Gas-liquid Two-Phase Flow In A Mini-Channel With Water Infiltrating Along The Flow Direction

In the process of operation, gas-liquid two-phase flow will be inevitably formed inside the cathode flow channel of the proton exchange membrane fuel cell(PEMFC). In this case, maintaining characteristics and the law of this kind of gas-liquid two-phase flow clear will be helpful to improve PEMFC’s water management level, which has important scientific significance and application value. In the related studies of PEMFC, water management is a very important issue, which aims to prevent flooding and guarantee proton conductivity of the electrolyte membrane at the same time. Although many research results on water management have been reported in recent years, but many deeper mechanism problems have not yet been resolved. This severely restricts the further study of related research. Based on this actuality, in this thesis, take the flow phenomenon of gas-liquid two-phase flow in PEMFC’s cathode flow channel as the research background, starting from the study of the characteristic of single droplet, and then take consideration of gas-liquid two-phase flow in the flow channel.The flow phenomenon of droplet in micro/mini-flow-channel has many application backgrounds, including chemical engineering, aeronautical engineering and many other fields, and the fuel cell is also one of the application backgrounds. For PEMFC, during operation, with the chemical reaction, small droplets will be emerged on the surface of gas diffusion layer, the droplets will gradually grow up and deformation will be occurred under the action of the air flow. In this thesis, this phenomenon is extracted to study the whole process of a single droplet’s emergence, growth, detachment and even attachment/broken under the action of the air flow on the surface of stainless steel plate.For convenience of the observation of single droplet, a visual flow channel was designed to observe the deformation process and detachment or attachment behavior of the single droplet under the action the upstream in the flow channel. High speed camera system was adopted to acquire the dynamic information of the transient process of droplet deformation, and then process the information. For different combination of experiment parameters, different experiments were done to study the relationship between droplet physical parameters including the advancing contact angle, receding contact angle, the difference between advancing and receding contact angle and droplet height, Reynolds and the droplet infiltration diameter. Specific analysis was done to analyze the cause of the phenomenon, and then the influence law of various physical parameters was summarized. Hydrophobic treatment was done on the surface of stainless steel to study the influence of surface characteristic on the existing state of single droplet by comparing with the ordinary surface.To facilitate the study of the gas-liquid two-phase flow in the mini-flow channel, a visualization experiment system was designed. This kind of gas-liquid two-phase flow was studied,and one side of the millimeter size flow channel was made by porous media(permeability wall made by carbon paper), water infiltrates the flow channel along gas flow direction, and the liquid infiltration amount increases gradually along the channel. Three kinds of flow channel dimensions were selected, multi groups of gas and liquid flow rate were selected respectively, and the combination of two types of data was done. Two kinds of porous media that used as permeability wall were selected, one is ordinary carbon paper, and an alternative is carbon paper with the PTFE infiltration treatment. Based on the results of visual observation, the flow patterns of this kind of flow were classified, and the influence of gas flow rate and water infiltration rate on flow patterns were also analyzed. Meanwhile, due to the infiltration of liquid water, the differentials in pressure at both ends of the channel demonstrate the characteristics of fluctuations. With the increase in the gas flow rate, the relative amplitude of differential pressure fluctuations decreases until the fluctuations are diminished and the amount of liquid infiltration has almost no effect on the amplitude of the fluctuations in pressure differentials. The main influence of the PTFE infiltration treatment process of permeability wall is not to change the species of flow patterns, but to vary the appearance position of specific flow patterns in the flow channel. Moreover, for both kinds of properties of permeability wall, the amplitude of pressure drop and the characteristics of pressure drop fluctuations are both significantly different. This reflects the difference of liquid discharge mechanism between the two kinds of wall surface.