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1、15th Australasian Fluid Mechanics Conference The University of Sydney, Sydney, Australia 13-17 December 2004Numerical simulation of drop formation in a T-shaped microchannelJ.L. Liow11Mechanical Engineering Discipline, School of EngineeringJames Cook University, Qld, 4811 AUSTRALIAAbstractThe format
2、ion of water drops of specific size when sheared by a tetradecane stream at a T-junction in a microchannel was in- vestigated numerically. The numerical results showed that thewater drop sizes decreased with increasing tetradecane flow rate and decreasing interfacial tension. This is in agreement wi
3、th published experimental results.During the formation of the water drops, two recirculating zones are formed in the water stream, a large one occupying most of the water drop, and a smaller one near the downstream corner of the T-junction. The smaller recirculating zone results in a high pressure s
4、pot.IntroductionIn the last two decades, the emphasis on fluid flow in MEMSdevices has been focused on the study of single fluid flows, in- volving only a gas or a liquid 4. In contrast, the dispersionof the droplets of one fluid in a second immiscible fluid has re- ceived less attention. Neverthele
5、ss, there is a wide range of ap-plications for multiphase flows in microchannels and such flows are found in naturally occurring systems. The ability to control and manipulate the drop size and frequency enables the drop formation process to be tailored. This is of immense impor- tance as it allows
6、the outcomes of the applications to be veryspecific. In recent years, the use of microchannels for the for-mation of drops of uniform and specific sizes has been shown experimentally to be feasible. However, there is a wide range ofmicrochannel configurations that have been published and the drop fo
7、rmation processes are quite different for different con-figurations. The simplest configuration is the T-junction whereone liquid shears a second liquid that flows in the perpendicular arm.Early work by Thorsen et al. 13 showed that aliphatic straight chained hydrocarbons (HC) with 2% SPAN 80 were a
8、ble toshear a normal flow of water to form a variety of drop shapes. The drop shapes were strongly dependent on the sharpness of the edges of the T-junction. Further work by Tabeling et al. 12 suggests that such drops can only be formed when the surfacetension of the HC is significantly lowered by S
9、PAN 80 used as the surfactant. The roughness of the channel surface also plays a critical role in the drop formation process. In contrast, Ni- sisako et al. 7 managed to form droplets without the use of surfactants albeit the channels they used were 100500 m in width and 100 m in depth compared to 3
10、5 m in width for the work of Thorsen et al. 13 and Tabeling et al. 12. Apart fromchannel size, there were a few significant differences between Thorsen et al. 13 and Nisisako et al. 7. First, Nisisako etal. 7 used a sunflower oil where the fluid properties were notmeasured but sunflower oil does hav
11、e a much higher kinematic viscosity than straight chained HC. Second, Nisisako et al. 7 produced their microchannels with a micromilling technique re- sulting in extremely sharp corners. Thorsen et al. 13 produced their microchannels from moulding with an acrylated urethane where the corners are muc
12、h smoother.Other more complex configurations include the use of multi-ple slits to focus the flow and produce drops that are muchsmaller than the slit width 1, or the use of fine pores and stepped structures to try and produce reproducible drop sizes 11. More complex network of T-junctions have been
13、 shown to enable drop size distributions to be controlled (Link et al. asreferenced in 10). The control of drop breakup is influencedsignificantly by the interfacial tension between the two fluids and Dreyfus et al. 3 have found that without the presence of Span80, drops do not form in microchannels
14、 of 20 m by 200 m cross section.In this study, a T-junction microchannel is numerically simu-lated using a volume of fluid (VOF) code in 2D to determinethe breakup of water drops entering into a flow of tetradecane. This is compared to the experimental result of Cole 2. Al- though the depth of the c
15、hannel may affect the drop formation, much information can be obtained of the behaviour of the drop break up characteristics in 2D prior to a full simulation in 3D.Mathematical formulationDetailed description of the multifluid VOF (MFVOF) code has been published elsewhere 5 and only a brief descript
16、ion will be included here.The distribution of the fluid species is tracked as a “colour” C where in the continuous limit within a computational domain represented by a 2D axisymmetric mesh,C(r,z) is the Heaviside functionC =1,if point (r,z) is occupied by C fluid; 0,otherwise.(1)All interfaces throughout the computational domain are de- duced from the spatial locations of the discontinuities in the distribution of C. The equations solved are:Equation of c
