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Experimental and Numerical Investigation on the Splitting of Microdroplets in Symmetrical and Asymmetrical T Junctions Using Magnetic Fields

Aboutalebi, Mohammad | 2022

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 54927 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Shafii, Mohammad Behashad; Kazemzadeh Hannani, Siamak
  7. Abstract:
  8. Ferrofluids are used in various fields such as drug delivery systems, heat transfer, biotechnology and separation processes. In this study, the splitting of ferrofluid microdroplets in symmetric and asymmetric T-junctions in the presence of a magnetic field has been investigated numerically and experimentally. These studies have shown that the creation of an asymmetric magnetic field can either affect the phenomenon of ferrofluid microdroplet breakup or the volume of microdroplets that are created after breaking and leaving each branch. Furthermore, more control can be applied to the phenomenon of microdroplet breakup. In the numerical study, the ferrofluid microdroplet breakup at different microdroplet lengths and velocities inside symmetrical and asymmetrical T-junctions (branches with unequal widths) in the presence of an asymmetric magnetic field is investigated. In these studies, 4 different regimes for ferrofluid microdroplet breakup have been observed while passing from symmetrical T-junction, as well as 1 regime of non-breakup. One of the current identified splitting regimes was observed in this study for first time, called splitting hybrid mode (SHM). In this regime, the dumbbell-shaped head close to the magnetic field is separated from the upper wall of the T-junction, while the dumbbell-shaped head on the opposite side is still attached to the wall. Other results showed that the increment of the strength of the magnetic field, the possibility of asymmetric splitting of the ferrofluid microdroplet increases in such a way that for high values of the magnetic field, it is possible to reach splitting ratios close to one. Splitting ratio refers to the volume ratio of the baby microdroplet inserted into the right branch (which has less resistance) to the total volume of the mother microdroplet. It was also observed that with an increment in capillary number and microdroplet length, the splitting ratio tends to 0.5. On the other hand, it was observed that by reducing the width ratio of the branches, the breakup ratio tends to 1. Furthermore, using the results obtained in different magnetic bond numbers and curve fitting, a regression relationship is obtained which can be used to accurately predict the boundary curves between breakup and non-breakup zones for symmetrical T-junction. Furthermore, using the results obtained in different magnetic bond numbers and curve fitting, a regression relationship was obtained which can be used to accurately predict the boundary between breakup and non-breakup zones of ferrofluid microdroplets in symmetrical T-junction for different values of the magnetic bond. During the experimental investigations, a fixed magnet was placed at different distances from the right side of the T-junction, creating an asymmetric magnetic field. Then, for different ranges of lengths and velocities of ferrofluid microdroplets, the effect of changing the magnetic flux density on the behavior of ferrofluid microdroplets at the T-junction is investigated. The results show that with increasing magnetic flux, the probability of non-breakup or asymmetric breakup of microdroplets to two baby microdroplets with different volumes increases. For example, in the absence of a magnetic field, the breakup ratio for a ferrofluid microdroplet with a dimensionless length of 1.34 is 0.5. However, with increasing the amount of magnetic flux to 100 and 170 mT, the breakup ratio increases to 0.669 and 0.74, respectively. It is also observed that in a constant magnetic flux, the ferrofluid microdroplet tends to break asymmetrically as the capillary number decreases. Finally, using the obtained results and performing the curve fitting, a regression relationship was obtained that can predict the changes between the breakup and non-breakup zones of ferrofluid microdroplets in different states.
  9. Keywords:
  10. Micro Droplet Breakup ; Ferrofluid ; T-Shape Connection ; Magnetic Fields ; Splitting Ratio ; Splitting Regime ; Symmetric T Junction ; Asymmetric T Junction

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