Study on Bubble Behaviors in Anode Channel of a Passive Direct Methanol Fuel Cell
|School||Zhejiang University of Technology|
|Keywords||passive direct methanol fuel cell two-phase flow bubble behavior CFD|
The performance of passive direct methanol fuel cell(DMFC) is greatly influenced by the removal of the carbon diocxide produced from the catalyst layer/ diffusion layer and the anode channel. Thus, the study of gas-liquid two-phase flow behavior taking place in the anode channel is necessary for the optimization of the passive DMFC.In this thesis, experiments combined with simulations were carried out to research on bubble behaviors in anode channel in an effort to improve the performance of the passive DMFC.In the experimental part, visualization methods are used to study bubble behaviors which can be described by detaching time, detaching diameter and bubble detaching frequency. The effects of the orifice submergence, gas flux, gas nozzle size, liquid concentration and the shape of the orifice on the performance of fuel cell were investigated. The process of the contact angle during the bubble growth and the path of bubble rising were recorded by using CCD camera. The results show that the bubble formation process has three stages:expansion, detachment and rising. During the expansion period, bubbles expand in the radial direction at first, afterwards it elongates vertically. The contact angle decreases sharply to 20°starting from initial value 180°when it changes the mode of spherical growth, then it increases to around 50°quickly until detachment. The bubble aspect ratio keeps constant after first decline with the rising of the bubble. Bubble rising velocity increases until reaching a fixed value of the process. Gas flux has much impact on the frequency of bubble formation instead of the size of the bubble. With the gas flux increases, the frequency of bubble formation shows a linear increase, having a decreasing trend in detaching time in every orifice submergence and gas nozzle size. As the gas nozzle size increases, the bubble detaching diameter increase significantly, the bubble frequency and the detaching time decrease. Static pressure will affect the size and shape of the final bubble. The bubble detaching time and detaching diameter decrease, the frequency of bubble formation increses with the increase of the orifice submergence and liquid concentration. With the increase of the flow flux, the bubble detaching diameter remains constant, but the detaching time decreses and then becomes constant in every concentration. Bubble generated from the tilting orifice is smaller than the flat situation and the bubble can detach from this kind of orifice easier.In the simulations, the software package FLUENT was used to simulate the formation of bubbles in various conditions. The results show that the convection cycles exsit in the bubble and the space between the bubble and the wall during the bubble formation. The external fluid pressure reaches the maximum value at the bottom of the computational domain and decreases along the bubble growth direction. The pressure inside the bubble keeps almost constant. The vortex generated by the bubble is able to account for why the rising path is instability. When the orifice is getting off from the diffusion layer surface, bubbles generated are smaller, more inerratic and can leave the orifice earlier. The detaching time of the first bubble from the same orifice is the longest and the size is the largest. With the gas nozzle size and the orifice edge angle increasing, the bubble can detach from the orifice more quickly, but the detaching diameters of bubble increase. With the increase of the orifice aziimuth, the bubble detaching time and diameter gradually decrease to a constant value. The detaching time and the diameter of bubble are decreased with the increase of the methanol concentration and temperature. Comparison of the experiments and simulations, the results are consistent and the model used in the simulation is suitable.The discharge processes of the bubbles form the adjacent orifices are also simulated. When the two same bubbles with moderate spacing rise synchronously, they may get closer and contact with each other to form a spindle which merge into an ellipse later. The velocity of the bubble rising is faster than that of the single bubble and the bubble deforms when it rises. The flow rate of the liquid around the bubbles and where is between the bubbles is much more quicker, resulting the symmetric vortex exsits in the bubble and the bubble tail. The vorticity distribution of every single bubble is no longer symmetrical, but the two bubbles are mirror symmetric. When the two bubbles is getting close to each other, they will go through the stage of "attract-repulsive-attract" during floating. If the distance between two bubbles is a little far, the repulsive phenomena might be observed. The two bubbles with different sizes are floating synchronously, if their space is small, they will close to each other and integrate as one, and if their space is a little far away, the small bubble might follow the big one during their rising and they would’t merge. When the two bubbles are rising in a non-synchronously way, the tail bubble is captured by the wake of the head one and they could merge as one bubble earlier and are able to leave the channel faster.