Technical challenges in numerical simulation of droplet behaviors with dynamic contact angle in microchannels

Droplet behaviors play a major role in water management of proton exchange membrane fuel cells. Contact angle, as one of the critical parameters in the boundary conditions for the droplet dynamics, can greatly affect the simulation results for droplet deformation and evolvement. Recently, the dynamic contact angle (DCA) model implemented with Hoffman function has been successfully validated in the simulation of droplet impact on surfaces. In this paper, the Hoffman function is further applied to simulate liquid water slug flow in a straight microchannel with the volume of fluid (VOF) method. It is found that the numerical results are difficult to well match the corresponding experimental results under the same reported experimental conditions. However, the numerical results with lower gas inlet velocity can significantly improve the comparison. It is indicated that the DCA model coupled with Hoffman function has limitations in the simulation of liquid water behaviors with surrounding flows and needs to be further developed. In addition, a series of numerical simulations are conducted with different air inlet velocities, surface tensions, and viscosities to investigate the effects of these factors on the droplet behaviors. The technical challenges in the current research progress for DCA simulation with Hoffman function and the VOF method are also proposed and discussed.