(186bg) Numerical Simulations of Dynamics of Drop Impact and Spreading on Cylindrical and Spherical Surfaces

The phenomenon of liquid spreading over solid surfaces is important to several applications in chemical process industry e.g. liquid spreading in trickle beds, packed columns, separation units, structured reactors, etc. Over last couple of decades, several numerical methods have been developed to simulate gas-liquid flows with moving interfaces. While these methods have been applied to simulate drop impact and spreading behavior on solid surfaces, most of the studies done so far were limited to drop impact and spreading on horizontal and inclined surfaces. It is not yet clear if the static contact angle (SCA) approach (single value of contact angle for horizontal surfaces and advancing (θ_A)/receding (θ_R) contact angles for inclined surfaces) are adequate to predict the phenomenon of drop impact and spreading or the dynamic contact angle (DCA) approach is needed. It appears that the progress in the numerical computations of drop impact and spreading is limited by the lack of appropriate models to account for wall adhesion effects (contact angle and surface roughness) and also by the lack of experimental data on dynamics of drop impact and spreading over surfaces of varying surface characteristics and geometries.

The present study aims at the development and experimental verification of computational models for simulation of drop impact and spreading on cylindrical and spherical surfaces. Measurements of dynamic contact angle and characterization of drop spreading on cylindrical and spherical surfaces of different materials (glass, acrylic, steel, Teflon and wax coated glass) were performed using a high-speed digital camera. The VOF simulations were performed using a commercial CFD solver (FLUENT v6.3) to verify the adequacy of SCA and DCA approach for simulation of drop spreading on surfaces with varying surface characteristics. The dynamic contact angles were implemented by using appropriate user defined functions. The predictions were verified using the experimentally observed drop shapes and measured drop spreading and apex height variations.

For the less wettable surfaces with a small difference in the static advancing and receding contact angles e.g. wax coated glass surface (θ_A=122^o, θ_R=85^o), a single value of contact angle is sufficient to predict the dynamics of drop impact and spreading satisfactorily. However, for the surfaces with large differences in the static advancing and receding contact angles e.g. Teflon (θ_A=135^o, θ_R=45^o), glass (θ_A=116^o, θ_R=12^o) and steel (θ_A=116^o, θ_R=45^o), the simulations carried out using a single value of contact angle failed to predict the dynamics of drop impact and spreading even qualitatively. However, when static values of both θ_A & θ_R were implemented, the predicted drop shapes and the spreading behavior were found to be in satisfactory agreement with the measurements. However if θ_A & θ_R vary strong with time, it was necessary to account for the time variation of θ_A & θ_R (DCA approach) in the numerical simulations.