| Proton exchange membrane fuel cell(PEMFC),as a green and pollution-free energy conversion device,has great application potentials in transportation and industry.Water management restricts the performance and reliability of fuel cells,and the dynamic behavior of water in fuel cell channels has important influence on water management.Based on ex-situ optical visualization method,this study investigates the dynamic behavior and detachment characteristics of droplets on the gas diffusion layer(GDL)surface in rectangular channel of fuel cells.The main work includes:(1)The experimental visualization system is designed and improved,including the design of a transparent cell model,the connection of gas and water supply system,the writing of temperature control and data acquisition script,and the design of pulsating flow control system.Based on C++ and Python,a data extraction program of droplet contour is developed,and a double-circle fitting algorithm for data extraction of droplet contour is improved,which greatly improves the speed and accuracy of image processing.The result shows that based on droplet size and secant angle,the improved double circle fitting algorithm greatly improves the fitting accuracy of the receding contact angle,and the fitting error is less than 5%.(2)The effects of GDL surface hydrophobicity,gas flow rate,and channel height on the movement pattern and detachment characteristics of droplets in the channel are studied.The variation of droplet height,chord,height/chord ratio,contact angle hysteresis with gas Reynolds number and the relationship between contact angle hysteresis and height/chord ratio are obtained.In addition,the effect of transparent model temperature on the dynamic characteristics of droplets is studied.The results show that with the increase of the gas Reynolds number,the droplet height,chord,height/chord ratio and contact angle hysteresis at the instant of droplet detachment decrease.With the increase of temperature,the droplet height,chord and contact angle hysteresis at the instant of droplet detachment increase,so the droplets are more difficult to be removed.A force analysis model of a single droplet in the channel is established and verified by the experimental data.It is found that with the increase of gas flow rate,the drag force and adhesion force of droplets decrease,so it is easier to remove droplets with larger gas flow rate.(3)A droplet removal method by pulsating flow is proposed for water management.The effects of pulsating frequency and gas flowrate on droplet removal time,height,chord and contact angle hysteresis at the instant of droplet detachment are studied.It is found that pulsating flow can promote droplet removal on the GDL surface.By analyzing the phenomenon of droplet centroid oscillation,the characteristics of centroid oscillation and the mechanism of water removal by pulsating flow are obtained.The results show that the droplet height,chord and contact angle hysteresis at the instant of droplet detachment are minimum at an optimum frequency and gas flowrate of pulsating flow,and the droplet is the easiest to be removed.Compared with untreated GDL,the optimum pulsating frequency for water removal on PTFE-treated GDL surface becomes smaller.Under the optimum pulsating frequency and gas flowrate,both the overall frequency of droplet centroid oscillation and the frequency of droplet centroid oscillation at the instant of droplet detachment are the highest.The pressure drop between both ends of the channel with pulsating flow is studied and obtained.The results provide important evidence for the feasibility of removing droplets by pulsating flow. |
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