Two-phase Flow Simulation Of Droplets Behavior In PEM Fuel Cell Microchannel Aided By Static And Dynamic Contact Angle Models

The application of the Proton exchange membrane(PEM)fuel cell is emerging as the alternative for the zero-emission power sources in automotive and backup power.The proton exchange membrane fuel cell is a device for converting chemical energy into electric power.It produces water and heat as byproducts during its operation.The produced water at the cathode side is essential for hydrating the membrane.Nevertheless,excessive water generated causes flooding in the flow channel,gas diffusion layer(GDL),and reaction zone hence impeding the oxygen supply.At high current densities and low operating temperature,water droplets accumulate on the cathode side of the PEM fuel cell.The concentration and temperature gradient,local pressure,and electro-osmotic drag causes the water to migrate via the polymer membrane into the cathode side.The water passes through the GDL and finally enters the cathode;furthermore,the fuel cell will operate at a lower efficiency.Water accumulation existed in the channel on the GDL surface causes flooding and leads to excessive pressure drop hence reduces overall performance.The static contact angle of 140 degreeswhich is a typical value for PTFE(polytetrafluoroethylene),treated carbon paper materials which usually employed as GDLs in PEM fuel cells.Numerical simulations using the Volume-of-Fluid(VOF)method presents to study and investigate the water dynamics in the droplet flow.VOF methods more utilized for the simulation of PEMFCs;hence it accounts for the surface tension and wall adhesion forces applied,thus influencing the formation of two phase-flow.The focus of the modeling is on methods that account for the dynamic nature of the contact line evolution.The simulation used for the studies relates to the static contact angle,Hoffman,and Qin’s models called dynamic contact angle formulations derived empirically.When the droplet rest on the surface and the gas-liquid interface is not varied,the contact angle is known as the static contact angle(SCA).The significance of the dynamic formulation,as well as the necessity for high numerical resolution,will beanalyzed and distinguished.The comparison between the three models would help better understand and visualize the relevant droplet motion mechanism and dynamics in relation to advancing and receding contact angle and periodicity of the emergence process.Many strategies have been analyzed and created to promote the removal of liquid water from the flow channels.The contact angle is the angle that the phase boundary makes relation to the solid surface,and it plays a vital role in water dynamics.Another strategy for increasing the removal of the water droplets comesfrom adjusting the flow channel dimensions to optimize the droplet removal.The most frequently used methods for determining the optimal solution are based on thus increasing the flow velocity or reducing the channel height.The main stumbling point in the CFD framework is in the application of the boundary conditions.The three-phase contact line presents an interesting numerical problem of a singularity arising from the no-slip boundary condition imposed at the moving contact line.The boundary condition results in extremely large tangential stress imposed by the fluid onto the solid surface due to the moving fluid and the stationary boundary condition.DCA model is more capable of predicting the droplet dynamics and simulating the gasliquid phenomenon.The difficulty in performing experiments to determine the actual dynamics of liquid water in a PEM fuel cell has motivated researchers to conduct computational modeling and simulation to obtain quantitative insight into the two-phase flow and its effects.The moving liquid-water droplets on the GDL and channel side and top walls numerically analyzed with the aid of SCA and DCA models in a microchannel.The droplet’s behavior and pattern resulting in detachment and deformation compared extensively.The DCA models used are Hoffman and Qin’s codes.DCA models gives a better outcome for water management studies in PEM fuel cells.However,the results indicated that Hoffman model presented an efficient dynamic for liquidwater droplet removal compared to that of Qin and SCA model from the microchannel.