Instability of a charged annular liquid jet in the presence of an axial magnetic field
Date Issued
2010
Date
2010
Author(s)
Chen, Li-Yen
Abstract
Subject to the electrically repulsive force, charged annular jets are more unstable than uncharged jets. The short-wavelength disturbances often nonlinearly grow due to the interaction between various instability mechanisms, and eventually leads to the breakup into micron-scale drops. The involving phenomena have important applications in various engineering and technologies over the past 20 years. For example, the performance of engine combustors strongly depends on the atomization process of fuel oils. A complete understanding of the mechanisms beyond the instability phenomena is essential to the practical applications. This study focuses on the effects of various physical mechanisms on the onset of instability of a charged annular jet. In addition, we also examine the stabilizing ability of the magnetic field to benefit the fabrication of hollow nanofibers, in which the instability is undesirable.
The presented theoretical model is a complex system coupled with electromagnetic field and flow motion, and the governing equations, including the Maxwell equations and the Navier-stokes equations with moving internal and external interface, make the mathematical procedure extremely difficult to handle with. Thus, simplicity with some reasonable assumption is necessary. In the context, we clearly reveal that when the fluid conductivity is limited, the mathematical model can be approximately simplified as a quasi-electrostatic form. After linearization and normal mode expansion, we obtain an eigenvalue problem in terms of ordinary differential equations, which are then solved by using Chebyshev collocation method. The results show that an annular jet usually possesses two unstable modes (i.e. para-sinuous and para-varicose modes) due to the presence of the inner surface, and the para-sinuous mode is found to be always more unstable than the para-varicose mode. A decreasing thickness of the annular jet enhances the growth rate of the para-sinuous mode while suppressing that of the para-varicose modes. In addition, we also found that the asymmetric mode with m=2 may dominate the instability in a small parameter range. This implies that the annular jet under some special conditions tends to split into multiple jets. Such a phenomenon has not been found in the system of round jets. For the cases with the magnetic effect, the para-sinuous and the para-varicose modes can transform with one another at a sufficiently high magnetic field strength. This phenomenon is possibly resulted from the twisted effect caused by the Lorenz force, which makes non-synchronized growth of the inner and outer surfaces.
This research devotes to the understanding of the physical mechanisms leading to unstable deformation of charged annular jets by carrying out a detailed parameters analysis. The resultant parameter maps, in which the boundaries between various instability types are clearly depicted, should be able to provide engineers a useful guideline for novel designs in future.
Subjects
Flow instability
Electrospray
Atomization
Hollow Nanofibers
Liquid jet
Electro-magneto-hydrodynamics
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-99-R97543071-1.pdf
Size
23.53 KB
Format
Adobe PDF
Checksum
(MD5):50af1514f29fb0df57bf16cf181bcd45