(Dr. James G. Hartley, advisor)

"Modeling the Impact of a Liquid Droplet on a Solid Surface"

Abstract

The normal impact of a liquid droplet on a horizontal heated surface has been investigated numerically through development of a computer program which accurately simulates the spreading of the liquid film formed upon impact and the heat transfer to that liquid. A level set method has been utilized to track the interface between the liquid and the surrounding air; numerical algorithms to solve the incompressible Navier-Stokes equations and energy equation in axisymmetric form were employed. Simulation of a liquid droplet impacting onto a surface at ambient temperature has demonstrated that an increase in Weber number (We) and a decrease in the contact angle (q) significantly increases the spreading ratio in the range of parameters studied. Spreading also increases with rising Reynolds number (Re), but the dependence on Re is not as great as that for We and q. A parametric study has been carried out with these variables to give relations for the spreading ratio and film height.

Modeling of droplet impact onto a heated surface has demonstrated that significant heat transfer and temperature variations can occur during the spreading process. These temperature variations in the liquid affect spreading by changing the viscosity and surface tension. Results illustrate that the spreading ratio of a water droplet can increase up to 10% when impacting a surface at 100°C as compared to impact at 20°C. When a solid wall made of either glass or aluminum was included under the liquid, it was found that considerable temperature gradients also developed in the glass, but the aluminum surface remained nearly isothermal. The decrease in temperature of the glass surface led to smaller spreading ratios than were found for impact onto the nearly isothermal aluminum surface. In both cases, heat transfer was considerable during the spreading period. Results demonstrate that heat conducted to the liquid from the solid can constitute up to 20% of the energy required to raise the entire droplet to the saturation temperature.