Theoretical and Experimental Study of Gas Bubbles Behavior

Transcription

1 Theoetical and Expeimental Study of Gas Bubbles Behavio Lucian Mândea, Gabiela Opina, aeș-andei Chihaia, Lucia-Andeea El-Leathey, and adu Miea Abstact The pape pesents theoetical consideations on gas bubbles behavio - analytic calculation of bubble time fomation, exta pessue needed fo the bubble fomation and detachment, bubble size and coelation between the exta pessue and the liquid supeficial tension. In ode to evaluate the theoetical elation fo exta pessue calculation, the esults of the tests pefomed on diffeent metallic pefoated plates Ø 6, with a specific numbe of oifices of diffeent diametes, wee used. The exta pessue needed fo bubble fomation vaies fom appoximately 171 Pa fo the MPP with.9 mm oifices to 5 Pa fo the MPP with. mm oifices. Index Tems Bubble size, bubble detachment, exta pessue, oifice. I. INTODUCTION Due to vaious applications in chemical units and waste wate teatment, bubbles and bubble columns have been lagely investigated duing the last decades. Thus, vaious studies have been pefomed egading the bubble fomation, both in still and flowing liquid. Some eseach focused on the fomation and evolution of single bubbles [1], while othes investigated bubble columns. Most of the studies found in liteatue wee pefomed in clea (tap) wate, and envisaged bubble fomation and detachment, bubble velocity, bubble size and bubble size distibution, bubbles flow egime, gas-liquid intefacial aea, gas-liquid hydodynamics, gas hold-up, volumetic mass tansfe coefficient etc. Also, othe studies wee pefomed in wate with sufactants []. Since the bubble behavio depends on the geneation type, the media in which bubbles ae geneated, gas flow ate, etc. the peviously developed studies achieved on dispesed aeation systems and mentioned in liteatue wee focused on the investigation of diffeent paametes influence on bubbly flows. Thus, some studies have mainly investigated the bubble size, bubble size distibution and flow egime tansitions in bubble columns []-[7]. Fo example, in [7], an expeimental study egading the local gas hold up of five types of pefoated membanes was caied out. The membanes have been investigated in two ectangula expeimental setups by vaying the following paametes: ai Manuscipt eceived Januay, 17; evised May 1, 17. L. Mândea is with the Faculty of Powe Engineeing, Politehnica Univesity of Buchaest, O ( mandea_lucian@hotmail.com). G. Opina is with the National Institute fo eseach and Development in Electical Engineeing ICPE-CA (Incdie ICPE-CA), Buchaest, O, omania ( gabiela.opina@icpe-ca.o). C. Băbuțanu,. A. Chihaia, and L. A. El-Leathey ae with the INCDIE ICPE-CA, Buchaest, O, omania ( coina.babutanu@icpe-ca.o, aes.chihaia@icpe-ca.o, andeea.elleathey@icpe-ca.o).. Miea is with the omanian eseach and Development Institute fo Gas Tubines, Buchaest, O, omania ( adu.miea@comoti.o). flow ate though the oifices, the diamete of the oifices and thei density on the membane suface. In the case of the membanes povided with low density of oifices, it was found that the bubble size inceases with the ai flow ate; in the case of the membane with the highest oifice density, an intense inteaction among bubbles was found, leading to a significant incease in the bubbles size in thei ising motion to the suface. It was also noticed that the influence of the oifices density on bubbles size is lowe at highe ai flow ates. The bubbly (homogeneous) flow is chaacteized by small bubbles and a naow distibution of the equivalent diametes; the shifting fom homogeneous to heteogeneous flow egime is achieved though the tansition egime, when the diametes vaiation domain becomes lage; when eaching heteogeneous egime, the flow is chaacteized by coalescence and by the coexistence of small and big defomed bubbles. The tansition between homogeneous to heteogeneous flow detemines adical modifications of the ai-wate dispesed system, eflecting simultaneously in the bubble size distibution, global gas hold up and the ising velocity of the bubbles [8]. Fig. 1. Vaiation of bubbles shape fom homogeneous (left) to heteogeneous egime (ight) [8]. Seveal othe numeical and expeimental studies focused on the investigation of the bubble column hydodynamics [9-1] and bubble oscillations. On a ange of ai flow ates, the bubble columns pesent an oscillating movement. The time inteval of this oscillation depends on the ai flow ate and on the tank geomety. Expeiments on a naow ectangula expeimental setup have been pefomed in [9] in ode to detemine the oscillation peiod of the bubble column. Also, in [1], some expeimental esults ae epoted, obtained by vaying diffeent paametes such as ai flow ate, liquid popeties, and poous diffuse; the study evealed that the bubble column oscillating peiod deceases with the incease of the ai flow ate. Some simila esults povided by numeical simulations, using the data fom a ectangula expeimental setup of.5 m height and a squae coss section. m x. m wee epoted in [11]. The simulations developed fo an ai flow ate of l/h evealed a bubble column oscillation peiod vaying fom 1.7 to 16 s. In ode to coelate the bubbles shape oscillation to thei tajectoies, DOI: 1.776/IJMO.17.V

2 . mm diamete isolated bubbles (geneated by capillay tubes) wee expeimentally studied in [1]. Along with the oscillations and tajectoies, the bubbles shape and liquid velocity field wee investigated. It was found that the bubbles pesent a ectilinea ising motion up to a height of 5 mm out of the mm height of the column; beyond this distance, complex shape oscillations appea, along with the tansition of the tajectoy to a spial. Investigations on the bubble column stability wee also pefomed in [8], by expeimentally studying the opeation of 5 mm diamete ceamic poous diffuses in a ectangula column; the behavio of the bubble plume was studied fo diffeent ai flow ates and the peiod of the diffeent identified oscillations wee detemined, anging fom 11 to a maximum of 1 seconds. Q = 78 l/h Q = 7 l/h Q = l/h Q = 8 l/h Q = 58 l/h Q = 6 l/h Fig.. Bubble column instability [8]. Diffeent studies wee pefomed on bubble fomation, gowth and detachment [], [1]-[19] in diffeent conditions. In [] expeimental studies wee pefomed on bubble gowth and detachment, bubble diamete and bubbles fequency in diffeent aqueous solutions fo the case of one igid oifice of.7 mm diamete and seveal flexible oifices with diametes anging fom.5 to.5 mm, depending on the ai flow ate. Fo both igid and flexible oifices, the study evealed that the bubble diamete is smalle in wate with sufactants than in tap wate; also, fo the igid oifice case, highe fequencies wee egisteed in aqueous solutions. Some studies caied out in flowing wate [1], [1] investigate the effect of seveal paametes on bubble fomation: nozzle diamete, liquid velocity and diection, gas chambe volume, gas flow ate etc. In [1], the study is focused on the mechanism of bubble fomation in concuently upwad flowing liquid. The esults following the study on a non-spheical bubble fomation model when consideing that the bubble suface is fomed by n elements, ae compaed to the expeimental esults egading volumes, shapes and gowth cuves of the bubbles eleased by nozzles with inne diamete of 1.19 mm,.9 mm and. mm and a length of 15 mm. Fo the pessue dop on the nozzle, thee coefficients wee defined: F, fom the Hagen-Poiseuille equation when the flow inside the nozzle is lamina, F' fom the Blasius's equation when the flow inside the nozzle is tubulent and K fo contaction and enlagement losses. Thus, fo lamina flow, the pessue inside the bubble, P i, was evaluated by: And fo tubulent flow by (1). () In equations (1) and (), Pc is the pessue in the nozzle's chambe and Q is the flow ate at the outlet of the gas chambe. Fo gas flow ates vaying between and m /s and fo liquid velocities vaying between and m/s, as well as fo a gas chambe volume fom. 1 6 to m it was noticed that: the bubble volume deceases with the incease of the liquid velocity and inceases with the gas flow ate when the liquid velocity is kept constant; also, since the liquid flow influences the detachment of the bubbles fom the nozzle, the bubble gowth time deceases with the liquid velocity incease. egading the bubbles shapes, a significant change with the modification of the liquid flow was not noticed. Having in mind the development in space technology applications, some studies investigated the bubble fomation unde vaious gavity conditions. In [15], the bubble detachment and fomation of new bubbles unde micogavity conditions was numeically investigated. By using a Lagangian smoothed paticle hydodynamics, the bubble detachment fom an oifice of 1.5 mm diamete, at a gas flow ate of m /s, was studied fo diffeent gavity conditions. Fo lowe gavity, it was noticed that the bubbles have a lage size than in nomal gavity conditions; they wee athe elongated and, afte detachment, combined to the next bubble that was in the pocess of fomation. The bubble detachment fom a submeged oifice in quiescent liquid unde nomal and educed gavity, as well as the bubble volume and fequency, wee numeically investigated in [16]. Fo a gas flow ate of. 1 6 m /s though an oifice of mm diamete and thee diffeent gavity conditions, the study found that the size of the detached bubbles is getting lage with the decease of the gavity and the bubble's contact aea with the oifice inceases, leading to a highe suface tension that delays the bubble detachment; also, along with the gavity decease, the bubble fomation fequency deceases and the bubble volume inceases. Moeove, fo vey low gavity (g 1 ), the bubbles do not detach fom the oifice. In [17], the fomation and natue of the bubbles detached fom igid and flexible oifices was expeimentally studied. The diamete of the igid oifice of the diffuse used in expeiments was equal to.7 mm, wheeas fo the flexible case a ubbe membane was used, with a single punctue in its cente. A.5 mm diamete of the bubbles detached fom the igid oifice was detemined; a discontinuity in the bubbles fomation and a low bubbles fequency espectively 16

3 wee found, most pobably due to the decease of the pessue in the gas chambe below the hydostatic pessue duing the bubble s detachment, followed by liquid entance into the oifice. A diamete below.5 mm was detemined fo the flexible membane case; the pessue inside the gas chambe emains constant, peventing the liquid to penetate the oifice. Oveall, the expeimental study evealed that the bubbles diamete at detachment and thei gowth ate do not depend on the gas flow ate in the igid oifice case, while they incease with the gas flow ate in the flexible oifice case. The cuent study is pefomed assuming that the compessed ai is povided by a compesso and the bubbles exit though a disk (pefoate plate) with the constuctive adius. In an ideal case, the bubbles ae chaacteized by the inteio adius at a cetain moment of time. Duing a small vaiation of time dt, the bubble adius inceases with d and, as a consequence, the bubble volume inceases with dv as in Fig.. The infinitesimal vaiation of the bubble volume can be expessed as a diffeence between the final bubble volume and the initial bubble volume: dv V f V i d d d d () Fig.. Gas bubble evolution. The bubble fomation fom a single oifice in aqueous solutions was investigated in [18]; the expeiments pefomed in diffeent opeating conditions and fo diffeent single oifices showed that the shape of the oifice influences the bubble fomation and that the volume of the bubble inceases with the gas flow ate, viscosity of the aqueous solution, suface tension and oifice diamete. Thee ae also numeous studies investigating bubble paametes and elated phenomena aiming eithe to evaluate the coelation between the bubble shape and pessue field duing gowth and detachment [19] o the bubbles fequency [], as well as the changes in bubble diamete and fequency fo diffeent gas popeties such as suface tension and gas density [1] and the influence of continuous (liquid) phase popeties (density, viscosity, suface tension) and geometical popeties on bubble behavio (bubble fomation, diamete, detachment and motion) []. The pesent study aims to povide an insight in the bubbles behavio by theoetically investigating the bubble fomation time, the exta pessue needed to ceate the bubble and its coelation to the liquid supeficial tension. In ode to evaluate the theoetical esults, tests wee pefomed on diffeent metallic pefoated plates (MPP) Ø 6 with diffeent oifice sizes anging fom. mm to.9 mm and diffeent numbe of oifices, vaying fom 1 to 918. II. THEOETICAL STUDY OF BUBBLES BEHAVIO A. Analytic Calculation of Bubble Time Fomation The theoetical study pesented in this pape takes into account the esults epoted in [], namely that, fo an oifice numbe highe than 15, the size of the bubbles is not influenced by the gas chambe volume. Fo igid oifices, the study [] suggests that the bubble inteaction duing the bubbles fomation pocess can be neglected fo a atio of pitch to oifice diamete below 8. The study epoted in [7] on seveal membanes, with diffeent pitch to diamete atios evealed that the finding/conclusion of the study [] is not valid in the flexible mateials case. Since d and d have low values, they can be neglected and the pevious elation becomes dv d () If the compesso volumetic flow ate is Q, then duing the same small vaiation of time dt, the bubble volume inceases with dv: dv Qdt Thus, fom () and (5), it esults: (5) dt ( / Q) d (6) The ai volumetic flow ate povided by the compesso may be expessed using the constuctive oifice aea and the ai nomal mean velocity though the oifice, v : Q v By substituting the flow ate fom (7) to (6), the following diffeential equation is obtained: dt v d If τ is the bubble detachment time and the coesponding bubble adius, then the pevious equation can be integated: dt v d Thus, the bubble detachment time is obtained: v (7) (8) (9) (1) 17

4 B. Pessue Loss on the Pefoated Plate A diffuse with a consideable numbe of oifices is consideed, with the global local hydaulic loss coefficient on the oifices and the ai density ρ. If S e is the emission (active) suface of the diffuse, fomed by n paallel capillay tubes, each having the inne adius, the pessue loss becomes: Q p S e v v 1 p (11) By substituting the mean ai velocity v fom (11) in (1), a new expession of the bubble detachment time is obtained p (1) This expession also epesents the duation needed fo an isolated bubble fomation, the bubbles emission fequency being f 1/ bubbles/min. The pessue loss on the poous plate, p, may be computed with the diffeence between the pessue inside the diffuse s capsule, p 1 (ead at the manomete), the hydostatic pessue detemined with the wate density, and the exta pessue needed to ceate the bubble: p p (1) 1 gh / C. Coelation between the Bubble Exta Pessue and the Liquid Supeficial Tension The mechanical wok necessay in ode fo the bubble to gow fom to can be computed by the following elation: V V p / / L p (1) s f i It can be also seen like the enegy used to incease the bubble volume: s E ps (15) The infinitesimal vaiation of the necessay enegy used to incease the bubble may be expessed as: de da d d d (16) l Consequently, fom elations (15) and (17) the coelation between the bubble exta pessue and the liquid supeficial tension is obtained as: p s III. EXPEIMENTAL STUDY OF BUBBLES BEHAVIO (18) In ode to evaluate equation (18), the esults of the tests pefomed on diffeent metallic pefoated plates (MPP) Ø 6 developed within a eseach contact [] wee used. These plates aim to impove the mass tansfe in aeation pocesses and theefoe ae chaacteized by a high numbe of oifices and low pessue loss. The numbe of oifices and thei aangement/layout on the suface of the plates wee sized and contolled by using the manufactuing technology discussed in [5]. The numbe of oifices was detemined by keeping a constant distance between them, big enough to pevent bubble coalescence, complying with the conditions epoted in liteatue []. In ode to neglect the contaction coefficient of the oifice, thickness t of 5 oifice diametes (5d) was chosen fo the plates; thus, a diffeent thickness fo each plate esulted, anging fom 1 mm to.5 mm. Table 1 shows the main chaacteistics of the pefoated plates: oifice diamete d, oifices numbe n, and distance among oifices l. TABLE I: PEFOATED PLATES CHAACTEISTICS Pefoated plate d [mm] n [ ] l [mm] MPP with oifices placed at a distance of 1d MPP.. 77 MPP MPP MPP MPP with oifices placed at a distance of 7d MPP MPP MPP MPP Using the technology descibed in [5], MPPs with l=1d and t=5d wee fistly manufactued, the oifices being disposed in a squae layout on the plate suface; theefoe, the MPP chaacteistics pesented in the authos pevious wok coespond to the fist type of pefoated plates developed unde the eseach contact []. In ode to impove the mass tansfe, fo the second seies of MPPs, a distance l=7d between oifices was used and a tiangula layout was consideed, thus obtaining an inceased numbe of oifices (Table I) that povides a highe intefacial aea. with the ai-wate suface tension. Because d has a educed value, it can be neglected and the vaiation of the enegy becomes de 8 d. Thus, the total enegy used to incease the bubble volume is E de (17) Fig.. Pefoated plate with. mm diamete oifices. 18

5 Fig. 5. Pefoated plates used in the expeimental eseach oifices of.,.5 and.9 diamete in tiangula layout. Fo the calculation of the exta pessue povided by equation (18), the bubble adius in the moment of detachment, fo each MPP, was fistly detemined. In the assumption that the bubbles ae emitted in cvasi-static egime (ai flow ate Q is lowe than the citical flow ate Q c defined in [6], was calculated using the following equation 1/ σ ρ g (19) The analysis of Fig. 6 shows that the pessue loss on the pefoated plates incease with the ai flow ate and a highe pessue loss on the MPP with oifices of. mm diamete can be noticed. This high may be given by the numbe of oifices, which is consideably highe than in the othe MPPs cases. The tests pefomed on the pefoated plates evealed the necessity of a minimum ai flow ate in ode to achieve a continuous bubbles fomation and detachment. Fo example, thee was identified that: in the case of the MPP with 61 oifices of.5 mm diamete an ai flow ate of minimum 1 L/h is necessay fo a full opeation, wheeas fo the MPP with 1 oifices of.9 mm diamete an ai flow ate of at least 16 L/h is equied. The modification of the flow egime has been expeimentally noticed but futhe investigations in ode to detemine the pecise flow ates, bubble size distibution and gas hold up have to be caied out. Fo ai flow ates supeio to 6 L/h changes in bubble shape can be noticed (Fig. 7), as well as the bubbles coalescence (Fig. 8). The exta pessue needed by the bubbles to detach fom the suface of the fou types of pefoated plates with oifice in tiangula layout, as well as the detachment adius of the bubbles ae shown in Table. TABLE II: EXTA PESSUES AND DETACHMENT ADII OF THE BUBBLES Pefoated plate [mm] [mm] p s [Pa] MPP MPP MPP MPP Secondly, in ode to evaluate equation (18), the pessue loss on the plates was expeimentally detemined in laboatoy, by immesing each pefoated plate in a ectangula tank with the coss section of. m x. m and the height of 1.1 m. The compessed ai necessay fo the opeation of the pefoated plates was povided by a compesso; in ode to delive a constant pessue into the ai chambe of the plates, a pessue egulato was attached to the compesso outlet. A manomete with a measuing scale of up to 1 L/min was used fo the plates opeation testing at lowe ai flow ates, wheeas a manomete with 1 to L/min scale was used fo the tests pefomed at highe ai flow ates. Fig. 6 shows the vaiation of the pessue loss with the ai flow ate and oifice diamete fo a (tap) wate column of.8 m. Fig. 6. Pessue loss vaiation with the ai flow ate and oifice diamete. Fig. 7. Coexistence of small and big bubbles specific to tansition egime. Fig. 8. Coalescence of bubbles. IV. CONCLUSION The pape pesents theoetical consideations on gas bubbles behavio - analytic calculation of bubble time fomation, exta pessue needed fo the bubble fomation and detachment, bubble size and coelation between the exta pessue and the liquid supeficial tension. The theoetical aspects have been evaluated by expeimental esults on diffeent plates with igid oifices. Duing the testing of the pefoated plates a discontinuity in the bubbles fomation was noticed at lowe ai flow ates, as well as a minimum flow ate necessay fo the MPPs to entiely opeate (i.e. to elease bubbles fom all oifices). This may be due to a low pessue in the gas chambe and its futhe decease when bubbles detach fom the MPP s 19

6 suface, as suggested by othe esults povided by the liteatue; also, the gas chambe (MPP s capsule) should be bette designed (a height of the chambe at least 5 times highe than its diamete) in ode to achieve an unifom pessue distibution inside the capsule. In ode to have a bette insight in the bubble fomation mechanism and behavio duing thei ising to the fee suface, futhe eseach must be conducted on bubble size distibution and gas hold up. EFEENCES [1] S. C.Geogescu, "Evolution d une bulle," Ph.D. dissetation, Institut National Polytechnique de Genoble and Politehnica Univesity of Buchaest, [] K. Loubiée and G. Hébad, "Influence of liquid suface tension (sufactants) on bubble fomation at igid and flexible oifices," Chemical Engineeing and Pocessing, pp ,. [] E. S. Gaddis and A. Vogelpohl, "Bubble fomation in quiescent liquids unde constant flow conditions," Chem. Engng. Sci., vol. 1, no. 1, pp , [] M. C. uzicka, J. Dahos, P. C. Mena, and J. A. Teixeia, "Effect of viscosity on homogeneous heteogeneous flow egime tansition in bubble columns," Chemical Engineeing Jounal, vol. 96, issues 1-, pp. 15-,. [5] M. Polli, M. Di Stanislao,. Bagatin., E. A. Bak, and M. Masi, "Bubble size distibution in the spage egion of bubble columns," Chemical Engineeing Science, vol. 57, no. 1, pp ,. [6] P. Chen, J. Sanyal, and M. P. Dudukovic, "CFD modeling of bubble columns flows: implementation of population balance," Chemical Engineeing Science, vol. 59, pp ,. [7] A. Hasanen, P. Oivuoi, and J. Aittamaa, "Measuements of local bubble size distibutions fom vaious flexible membane diffuses," Chemical Engineeing and Pocessing, vol. 5, pp. 91, 6. [8] G. Opina., I. Pincovschi, and G. Băan, "Hido-gazo-dinamica sistemelo de aeae echipate cu geneatoae de bule," (Hydo-gas-dynamics of aeation systems equipped with bubble diffuses), Ed. Politehnica Pess, Buchaest, ch. 5, pp [9] S. Becke, H. D. Bie, and J. Sweeney, "Dynamic flow behaviou in bubble columns," Chemical Engineeing Science, vol. 5, pp , [1] V. V. Buwa and V. V. anade, "Dynamics of gas-liquid flow in a ectangula bubble column: Expeiments and single/multi-goup cfd simulations," Chemical Engineeing Science, vol. 57, pp ,. [11] K. Bech, "Dynamic simulation of a D bubble column," Chemical Engineeing Science, vol. 6, pp. 59-5, 5. [1] G. Benn, V. Kolobaic, and F. Dust, "Shape oscillations and path tansition of bubbles ising in a model bubble column," Chemical Engineeing Science, vol. 61, pp , 6. [1] K. Teasaka, H. Tsuge, and H. Matsue, Bubble fomation in cocuently upwad flowing liquid, The Canadian Jounal of Chemical Engineeing, vol. 77, pp. 58-6, June [1] H. K. Naha, Y. Kamotani. (). Pediction of bubble diamete at detachment fom a wall oifice in liquid coss flow unde educed and nomal gavity conditions. [Online]. Available: [15] A. K. Das and P. K. Das, Numeical study of bubble fomation fom submeged oifice unde educed gavity condition, Pocedia IUTAM, pp. 8-17, vol. 18, 15. [16] I. Chakaboty, B. ay, G. Biswas, F. Dust, A. Shama, and P. S. Ghoshdastida, Computational investigation on bubble detachment fom submeged oifice in quiescent liquid unde nomal and educed gavity, Physics of Fluids, vol. 1, 9. [17] K. Loubiée, G. Hébad, and P. Guiaud, Dynamics of bubble gowth and detachment fom igid and flexible oifices, The Canadian Jounal of Chemical Engineeing, vol. 81,. [18] N. Dietich, N. Mayoufi, S. Poncin, and H. Z. Li, "Expeimental investigation of bubble and dop fomation at submeged oifices," Chemical Papes, vol. 67, pp. 1-5, Mach 1. [19] S. Di Bai and A. J. obinson, "Expeimental study of gas injected bubble gowth fom submeged oifices," Themal and Fluid Science, vol., pp. 1-17, 1. [] A. K. Das, P. K. Das, and P. Saha, "Fomation of bubbles at submeged oifices Expeimental investigation and theoetical pediction," Expeimental Themal and Fluid Science, vol. 5, pp , 11. [1] P. Hanafizadeh, J. Eshaghi, E. Kosai, and W. H. Ahmed, "The effect of gas popeties on bubble fomation, gowth, and detachment," Paticulate Science and Technology, pp. 1-7, 15. [] P. Zahedi,. Saleh,. Moeno-Atanasio, and K. Yousefi, Influence of fluid popeties on bubble fomation, detachment, ising and collapse; Investigation using volume of fluid method, Koean J. Chem. Eng., vol. 1, issue 8, pp , 1. [] T. Miyahaa, Y. Matsuba, and T. Takahashi, The size of bubbles geneated fom pefoated plates, Int. Chem. Eng., vol., pp , 198. [] eseach contact no. PN 9 5/9, eseach theme Systems fo micobubbles geneation. [5] G. Opina, G. Băan, F. Bunea, M. Popa, and C. Ilie, Detemining the global coefficient of aeodynamic dag on pefoated plates, A şasea Confeinţă a Hidoenegeticienilo din omânia, Doin Pavel, pp. 51-6, 1. [6] H. N. Oguz and A. Pospeetti, Dynamics of bubble gowth and detachment fom a needle, Jounal Fluid Mech., vol. 57, pp , 199. L. Mândea was bon in Buchaest (O), on Oct. 1, He gaduated "Politehnica Univesity of Buchaest, Faculty of Powe Engineeing, (O) in 198 and in 199 he eceived the PhD degee fom the same univesity in the field of powe engineeing. The autho s majo field of study is fluid mechanics. He is an associate pofesso at Politehnica Univesity of Buchaest, Faculty of Powe Engineeing, Depatment of Hydaulics, Hydaulic Machineies and Envionmental Engineeing. He published moe than 8 aticles and communications, 8 books and 1 book chapte. His main eseach inteests concen fluid mechanics, hydo enegy, sediment tanspot, human enegy, numeical methods. D. Mândea is a membe of the pofessional society the Geneal Association of the Enginees in omania (AGI). G. Opina was bon in Campina (O), on Nov., She gaduated "Politehnica Univesity of Buchaest, Faculty of Powe Engineeing, (O) in and in 8 she eceived the PhD degee fom the same univesity in the field of powe engineeing. The autho s majo fields of study ae hydodynamics and convesion of enewable enegy souces. She is senio scientific eseache at INCDIE ICPE-CA Buchaest, Depatment of Efficiency in Enegy Convesion and Consumption and the secetay of the Scientific Council of this institution. She published moe than 7 aticles and communications, books and 1 book chapte. He main eseach inteests concen fluid mechanics, dispesed aeation systems, enewable enegy souces, enegy convesion, envionmental engineeing. D. Opina is membe of the pofessional societies Intenational Association fo Hydo-Envionment Engineeing and eseach (IAH) and of The Geneal Association of the Enginees in omania (AGI). In 1 and 1 she was eviewe fo Chemical Engineeing Science Jounal.. Chihaia was bon in Buchaest (O), on Sept., He gaduated "Politehnica Univesity of Buchaest, Faculty of Powe Engineeing, (O) in 9. In 11, he obtained a Maste s Degee in Hydopowe Engineeing and Technical Hydaulics. In 15 he eceived the PhD degee fom Technical Univesity of Civil Engineeing Buchaest in the field of civil engineeing hydotechnical engineeing. The autho s majo field of study is Powe Engineeing and enewable Enegy Souces. He is a scientific eseache at INCDIE ICPE-CA Buchaest, Depatment of Efficiency in Enegy Convesion and Consumption. He published 6 aticles and communication. His cuent wok focuses on the use of enewable enegy souces, eseach, development and design of innovative poducts with high efficiency designed fo wind and wate enegy convesion. D. Chihaia is a membe of the pofessional society The Geneal Association of the Enginees in omania (AGI). 15

7 A. El-Leathey was bon in Buchaest, omania, on Octobe 1st, In 1, she eceived the B.S. degee, while in 1, she eceived the M.S. degee, both in the field of electic powe engineeing fom Univesity POLITEHNICA of Buchaest, omania. In 16, she gaduated the Ph.D. degee in powe engineeing within Univesity POLITEHNICA of Buchaest, Doctoal School of Powe Engineeing. Fom 11 up to pesent, she activates as Scientific eseache within the National Institute fo eseach and Development in Electical Engineeing ICPE-CA fom Buchaest, omania, Depatment fo Enegy Efficiency in Convesion and Consumption (ECCE). He eseach inteests include: the use of enewable enegy souces (sola enegy convesion, wind and hydo enegy convesion) fo electical powe geneation, micogids modeling, simulation and implementation as well as smat gids. Miea was bon in Pitesti (O), on Apil 19, 198. He gaduated Politehnica Univesity of Buchaest, Faculty of Applied Chemisty and Mateial Science (O), in and in 1 he eceived the PhD diploma fom the same Univesity in the field of chemical engineeing. His main fields of study ae advanced cabonic mateials, enewable enegy souces and pollutants assessment and contol in the field of tubo engines and gas tubines. He is a senio eseache at INCDT COMOTI, Depatment of Physical-Chemical Investigations. As Technological Development Enginee, his scientific wok consists of: 1 technology and 1 poduct to be industially implemented, 11 expeimental models, 8 scientific aticles as follows: 1 ISI aticles, 5 BDI aticles, 11 Intenational Confeences papes. Also he has awaded patens and 1 patent equest. As eseache he paticipated in 8 eseach pojects, 7 being intenational pojects. He was poject manage of and eseach team membe of othe. 151