors, an the separation istance between the two electroes. A ielectric material place between two conuctor plates acts as an insulator to increase the charge storage capabilities because the ielectric contains charge molecules that are ranomly oriente. When an external fiel is applie across the two plates, the charge molecules align themselves with the electric fiel an prouces ipoles, where the positive charges of each molecule are in the irection of the applie fiel an the negative charges oppose the fiel. An internal Elec (2) Elec () Elec () Capacitance meter Elec (2) Capacitance meter a) Ring type b) Concave type Fig. (). Schematic of electroes configurations.

4 4 Recent Patents on Electrical Engineering, 28, Vol., No. Ahme an Ismail electric fiel, opposite in irection of the external electric fiel, will result with a consequent reuction of the overall electric fiel an the overall potential. It is important to unerstan the theory of the capacitance sensor technique to properly optimize the sensor esign. For simplicity, this can be illustrate using a simple parallel plate capacitor to show the effect of a ielectric material on the capacitance. Neglecting ege effects, the uniform electric fiel between the two plates of the capacitor in vacuum space without the ielectric meium, with charge ensity f an the permittivity of free space, o = 8.854x -2 C 2 N - m -2, can be written as: E = f o () When a ielectric material is inserte to fill the entire space between the plates, the ielectric is polarize, an a polarization charge of ensity p appears on the two surfaces. For a uniform electric fiel out from a plane, the electric fiel can be written as: ( ) E = f + p o (2) The polarize charge ensity is: p = o E (3) where is the electric susceptibility of the material. Finally we can write the capacitance as: C = o ( )A (4) an the ielectric constant is efine as: = ( ) (5) The permittivity is sometimes use to characterize the ielectric behavior of the matter an is relate to the permittivity of the free space as = o (6) In orer to mathematically represent the output signal as a function of the voi fraction, the capacitance measure by a transucer for two-phase flow can be treate as an approximation of a parallel or series combination of capacitors with ifferent ielectric constants. In this work, the performance of two ifferent types of sensor configurations is investigate, an a esign metho to optimize the sensor performance is evelope. The sensor output is sensitive to the flow pattern an 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 consiere here. For horizontal pipes: bubbly, stratifie, slug, an annular flow are use an for vertical pipes: bubbly, slug, an annular are consiere. Churn flow is ifficult to simulate in our analysis. 4.. Ring Type Sensors If we consier only stratifie, annular an long slug flow patterns among the basic flow patterns liste before, the electric fiel between two ring electroes in free space can be schematically presente in Fig. (2a). Assuming the electrical fiel is shiele outsie the tube an neglecting the raial electric fiel, the istribution can be approximate to that shown in Fig. (2b). Therefore the electric fiel can be assume approximately constant in the axial irection an the two rings equivalent to parallel flat isks. The equivalent capacitance circuit metho can be use to analyze this problem []. In this metho, the two phases are moelle as series or parallel capacitors between the electroes. This equivalent circuit is base on the istribution of the two phases insie the channel an is illustrate in Fig. (3). By consiering the two electroes as two imaginary isks in parallel with a separation istance (), the two phases istribute horizontally will be equivalent to two capacitances in series Fig. (3a), while the two phases istribute in the vertical irection will be equivalent to two parallel capacitances Fig. (3b). The equivalent circuits for ifferent twophase istributions are shown Fig. (4a) through as iscusse by Chang et al. []. The theoretical output Imaginary planes Electroe Electroe Electroe 2 Electroe 2 a) Actual b) Assume Fig. (2). Schematic of electrical fiel profile generate by two ring electroes.

5 Innovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 28, Vol., No. 5 Ag AL x CL Cg Gas Liqui Cg CL a) Series circuit b) Parallel circuit Fig. (3). Capacitance circuit equivalent to two-phase flow istribution. CG a) Annular flow b) Stratifie flow c) Bubbly flow ) Slug Flow Fig. (4). Equivalent capacitance circuits for typical flow regimes (Aapte from Chang et al. []). capacitance as a function of voi fraction for annular, stratifie an long slug flow can be obtaine by consiering the equivalent circuit metho represente in Fig. (4a & b), an the total equivalent capacitance for both circuits can be calculate as: = C g + C L + C w (7) For annular or stratifie flow = K g A g = K g A g + K L A L ( ) + K L A K w g + 4 The voi fraction is efine as = V G V T + K w A w (8) (( + t) 2 ) 2 (9) = A G A T = A G A T () where V G an V T are the volume of gas an total volume respectively. By iviing equation (9) by the cross-sectional area of the pipe results in = 2 o 4 K g + K ( L )+ K + t 2 w () where, t is the pipe wall thickness an w is the ielectric constant of the pipe material Concave Type Sensors A similar analogy is use for the concave type sensors, however, an equivalent geometrical shape is introuce (Fig. 5a through c), where the change of the electrical fiel ue to the curvature of the electroe is neglecte [26]. Then, the total capacitance for stratifie or annular flow pattern can be written as:

6 6 Recent Patents on Electrical Engineering, 28, Vol., No. Ahme an Ismail 63.5 mm Cw a t CL2 z Elect() e f Cg CL CL,2 CL2 Elect(2) h Cw (a) (b) (c) Fig. (5). (a),(b) Geometrical simplification of the concave type capacitance sensor, (c) Equivalent capacitance circuits for annular an core flow regimes. = C W C L2 C g + C L + C L C W C L2 = (2) The equivalent geometrical parameters can be efine as ( 2 t) e = = ( 2 t) f = ( 2 ( 2 t )) a = 2 z 2 ( ) (3) Base on the two phases istribution shown in Fig. (5), an using the capacitance circuit analogy, the capacitances in equation (2) can be written as: C w = w o a L t C g = g o h L e C L = L o a L e C L2 = L o a L f (4) The above analysis may be use to estimate the capacitance of such a sensor by substituting the permittivity of the free space an the ielectric constant of the liqui as well as the geometrical parameters liste in equation (3) as a function of the voi fraction. The set of equations (2) to (4) relate the capacitance to the voi fraction an can be use to investigate the sensitivity an/or to compare ifferent esigns esign Metho Applie to ifferent Flow Patterns Although the theoretical moel iscusse above consiere only stratifie an annular flow, the analysis coul be extene to other flow patterns in orer to esign an optimize the capacitance sensor. As an example, plug flow is consiere, an in particular the case where an elongate bubble with a length l G in a horizontal pipe is passing through a pair of ring type sensors as shown in Fig. (6a). The bubble can be assume as a horizontal cyliner with cross sectional area AG an occupying a length l between the two electroes. Assuming a constant electric fiel between the electroes, the equivalent capacitance circuit using the same analogy as before is shown in Fig. (6). In this case = C g + C L C g C L + C L2 + C w (5) an the volumetric voi fraction is ifferent from the cross sectional voi fraction an can be calculate as = V G V T = A G l G A T The capacitances in equation (5) can be written as w o (( + t) 2 ) 2 C w = 4 C L2 = L o ( A t A g ) C L = L o A g l (6) C g = g o A g l (7) The corresponing relation between the voi fraction an the output signal of the capacitance sensor in the case of elongate bubble can be written as Elect2 Elect t CW AG AL Gas Liqui CG CL CL2 l (a) Elongate bubble simulation b) Equivalent circuit Fig. (6). a) Geometrical simulation of elongate bubble in ring type sensor. b) Equivalent capacitance circuit.

7 Innovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 28, Vol., No. 7 = o 2 4 w + + L t l + l 2 + g l 2 l, L = 5.8 mm Elct() A 63.5 mm where l >. The above analysis can be extene to ifferent flow regimes, where the level of complexity of the analysis epens on the complexity of the flow regime an the require accuracy of the moelling. 5. ESIGN AN CHARACTERIZATION OF THE SENSORS Base on the theoretical analysis, ring an concave type sensors were esigne an constructe for a tube having insie an outsie iameter of 2.7 an 5.8 mm, respectively. The schematic iagrams with the basic imensions of the sensors are shown in Figs. (7 & 8). The ring type sensor consists of two pairs of active electroes mae of 5 mm brass strips space 2 mm apart to increase the signal to noise ratio by amplifying the absolute value of the capacitance circuit. This geometry provies sufficient volumetric resolution to resolve the characteristics of the slug flow regimes. Two separate half hollow cyliners mae of acrylic are use for housing the electroes as show in Fig. (7a & b), an the unit is shiele using a.5 mm thick groune brass electroe to eliminate stray capacitance between any of the electroes, circuit an the wires. The housing is sanwiche together over the test section pipe an fastene using acrylic screws to make sure the electroes are firmly in contact with the tube surface. The noise on the signal ue to the surrounings an orientation of the connection wires was eliminate in the present esign by increasing the separation istance between the electroes an the shiel relative to the separation istance between electroes as suggeste by Elkow an Rezkallah [23]. The length of the electroe for the concave type sensor is taken as mm, an both electroes are equal in length to ecrease the nonlinearity in the sensor response as recommene by Elkow an Rezkallah [23]. Two pairs of electroes were use with spacing of 6 mm to have the same spatial resolu-tion as the ring type for comparison. In the experimental work reporte here, the istance between the electroes an the shieling was experimentally optimize to reuce the noise ue to groun current an electric fiel leakage. The output capacitance from the sensors is measure using a Boonton 72B capacitance meter operate at an excitation frequency of MHz. The accuracy of the meter is + 4% in the pf range with meter resolution of at least. of the range. The analog output voltage signal from the capacitance meter is sample using a 6-bit A/ converter at a sampling rate up to 2 KHz. The sensors were calibrate over the full range of voi fractions to verify the theoretical esign approach. A static calibration was performe using simulate stratifie an annular flow patterns, an a ynamic in situ calibration was performe to investigate the effect of the noise from the surrounings such as from other operating evices on the output signal of the sensors. It shoul be note that the effect Elct(2) A = 5 mm 5.8 mm a) Schematic of the ring type capacitance sensor. b) Soli moel. Fig. (7). Ring type capacitance sensor. z Elect() Elect(2) Sec A-A 63.5 mm Fig. (8). Geometrical simplification of the concave type capacitance sensor. of the operating temperature was taken into consieration in the calibration tests. The calibration tests were use to compare the performance of the two types of sensors base on better sensitivity for the same spatial resolution as escribe in etails by Ahme [3]. 5.. Sensor Calibration Canvas base Phenolic ros with the same ielectric constant as oil (=5.45) are use to represent the liqui in the static calibration at ambient temperature. The soli ros with

8 8 Recent Patents on Electrical Engineering, 28, Vol., No. Ahme an Ismail a iameter equal to the insie pipe iameter of the test section (2.7 mm) are machine to mimic ifferent flow regimes. The lengths of these pieces are the same an equal to 3 cm. Calibration was carrie out for two ifferent flow patterns, namely annular an stratifie flow. To represent annular flow, holes of ifferent iameter were rille through the center of each piece. Nine pieces were generate to simulate ifferent voi fractions. For stratifie flow, eleven separate sections were mille off the ro. The value of voi fraction for each piece can be calculate using the geometry. A comparison between the results from the theoretical moels an the experimental ata for the off-line calibration is presente in Figs. (9a & b) for both ring an concave sensors. The experimental results show that the relation between the capacitance an voi fraction for both types of sensors is approximately linear. The eviation between the moel an experimental results is ue to the assumption of Capacitance (pf) Capacitance (pf) Voi fraction (%) a) Static calibration for ring type capacitance sensor Moel Annular simulation Stratifie simulation Moel Annular simulation Stratifie simulation Voi fraction (%) (b) Static calibration for concave type capacitance sensor. Fig. (9). Static calibration for capacitance sensors. perfect shieling an also ue to a non-uniform electrical fiel between the electroes, with the iscrepancy being higher for the concave type sensor. This is because the assumption of a constant electric fiel between the electroes is more applicable for the ring type sensor than for the concave type. The non-linearity of the moel results for the concave type sensor is ue to the electrical circuit analogy which gives rise to a series capacitance circuit, where the total capacitance is a non-linear function of the iniviual capacitances. For the ring type sensors, the total capacitance between the electroes is a summation of capacitances in parallel. The calibration ata showe a correlation coefficient of.928 an a precision error of.4 with 95% confience level for the ring type sensor, while the agreement between the experiments an the theory for the concave sensor was within 8%. Measurements were performe with the sensors installe on the air-oil two phase flow loop which iscusse in etails by Ahme [32]. The ensity an viscosity of the oil use are kg/m 3 an.26 Pa.s, respectively. The air an oil are then mixe through an annular mixer. The oil flows on the outsie of an inner perforate pipe, while the air flows in the inner pipe an enters the oil stream through mm iameter perforations. The air-oil mixture passes through the horizontal test section, which has a total length of 3.5 m. Tests were performe with single phase oil (a=) an single phase air (a=) to check the range of the output signal of the sensors. A number of tests were performe for stratifie flow by ajusting the epth of the oil in the pipe to check the linearity of the capacitance-voi fraction relation. Results show that the sensitivity is the same for both the in situ an static calibrations, however, the absolute values of the capacitances at zero an % voi for the two cases are off by 5 to %. Therefore, the calibration curves were checke against the ynamic results before the experimental ata were taken. Repeatability tests were also performe an the results inicate that the variation in voi fraction is between 2% to 3.5%. The capacitance measurements in the air-oil flow tests are in the range. to 5 pf, an proper shieling against stray capacitance is require to obtain a goo signal to noise ratio (SNR). The range of the noise frequencies ue to the equipment an electrical evices near the test rig were etermine using a frequency analyzer an was foun to be lower than Hz in this case. The electronic circuit was moifie to eliminate the environment noise using a high pass filter. The voi fraction can be relate to the normalize capacitance (C n ) efine as: C n = C alloil C measure alloil allair (8) The correlation between the voi fraction an normalize capacitance is foun by applying a linear regression to the calibration ata for the ring type sensor an is given by, =.756 C n (9) with a correlation coefficient of.95.

9 Innovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 28, Vol., No Time Constant 5 The efinition of the time constant of the capacitance sensor here is referre to the time interval require for the sensor-meter to change 7% from one state or conition to another. The time constant for both types of capacitance sensors was obtaine experimentally by applying a unit step signal an recoring the sensor response Fig. (). The time constant for both types of capacitance sensors using equation (2) is foun to be approximately 4 μs, corresponing to a ynamic response of 25 khz. Fast Fourier transform analysis of the current flow fluctuations in the present stuy showe that the major fluctuations in the voi fraction were below khz, thus the response of the sensors is more than aequate Sensitivity (pf) 5 Elct( Elct(2) Y t Y intial Y max Y intial = e T 5.3. Sensitivity The sensitivity of the sensor can be efine as: (2) / Fig. (). Effect of electroe spacing on the sensitivity of ring type sensor. Senistivity = C allliqui C allair = C allliqui C allair allliqui allair (2) an nees to be maximize. The sensitivity epens on the geometrical shape an gap between the electroes, which also affects the spatial resolution of the sensor. The effect of the esign parameters such as the electroe with an spacing were stuie using equations () an (2) for both sensors. For the ring type sensor, the main imension that affects the sensitivity is the spacing between the electroes (). The results show that the sensitivity increases as the spacing ecreases Fig. (), 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 electroe separation (z) ecreases Fig. (2a), an the electroe length increases. However, increasing the electroe length leas to a poor spatial resolution as shown in Fig. (2b). In general, the sensitivity of the ring type sensor is foun to be higher than the concave type for the same spatial resolution. The sensitivity of the ring sensor was foun to be approximately.75 pf, while for the concave sensor it was.6 pf. Sensitivity (pf) z Elect( Elect(2 z/ (a)effect of electroe separation. Volt (v) 2 Sensitivity (pf) z Elect( Elect( Time (µs) Fig. (). Capacitance Sensor response to an input step function (b) Effect of electroe length. Fig. (2). The effect of sensor imensions on the sensitivity for concave type sensor. z/

10 Recent Patents on Electrical Engineering, 28, Vol., No. Ahme an Ismail The effect of the flow regime on the sensitivity of the capacitance sensor can b estimate. For example, in the case of elongate bubbles, the sensitivity can be calculate using equation (8) along with equation (2). The sensitivity as a function of voi fraction for ifferent bubble length to electroe spacing length ratios for an electroe spacing of mm is shown in Fig. (3). It shoul be note that the sensitivity is also affecte by the electroe spacing an for the ring type sensor an electroe spacing of less than 2 mm is require for a high sensitivity. For this electroe spacing, the effect of ( l X ) is foun to be neglecte, which means that the sensor gives an approximate volumetric voi fraction equal to the cross sectional value for bubbly flow. Sensitivity (pf) l =.7 l =.6 l = Voi fraction Fig. (3). The sensitivity of ring type sensor in case of elongate bubble flow Measurement Uncertainty The total flow rate through the test section was maintaine within +2% of the average value while the voi fraction measurements were taken. The main source of uncertainty in the voi fraction measurement was the noise in the sensor signal from the surrouning equipment such as the oil pump an the biases of the voltage signal, sensor spacing an position. This uncertainty was calculate to be in the range of +6% over the entire range of voi fraction. 6. FLOW PATTERN IENTIFICATION Flow pattern ientification can be performe either by visual inspection of the flow in a transparent pipe or by measuring an quantifying the fluctuations of the flow parameters such as voi fraction (Jones an Zuber [3]) or ynamic pressure (Keska an Williams [33]), which reflect the flow structure. The flow pattern recognition from the signal fluctuations can be one by analyzing the probability ensity function (PF) or power spectral ensity function (PS) of the time trace signal [3]. In the present stuy, the applicability of the capacitance sensors for flow pattern ientification was investigate. Only the ring type sensor is consiere here since it has a better sensitivity than the concave type for the same spatial resolution. The time average voi fraction an the probability ensity function (PF) of the voi fraction signal is use in this instance to ientify the flow pattern. The flow regimes are also obtaine using a high spee camera for valiation. Three main flow regimes an their corresponing PF istributions are iscusse here for two-phase air-oil through a horizontal pipe. These three basic flow regimes are: elongate bubble, slug an annular flow. All the ata presente was collecte at 2 khz over a perio of 5 sec. Figures (4a,b & c) illustrate the time trace, PF an voi fraction signal respectively for a typical slug flow while an image of the corresponing flow pattern using the high spee vieo system is presente in Fig. (4). The time trace is characterize by an intermittent voi fraction signal that fluctuates between a high an low value. The high voi fraction (a=.29) correspon to the passage of a long gas bubble, while the low voi fraction inicates the passage of the liqui slug. Small bubbles in the tail of the long bubbles gives rise to the low voi fraction (a=.4). Although the capacitance sensor measures the volumetric voi fraction over a tube length of.65, the sensor seems to be sensitive to bubbles that are entraine in the liqui slugs as can be seen in the time trace signal. In orer to istinguish between two signals having ouble peaks on the PF iagram, the power spectral ensity (PS) was use. In this case, aitional information about the flow structure was obtaine such as the bubble an liqui slug frequencies. The power spectral ensity (PS) for the slug flow signal shown in Fig. (4c) is characterize by two ominant frequencies, corresponing to the liqui slug an the gas bubble frequencies. In Fig. (4c), the slug frequency is approximately 8 Hz, while the frequency of the small bubbles is 228 Hz. For elongate bubble an slug flows, a ouble peak PF is obtaine, one at low voi fraction corresponing to the liqui slugs an the other at high voi fraction ue to the gas slugs. The time trace an PS in this case are use to istinguish between the two cases. For elongate bubbly flow, the voi peaks on the time trace signal occur at a higher frequency than for slug flow (Fig. (5a) through ). This is inicate in the power spectral ensity (PS) of the signal where a higher frequency of 394 Hz is obtaine for the case of elongate bubble flow Fig. (5c), while a frequency of 228 Hz is obtaine for the case of slug flow Fig. (4c). The PF for annular flow is characterize with a single peak at high voi fraction as shown in Fig. (6b), while the time trace shows small oscillations aroun a high mean value of voi fraction (a=.54) inicating the unsteay surface waves of the liqui film. It shoul be note that, as the peak in the PF becomes narrower, the liqui film thickness becomes more nearly constant. 7. CURRENT & FUTURE EVELOPMENTS Electrical capacitance is an innovative technique for measurement of two-phase flows. In this paper, a review incluing most recent patents evelope for two-phase flow measurements using capacitance-base sensor was presente. Also, in this work, a novel systematic metho for the esign of capacitance sensors for voi fraction measurement an flow pattern ientification was evelope an iscusse in etail. Two types of capacitance sensors, ring an concave type, have been analyze. The phase or voi istribution is represente by an equivalent capacitance circuit to estimate

11 Innovative Techniques For Two-Phase Flow Measurements Recent Patents on Electrical Engineering, 28, Vol., No. 3.8 (a) 25 (b) Voi Fraction Time (Sec) PF (%) Voi Fraction (c) () PS Freqency (Hz) in Fig. (4). (a) Time trace signal, (b) PF. (c) PS an () Flow image for intermittent (slug) flow regime (G L = 65 kg/m 2.s an x=.77). Voi Fraction (a) PF (%) (b) Time (Sec) (c) () Voi Fraction PS F (Hz) in Fig. (5). (a) Time trace signal, (b) PF, (c) PS an () Flow image for elongate bubble flow regime (G L = 79 kg/m 2.s an x=.).

12 2 Recent Patents on Electrical Engineering, 28, Vol., No. Ahme an Ismail 3 Voi Fraction (a) PF (%) (b) 5 Time (Sec).5 Voi Fraction (c) () PS Frequency (Hz) in Fig. (6). (a) Time trace signal, (b) PF, (c) PS an () Flow image for annular flow regime (G L = 395 kg/m 2.s an x=.5). the sensitivity of both concave an ring type sensors. The sensitivity of the ring type sensor increases as the separation istance between the electroes ecreases, which also increases the spatial resolution of the sensor. The sensitivity of the ring type sensor is foun to be higher than the concave type for the same spatial resolution. Both types of sensors were fabricate an teste in an air-oil flow loop. The voi fraction preictions from the theoretical moels for the sensor esign are within 5% of the experimental ata. Calibration of the sensors shows that the output capacitance is linearly relate to the voi fraction for both types of sensors. The volumetric average voi fraction is inepenent of the flow pattern for both ring type an concave type sensors, which is useful to obtain accurate measurements of average voi fraction. The capacitance sensors are very sensitive to flow regime change an can be use to ientify the flow pattern. The mean value an the probability ensity function of the instantaneous voi fraction signal is use to ientify the flow regime. Both the slug an elongate bubble flow patterns are characterize by a ouble peak PF with one peak at a high voi fraction an one at a low voi fraction. However, the time trace signal an the power spectral ensity (PS) can be use to istinguish between the two flow regimes. The time trace signal shows that, for elongate bubble the fluctuations occur at a higher frequency than for slug flow, which is reflecte in the power spectral ensity (PS). A single sharp peak in the PF at high voi fraction characterizes the annular flow pattern. NOTATION A = Area = iameter (mm) E = Electric fiel = Voi fraction l = Electroe length (mm) t = Pipe wall thickness (mm) T = Time (sec) V = Potential ifference (volt) = Time constant (sec) = ielectric constant SUBSCRIPT o = Refer to free space c = Charge P = Polarization charge L = Liqui G = Gas T = Total w = wall REFERENCES [] Lowe C, Rezkallah KS. A capacitance sensor for the characterization of microgravity two-phase liqui-gas flows. Meas Sci Technol 999; : [2] Boothroye RG. Flowing gas-solis suspensions. Chapman & Hall, Lonon, 97. [3] Green RG, Thorn R. Sensor systems for lightly loae pneumatic conveyors. Power Technol 998; 95: [4] Jaworski AJ, yakowski T. Application of electrical capacitance tomography for measurement of gas-solis flow characteristics in a pneumatic conveying system. Measurem Sci Technol 2; 2: 9-9. [5] Ostrowski K, Luke SP, Bennett MA, Williams RA. Real-time visualization an analysis of ense phase power conveying. Power Technol 999; 2: -3. [6] Sen S, as PK, utta PK, Maity B, Chauhuri S, Manal C, Roy SK. Pc-base gas-solis two-phase mass flow meter for

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