Linear stability of confined coaxial jets in the presence of gas velocity oscillations with heat and mass transfer

We numerically study the dynamics of axisymmetric compound liquid threads with a phase-field model. Compound threads consist of a middle annular thread enclosing an inner core liquid and surrounded by an outer immiscible liquid. The model is composed of the Navier–Stokes equation, including a surface tension force term, and the convective ternary Cahn–Hilliard system. The finite difference method is used to discretize the governing equations and the resulting discrete equations are solved by using a multigrid method. A variety of numerical tests are performed to investigate the effects of inner and middle liquid radii, viscosity ratio, surface tension ratio, initial amplitude, and evolution mode on the dynamics of the axisymmetric compound liquid threads. We find that a larger inner fluid radius delays the evolution, a larger middle fluid radius suppresses the formation of double droplets, the evolution of compound liquid threads is delayed if we increase the viscosity ratio or surface tension ratio. Furthermore, a squeezing mode produces a more complex evolution process than a stretching mode.

Numerical simulations of the dynamics of axisymmetric compound liquid threads with a phase-field model

Instability and energy budget analysis of viscous coaxial jets under a radial thermal field

The temporal instability of a confined viscous liquid jet surrounded by high speed co-flowing viscous gas phase is studied in this work. The effect of the longitudinal acoustic oscillations, which is regarded as gas axial velocity oscillations, is also considered. The heat and mass transfer is characterized by the ratio between conduction heat flux and the evaporation heat flux, then an explicit dispersion relation equation is obtained. The results suggest that more than one unstable region appears because of the gas velocity oscillations, including Kelvin-Helmholtz (K-H) instability and parametric instability regions. Increasing the forcing frequency enhances the K-H instability, while it has a stabilizing effect on the parametric instability. In addition, the liquid jet tends to be more unstable in non-axisymmetric modes when the gas rotating strength is strong. Although the gas viscosity has a destabilizing effect on the gas-liquid interface, the destabilizing effect is weak due to the low viscosity of gas phase. According to the linear instability theory, the dominant wavenumber will locate in the most unstable region. Moreover, the parametric instability in non-axisymmetric modes may be observable when the Weber number is large.

Temporal instability of confined three-dimensional liquid jet with heat and mass transfer under longitudinal acoustic oscillations

This paper investigates the temporal instability of an eccentric compound liquid thread. Results of linear stability are obtained for a typical case in the context of compound threads in microencapsulation. It is found that the disturbance growth rate of an eccentric compound liquid thread is close to that of the corresponding concentric one, in terms of both the maximum growth rate and the dominant wavenumber. Furthermore, linear stability results over a wide parameter range are obtained and the conclusion is basically unchanged. Energy balance of the destabilization process is analyzed to explain the mechanism of instability, and it is found that although the disturbance growth rate of an eccentric compound thread is close to that of the corresponding concentric thread, their energy balances are distinctively different. The disturbance interface shape and disturbance velocity distributions are plotted. It is found that the behavior of the disturbance velocity in the cross section plane is different from...

Jie Peng

Papers

Instability of eccentric compound threads