Transient Two-Phase Flow Pressure Drop During Droplet Emergence and Growth in Gas Flow Channels

Abstract

The water produced during the operation of proton exchange membrane fuel cells can pass the electrode and emerge from the surface of the gas diffusion layer (GDL), forming liquid-gas two-phase flow in the channel. It is of great importance to be able to precisely quantify the water content in the channel. One method to do so is to measure the pressure drop in the channel and correlate the two-phase flow pressure drop to the accumulated water in the channel. The topic of two-phase flow pressure drop in PEM fuel cell flow channels is investigated in the literature. However, the main focus has been put on the pressure drop in the steady operating condition of the cell. The current study evaluates variation of the pressure drop in the channel while a droplet is emerged from the surface of the GDL. This corresponds to the early operation of the cell. The pressure drop data during droplet growth was acquired in a transparent ex-situ test section while images of droplets were taken in real time with a highspeed camera. The combination of pressure drop data and the size of the droplet obtained from images were used to obtain the drag force applied on the growing droplet. Experimental results showed that the droplet detachment height decreased from 1.54 mm to 1.14 when the superficial gas velocity increased from 4.17 m/s to 10.76 m/s. This decrease in the droplet detachment size reduced the drag force needed to detach the droplet by overcoming the pinning forces. For the same increase in the superficial gas velocity, the drag force decreased for around 50%. Moreover, evaluation of droplet profiles at 10.76 m/s superficial gas velocity indicated that contact line expanded in the downstream of the droplet and the advancing contact angle increased for around 40° from droplet emergence to its detachment.