当前位置：文档之家 › Topological model of flow regimes in the plane of symmetry of a surface-mounted obstacle

Topological model of flow regimes in the plane of symmetry of a surface-mounted obstacle

  • 格式：pdf
  • 大小：1017.50 KB
  • 文档页数：8

Topological model of flow regimes in the plane of symmetry of a surface-mounted obstacleM.Javed KhanDepartment of Aerospace Science Engineering,Tuskegee University,Tuskegee,Alabama 36088Anwar Ahmed a ͒Department of Aerospace Engineering,Auburn University,Auburn,Alabama 36849͑Received 16November 2004;accepted 4January 2005;published online 10March 2005͒Flow visualization between Reynolds numbers of 2000and 6500in the end-wall region of an obstacle mounted on a flat plate revealed presence of a multiple vortex system.Three Reynolds-number dependent regimes were identified.With increasing Reynolds number the vortex system transitioned from a static system to a to-and-fro oscillating system and finally to a shedding-splitting system.These observations have been used to infer flow topologies in the plane of symmetry of the juncture from start-up through all three regimes.©2005American Institute of Physics .͓DOI:10.1063/1.1864072͔I.INTRODUCTIONExplanation of three-dimensional separation using math-ematical theory of singular points in a continuous vector field has allowed the use of topological concepts to study compli-cated fluid-flow phenomena.It was pointed out by Lighthill 1that the singular points in the limiting streamlines patterns on a surface obey the topological rule⌺N −⌺S =0,͑1͒where N denote “nodes”and S denote “saddles.”Legendre 2stated that most of the fluid-flow phenomena could be mod-eled by the use of few elementary singular points that obeyed simple topological rules.Since then,topological concepts are increasingly being used.Perry and Fairlie 3used the concepts of “phase space”and “phase plane”to reproduce Lighthill’s 1results.They ex-tended the approach to inviscid rotational flow to model tur-bulent boundary layers approaching surface-mounted ob-stacles.Hunt et al.4introduced the concept of “half nodes”͑N Ј͒and “half-saddles”͑S Ј͒which are nodes and saddles at the intersection of a surface and plane normal to ing topological arguments they showed that for an n -tuply con-nected body⌺͑N +12N Ј͒−⌺͑S +12S Ј͒=1−n .͑2͒Flow in the end-wall region of a surface-mounted ob-stacle,which is commonly referred to as juncture flow,ischaracterized by a complicated system of vortices and coun-terrotating structures as explained by Khan et al.5It may be noted that the dynamic nature of the juncture vortex has also been observed in the juncture of a rectangular/flat plate by Seal et al.6This system is a consequence of skewing of streamlines due to the presence of the surface-mounted ob-stacle which introduces three dimensionality ͑pressure driven ͒in the nominally two-dimensional approach bound-ary layer.Additionally,the adverse streamwise pressure gra-dient imposed by the obstacle causes the flow to decelerateupstream of it resulting in separation of the boundary layer ͑Fig.1͒.This separation has the characteristics of a classical three-dimensional separating boundary layer with a combi-nation of “singular points”located upstream of,and “ordi-nary”separation/attachment lines around the obstacle.Such a configuration forms a simply connected body ͑i.e.,n =1͒and the topological rule takes the form⌺͑N +12N Ј͒−⌺͑S +12S Ј͒=0.͑3͒Hunt et al.4applied this rule to flow around a surface-mounted obstacle and inferred the streamline patterns from flow-visualization results.However,they did not elucidate as to whether the inferred topology was for static or a dynamic system of vortices.Baker 7studied the dynamic nature of the juncture vortex system in some detail.He reported a static system transitioning to an oscillating system above a Rey-nolds number of 5000in his experiments on laminar flow in the juncture of cylinder-flat plate configuration.He put for-ward streamline patterns for a three-vortex system inferred from flow visualization,which conformed to the criterion of Hunt et al.4Baker,7however,also did not elaborate on whether the topology was for a static system or an oscillating system.In a later paper,Baker 8proposed a two-vortex model for the turbulent juncture vortex.He invoked the topological criterion of Hunt et al.4to argue the presence of a nodal point of attachment between the primary and secondary saddle points of separation;however,the plane of symmetry stream-line pattern inferred by him from surface flow-visualization results does not follow the same topological criterion ͓Fig.2͑a ͔͒.Lai and Makomaski 9proposed a single vortex model based on computational studies of juncture flow,but their model does not follow the criterion proposed by Hunt et al.4either.Tobak and Peake 10have topologically shown the evolu-tionary process of the juncture vortex system.They used the flow-visualization data of Schwind 11and the topological cri-a ͒Author to whom correspondence should be addressed.Telephone:͑334͒844-6817.Fax:͑334͒844-6803.Electronic mail:aahmed@PHYSICS OF FLUIDS 17,045101͑2005͒1070-6631/2005/17͑4͒/045101/8/$22.50©2005American Institute of Physics17,045101-1terion of Hunt et al.4to infer flow development from start-up until the “shedding”and the final “dying”phase.They showed the unsteady system to be oscillating between one and two vortices when the flow finally established in the juncture.The connectivity of the vortices in Baker’s 7topo-logical model is observed as an intermediate step in the evo-lutionary process proposed by Tobak and Peake.10It is inter-esting to note that although Schwind 11reported to-and-fro oscillating vortices also,Tobak and Peake 10have not referred to them.Streamline patterns for a to-and-fro oscillating sys-tem inferred from flow-visualization observations were pre-sented by Thomas,12but he did not attempt to discuss them in terms of topological concepts.Visbal 13has put forward a model for laminar juncture flow based on computational work,showing that the singular point upstream of the obstacle is a saddle of attachment in-stead of a saddle of separation ͓Fig.2͑b ͔͒for Reynolds num-bers as high as 5000.This possibility had been mathemati-cally observed earlier by Perry and Fairlie 3who noted that the controlling parameter which determined the type of sin-gular point was the ratio of the gradient of vorticities ץ⍀z /ץx and ץ⍀x /ץz where ⍀z and ⍀x are the spanwise and stream-wise vorticities,respectively.For values greater than 1for this ratio,the singular point was observed by them to be a saddle of separation while for values between 1/3and 1,the singular point was an attachment putations by Visbal 13for a much thinner boundary layer with a ratio of 1.2for the controlling parameter resulted in a streamline pattern in which the primary singular point was observed to be a saddle of separation.Hung et al.14have carried out similar computations.For subsonic flow,the computed topology showed that the pri-mary singular point was an attachment point.Additionally,their computations showed that the singular point on the cyl-inder was also a node of attachment ͓Fig.2͑c ͔͒.Streamlines patterns computed by them for supersonic flow are similar to that of conventional topology with a separation point;how-ever,they conclude that the primary singular point was still an attachment point.They also commented that in case the saddle point in the flow field becomes coincident with the node of attachment on the body surface,the critical point will become a conventional saddle of separation.Puhak etal.15have shown that indeed the computed node of attach-ment changes its character to that of a saddle of separation as the solution progresses in time.Visbal 13has cited experimental results of Kawahashi and Hosoi 16as a verification of his computational model.How-ever,Kawahashi and Hosoi did not give their interpretation of the flow or complete details of the test conditions,as the objective of the research was the experimental technique and not the flow physics.The data in the paper by Kawahashi and Hosoi,consisted of single photographic frame of a laser specklegram and did not resolve any near-wall structure or the singular point.The work of Coon and Tobak 17also has low resolution of flow in the near-wall region.Thus,becauseFIG.1.Details of flow around a surface mountedobstacle.FIG.2.Topological models in the plane of symmetry of a surface mounted obstacle:͑a ͒Baker ͑Ref.7͒,͑b ͒Visbal ͑Ref.13͒,and ͑c ͒Hung et al.͑Ref.14͒.045101-2M.J.Khan and A.Ahmed Phys.Fluids 17,045101͑2005͒of the inherent difficulties in resolving the near-wall flow in the juncture region,topological models suggesting the pri-mary critical point to be a singular point of attachment are yet to be verified experimentally.Although end-wall flow changes its character with in-creasing Reynolds number and consists of multiple flow re-gimes,most of the topological studies have addressed this flow phenomenon only partially.This paper looks at various regimes of the juncture vortex system more comprehensively and proposes topological models based on detailed experi-mental studies.II.DESCRIPTION OF EXPERIMENTSAn extensive series of tests were conducted to qualita-tively and quantitatively understand juncture flow.The ob-stacle model used in the present study was a wing with a NACA 0020airfoil section and a 3:2elliptic nose joined at its maximum thickness location.The ratio of span to maxi-mum airfoil thickness ͑t max =60mm ͒was Ϸ10.A large as-pect ratio was chosen to ensure that the wing-tip effects do not influence the juncture region as such an influence has been reported by Khan and Ahmed.18Flow visualization tests were conducted in a 60ϫ90cm 2cross-section test sec-tion water tunnel with a maximum velocity of 1m/s and free stream turbulence intensity of 1%at peak velocity.The wing was mounted vertically on a 1.5m long and 46cm wide plate which had an elliptical leading edge and a tapered trailing edge.The plate was installed between two-dimensional inserts that protruded above the free surface ͑Fig.3͒.A variety of techniques consisting of hydrogen bubble and fluorescent dyes were ser light sheets generated from a 4W argon ion laser and a combination of cylindrical and spherical lenses illuminated regions of inter-est in the flow field.Finer details were observed with the help of a laser fiber illuminator.A set of charge-coupled de-vice cameras and image processing techniques were em-ployed to analyze recorded video data.Test Reynolds num-bers ͑based on the maximum thickness of the airfoil ͒ranged from 1900to 6500.A single component laser Doppler ve-locimeter ͑Aerometrics Inc.,͒was used to measure the boundary layer profiles on the plate at a distance of 75cm from the leading edge,where the leading edge of the airfoilmodel was later minar boundary layer thickness ␦varied from 25.5mm to 11mm for the range of Reynolds number tested.Corresponding range of Reynolds numbers ͑Re ␦*͒based on the boundary layer displacement thickness therefore varied from 218to 319͑Re tmax =1900–6500͒.Ad-ditional details of test instrumentation flow-visualization and image-processing techniques can be found elsewhere.5III.RESULTS AND DISCUSSION A.Flow visualizationFlow visualization of the phenomenon very clearly brought out the static and dynamic characteristics of the juncture vortex system.Because of the high shear in the juncture region due to wall-vortex interactions,residence time of the fluorescent dye was observed to be very short,which required continuous injection of fluorescent dye in the juncture rge amount of dye resulted in “optical noise”that lowered the contrast between laser-illuminated region of interest and the surrounding.To overcome this problem a rather simple technique was employed to visualize and document the dynamics of foci structures in detail.So-dium fluorescein was mixed with corn syrup and allowed to dry.Dry crystals were thereafter sprinkled in the region of interest when the flow was at rest.As the flow was brought to the required test conditions,fluorescent dye slowly dis-solved,which significantly improved quality of the video recordings.In the presence of hydrogen bubbles,this tech-nique yielded additional finer details of near-wall and outer flow.The hydrogen bubble probes consisted of single and multiple platinum wires spaced 2mm apart and strung across an insulated bow shaped support.V oltage to each wire could be individually controlled.Both continuous and pulsed bubble sheets were used;however only the images of con-tinuous bubble sheets were recorded and analyzed.V ortex trajectory and periodic behavior were determined from the analyses of the digitized video images using the Expertvision Motion Analysis System.An average of 200video frames were digitized for the application of autonomous and re-stricted blob/centroid tracking techniques.A typical digitized image is shown in Fig.4.The starting flow is shown in the sequence of photo-graphs of Fig.5.Although the phenomenon was videotaped at a known frame rate,timing markers have not been in-cluded in the sequence of these figures as its purposeisFIG.3.Experimental setup of the model and boundary layerplate.FIG.4.Example of digitized image of a single video frame for motion analysis.045101-3Topological model of flow regimes Phys.Fluids 17,045101͑2005͒purely qualitative.After the flow accelerates from rest,the obstacle induces a backflow and as a result a separation point ͑S ͒upstream of the obstacle is set up on the body surface and a weakly rotating flow ͓labeled as N in Fig.5͑a ͔͒is observed which is soon discernable as a vortex v 1in subsequent fig-ures.A counterrotating structure f 1due to secondary separa-tion is then established close to the body surface and up-stream of v 1͑not visible in this sequence ͒along with formation of another vortex v 2further upstream.This is fol-lowed by the formation of a second counterrotating structure f 2upstream of v 2and a third vortex v 3as shown in Fig.5͑c ͒.It can be observed that as the system developed,it moved upstream and closer to the body surface.Further-more,the primary singular point S identifiable as a separa-tion point also continues to move upstream to an equilibrium position.In this three-vortex system,the vortices and accom-panying foci f 1and f 2were observed to be temporally and spatially locked in one position and was termed as regime I.This regime was visible up to a Reynolds number of Ϸ2500.With an increase in Reynolds number,the vortex system transitioned to a to-and-fro oscillating system termed as re-gime II and is shown in a sequence of photographs in Fig.6.In this sequence the vortex closest to the obstacle has started its upstream movement ͓Fig.6͑a ͔͒,and can be seen wrapping around and finally amalgamating with vortex following it ͓Figs.6͑b ͒–6͑f ͔͒.After being fully merged,the new vortex continues its journey towards the obstacle ͓Figs.6͑g ͒–6͑k ͔͒and after reaching a maximum downstream position;it un-dergoes stretching ͓Fig.6͑i ͔͒and then starts its upstream motion to be consumed once again.The cycle thus continues.These vortices are accompanied in their excursions by the counterrotating foci structures,hereafter simply referred to as foci f 1,f 2,etc.V ortex v 1can be seen riding focus f 1as the system moves downstream in Fig.7͑a ͒.Focus f 2is vis-ible in Figs.7͑b ͒and 7͑c ͒.During the upstream movement focus f 1is flattened by vortex v 1͓Fig.7͑c ͔͒,and finally its vorticity is distributed between the new vortex v 1͑previ-ously v 2͒and focus f 2͑which is now the new focus f 1͒as seen in Figs.7͑d ͒–7͑f ͒.The cycle is then repeated.These foci as described earlier were made visible by laser-induced fluo-rescence.The system would oscillate between three and fourvortices as shown in Fig.8where vortex v 1can be observed merging with vortex v 2.Thus the streamline pattern trans-formed into a moving reference frame as reported by Seal et al.19has been vividly captured to a much greater detail in the flow-visualization sequences of this investigation as de-scribed above.As the Reynolds number was further increased,the vor-tex system transitioned to regime III ͑Fig.9͒.In this regime,which was stable for Reynolds numbers greater than 4500,the vortex closest to the obstacle would convect downstream by a certain maximum distance while stretching intensely.This vortex would split symmetrically into its two legs and convect away.It was observed that while the vortex was being intensely stretched a new vortex would form,increas-ing the number of vortices to four.After v 1would split the number of vortices would be three again.Additional details of this system of vortices are given in Khan et al.5For the entire range of Reynolds numbers a small weakly rotating structure was observed along the attachment line of the wing.B.Proposed topological modelsThe proposed models are based on detailed analyses of flow-visualization results.The topological criterion of Hunt et al.4has been invoked in inferring the topology.StandardFIG.5.Flow visualization of regime I—static system of vortices;͑a ͒flow starting from rest,͑b ͒upstream movement of primary singular point,and ͑c ͒established flow ͑␦*=7mm ͒.FIG.6.Flow visualization results of regime II—oscillating system of vor-tices ͑␦*=5mm,field of view is 3.6␦horizontal and 1.5␦vertical ͒.045101-4M.J.Khan and A.Ahmed Phys.Fluids 17,045101͑2005͒terminology for nodes and saddles is used to describe the models.To be consistent with the flow-visualization results discussed earlier,some subscripts have been added.Thus N v1and N f1are nodes in the velocity field corresponding to vor-tex v 1and focus f 1,respectively.Similarly all “super-scripted”saddles are half saddles in the shear stress field and represent saddle of attachment S a Јor saddle of separation S s Ј.Thus S ab Јis half saddle of attachment of the bifurcation streamline and S sp Јis the half saddle due to the primary sepa-ration.Unsuperscripted saddles S are full saddles in the ve-locity field.1.Regime I:Static system of vorticesAs the flow starts from rest,a bifurcating streamline ͑S ab Ј͒attaches to the leading edge of the obstacle.The near-wall flow exhibits a separation point S sp Јand formation of a rotating flow N v1upstream of the wing as shown in Fig.10͑a ͒;also refer to “N ”and “S ”in Fig.5͑a ͒.With the flow continuing to accelerate,it separates from the leading edge of the obstacle ͑S sw Ј͒forming a focus N vw .The flow reattaches on the body surface ͑S a1Ј͒as shown in Fig.10͑b ͒.Soon a secondary separation S s1Јis established on the body surface as well,forming a counterrotating focus N f1.Flow into these nodes is assured by the establishment of an additional saddle of attachment S a2Ј͓Fig.10͑c ͔͒.Continued acceleration of the flow results in the establishment of a stagnation or saddle point S 1in the flow and a new node N v2͓Fig.10͑d ͔͒.As the approaching boundary layer thickens,vorticity in the outer layers increases,resulting in the formation of another saddle point S 2in the flow field and a third node N v3͓Fig.10͑e ͔͒.The flow moving upstream of S a2Јseparates to form the coun-terrotating focus N f2upstream of which the flow reattaches ͑S a3Ј͒;thus finally a system of three vortices with two accom-panying counterrotating foci is established upstream of the obstacle ͓Fig.10͑f ͔͒.This topological representation corre-sponds to the flow visualization shown in Fig.5͑c ͒.The difference between Baker’s 7model and the pro-posed model primarily is the topology of the attachment streamlines on the body surface between the vortices ͑N v2and N v3͒.A similar comment can be made about the com-puted topology by Visbal.13The flow-visualization photo-graph shown in Fig.11confirms the present topological model,on which the final stage of the evolving topology ͓Fig.10͑f ͔͒has been based.Though Coon and Tobak 17con-jecture connectivity similar to Visbal 13in the plane ofsym-FIG.7.Flow visualization of foci accompanying vortices in regime II.Note that some fluorescent dye used to visualize near-wall flow has been en-trained by vorticesalso.FIG.8.Hydrogen bubbles marking the four vortices of regime II vortexsystem.FIG.9.Plane of symmetry view of regime III vortex system.Only one cycle is shown.Note the intense stretching of the vortex near the leading edge of the obstacle.͑␦*=3mm,field of view is 2␦horizontal and 0.5␦vertical.͒045101-5Topological model of flow regimes Phys.Fluids 17,045101͑2005͒metry;however,in the 30°radial plane,the connectivity of the two vortices is not shown to be the same as that in the plane of symmetry,i.e.,through the saddle in the flow.The shear stress field on the body surface is shown in the Fig.12͑a ͒and the corresponding cartoon in Fig.12͑b ͒.The skin friction lines of Fig.12͑a ͒were visualized using dry crystals of fluorescent dye sprinkled in the region of interest when the flow was at rest.In this case the pattern certainly is for a flow where the primary critical point is a saddle of separation as is evident from the flow visualization of Fig.5.The topological evolution of flow depicted in Fig.10satis-fies the constraint of Eq.͑3͒.2.Regime II:Oscillating system of vorticesAs mentioned earlier,at higher Reynolds numbers,the system is no longer static,but rather oscillates to and fro.In this case the original three-vortex system is shown in Fig.13͑a ͒.As the Reynolds number increases N v3convects down-stream and is stretched in the process,along with its accom-panying focus N f1.While N v3is stretching a topological change occurs upstream;the saddle S 1establishes a connec-tion with the half saddle of attachment S a3Ј͓Figs.13͑b ͒and 6͑a ͔͒.Continuing the upstream motion by the couple,a saddle in the flow field is established,connecting N v3and N v2͓Figs.13͑c ͒and 6͑b ͔͒.As node N v3wraps around N v2͓Fig.6͑c ͔͒,the upstream part of the system regenerates itself and a new node N v4is established in the flow field resulting in the topology of Fig.13͑d ͒with four vortices.N v3and N v2merge and become the new N v3͓Fig.13͑e ͔͒.For consistency N v4is renamed N v2.Meanwhile N f1undergoes straining in the vertical plane ͓Fig.6͑d ͔͒.The vorticity contained in N f1is distributed to N f2and N v1through the topological connec-tion shown in Fig.13͑f ͒.This is graphically visible in the photographic sequence of Fig.7.Visbal 13has computation-ally demonstrated a similar phenomenon for the connectivity between the vortex and the counterrotating focus.In fact,theFIG.10.Topological model of regime I vortex system.Note the upstream migration of the primary singular point and evolution of secondary nodes andfoci.FIG.11.Flow visualization record of the final stage of evolving topology described in Fig.10͑f ͒.FIG.12.Trajectory of limiting streamlines;͑a ͒flow visualization and ͑b ͒cartoon of near-wall flow.045101-6M.J.Khan and A.Ahmed Phys.Fluids 17,045101͑2005͒topology for the unsteady case shown by Visbal 13actually confirms our observation about the connectivity of the vorti-ces as well.The flow then reestablishes as the three-vortex system ͓Fig.13͑a ͔͒to repeat the cycle.The system thus os-cillates between a three-and four-vortex system.As can be readily observed,this sequence also follows the topological constraints on the number of critical points.3.Regime III:Shedding-splitting system of vorticesWith further increase in the Reynolds number ͑Ͼ3500͒,the three-vortex system of Fig.14͑a ͒essentially initiates a motion similar to regime II.However,increased flow veloci-ties result in an intense stretching of N v3as it convects down-stream ͑Fig.9͒.This results in large vorticity gradients caus-ing viscous dissipation of the vortex.Downstream motion of the paired focus N f1although observed could not be resolved by the video.It is conjectured that in this process focus N f1pairs with N v3,forming the topology of Fig.14͑b ͒,similar to that computed by Visbal 13for the unsteady case.The system regenerates upstream of N v2in the classical manner into a four-vortex system.Finally,as N v3dissipates ͑splits ͒in front of the obstacle,the classical three-vortex system is reestab-lished to repeat the process ͓Fig.14͑a ͔͒while observing the constraint of Eq.͑3͒.IV.CONCLUSIONSEnd-wall flow of a surface-mounted obstacle consists of three Reynolds-number dependent regimes.These being a static system of three vortices,a to-and-fro oscillating system in which the number of vortices varies between three and four,and a system in which a vortex sheds and convects towards the obstacle,gets stretched and splits in the plane of symmetry with the number of vortices varying between three and four in this regime also.While earlier investigators have proposed “three-vortex”topologies,the present study has proposed topological mod-els for a wide range of Reynolds numbers inferred from these observed regimes.These models include transitions from a single to a three-vortex system and the oscillation between three and four vortex systems.These topologies conform to the established topological rules for fluid flows.For regime I,starting from rest,the flow field developed into a topology of two saddles.In regime II the saddles in the flow field oscillated between two and one.While in regime III the numbers of saddles increased from two to three and then back to two.It is therefore further concluded that the periodicity of flow can be linked directly to the fluctuations in the number of saddles present.The proposed topology for regime I is supported by flow visualization and is different than the attachment line topol-ogy suggested by Baker.7Topological model for the oscillat-ing system has been proposed based on flow visualization that has also been observed computationally.131M.J.Lighthill,“Attachment and separation in three-dimensional flow,”in Laminar Boundary Layers ,edited by L.Rosenhead ͑Oxford University Press,Oxford,1963͒,Sec.II,V ols.2and 6,pp.72–82.2R.Legendre,“Lingnes de Courant d’un Ecoulement Permanent:Decolle-ment et Separation,”La Recherche Aerospatiale,Report No.1977-6,1977.3A.E.Perry andB.D.Fairlie,“Critical points in flow patterns,”Advances in Geophysics ͑Academic,New York,1974͒,V ol.18b,pp.229–315.4J.C.R.Hunt,C.J.Abel,J.A.Peterka,and H.Woo,“Kinematical studies of flows around free or surface mounted obstacles;applying topology to flow visualization,”J.Fluid Mech.86,179͑1978͒.5M.J.Khan,R.J.Trosper,and A.Ahmed,“On the dynamics of wing-body juncture flow,”AIAA J.33,1273͑1995͒.6C.V .Seal,C.R.Smith,O.Akin,andD.Rockwell,“Quantitative charac-teristics of a laminar,unsteady necklace vortex system at a rectangular block-flat plate juncture,”J.Fluid Mech.6,117͑1995͒.7C.J.Baker,“The laminar horseshoe vortex,”J.Fluid Mech.95,347͑1979͒.8C.J.Baker,“The turbulent horseshoe vortex,”J.Wind.Eng.Ind.Aerodyn.6,9͑1980͒.9K.Y i and A.H.Makomaski,“Three-dimensional flow pattern upstream of a surface-mounted rectangular obstruction,”J.Fluids Eng.111,449͑1989͒.10M.Tobak and D.J.Peake,“Topology of two dimensional and threedi-FIG.13.Topological model of the to-and-fro oscillating regime II vortexsystem.FIG.14.Topological model of regime III vortex system.045101-7Topological model of flow regimes Phys.Fluids 17,045101͑2005͒mensional separatedflows,”AIAA12th Fluid and Plasma Dynamics Con-ference,Williamsburg,V A,23–25July1979,AIAA Paper79-1480.11R.Schwind,“The three dimensional boundary layer near a strut,”Gas Turbine Laboratory,Massachusetts Institute of Technology,Report No.67, 1962.12A.S.W.Thomas,“The unsteady characteristics of horseshoe vortex sys-tem,”Phys.Fluids30,283͑1987͒.13M.R.Visbal,“Structure of laminar junctureflows,”AIAA J.29,1273͑1991͒.14C.Hung,C.Sung,and C.Chen,“Computations of saddle point of attach-ment,”AIAA J.30,1561͑1992͒.15R.I.Puhak,A.T.Degani,and J.D.A.Walker,“Unsteady separation andheat transfer upstream of obstacles,”J.Fluid Mech.305,1͑1995͒.16M.Kawahashi and K.Hosoi,“Beam-sweep laser speckle velocimetry,”Exp.Fluids8,109͑1989͒.17M.D.Coon and M.Tobak,“Experimental study of saddle point of attach-ment in laminar junctureflow,”AIAA Paper95-0785,1995.18M.J.Khan and A.Ahmed,“Influence of aspect ratio on the dynamic characteristics of end-wallflow,”Proceedings of the First International Conference on Heat Transfer,Fluid Mechanics&Thermodynamics,South Africa,April2002.19C.V.Seal,C.R.Smith,and D.Rockwell,“Dynamics of vorticity distri-bution in endwall junctions,”AIAA J.35,1041͑1997͒.045101-8M.J.Khan and A.Ahmed Phys.Fluids17,045101͑2005͒。

合集下载

土壤多组分重金属吸附运移过程的格子Boltzmann模型研究

土壤多组分重金属吸附运移过程的格子Boltzmann模型研究


王飞 等
Abstract
In order to solve the problem of soil and groundwater heavy metal pollution more scientifically and effectively, it is necessary to establish a mathematical model to quantitatively study the transport characteristics of multi-component heavy metals in soil and groundwater. Combined with the multi-component reaction heavy metal solute competition adsorption kinetic model and lattice Boltzmann model method, a multi-component heavy metal adsorption transport lattice Boltzmann percolation chemical model and its numerical simulation method are proposed to quantitatively study the ion exchange of multi-component heavy metals in soil. The characteristics of concentration coupling change during the process of reaction, competitive adsorption, degradation and migration, combined with the application analysis of engineering examples, provide a reliable basis for the prediction and control of heavy metal pollution.
考虑颗粒旋转的颗粒动力学模拟提升管气固两相流动特性

考虑颗粒旋转的颗粒动力学模拟提升管气固两相流动特性


动特性。计算结果表 明提升管 内中心 区域为低浓度． 的颗粒 上升流 动、壁 面区域 为高浓度 ． 的颗粒下降流动 高速 低速 分析了颗粒粗糙度系数对颗粒相能量耗散、颗粒平动温度和黏度的影响。随着颗粒 粗糙度 系数的增加，颗粒碰撞能量 耗散先逐渐增加后减小。颗粒平动温度和黏度的变化趋势是相反的，表明颗 粒旋 转产生摩 擦将 导致颗 粒旋转脉动能量
文章编号 ：１０ －０ ５２ １ ）５０ ７ －７ ０ ３９ １ （０ ００ －７ ６０
考虑颗粒旋转 的颗粒动 力学模拟提升管气固两相 流动特 性
郝振 华， 陈 巨辉， 白颖华， 王 帅， 徐鹏 飞， 陆慧林
（ 尔滨工业 大学 能源科 学与工程 学院， 黑龙江 哈 尔滨 １００） 哈 ５０ １
ｔ ｅ ｒ ｆｇ ａ ｕ ａ ｏ Ｉ ｈ ｓｓｕｄ ，ｕ ｉｇ ａｔ — ｕｄ ｍｏｄ ｌｃ ｍ ｂ ｎ ｎ ｔ ｈ ｙ ｍ ｉ ｅ ｒ ｆｇ ａ ｕ ａ ｈ ｏ ｙ ｏ ｒ ｌ ｆ ｗ． ｎ ｔ ｉ ｔ ｙ ｓｎ ｗｏ ｆ ｉ ｎ ｒ ｌ ｌ ｅ ｏ ｉ ｉ ｇ ｗｉ ｔ ｅｄ ｎａ ｃｔ ｏ ｏ ｒｎ ｌ ｈ ｈ ｙ ｒ ｌ ｗ，ｔ ｕ ｒｃ ｌａ ｌ ｓ ｓｏ ｆ ｃ ｆｔ ａ ｔｃｅ ｒ ｔｔｏ ｎ ｇ ｐａｔｃｅ ｔ － ａ ｅ ｆ ｗ ｅ ａ ｏ ｎ ａ ｆｏ ｈ ｅｎ ｍ ｅ ｉａ ａｙ ｅ ｎ ｔ ｅ ｅ ｅ ｔｏ ｅ ｐ ｒｉｌ ｏ ａｉ ｎ ｏ ａ － ｒｉｌ ｗｏ ｐｈ ｓ ｏ ｂ ｈ ｖｉｒｉ ｎ ｈ ｈ ｓ ｌ ｌ ｉ ｚｎｇｒｓ ｒｗｅ ｅｃ ｎ ｕ ｔ ｄ Ｖｉ ｏ ｇ ｎ ｓ ｏ ｆ ｉｎ ，ｈｅｅ ｅ ｇ ｄｉｓｐ ｔ ｕ ｏ ｔ ａ ｔ ｅ ｒ ｔｔｏ ｆｕｄｉｉ ｉｅ ｒ ｏ ｄ ｃｅ ． ａｔ ｅｒ ｕ ｈ ｅ ｓｃ ｅ ｃｅ ｔ ｔ ｎ ｒ ｓ ｉａｉ ｎ ｄ ｅｔ ｅｐ ｒｉｌ ｏ ａｉ ｎ ｈ ｉ ｙ ｏ ｈ ｃ
Experimental investigation of air–water, two-phase flow regimes in

Experimental investigation of air–water, two-phase flow regimes in

Scientia Iranica B(2011)18(4),923–929Sharif University of TechnologyScientia IranicaTransactions B:Mechanical EngineeringExperimental investigation of air–water,two-phase flow regimes in vertical mini pipeP.Hanafizadeh,M.H.Saidi∗,A.Nouri Gheimasi,S.GhanbarzadehSchool of Mechanical Engineering,Sharif University of Technology,Tehran,P.O.Box11155-9567,IranReceived8November2010;revised28April2011;accepted12June2011KEYWORDSMini pipes;Two-phase flow; Flow pattern; Visualization; Flow pattern map.Abstract In this study,the flow patterns of air–water,two-phase flows have been investigated experimentally in a vertical mini pipe.The flow regimes were observed by a high speed video recorder in pipes with diameters of2,3and4mm and length27,31and25cm,respectively.The comprehensive visualization of air–water,two-phase flow in a vertical mini pipe has been performed to realize the physics of such a two-phase flow.Different flow patterns of air–water flow were observed simultaneously in the mini pipe at different values of air and water flow rates.Consequently,the flow pattern map was proposed for flow in the mini-pipe,in terms of superficial velocities of liquid and gas phases.The flow pattern maps are compared with those of other researchers in the existing literature,showing reasonable agreement.©2011Sharif University of Technology.Production and hosting by Elsevier B.V.All rights reserved.1.IntroductionGas–liquid,two-phase flow in micro structures has played an important role in several industrial and medical applications, such as micro heat exchangers,lab-on-chips,bio-MEMS and micro cooling electronics.Physical perception of micro flows is critical in order to optimize and develop the design of such devices.Two-phase flows at mini and micro scale have recently attracted the attention of scientists as a result of its wide usage in advanced science and technology,namely Micro-Electro-Mechanical Systems(MEMS),chemical engineering, bioengineering,medical devises,micro cooling systems,micro structures in computers,etc.The literature survey on this issue has been categorized into adiabatic and phase change work, which has been summarized in this paper.∗Corresponding author.E-mail address:saman@(M.H.Saidi).1.1.Adiabatic worksThe works of Suo and Griffith[1]were among the first studies concentrating on flow patterns in microchannels.They detected three different flow patterns,namely,bubbly/slug, slug and annular flow,in their studies,using channels with widths in the range of0.514–0.795mm.Sadatomi et al.[2] proposed flow regime maps in vertical rectangular channels and indicated that channel geometries have little influence in noncircular channels with large hydraulic diameters greater than10mm.Xu et al.[3]investigated concurrent vertical two-phase flow in a vertical rectangular channel with a narrow gap, experimentally.They reported that with a decrease in channel gap,the transition from one flow regime to another occurs at smaller gas flow rates.They developed a new criterion to predict transition from annular flow,as well.Hestroni et al.[4] performed experiments for air–water and steam–water flow in parallel triangular micro-channels,developed a practical modeling approach for two-phase micro-channel heat sinks and considered the discrepancy between flow patterns of air–water and steam–water flow in parallel micro-channels. Fukagata et al.[5]simulated an air–water two-phase flow in a20µm ID tube,numerically,with focus upon flow and heat transfer characteristics in the bubble train flows.He and Kasagi[6]simulated numerically adiabatic air water slug flow in a micro tube.They focused on pressure drop characteristics and their modeling.They found that the total pressure drop of a slug flow can be decomposed into a frictional pressure drop and a pressure drop over the bubble itself.Carlson et al.[7]1026-3098©2011Sharif University of Technology.Production and hosting by Elsevier B.V.All rights reserved.Peer review under responsibility of Sharif University of Technology.doi:10.1016/j.scient.2011.07.003924P.Hanafizadeh et al./Scientia Iranica,Transactions B:Mechanical Engineering18(2011)923–929investigated characteristics of multiphase dynamics,especially two-phase gas–liquid flow,by means of advanced numerical simulations.They compared two Computational Multi-Fluid Dynamic(CMFD)codes,Fluent and TransAT,and reported a prediction of recirculating flow in the bubbly flow case using TransAT,while significant recirculation was not observed in the solution using Fluent.Saison and Wongwises[8]performed a series of experiments in a horizontal circular micro channel with an inner diameter of0.15mm.They presented a flow pattern map in terms of the phase superficial velocities, and proposed a new pressure drop correlation for practical application.1.2.Phase change worksThe pool boiling heat transfer,in a vertical narrow annular with closed bottoms,was observed through a transparent quartz shroud by Yao and Chang[9],and stages of evolving boiling phenomena with an increase in heat flux were reported. Several researchers observed three basic flow patterns,namely, bubbly,slug and annular flow,in the mini pipe and channel. Damianides and Westwater[10]performed experiments with a1mm tube,and Mertz et al.[11]and Kasza et al.[12] studied the flow visualization of water nucleation in a single rectangular channel of2.5mm by6mm.Lin et al.[13]used a single round tube with2.1mm inside diameter for their experiments,and compared the flow transitions with those predicted by Bernea et al.[14].Sheng and Palm[15]performed their experiments with1–4mm diameter tubes.Cornwell and Kew[16]found three different flow patterns for R-113, namely,isolated bubbles confined bubbles and slug/annular flow,in rectangular channels with cross sectional areas of 1.2–0.9mm and3.5–1.1mm.Ory et al.[17]considered the effects of capillary,inertia,friction and gravity forces on the velocity distribution and temperature field along a single capillary two-phase flow in a heated micro-channel.Research dealing with gas–liquid,two-phase flow in micro-channels,in situations where fluid inertia was significant in comparison with surface tension,was reviewed by Ghiaasiaan and Abdel-Khalik[18].Jiang et al.[19]studied the boiling of water in triangular micro-channels,having widths of50and100µm. They observed individual bubbles at low heat fluxes,and an abrupt change in flow pattern to an unstable slug flow with increasing heat flux.Chedester and Ghiaasiaan[20]addressed the hydro-dynamically controlled Onset of a Significant Void (OSV)in heated micro tubes.They derived a simple semi-empirical correlation for the radius of departing bubbles at the OSV point to show the accuracy of their hypothesis.Some experimental studies have been reported on gas liquid two-phase flow in mini and micro conduits by Kandlikar[21],Lee and Mudawar[22]and Serizawa et al.[23].The three zone boiling heat transfer model was developed by Thome et al.[24]. Revellin and Thome[25]used an optical measurement method for two-phase characteristics of R-134a and R-245fa,in0.5mm and0.8mm diameter channels,to determine the frequency of bubbles existing in the microevaporator.They detected four flow patterns,namely,bubbly,slug,semi-annular and annular flow,whose transitions were not well compatible with neither the macroscale map of refrigerants nor the microscale map of air–water flow.Sobierska et al.[26] experimentally investigated the water boiling phenomena in a vertical rectangular microchannel,with a hydraulic diameter of0.48mm.They observed three main flow patterns,namely, bubbly,slug and annularflow.Figure1:Schematic of test apparatus.Due to the effects of surface tension,two-phase flows at mini and micro scale have different behavior in comparison with the macro scale.The aim of the present work is to visualize flow regimes in air–water two-phase flows and propose a flow regime map for such flows in vertical mini pipes.The neural network technique is implemented to recognize and predict a gas–liquid,two-phase flow pattern in mini tubes,having diameters of2,3and4mm.2.Experimental setupThis study is carried out by experimental apparatus schematically shown in Figure1.Air and water are used as gas and liquid phases in the experiments.The water flow rates are regulated by the needle valves and are measured by the cali-brated rotameter.Air and water are mixed together in a mixer made of acrylic glass and placed at the bottom of the riser pipe. The compressed air is fed by the compressor via an air injec-tor,which is schematically depicted in Figure2.The water flows from the center hole of a mixer,with a diameter of2mm,while air is injected into the holes around the center hole,each having 1mm diameter.The air flow rates are set by the regulator valve and are continuously measured by the calibrated gas rotameter. The overall height and inside diameter of the riser pipe are sum-marized in Table1.In order to have the opportunity to visually observe the two-phase flow patterns,the riser pipe was made of transparent glass.The air water mixture was directed upward through the riser,separated in the separation tank at the top of the riser and the air was discharged into the atmosphere.Differ-ent flow regime images were captured by a digital high speed camera,with a frame rate of1200fps,from the test section of the upriser.The test section is placed after the entrance section to diminish the effect of the entrance region.The length of the entrance section is about500mm.The superficial air and water velocities are0.5–10m/s and0.05–1m/s,respectively.P.Hanafizadeh et al./Scientia Iranica,Transactions B:Mechanical Engineering 18(2011)923–929925Figure 2:Schematic of air and watermixer.Figure 3:(a)RGB picture;(b)gray picture;and (c)subtracting and median process of flow in the pipe (3mm diameter).3.Experimental results 3.1.Image processingImage processing techniques must be performed in order to extract features from the images of the two-phase flow.Each picture has 8bit RGB (red,green and blue)color format,being converted from RGB to a grey scale mode.The output image has 256grey levels from 0(black)to 255(white).It is difficult to extract the bubbles directly from an original digital image and therefore preprocessing procedures must be undertaken to reduce noise and improve the quality of the images.An image-subtracted algorithm was used to reduce background noise by subtracting the background image from each dynamic image.In order to smooth the image border,a median filter was also used.A sliding window (3×3)was used in this process,and the median gray level of the pixels in the window was ter,the gray level of the pixels located at the center of the window was replaced by the median.The result of these processes is shown in Figure 3.3.1.1.Inverting binary imageThe images were converted from grayscale to binary mode by threshold segmentation,and an iterative procedure was used to calculate the optimizing threshold as follows [27]:Figure 4:Binary image of two-phase flow in the mini pipe.(a)The minimum and maximum of the gray level,namely Z land Z k ,are found in the image,and the initial value of the threshold is derived from their arithmetic average as:T 0=(Z k +Z l )/2.(1)(b)According to the initial value of threshold T K ,the imageis divided into two parts,namely,object and background,and the average value of the gray level in each part is calculated as:Z O =−Z (i ,j )<T kZ (i ,j )N O,(2)Z B =−Z (i ,j )>T kZ (i ,j )N B,(3)where Z (i ,j )is the gray level of the pixel (i ,j )in the image,N O is the number of the pixels in which Z (i ,j )is less than T K ,and N B is the number of pixels in which Z (i ,j )is more than T K .(c)The new threshold is calculated based on the arithmeticaverage of the object and background segments of the image as:T k +1=(Z O +Z B )/2.(4)If T K =T K +1,then the algorithm is finished,else K ≪=K +1,and turn to step (b).The binary image of the bubbles in the vertical pipe,which is the result of the above procedure,is shown in Figure 4.3.1.2.Image morphology processingSome morphological functions,such as dilation,erosion,opening and closing operations,were applied to modify the shapes of bubbles.Dilation adds pixels to the boundaries of the objects in an image,while erosion removes pixels on the object boundaries.The definition of a morphological opening of an image is erosion followed by dilation,using the same structuring element for both operations.The related operation,morphological closing of an image,is the reverse.It consists of dilation followed by erosion,with the same structuring element.Both of them do not significantly alter the area or shape of objects.The opening operation removes small objects and smoothes boundaries.Borders removed by erosion are restored by dilation,but small objects that were absorbed during erosion do not reappear after dilation.The closing operation was used to fill tiny holes and smooth boundaries.Objects were expanded by dilation and then reduced by erosion,so borders were smoothed and holes were filled [28,29].After926P.Hanafizadeh et al./Scientia Iranica,Transactions B:Mechanical Engineering 18(2011)923–929Figure 5:Final image of two-phase flow in the mini pipe.these operations,the result of image processing is shown in Figure 5.Bubble images of two-phase flow were clear using the above image processing,and it prepared bubbles for quantitative analysis,such as measuring area,perimeter and diameter.3.2.Flow pattern mapIn the experimental procedure while varying gas or liquid mass flow rate,a 10s film was recorded from the flow regime at a speed of 1200fps.The recorded film was replayed in slow motion for recognition of flow regimes.Each film converted to separate frames in a picture format using Adobe Premiere software.The achieved pictures were used as inputs of image processing techniques.The final binary pictures were used for the mentioned post processing procedure,such as flow regimedetection,void fraction and bubble velocity calculation,etc.Figure 6shows those typical flow regimes observed in the vertical,co-current,air–water,two-phase flows,in the 3mm mini pipe.Four basic flow patterns,namely,bubbly,slug,churn and annular,accompanied by their transitions,are illustrated in these figures.The visualization shows that air–water two-phase flows in mini pipes do not have three dimensional behaviors,especially in bubbly and slug flows.The final processed images of different flow regimes in air–water,two-phase flow in mini pipes have been presented in Figure 7.Figures 8–10show the flow pattern map for a vertical round tube with inner diameters of 2,3and 4mm,respectively.The proposed maps are in terms of superficial velocities of phases,and the four main flow patterns are depicted in these maps.In Figure 11,the achieved flow pattern for the pipe with 2mm ID was compared to the work of Ide et al.[30],shown by a solid line.They divided the flow pattern map into the four main regions,namely,dispersed bubbly flow,intermittent flow,churn flow and annular flow.The comparison shows that the bubbly and annular flows in the present work are not well in accordance with those of Ide et al.In the present work,the dispersed bubbles were not seen,because the air bubble injector did not have very thin holes.As a result,the created bubbles mostly have diameters in the range of the pipe diameter.Even the existence of air injectors with thin holes cannot guarantee the creation of bubbly flow.In the case of small bubbles occurring,as a result of thin holes in the air injector and the developed two-phase flow,they would collapse,resulting in large bubbles know as intermittent flow.Bubbly flows are mainly promoted by bubble breaking mechanisms,due to turbulence effects.It seems that in small diameter pipes,the formation of a specific flowpatterns(a)Bubbly.(b)Bubbly-slug.(c)Slug.(d)Messy-slug.(e)Churn.(f)Wispy-annular.(g)Ring.(h)Wavy-annular.(i)Annular.Figure 6:Different flow patterns in a vertical pipe with 2mm diameter.P.Hanafizadeh et al./Scientia Iranica,Transactions B:Mechanical Engineering18(2011)923–929927(a)Bubbly.(b)Slug.(c)Messy-slug.(d)Churn.(e)Ring.(f)Wavy-annular.Figure7:Final processed image of different two-phase flow regimes in the minipipe.Figure8:Flow patterns for2mm innerdiameter.Figure9:Flow patterns for3mm inner diameter.mainly depends on mixer configuration.The radial air supplierused in this study makes intermittent flow patterns,such asslug and churn flows,while the air supply in the tube centerfavors annular flow.This can be the reason for an absence ofannular flow in the proposed flow patterns.The comparison offlow patterns also reveals that the slug,messy slug andsemi-Figure10:Flow patterns for4mm inner diameter.annular flows in the proposed map are in accordance with theintermittent flow of Ide et al.[30].In the present study,a noticeable difference between flowpattern maps for vertical pipes with various diameters of2,3and4mm is not seen.This can be justified in regard tothe fact that the dominant forces acting on the air–watermixture in the small diameter pipes,namely,gravitation,inertia,surface tension and buoyancy forces,are in the sameorder of magnitude.This concept clearly indicates that thesethree flow patterns can be combined to form a new flow patternfor the gas–liquid,two-phase flow in small diameter pipes.A combination of these three flow patterns results in a newflow pattern map,which is illustrated in Figure12.A FuzzyC-Means clustering technique(FCM)was used to classify theflow patterns.The solid lines in the figure show the transitionregion of the flow patterns.This figure shows the achieved flowmap for mini pipes with diameters in the range of2–4mm.4.ConclusionIn this paper,air–water,two-phase flow patterns wereinvestigated experimentally for mini pipes with diameters of2,3and4mm.An image processing technique was used fordetection of flow patterns from pictures derived from filmsrecorded with a high speed camcorder.The obtained flowpatterns reveal that there is no noticeable difference between928P.Hanafizadeh et al./Scientia Iranica,Transactions B:Mechanical Engineering 18(2011)923–929Figure 11:Comparison between the achieved flow patterns with the work of Ide et al.[30]for a pipe with diameter of 2mm.Figure 12:Proposed two-phase vertical upward flow pattern map.two-phase,upward flow patterns in this range of diameters.A new flow pattern map was achieved for vertical mini pipes,due to a comparison of the flow patterns of these three diameters of pipe.The proposed map was compared with existing research.A comparison of the present work and previous research shows that the flow patterns of slug,messy slug and semi-annular in the present work are compatible with the intermittent flow pattern of Ide et al.[30].However,in the present study,the annular flow is seen at a lower superficial air velocity than that in the work of Ide et al.[30].AcknowledgmentsThis research was funded by Iran Supplying Petrochemical Industries,Parts,Equipment and Chemical Design Corporation (SPEC),as a joint research project with Sharif University of Technology (project No.KPR-8628077).References[1]Suo,M.and Griffith,P.‘‘Two-phase flow in capillary tubes’’,Int.J.Basic Eng.,86,pp.576–582(1964).[2]Sadatomi,Y.,Sato,Y.and Saruwatari,S.‘‘Two-phase flow in verticalnoncircular channels’’,Int.J.Multiphase Flow ,8,pp.641–655(1982).[3]Xu,J.L.,Cheng,P.and Zhao,T.S.‘‘Gas–liquid two-phase flow regimes inrectangular channels with mini/micro gaps’’,Int.J.Multiphase Flow ,25,pp.411–432(1999).[4]Hetsroni,G.,Mosyak, A.,Segal,Z.and Pogrebnyak, E.‘‘Two-phaseflow patterns in parallel micro-channels’’,Int.J.Multiphase Flow ,29,pp.341–360(2003).[5]Fugakata,K.,Kasagi,N.,Ua-arayaporn,P.and Himeno,T.‘‘Numericalsimulation of gas liquid two-phase flow and convective heat transfer in a micro tube’’,Int.J.Heat and Fluid Flow ,28,pp.72–82(2007).[6]He,Q.and Kasagi,N.‘‘Numerical investigation on flow pattern andpressure drop characteristics of slug flow in a micro tube’’,6th Int.ASME Conf.on Nanochannels,Microchannels and Minichannels ,Darmstadt,Germany,pp.24–35(2008).[7]Carlson,A.,Kudinov,P.and Narayanan,C.‘‘Prediction of two-phase flowin small tubes:a systematic comparison of state-of-the-art CMFD codes’’,5th Europe Thermal-Sci.Conf.,The Netherlands,pp.138–150(2008).[8]Saisorn,S.and Wongwises,S.‘‘An experimental investigation of two-phaseair–water flow through a horizontal circular micro-channel’’,Exp.Thermal Fluid Sci.,33,pp.306–315(2009).[9]Yao,S.C.and Chang,Y.‘‘Pool boiling heat transfer in a confined space’’,Int.J.Heat Mass Transf.,26,pp.841–848(1983).[10]Damianides,D.A.and Westwater,J.W.‘‘Two-phase flow patterns in acompact heat exchanger and in small tubes’’,2nd UK National Conf.on Heat Transf.,11,United Kingdom,London,pp.1257–1268(1988).[11]Mertz,R.,Wein,A.and Groll,C.‘‘Experimental investigation of flow boilingheat transfer in narrow channels’’,Calore e Technologia ,14(2),pp.47–54(1996).[12]Kasza,K.E.,Didascalou,T.and Wambsganss,M.W.‘‘Microscale flow visu-alization of nucleate boiling in small channels:mechanisms influencing heat transfer’’,Int.Conf.on Compact Heat Exchanges for the Process Indus-tries ,New York,USA,pp.343–352(1997).[13]Lin,S.,Kew,P.A.and Cornwell,K.‘‘Two-phase flow regimes and heattransfer in small tubes and channels’’,11th Int.Heat Transf.Conf.,Kyongju,Korea,2,pp.45–50(1998).[14]Barnea,D.,Luninsky,Y.and Taitel,Y.‘‘Flow pattern in horizontal andvertical two-phase flow in small diameter pipes’’,Canadian J.Chem.Eng.,61,pp.617–620(1983).[15]Sheng, C.H.and Palm, B.‘‘The visualization of boiling in small-diameter tubes’’,Int.Conf.on Heat Transport and Transport Phenomena in Microsystems ,Banff,Canada,pp.44–53(2001).[16]Cornwell,K.and Kew,P.A.‘‘Boiling in small parallel channels’’,CEC Conf.on Energy Eff.in Process Tech.,Athens,Greece,pp.624–638(1992).[17]Ory, E.,Yuan,H.,Prosperetti, A.,Popinet,S.and Zaleski,S.‘‘Growthand collapse of a vapor bubble in a narrow tube’’,Phys.Fluids ,12,pp.1268–1277(2000).[18]Ghiaasiaan,S.M.and Abdel-Khalik,S.I.‘‘Two-phase flow in micro-channels’’,Adv.Heat Transf.,34,pp.145–253(2001).[19]Jiang,L.,Wong,M.and Zohar,Y.‘‘Forced convection boiling in a micro-channel heat sink’’,Int.J.Micro-Electro-Mech.Sys.,10,pp.80–87(2000).[20]Chedester,R.C.and Ghiaasiaan,S.M.‘‘A proposed mechanism for hydrodynamically-controlled onset of significant void in microtubes’’,Int.J.Heat Fluid Flow ,23,pp.769–775(2002).[21]Kandlikar,S.G.‘‘Fundamental issues related to flow boiling in minichan-nels and microchannels’’,Exp.Therm.Fluid Sci.,26,pp.389–407(2002).[22]Lee,J.and Mudawar,I.‘‘Two phase flow in high heat flux micro channelheat sink for refrigeration cooling applications’’,Int.J.Heat Mass Transf.,48,pp.928–955(2005).[23]Serizawa,A.‘‘Gas liquid two-phase flow in microchannels’’,In MultiphaseFlow Handbook ,C.T.Crowe,Ed.,2nd ed.,pp.830–887,CRC Press (2006).[24]Thome,J.R.,Dupont,V.and Jacobi, A.M.‘‘Heat transfer model forevaporation in micro channels’’,Int.J.Heat Mass Transf.,47,pp.3375–3385(2004).P.Hanafizadeh et al./Scientia Iranica,Transactions B:Mechanical Engineering18(2011)923–929929[25]Revellin,R.and Thome,J.R.‘‘Experimental investigation of R-134a andR-245fa two-phase flow in microchannels for different flow conditions’’, Int.J.Heat Fluid Flow,28,pp.63–71(2007).[26]Sobierska, E.,Kulenovic,R.and Mertz,R.‘‘Heat transfer mechanismand flow pattern during flow boiling of water in a vertical narrow channel experimental results’’,Int.J.Thermal Sci.,46,pp.1172–1181 (2007).[27]Shi,L.‘‘Fuzzy recognition for gas–liquid two-phase flow pattern based onimage processing’’,Proc.of13rd IEEE Int.Conf.on Control and Automation, pp.1424–1427(2007).[28]Heijmans,H.J.A.M.,Morphological Image Operators,Academic Press,NewYork(1994).[29]/help/toolbox/images/index.html.[30]Ide,H.,Kariyasaki,A.and Fukano,T.‘‘Fundamental data on the gas–liquidtwo-phase flow in minichannels’’,Int.J.Thermal Sci.,46,pp.519–530 (2007).Pedram Hanafizadeh received his M.S.and Ph.D.Degrees in Mechanical Engineering from the Centre of Excellence in Energy Conversion at Sharif University of Technology,Tehran,Iran,in2005and2010,respectively.His work is mainly concentrated on the field of Multiphase Flow,Experimentally, Numerically and Analytically.His research interests include Characteristics of Multiphase Flow,Heat Transfer,Boiling and Condensation,Instrumentation in Fluid Flow,Image Processing for Flow Field Analysis,and Industrial and Applicable Usage of Multiphase Flow.Mohammad Hassan Saidi is Professor and Chairman of the School of Mechanical Engineering at Sharif University of Technology,Tehran,Iran. His current research interests include Multiphase Flows,Heat Transfer Enhancement in Boiling and Condensation,Modelling of Pulse Refrigeration, Vortex Tube Refrigerator,Indoor Air Quality and Clean Room Technology, Energy Efficiency in Home Appliances and Desiccant Cooling Systems.Arash Nouri Gheimasi obtained his B.S.Degree in Mechanical Engineering in2010,and is currently an M.S.student at the Centre of Excellence in Energy Conversion at the School of Mechanical Engineering,Sharif University of Technology,Tehran,Iran,under the supervision of Professor Saidi.His B.S. thesis involved work on the Characteristics of Gas-Liquid Two-Phase Flow in Mini Pipes and he is now working on Application of Visual Techniques in Two-Phase Flow.His research interests include the area of Two Phase Flow and Its Industrial Applications.Soheil Ghanbarzade received his B.S.and M.S.Degrees in Mechanical Engineering from the Centre of Excellence in Energy Conversion at Sharif University of Technology in2008and2010,respectively.Since then he has worked under the supervision of Professor M.H.Saidi as research staff in the Multiphase Group.His research interests include:Analytical,Numerical and Experimental Methods to Study Characteristics of Large Scale and Mini Scale Air-Water,Two-Phase Flows.He holds a Gold medal from the13th National Olympiad of Mechanical Engineering in Iran,and is currently a Ph.D.student of Petroleum Engineering at the University of Texas,Austin,USA.。

阶梯消能工

阶梯消能工

EXPERIMENTATION
The experiments are realized by means of the complex fluid measurement installation in the hydraulic institute of Ain el Hadjar (Saida, Algeria). It concerns an installation allowing fluid circulation in a variable slope glass channel. The installation allows mainly observing the different flow regimes.
The adaptation of this kind of technique in the
Accepted for Publication on 15/1/2010.
hydraulic structures is of a great interest to both the structure and the environment. The spillways present in addition to good structure stability, water aeration during its passage over the spillway. The spillway technique became mainly an economical mean. The gain on the project cost is about 20% compared to other techniques (e.g., inclined spillways, vertical full spillways), (Peyras et al., 1992).