Schlieren Visualization of Marangoni Effect in Gas-Liquid Systems

Transactions of Tianjin UniversityIssN1006-4982pp235·241Vol 12 No 4Aug.2006Schlieren Visualization of marangoniEffect in Gas-Liquid SystemsYU Liming(余黎明), ZENG Aiwu(曾爱武), YU Kuo Tsung(余国琮)State Key Laboratory of Chemical Engineering, School of ChemicalEngineering and Technology, Tianjin University, Tianjin 300072,ChinaAbstract: This paper is focused on the Marangoni effect in the gas-liquid mass transfer systems. Aseries of experiments were conducted to observe Marangoni effect by a laser Schlieren system. Ex-perimental investigations of the occurrence of Marangoni convection were presented. The typicalpolygonal patterns and even the reaching of chaotic interfacial flow were observed. the visual evi-dences were discussed and the characteristic time and scale of Marangoni convection were obained approximately as 0. 5 s and 1 mm according to the Schlieren images. From the perspective ofhydrodynamic instability, the mechanism of the Marangoni convection was investigated. Thoughmany external factors have influence on the interfacial instability, the local surface-tension gradientis the primary reason for the Marangoni convection. The small-scale interfacial flow increases thesurface renewal rate. Consequently, due to the occurrence of the Marangoni effect, the masstransfer rate can be significantly enhancedKeywords: mass transfer; Schlieren system; Marangoni; gas-liquid systemsIn numerous gas-liquid and liquid-liquid chemi- was carried out. In this paper, experimental investical processes, the mass transfer is usually accompa- gations of the occurrence of Marangoni convection arenied by an interfacial instability arising from mutual presented. Orderly polygonal and curved roll convec-molecular interactions. One of the interactions is the tion patterns and the turbulence interfacial flows weresurface tension gradient and the induced convection is observed using Schlieren photography. The fundamen-usually termed as Marangoni convection. This kind of tal characteristics of Marangoni convection in the gasconvection is of interest to chemical engineers in both liquid mass transfer process were studied. Experimentheory and practice[I-ll. The interfacial convection tal results manifest that Marangoni convection occursis capable of increasing the mass exchange rate, and during mass transfer between phases, and some fac-results in the intensification of separation processes, tors, for example, the surface tension concentrationsuch as distillation, extraction, absorption and de- gradients), the liquid film thickness, gas and liquidsorption,etc. Under such conditions, the intensifica- velocity, etc. have remarkable influence on the Mation is driven by the internal energy of the system and rangoni convectiondoes not require an additional supply of energy to re-duce the mass diffusion resistance1 ExperimentThe Marangoni convection has been extensively 1.1 Experimental apparatus and procedurestudied in the liquid-liquid system(12-17.AlthoughThe Schlieren optical arrangement is shown inthe excess values of gas- liquid mass transfer coeffi- Fig. 1. Schlieren system consisted of two lenses withcients were often attributed to the Marangoni convec- diameters of 150 mm and focus lengths of 1 500 mmtion,little systematic research in gas-liquid systems The other components include a 40 mW He-Ne laserAccepted date: 2005-11-29中国煤化工YU LiCorrespondingtoZENGAiwu,E-mail:awzeng@tju.edu.cn.CNMHGported by National Natural Science Foundation of China( No. 20136010)Transactions of Tianjin University Vol 12 No 4 2006nd two flat mirrors. The Marangoni effect was visual- 1. 2 Marangoni and Rayleigh-Benard effectized with a parallel beam passing through the gas-liqConvection initiated at the gas-liquid interfaceuid contactor. The picture of gas-liquid interface was was governed by the differences in surface tensionsprojected onto the screen and recorded by digital cam- and densities between the liquid on the surface anderasthat in the bulk. These disturbances can lead to theThe configuration and the structure of the gas-liq- surface tension or density fluctuations and result in theuid contactor are shown in Fig. 2. The length, width Marangoni convection and Rayleigh-Benard convecand thickness are 190 mm, 190 mm and 40 mm, res- tion. The Marangoni and Rayleigh numbers are de-ectively. Liquids under investigation were placed on- fined as[ 16, 181to a quadrate optical plate A continuous and uniformSAoliquid film on the plate surface was maintained andDthe channel effects were avoided(8,/W<0. 05). Sol8vents were ethanol, acetones isopropanolol, diethyl(2)ether, ethyl acetate and C2 HCl3, which were analytical reagents. The gas was pure nitrogen and carbonIf Ao >0 and Ma>0, the system is Marangonidioxide.Before entering the gas-liquid contactor, the instability; if Ap>0 and Ra >0, the system is Ray-gas was saturated with the working solution to preventleigh instability. Theite inequalities, Ap 0and Ma <0)at different liquid thicknesses were con-ducted. As shown in Figs. 3(a) and 3(b), the typical plumes pattern appears in the gravity directionObviouslyin, terms of the size or the intensity,the raylei中国煤化工 decrease of the luid film thCNM Gder the same oper-Fig 2 Ental apparatus for gas liquid contactoration conditions, the Rayleigh effect is weak in the236一YU Liming et al: Schlieren Visualization of Marangoni Efect in Gas-Liquid Systemsthin liquid film. Consequently, under our experimen- a well-known way to reduce the rate of evaporation isal condition(8<10 mm), the Marangoni effect is to pre-saturate the gas by bubbling it through thedominantworkIrng solvent. Consequently, in our experimentsOn the other hand due to evaporation effect the evaporation effects can be dramatically reduced andthermal convection thus induced interferes with the marangoni effect is substantial in creating convectiveother types of convection. In pure water and dry nitro- structuresgen system, the evaporation effect is remarkably 2 Experimental results and discussionweak. Even for high gas flows and up to the powhere rippling begins Re, >200), there is no visi- 2.1 Optical experimental observationble signs of convection, as shown in Fig 4.MoreoverFig 5 shows the Schlieren photos of dioxide car-bon desorbing from acetone, ethyl acetate, C2HCI3and ethyl ether, respectively. The chief convectivestructure is the polygonal cell. However, the characteristic size and the convective patterns are not totallye same indifferent systems. In Fig. 5(a), the pat-terms are mainly quadrangular and hexagonal cellswith center-to-center distance of 4-8mm(a)b1=20CO2-ethyl acetate system, as shown in Fig. 5(b), thepatterns are the pentagonal and hexagonal cells andthe characteristic length is 5-8 mm. In Fig. 5(c)the marangoni convective structures are the quadragular and hexagonal cells, whose characteristic sizesare 3-6 mm in the CO2-C2HCI3 system. In Fig. 5(d), the distinct flow pattern is the curved roll withhe 2-4 mm axial distance in dioxide carbon and ethyl ether system(b)81=10mmIn Fig. 6(a), the distances of interfacial turbulence structures are mainly 4-8 mm when the ethanolmass fraction is 5%. Under the same operation condi-tions with the increase of ethanol mass fraction theintensity of the marangoni convection intensifies. Asshown in Fig. 6(b), the convective pattern distancesare 2-6 mm(w%=10%). When the ethanol concen-tration is high enough, the Marangoni structures tend(c)8=5 mmto break up and shrink and the intensification of theFig 3 Schlieren image taken vertically to the interface convection increases. The experimental results showin the absorption of CO, by ethanolthat the concentration gradient influences the Ma-rangonI convectionFig. 7 shows the polygonal pattern of desorbingdioxide carbon from C2 HCI3(8= 10 mm)center-to-center distances are 8-10 mm which arergerthan those shown in Fig. 5(c)(81=5mmWith the increase of the liquid film thickness, theborders of pentagonal cells become dark, accompa-nied by some pin flow structures. It means that astratified and diregime is developing. Consequently, the中国煤化工 fluences the siand patternFig 4 Schlieren image of pure water evaporationCNMHG237一Transactions of Tianyin University Vol 12 No 4 2006t、(a)Acetone(a)w%=5%(b)Ethyl acetate(b)%=10%Fig6 Schlieren photography of ethanol desorption(81=5mm,Ren=50)(c) C2HCIFig 7 Schlieren photography of carbon dioxide desorbedfrom CHCI, (8=10 mm, Re,=50To investigate the influence of the relative velocity between phases on the Marangoni effect, Schlierenimages were obtained at the different gas flow veloci-ties. In the CO2- ethyl acetate system, the marangoni(d)Ethyl etherconvection is the typical quadranglar pattern,asFigs Schlieren photography of carbon dioxide desorbed shown in V凵中国煤化工 ncrease of the gasfrom acetone, ethyl acetate, CHCI, andflow theethyl ether(8=5 mm, Re,=50)CNMHG to stretch out,asshown in Hg.o(a,. in rigo(D/, when the relative238YU Liming et al: Schlieren Visualization of Marangoni Effect in Gas-Liquid Systemsvelocity is high enough, the cells tend to break up andBesides polygonal cell and curved structuresally the curved rolls appear( Reg =150Marangoni effect leads to the interfacial turbulenceOwing to the interfacial inhomogeneous, the MWhen the concentration differrencerangoni convection is triggered locally, and then and liquid phases is sufficiently large, cellular con-stretched out, as shown in Fig 9. The pentagonal cells vection can be developed to the random or chaoticalong the boundary of the gas-liquid contactor were state. Fig 10 gives an idea of this kind of convectionthe seeds of marangoni convection, and finallyevolved to cover the whole interface area. Similar phe-nomena were observed in the other experiments andthe characteristic time of the onset of marangoni effectconvection in the horizontal direction was much lessthan 0. 5 s. Most probably, it started even earlierbut extremely small dimensions of the patterns madedirect visualization of the phenomenon impossibleConsequently, it is believed that very short gas-liquidtime is required to produce Marangoni effect and alsothe growth of such interfacial convection is extremely(a)【=2sFig 9 Schlieren photography of ethanoldesorption(w%=5%, 81=5 mm)(b)Reg=150Fig 8 Schlieren photography of carbon dioxide中国煤化工thanol desorptiondesorbed from ethyl acetate(8,=5 mm)CNMHG239Transactions of Tianyin University Vol 12 No 4 2006during desorbing ethanol. It can be clearly seen that rangoni convection, which can be approximately ob-the small-scale interface convection is chaotic, and tained from the movement of the stripes in the Schlierthe characteristic size of the turbulent cells is less en images; D, is the liquid diffusion coefficient. Thethan 1 mmmost common basis for estimating the liquid diffusion2.2 Marangoni convection mechanismcoefficient is the following Stockes-Einstein equationThe Schlieren images provide convincing evi-dence to verify the existence of Marangoni convectionDI(4)tUrOThe experiments show that Marangoni convection isFrom Eqs. (3 )and(4), the diffusion coeffiqualified on small-scale and its characteristic dimen- cients of co2 desorbed from acetone, ethanol, ethylsion and time is approximately 1 mm and 0. 5 s re- acetate, ethyl ether and C2 HCla are 3.52 x10-9m2/spectively. The experimental results show that in themass transfer system, the primary reason for produ-$,9.80 x 10-10 m2/s, 2.84x10-9 m2/s, 4.71xcing Marangoni effect is the surface-tension gradient, 0.22 mm,0.38 mm, 0.49 mm and 0.31 mm,re-mmwhich depends on the local value of surface concentra-tion. These surface forces are mainly responsible for spectively. This means that the desorption is the mostthe establishment of a dynamic equilibrium between intensive in the vicinity of the interface, where thethe interactive transfer and hydrodynamic processesgradient of concentration attains their highest vaThe liquid surrounds ng the surface will be dragged to- and produces diffusional mechanism. Thoughward this high surface tension region. The liquid mi-thickness is small, the Marangoni convection is thegrating from the surrounding area pushes the local sur- secondary How between the surface and the liquidface upward and thereby creates surface ripples and bulk, as shown in Fig. 11. Consequently, the masscorrugations. Meanwhile, the surface flow is commu-transfer behaviors cOIipled with the diffusion andnicated to the bulk of the fluid as a result of viscous convection, which is the reason why some mass trans-effects and drags part of the bulk fluid upward. Thefer processes in some gas-liquid systems are signififluid transiting across the surface is diluted drIng Its cantly intensifiedjourney and will sink in the region of the lower con- 3 Conclusionscentration and a cellular pattern will be establishedas shown in Fig. 11. Consequently, the MarangoniThe investigations were focused on the Marangoniconvection can enhance the local surface renewal rate effect. Usinng a laser Schlieren system, Marangoniand amplify the local mass transfer rate. Hozawa convection was visualized for a series of gas-liquidet alLi] demonstrated that the amplitude of oscilla- systems. Some typical Schieren images of the Ma-tions of the mass transfer coefficient was characterizedrangoni patterns were obtained, such as the orderlypolygonal pattern, roll structure and even the turbuby a factor of 2/3lence interfacial flow. Based on the experimental results, the mechanism of the Marangoni convection wasGas phaseinvestigated. The surface-tension gradient is the pri-Gas-liquidGas flowmary reason for the Marangoni convection. The sponSurfacetaneous circulation flux may be developed in the vicinity of the interface The convection enforces mass exLiquid phasechange between the interface and the liquid bulkConsequently, mass transfer in the liquid phase canFig 11 Marangoni convection in thebe greatly enhancedvicinity of the interfaceNomenclatureIn order to estimate the extent of the marangoniD— -Diffusivity,m2/s;convection the characteristic depth of Marangoni conGravitational acceleration. m/svection. Ah, can be estimated asMmconstant中国煤化工 tensionless;△h=√DtMCNMHGionlesswhere tm is the surface renewal time causedRe--neynolds number, dImensionlessYU Liming et al: Schlieren Visualization of Marangoni Efect in Gas-Liquid SystemsR0— solute radius,m;Heat and Mass Transfer, 2005, 32(5): 677-684K[8 Michiel T Kreutzer, Freek Kapteijn, Jacob A Moulijnt-Timeset al. Inertial and interfacial effects on pressure dropW-Width of the contactor, m;of Taylor flow in capillaries J]. AlChE Journalw-Mass fraction in liquid2005,51(9):2428-2440[9 Post S, Urukova I, Tsotsas E. Intterftacia convectionGreeksduring evaporation of binary mixtures from porous ob8— Film thickness,m;stacles[ J]. AIChE Journal, 2005, 51( 12 ) 3257[10 Tan Ka Kheng. Predicting Marangoni convectionurface tension. N/mcaused by transient gas diffusion in liquids[J].Inter-national Journal of Heat and Mass Transfer, 2005, 48Subscripts(1):135-144[11] Zhang F, Zhang Z B, Geng J. 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