DECHAMP, L

JANSSENS-MAENHOUT, G

English

Ghent University Academic Bibliography

EJTPFGM 18/09/03GJM 1/8EUROPEAN-JAPANESE TWO-PHASE FLOW GROUP MEETINGCertosa di Pontignano, 21-27 September 2003VISUALISATION OF THE BUBBLE DETACHMENT AT A DIP TUBEG. Janssens-Maenhout, L. DechampJoint Research Centre IspraVia Fermi, 1, I-21020 ISPRA, ItalyABSTRACTIn accountancy tanks of reprocessing facilities the pneumercator known as dip tube techniqueis commonly used for level measurements, because the pneumercator delivers accuratemeasurements and the dip tubes are robust and easy in maintenance. The traditionalpneumercator consists in three dip tubes of steel with a flat end and different lengths, allconnected to the same air flow supply. The liquid level is measured by means of thehydrostatic head at the bubbling tip of the immersed dip tube.The internal diameter has a major impact on the pressure build up on one hand and on theclogging of the tips on the other hand. Blockage or plugging of the dip tubes usually occurs intanks with highly concentrated solutions. Therefore a regular check of the operation of the diptube at very small gas flow rate is performed. Under these conditions, hereafter called slowbubbling mode the bubble formation and especially the bubble detachment gives detailedinformation on the status of the tip. As the dip tube starts clogging, smaller bubbles aredetaching with a higher frequency. This can only be visualized by the pressure signal if thedetachment frequency is low enough. For carrying out this check in slow bubbling mode thepneumercator has to be taken out of normal operation for a longer period of time. Analternative method for checking the tip’s status is by recording the bubble detachment in thenormal fast operation mode with an electrical conductivity signal. Both methods are evaluatedin this paper.The bubble formation and detachment have been experimentally investigated for slow andfast mode by analysing the traditional pressure signal and the alternative electricalconductivity signal, acquired with a data rate of 10kHz and in synchronisation with camerapictures. In the case of slow bubbling mode the pressure signal yields most information on thebubble detachment, whereas in the case of fast bubbling mode the electrical conductivity is tobe preferred. Influence of the supplied air flow rate, of the solution’s salt concentration anddensity and of the tip’s depth have been taken into account.1. INTRODUCTIONFor safeguards reasons all nuclearmaterial in a nuclear facility has to beaccounted for and inspected by Euratomor the IAEA. A reprocessing plant treatsthe nuclear material in different forms(solid, liquid or vapour) and the inventoryis made at well-defined key measurementpoints, such as the accountancy tank forthe input and output solution. Eachinput/output accountancy tank is equippedwith a pneumercator system to monitor thevolume and density of the nuclear solution.The traditional configuration of apneumercator system at a reprocessingtank consists in three dip tubes of steelwith a flat end and different lengths. Theexternal diameter of each dip tube istypically 10mm and the internal diameter6mm. The level dip tube is the longestone, the density dip tube ends on a well-defined probe separation distance Sabove the level dip tube and the referencedip tube never dips into the solution. Inmost cases absolute pressures at the diptube’s bubbling tip, respectively pL, pD andpR are measured by three separatepressure transducers, although also thedifferential configuration exists where twopressure transducers measure pL-pR andpL-pD. The density rL is then derived byEJTPFGM 18/09/03GJM 2/8(pL-pD)/(gS), and the level l by (pL-pR)/(rg),and the volume is derived with themeasured level and temperature, utilisingthe calibration curve of the tank V(l, T).The flat shape of the tip of the dip tube hasbeen selected in most cases, since studieson other shapes of the dip tube's tip, suchas performed by Uchikoshi (1996), did notshow a clear advantage.The air bubbling causing up and downmovements of the solution in the lowestpart of the dip tube can cause drying out ofthe thin liquid films. Hence precipitation ofsalts of the concentrated solution occursand solid particles deposit on the internalwall of the dip tube. Clogging of dip tubesis partially anticipated by installation ofinstrument air humidifiers after the air flowmeter on the air supplying line. Howeverthe status of the dip tube's tip remains tobe surveyed in order to keep the pressuremeasurements accurate.This paper investigates various methodsfor checking the tip’s status by recordingthe bubble detachment in slow, mediumand fast operation mode. Three types ofsignals are utilised to experimentally investthe detachment of a bubble at the diptube’s tip.2. BUBBLE DETACHMENTPHENOMENA2.1  Experimental SetupThe experiments have been carried out inan open plexi-glass cylinder of 280mminternal diameter and filled up with tapwater up to 450mm height. The water bathwas located in an air-conditioned room at20ºC. A steel dip tube with internaldiameter of 6mm and external diameter of10mm, which was insulated by apolyethylene cover of 3mm thickness, wasdipping into the water. Normal dry air wassupplied to the dip tube with an air flowrate varying between 3l/hr and 15l/hr.Fig. 1: Experimental Setup for bubble detachment measured by pressure and electricalconductivity signal.An electrical circuit has been constructedwith an external resistance of 1.5kW, asindicated by the sketch of the experimentalsetup in Fig. 1. The electrical conductivityof the water was estimated to be 50mS/cm.As long as the water was in contact withthe inner steel tube, the voltage measuredover the resistance was maximal. Once abubble was formed with a diameterexceeding 10mm, the voltage droppeddown dramatically.The pressure on the dip tube wasmeasured by a small pressure transducer,based on the principle of strain changes ina Wheatstone bridge configuration. Thispressure transducer was operated with adata acquisition rate of 10 kHz and anaccuracy of 2Pa. The performance of thepressure transducer, especially thestability and reproducibility of themeasurements, was checked against thewell performing Digital Pressure Module1,which measures the pressure based onthe vibrating cylinder principle.The voltage related to the electricalconductivity was visualised by anoscilloscope2 with an acquisition rate up to10kHz and a relative accuracy of 1%. Thedetachment of the bubble could befollowed accurately by the very steepslope of the electrical signal without delay.However if the bubble was detaching                                                          1 The Digital Pressure Module is manufactured by theWeston Aerospace Co.2 The Gage Scope System of the TektronixTechnology Co. was utilised as software forcontrolling the oscilloscope and data-acquisitionboards.EJTPFGM 18/09/03GJM 3/8before reaching the size with meandiameter of 10mm, the bubble was notdetected. The internal PC clock has beenused to allocate the time for bothmeasurements and to synchronise theelectrical signal with the pressure signal.2.2  Modelling of the BubbleDetachmentThe pressure behaviour allows to followthe bubble formation and detachment atthe tip of the dip tube. The bubbleformation is traditionally visualized by asmooth pressure increase, whereas thebubble detachment corresponds to anabrupt pressure drop (see Fig. 1). Thisyields the typical pressure signal of Fig. 2,an example recorded with the pressuretransducer.Fig. 2: Typical signal for the pressure recorded at the tip of a bubbling dip tube, for differentgas flow rates.Both the amplitude as well as thefrequency of the periodical pressure signaldepend on the gas flow rate and on themaximum bubble volume. The influencingparameters of the gas-liquid system arethe liquid properties with density rL, withdynamic viscosity m and with surfacetension s, the dip tube's geometry withinternal diameter d and external diameterD, and with the supplied volumetric gasflow rate Q. According to Georgescu(2001) the maximum bubble volume V,which is reached at the moment ofdetachment can be well derived with themean detachment bubble diameter or so-called Sauter diameter SB in therelationship of Tate (1864):sprr BGL SgV =- )( (1)if a quasi-static detachment occurs. Thisequation expresses that under quasi-staticconditions, which are only fulfilled in thelimited case of extreme slow bubblingmode, the Archimedes force and thesurface tension force are in balance at thepoint of detachment.With the coordinates as given in Fig. 1 thebubble at its maximum size can beassumed to be an ellipsoid, which issymmetrically around the vertical axis withwidth DB and with height z With thebalance of Eq. 1 the height or distance zbelow the dip tube's tip is determined by( ) srr BGLB SgzD 62 =- (2).The height z can be measured by themaximum pressure variation. If themaximum bubble width DB is assumed toreach the outer diameter of the dip tube Dwithout depassing it, then the Sauterdiameter SB can be estimated with Eq. 2and the volume V can be calculated withEq. 1. The derived maximum bubblevolumes for the different pressuremeasurements are in agreement with theresults of Platzer et al. (1985). The quasi-static assumptions are acceptable.EJTPFGM 18/09/03GJM 4/8Of course the bubble detachment is adynamic process, and in particular aninstability problem. The form of the bubblecan be calculated by considering thevelocity potential of an irrotational liquidflow around the expanding bubble. Sincethe flow is symmetric around the dip tube'saxis, we concentrate on the normalcomponent of the velocity potentialnvn ¶¶=j . The gas velocity at the tip ofthe dip tube is determined by:( )24 dQu p=             (3).After introducing the following scaling:2,2,'2****dzzdtutuvvdglpp nnL===-=sr(4)and applying the Bernoulli equation for thedimensionless normal component of thevelocity potential j* yields for thedimensionless Sauter diameterdSS BB =*  the following four terms:).5(221122**22*******nCatWezEopSB¶¶+÷÷øöççèæÑ+¶¶++=jjjThe first term on the right hand side of Eq.5 describes the ratio of the inertialpressure force to the surface tensionforce, the second term indicates thegravity importance against the surfacetension force with the Eötvös Number,defined as ( )sr 22gdEo L= . The thirdterm addresses the instability problem withthe Weber Number given bysr duWe L2= and the last termdescribes the ratio of the viscosity forcesto the surface tension forces with theCapillary Number given by smuCa = .For the investigated case of an air-watersystem and the typical gas flow ratevarying from 3l/hr to 15 l/hr through a diptube with internal diameter of 6mm theorder of magnitude for the Eo, We and Canumbers is given in Table 1.d (mm) Q (l/hr) Eo We Ca6 3 2.26 0.07 0.000406 9 2.26 0.64 0.001216 15 2.26 1.78 0.002024 3 1.00 0.24 0.000904 9 1.00 2.16 0.002734 15 1.00 6.01 0.00454Table 1: Eötvös, Weber and Capillary number for an air bubbling dip tube in water withdiameter d and gas flow rate Q.Under the conditions considered in thispaper, the Capillary Number is so smallthat the last term in Eq. 5 can beneglected and the problem can be treatedin the inviscid limit.As already noticed by Rayleigh (1899)sufficient time must be allowed for theformation of the bubbles. If the time is notsufficiently long, instead of bubble bybubble release, one observes theformation of a bubble jet. The transitionfrom single bubbling to jetting appears tooccur at the critical Weber number WeC.This has been investigated for liquid-liquidsystems by Clift (1978) and for liquid-gas(droplet) systems by Clane (1999). Theinstability problem at the tip of a hangingtube for the droplet system differs from theone for the bubble system because theRayleigh-Taylor instability and thetherewith related necking is absent in thelatter case.However concerning the transition frombubbling to jetting and back to bubbling asimilar hysteresis effect was observed asin the case of transition from dropping tojetting and back to dropping. After abubble is released from the tube, the edgeof the remaining liquid after the pinchingd (mm) D (mm) We WeC4 8 2.16 76 10 0.64 56 12 0.64 8Table 2: Weber number and critical Weber number for a bubbling dip tube in water withinternal diameter d, external diameter D and an air flow rate of 9 l/hr.EJTPFGM 18/09/03GJM 5/8off begins to recede under the strong pullof surface tension forces. For the bubblesystem a longer period of interfaceoscillation is observed than for the dropletsystem. The dynamics of recession canhowever in both cases be described withthe extension model of Taylor (1959) forthe recessing motion of a rim. Based onthis model Clane derived the criticalWeber Number as a function of theclassical Eötvös number, built with theinternal diameter and the modified Eötvösnumber built with the external diameter ofthe dip tube D. The order of magnitude ofthe critical Weber number for the abovementioned case has been given in Table2. The normal operation conditions lead toa Weber number, which is smaller than thecritical Weber number. Therefore no jettingis further considered here.3. VISUALISATION OF THE BUBBLEDETACHMENTWith a high speed camera a sequence ofbubble formations and detachments hasbeen analysed for slow bubbling modeand the recorded pressure signal for onebubble has been interpreted. In Fig. 3 anexample is given of the pressure recordedwith 10 kHz for one bubble, from formationtill detachment, as shown in the camerapictures, taken with a rate of 30 frames/s.Fig. 3: Visualisation of a bubble formation sequence until detachment with camera.inside and outside at the exit or tip of thedip tube. Then the dynamic bubble growthis gradually performed by a steadilyincreasing pressure build-up with smalldynamic steps, which may correspond tothe bubble oscillation. The pressureincrease corresponds to the expansion ofthe gas/liquid-interface downwards the diptube’s tip. After the maximum height z (forthe given air flow rate) is reached, thebubble starts enlarging (with increasingDB), increasing its volume V withoutenhancing the mean internal pressure. Inthe contrary the bubble contracts in thevertical direction slightly to expand more inthe horizontal direction and a pressuredecrease is noticed. The bubble with alarge width DB becomes instable, and thebubble detachment is finally induced.4. MEASURED PRESSURE ANDCONDUCTIVITY SIGNAL4.1  Comparison of both signalsAs presented in the previous section thepressure signal clearly records thecomplete bubble dynamics in slowbubbling mode, i.e. for small gas flow rate.However Fig. 2 shows that for increasinggas flow rate the pressure signal is not anymore representing a reliable, reproduciblepicture of the bubble behaviour, despite ahigh recording rate of the measurements.EJTPFGM 18/09/03GJM 6/8Fig. 4: Pressure and conductivity signal for gas flow rates of 9l/hr (A), 4 l/hr (B) and 13l/hr (C).EJTPFGM 18/09/03GJM 7/8At high air flow rate with a high bubbledetachment frequency, the so-called fastbubbling mode, the phenomena of necking anddetaching or pinching off can not precisely beindicated. Instead the electrical conductivitysignal still allows to follow the detachment fromthe external, electrically isolated tube anddetects all pinching off points as represented inFig. 4.4.2  Sensitivity of ConductivitySignal on the Gas/LiquidProperties.Not only the gas flow rate but also the saltconcentration of the liquid solutioninfluence the electrical conductivity signal.The impact of both parameters on therecorded conductivity signal has beenevaluated and the results are representedin Fig. 5. The sensitivity study yields thefollowing validity range for the electricalconductivity signal: solution concentrationsof 0.1g/l<c<1g/l and gas flow rates7l/hr<Q<12l/hr. Under these operationconditions the conductivity measurementsindicated more precisely the bubbledetachment or pinching off points thanpressure measurements.5. CONCLUSIONTwo methods for checking the status of adip tube’s tip, by pressure recording andby conductivity recording, have beenexperimentally investigated under slow,normal and fast bubbling mode. Althoughthe pressure signal visualises clearly thedynamic bubble growth with gas/liquidinterface oscillations in the slow bubblingmode, the signal can not longer beinterpreted unambiguously in the fastbubbling mode. Instead the electricalconductivity signal detects all pinching offpoints.Therefore the electrical conductivity signalcan be utilised to follow the bubbledetachment at dip tubes with an air supplyof 7l/hr<Q<12l/hr and immersed inaqueous solutions with a concentration of0.1g/l<c<1g/l. This alternative method byrecording the electrical conductivity will beapplied to analyse the pressure signal infast bubbling mode. The meandetachment bubble diameter andcorresponding detachment frequency,which can be unambiguously determinedfrom the conductivity signal allows to setupa bubble growth model at the tip of a diptube. This can be utilised to interpret thepressure signal in a reprocessing plant infast bubbling mode.6. REFERENCESClanet, C. and Lasheras, J. C., TransitionForm Dripping to Jetting, J. Fluid Mech.,383, pp.307-326, (1999)Clift, R. and Grace, J.R. and Weber, M.E.,Bubbles, Drops and Particles, Acad.Press, (1978)Georgescu, S.-C. and Achard, J.-L., Loi deTate: Analyse Critique et Restrictions,Proc. XVieme Congres Français deMécanique, pp. 113-120, (2001)Platzer, R. and Neuilly, M. and Carrier, M.and Dedaldechamp, P., Mesures dedensité et de volume a l’entrée des usinesde retraitement, Proc. ESARDASymposium, (1985)Rayleigh, Lord, Investigations inCapillarity: The Size of Drops. - TheLiberation of Gas from SupersaturatedSolutions. - Colliding Jets. – The Tensionof Contaminated Water-Surfaces. - ACurious Observation, Phil. Mag., 48, pp.321-337, (1899)Tate, T., On the Magnitude of a Drop ofLiquid Formed under DifferentCircumstances, Phil. Mag., 27, pp. 176-180, (1864)Tayor, G.I., The Dynamics of Thin Sheetsof Fluid. {III}. Disintegration of FluidSheets, Proc. R. Soc. Lond., 253, A, pp.313-321, (1959)Uchikoshi, S. and Watanabe, Y. and Yasu,K. and Matsuda, Y., Fundamental Studyfor Bubble Formation on Dip-Tube, Proc.ESARDA Conf., pp.1-6, (1996)EJTPFGM 18/09/03GJM 8/8Fig. 5: Influence of the salt concentration on the conductivity signal: 0g/l (A), 0.3g/l(B), 3g/l (C)

Visualisation of the Bubble Detachment at a Dip Tube

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Visualisation of the Bubble Detachment at a Dip Tube

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