Two-phase Flow And Transport Characteristics In Liquid Feed Direct Methanol Fuel Cells

The increased functionality demands for present and future create a critical need for smaller, less-costly, environmentally safe, highly efficient and long-lasting power in portable energy applications. However, the performance capability of conventional electrochemical battery is unlikely to keep pace with the expanded power requirements. Direct methanol fuel cells (DMFCs) has inherent advantages over other battery system in particular for portable electronics application for the view point of using aqueous methanol solution as fuel, high energy density, rapid start-up and response, low operating temperature, zero emission and easy-to-recharge character. It has been widely recognized as the most promising power sources for the next generation portable electronic equipment. Consequently, a large number of research works focus on the development of DMFCs.Based on the literature survey, it is easy to find that many researches focused on the chemicals, materials and optimization of system performance. However, systematic experiments on the transport mechanisms inside it is still lack. Therefore, a homemade transparent DMFC was developed to visualize the two-phase flow and transport in the anode and cathode flow field. The main contents of this thesis include: The design and manufacture of a liquid feed DMFC; Test bench setup and experimental study of the cell performance; Two-phase flow and transport characteristics in the anode flow field; Visualization of water flooding and characterization of flow resistance in the cathode flow field; Building a pressure drop model for two-phase flow in the single serpentine anode flow field; Experimental study on the gas-liquid countercurrent flow in porous diffusion layer of DMFCs. The main results are summarized as follow:(1) For the anode using a parallel flow field, it is observed that the pores around the corner of the channel ribs and the intersection of the carbon cloth fibres were favorable sites for the emergence of CO₂ gas bubbles under constant current mode. When the gas slugs are removed, the fraction of gas coverage reduced gradually. The growth and departure of CO₂ bubbles are mainly controlled by the balance between buoyant, drag, shear lift and surface tension force in the horizontal flow channels of the vertical DMFC. Both a higher current density and a bigger diameter of contact ring induce the growth of CO₂ bubbles. The bubble departure diameter decreases with the increasing in methanol flow rate, methanol concentration and operating temperature. A more hydrophilic gas diffusion layer usually lead to smaller bubble departure diameters.(2) Increasing the flow rate of the methanol solution, the individual CO₂ bubbles emerging into the channels become smaller, and the coalescent gas slugs become shorter and less as well. This results in an improved methanol supply from the flow field to the catalyst layer, and consequently the cell performance. However, further increase in the methanol flow rate over a particular value does not result in the improvement of the cell performance. The temperature change of the methanol solution shows no influence on the formation and coalescence of CO₂ gas bubbles. There are more CO₂ bubbles and larger gas slugs can be found in the channels when the methanol concentration and the pressure difference between the anode and the cathode increases. Feeding methanol from lower and exhausting CO₂ gas from upper enhanced the removal of CO₂ bubbles, and hence the cell performance.(3) It is also found that the pressure drop between inlet and outlet is larger in serpentine flow field than that in parallel flow field under the same operational parameters. With the increase of current density, methanol solution flow rate and temperature, the pressure drop of anode flow field increases. The pressure drops in serpentine and parallel flow field decrease slightly with the increasing of methanol concentration when the DMFCs are operated under the same current density. The result from the parallel flow plates with the same channel depth and different channel widths shows that the pressure drop increases with the decrease of channel width. On the other hand, it is found that the pressure drop increases with the decrease of channel depth when the same channel width and different channel depths are used. It is clear from the experimental data that both the channel equivalent diameter and the open ratio plays an important role on the pressure drop.(4) A homogenous model is developed for the first time for the calculation of the two-phase flow pressure drop in the serpentine flow field of a DMFC. The results show that the relationships between pressure drop and methanol solution flow rate and current density agree with those of experiences, but the relative error is not neglectable. The separated model based flow pattern in mini-channels is also built for the improvement of the predictive validity. Although the results also agreed with the experimental results and the relative error less than homogenous model, satisfied accuracy are still not achieved. The dynamics of CO₂ bubbles has a great influence on the frictional pressure drop of the two-phase flow in flow channels. Therefore, the attached CO₂ bubbles can be treated as continued repeating rough units, and using a factor k to characterize the effect of discharge current density on the distribution of bubbles, an improved method for calculating the frictional pressure drop of the two-phase flow is proposed. The results show that the relative error is smaller than the homogenous model and the separated model, the predictive validity is good when using the relation of Beattie & Whalley to calculating two-phase homogeneous viscosity.(5) It was observed that the first droplet usually emerged in the up-right region of the parallel flow field and most liquid droplets usually appear in the downstream of channels. The growth of new droplets has the instant effusing character and they usually appear from the pores around the corner of the channel ribs and the intersection of the carbon cloth fibres. The droplets has the character of pulsed growth, the land-touching droplets developing on each side of the channel and water columns contacting the channel wall grow faster than those far from the wall. In addition, it is interesting to note that some water columns grow in a direction opposite to the oxygen flow. Moreover, it is shown that with a higher oxygen flow rate and a higher inlet oxygen temperature usually lead to a much more forming time in for a water droplet on the surface of gas diffusion layer, whereas droplets can be more easily removed from the wall. These enhance the mass transfer of the oxygen, and thus improve the cell performance. The pressure drop significantly affects the liquid water in the flow field. Under the constant current discharge operating mode, the pressure drop increases with the increase in the flow rate of the oxygen or at a lower temperature of the cell. Increasing the temperature of the oxygen leads to the decrease of the pressure drop.(6) From results of two-phase counter flow in the porous layer of DMFCs, it is found that the formation of the multiple bubbles usually accompanies with the bubble retrieve phenomenon. Water invasion process can be observed when the bubble is at the rapid growth stage. During the water invasion process, snap-off occurs at the junction between the pore and throat due to the velocity difference between liquid and gas phases. Water invasion in the network is rather complex. In throats, displacement is piston-like. Three types of invasion can be observed in the pores: (1) two neighboring throats are filled by gas and the rest two are filled by liquid, forming meniscus-like invasion (2) crown-like invasion under the condition that three throats are filled by liquid and the last one is gas-filled.; (3) snap-off invasion under the situation that the opposite two throat are liquid-filled while the other two are filled by gas. There are three steps for the gas invasion in the network: (1) piston-like displacement in the throats due to the pressure effects; (2) crown-like invasion in the pores; (3) pore filling process. The stable displacement occurs at a smaller air flow rate. With the increase in the air flow rate, air distribution in the network can be transformed to dendrite. With increasing the gas and liquid flow rate, gas pressure increases in the network. The largest invasion height of the liquid increases with the increasing in liquid flow rate. The gas saturation in the network slightly reduces with the increase in the liquid flow rate.