Instability of eccentric compound threads

In this work, the linear temporal stability of a confined coaxial jet has been examined in the presence of gas velocity oscillations with heat and mass transfer. The viscous potential flow theory was applied to account for the liquid and gas viscosities. Results suggest that gas velocity oscillations have a destabilizing effect. The forcing frequency restrained the instability in the parametric unstable region but enhanced the instability in the Kelvin-Helmholtz (K-H) unstable region. Heat and mass transfer decreased the curvature of the surface wave directly and enhanced the hydrodynamic force via the phase change. Heat and mass transfer had a stabilizing effect on the capillary instability, and a dual effect on K-H instability without oscillations. Results similar to the K-H instability were discovered when the oscillations were considered. Gas viscosity played a destabilizing role with the effect of heat and mass transfer, especially reducing the critical velocity for the appearance of the instability; moreover, the liquid viscosity had a stabilizing effect for all the cases discussed.

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

Jia,

Xie,

Cui

et al. 2019

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

Qiao

Luo

et al. 2020

Temporal linear instability of viscous coaxial jets under a radial thermal field is carried out by considering axisymmetric and non-axisymmetric disturbances. The interfacial tensions of different fluids are taken to be temperature dependent. The para-sinuous, para-varicose, and helical unstable modes are identified in the Rayleigh regime. The energy budget is also employed to explore the relative importance of thermal-induced stresses on the jet instability at the most unstable wavenumber by changing the dimensionless parameters. It is shown that decreasing the temperature ratio of inner fluid to surrounding fluid (T13) promotes the jet instability. For coaxial jets at T13 > 1, the Marangoni flow makes coaxial jets more stable, and increasing the fluid thermal conductivity suppresses the jet instability. For coaxial jets at T13 < 1, however, their influences on the jet instability are opposite. Compared with the thermal-induced stresses at the inner and outer interfaces, the inner interfacial tension is the main factor dominating the flow. Increasing either inner interfacial tension or outer surface tension and decreasing viscosity of any fluid can promote the instability of coaxial jets. The variations of thermal conductivity and specific heat capacity of either inner or surrounding fluids apparently influence the jet instability of the para-varicose mode, but hardly influence that of the para-sinuous mode. This work would provide great insight into the physical mechanism of thermal jet instability in various applications.

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

Deng

Wang²,

Cui³

et al. 2022

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.