فهرست مطالب کیوان فلاح

Droplet based microfluidic systems have found versatile industrial and scientific applications, like biological engineering, lab-on-chip, drug delivery, and encapsulation. In the current study, the breakup of mother droplets in a symmetric Y-junction microchannel under an electric field is numerically investigated. To this approach, a 2D numerical model base on the level set method is performed by the COMSOL Multi-physics software. Two methods for applying the electric field are considered: a symmetric and an asymmetric electric field. In the symmetric electric field, it is observed that the mother droplet splits faster in the presence of an electric field when compared with the case without the electric field. Also, the mother droplet is divided into two daughter droplets with equal sizes. For the asymmetric electric field, the results reveal that the mother droplet splits into two daughter droplets with unequal sizes. Furthermore, a novel flow pattern called hybrid pattern is determined in the asymmetric splitting of droplets. In addition, there is a critical electric capillary number above which a non-splitting pattern.

Motion of drops in microchannels is encountered in a wide range of applications, such as encapsulation, DNA analysis, and drug delivery. In this study, the formation of drops in a Y-junction microchannel is numerically investigated. To this approach, the Ansys Fluent software is used. To verify the simulation, results are compared to the previous experimental data. The comparison depicts that the current simulation has a deviation of less than 10% with previous experimental results. Effect of change in the flow rate ratio (0.6≤Q≤25.2) on the flow pattern, size of drops, the distance between them, and the breakup time are studied for the four junction angles of the microchannel (45, 90, 135 and 180 °) in details. Three flow regimes are observed: parallel, squeezing and dripping flow. Results reveal that the junction angle of 135° is the optimum angle for the formation of the smallest drops in the microchannel. It is also observed that, at the optimum angle, increasing the flow rate ratio from 0.6 to 25.2 decreases the size and time of drop formation by factors of 3.83 and 3.33, respectively. However, it has an adverse effect on the distance between the drops which results in 4.33 time larger distance between formed drops.

An appropriate technique for separation of particles in microfluidic system is the use of dielectrophoresis (DEP), a phenomenon referring to the motion of a particle in a non-uniform electric field due to the unbalanced electrostatic forces on the particle’s induced dipole. Therefore, in current study, separating platelet cells from red blood cells in a continuous flow using dielectrophoretics (DEP) mechanism is numerically investigated in microfluidic device. To this approach, COMSOL Multiphysics software is used. To verify the simulation, the results are compared to previous experimental data. The effects of different parameters including voltage, flow speed and frequency on the separation of platelets are studied in details. The results reveal that successful separation of platelets from blood cells are occurred by applying voltages. Low voltage causes platelet cells and red blood cells to exit from one side of the channel while high voltage causes the red blood cells to collide with the channel wall. According to condition, a range of voltage is suitable for current device to efficiently separate red blood cells and platelets. Also, with increasing particle inlet velocity, the separation takes place farther from the inlet, and the distance between the two consecutive isolated particles of red blood cells or platelets increases.

Droplet motion in microchannels is observed in versatile industrial and scientific applications, such as biological engineering, drug delivery, and encapsulation. In current investigation, breakup of mother droplet in a T-junction microchannel is simulated using the Gerris open-source software. To validate current simulation, results are appraised by the published literatures. The comparison depicts that the current results are in good agreement with previous studies. The effect of the presence of a semi-circular obstacle on the flow pattern and the breakup time of mother droplet is investigated for different Capillary numbers (0.006 Ca 0.019) and the non-dimensional initial length of droplet (L*=2, 3, and 4). The results reveal that by simple modifications on the symmetry T-junction, the mother droplet splits faster in the comparison of the ordinary symmetry T-junction geometry under the same conditions. For instance, the breakup time of mother droplet at Ca=0.01 for L*=2, 3, and 4 approximately decreases 75%, 45%, and 32% in the presence of obstacle, respectively. Furthermore, the breakup time of the drop decreases by increasing the Capillary number.

In the current study, the non-Newtonian fluid over a stationary circular cylinder has been simulated using the lattice Boltzmann method. The regime of 2D and unsteady flow (Re=100) for different non-newotonian power-law indices (0.4≤n≤1.8) and four different aspect ratios of 2, 4, 6 and 8 has been investigated. The power-law model was used for describing the non-Newtonian behavior of the fluid. The results have been compared with previous experimental and numerical results. The effects of the power– law index and aspect ratio on the vorticity contour, average drag coefficient, fluctuations of lift coefficient and time averaged of pressure coefficient have been studied in detail. The results show that the steady or unsteady fluid behavior depend on power-law index and aspect ratio. Generally, the average of drag coefficient deacreases by increasing aspect ratio at fixed power-law index. For all considered aspect ratio, except B=2, the average of drag coefficient increases when the fluid behavior changes from shear thining to shear-thickening.

In this study, falling droplet in the 2-D channel is numerically studied using the lattice Boltzmann method with the pseudopotential model. Quantitative information about the flow variables such as behavior of droplet, stream line and wake of behind of droplet for two cases is investigated: a) of falling of single droplet and b) falling of two droplets in tandem arrangement. To validate the current numerical simulation, the result are compared with previous literature. The comparison shows a good agreement. The results show that wake region at the back of the droplet increases by increasing Eovos number while a reverse trend can be seen by increasing of Ohnesorge number. In the case of falling of two droplets in tandem arrangement, the droplets collide and soon after form a larger droplet with twice the diameter as the initial droplet. Eventually, the droplet breaks up due to high inertia of formed droplet.

In the current study, the non-Newtonian flow over a cylinder is simulated using the lattice Boltzmann method. Two-dimensional unsteady flow at Re=100 and Pr=20 for different non-Newtonian power-law indices (0.4≤n≤1.8) and rotational ratios (0≤β≤3) is investigated. The effects of dimensionless rotational ratio and the power–law index on the vorticity contour, isotherm contour, local Nusselt number, average Nusselt number, periodic average Nusselt number and periodic-surface average Nusselt number are studied in detail. The results show that the fluid behavior changes from Newtonian to shear-thickening, the isotherms contour become elongated and wide, while the change in the liquid behavior from Newtonian to shear-thinning shows the reverse influence on the isotherm patterns. As dimensionless rotational ratio increases, the fluid and the thermal fields become steady. This behavior occurs in both Newtonian and non-Newtonian fluids. It is found that the increment of the shear-thinning and shear thickening of the fluid leads to the increase and decrease of heat transfer rate of circular cylinder surface, respectively. Results also show that the heat transfer rate of circular cylinder surface decreases with increasing of dimensionless rotational ratio.

In the current study, force convection heat transfer from a stationray circular cylinder in non-newtonian fluids has been simulated using the lattice Boltzmann method in 2D and unsteady flow regime. The simulations were performed for three Reynolds numbers (80, 100 and 120), with Prandtl numbers 10 and 20, the different non-newotonian power-law index in the range of 0.4 to 1.8 while varying the Brikman number from 0, 1 to 3. The effects of different Parameters on the vorticity contour, isotherm patterns, local Nusselt number and periodic-surface average Nusselt number is studied. The results show that viscous dissipation plays main when the operating fluid is non-newtonian. As the fluid behavior changes from shear-thinning fluids to Newtonian and then to shear-thickening fluids, the length and width of the wakes increase and sepration between the isotherms contours deacreases. Broadly, the rate of heat transfer deacreses with decreasing Reynolds and Prandtl numbers, and with increasing value of the Brikman number and the power-law index.