Stability Of Liquid Film Flow Research Articles

Effect of anisotropy and inhomogeneity on the stability of liquid film flowing down a porous inclined plane

We examined the linear stability of a Newtonian liquid film flow past a porous inclined plane. Falling film on inclined permeable planes displays three instability modes: surface, shear, and porous mode. Most of the previous studies have examined the film flow past an isotropic and homogeneous porous medium. We could find only one study that examined the film flow past an anisotropic and inhomogeneous porous inclined plane; however, it focused on parameter regimes where the porous mode remains stable. Here, we explored the parameter regimes where all three modes become unstable and investigated the effect of the anisotropic and inhomogeneous variations in the permeability of the porous medium. The generalized Darcy model is used to describe the flow through the porous medium along with the Beavers–Joseph condition at the fluid–porous interface. We show that there is a switching of dominant instability mode from surface mode to porous mode with variation in anisotropic parameter. Our results clearly show that for a given Darcy number, the surface mode is the critical instability mode for isotropic and homogeneous porous media. However, when an anisotropic porous medium is considered with high wall-normal permeability than the wall-parallel permeability, the porous mode becomes the most unstable mode. We show a similar exchange of dominant instability mode from fluid mode (i.e., surface or shear mode) to porous mode with variations in inhomogeneity parameter. This switching of the most unstable mode from fluid mode to porous mode with variation in anisotropy and/or inhomogeneity in permeability has not been demonstrated in any of the earlier studies in the context of film flows. We also present an energy budget analysis to decipher the mechanism responsible for making the three modes unstable.

Effects of viscosity variation on the stability of film flow down heated or cooled inclined surfaces: Long-wavelength analysis

The linear stability of liquid film flow, when the viscosity depends exponentially on temperature, is investigated here for the case of heating or cooling from below by means of an analysis valid for long waves. It is shown that cooling stabilizes the flow, while heating destabilizes it. For the case of cooling, a cutoff Prandtl number exists, above which the flow is stable with respect to long waves.

Two-fluid modeling in analyzing the interfacial stability of liquid film flows

The area-averaged two-fluid model formulation of a separated two-phase flow system is used to investigate interfacial stability of liquid film flows. The analysis takes into account the effects of phase change at the interface as well as the dynamic effects of the adjacent vapor flow on the interfacial stability. Wave formation and instability criteria are established in terms of the generalized fluid and flow parameters. The criteria are applied to investigate the stability of laminar liquid film flow with interfacial shear and phase change. The influence of various dimensionless parameters characterizing film thickness, gravity, phase change and interfacial shear are studied with respect to the neutral stability, temporal growth factor and the wave propagation velocity. The results of the present study indicate that the interfacial stability analysis developed within the frame of the two-fluid model formulation proves to be quite accurate as judged by comparing its results with the available experimental data and with the results of much longer and more complex analytical investigations which are valid only for the liquid film free of interfacial shear.

Linear stability of liquid films with interfacial phase change

The influence of the interfacial phase change on the stability of liquid film flow over a plane is studied. It is significant in the interfacial mass balance equation only. Evaporation has a destabilizing influence and condensation a stabilizing one. The thickness variation caused by the phase change is taken into account. Its effect on the stability is the opposite of the phase change effect.

Stability of Liquid Film Flow down an Inclined Plane with Oblique Airflow

The hydrodynamic stability of a liquid film of uniform thickness and constant viscosity flowing down an inclined plane is examined, subject to a wind stress in some direction on its surface and having insoluble surface contaminants present. The intention is to discover how these two instabilities—that due to gravity and that due to wind stress—are related for a three-dimensional primary flow and to find criteria for the onset of instability. Previous work is discussed in relation to the results.