Study For Two-phase Flow Of Gas And Liquid In Anode Channels Of DMFC

Fuel cell (FC) is a device which could continuously convert chemical energy stored in fuel and oxidant directly into electrical and thermal energy through electrochemical reactions. Compared with traditional heat engine, FC, which takes on numerous advantages as environmental friendly, higher reliability, better flexibility and lower noise, is much more efficient. Attributed by the low market price of methanol, the fast recharge of methanol, the short start-up time and the operation under low temperature and pressure, Direct Methanol Fuel Cell (DMFC) has played an important part in the domain of FC, and has been considered as a power source which deserves further exploration in many countries.A homemade transparent DMFC is developed to visualize experimentally the two-phase flow of aqueous methanol solution and CO₂ gas bubbles. The dynamic behavior of CO₂ gas bubbles including nucleation, growth, coalescence, and removal in the parallel anode channels of the operating transparent DMFC are recorded in situ. The two-phase flow with chemical reactions in the anode channels and the performance of the DMFC are studied under different operating conditions, and the flux of the CO2 gas is measured at the same time. Thus, preliminary experimental evidence is provided for optimizing running condition and flow-field design which in turn enhances the interior mass transfer process of DMFC. The characteristics of flow resistance in the anode channel are also discussed. A mass transport model on the pressure drop of the two-phase flow in the anode is also developed to analogize the different factors influencing it, and the calculating results are also verified through experiment. The significance of this research lies in that it provide an analytical scheme not only for a deeper understanding on the flow characteristic of the fluid in the anode channels, but also for the direct gas management on DMFC.The main results are presented as follows:1. A DMFC was in-housed designed to visualize experimentally the two-phase flow in the anode channel. 2. An experimental system for the DMFC test was set up.3. By adopting the visualized experiment on the parallel flow channels of DMFC, the writer of this paper test different current density, the concentration of methanol solution in the anode channel, methanol solution flow rate in the anode channel, temperature and the performance of FC while the gas does not participate in the electrochemistry response. Besides, the two-phase flow in the parallel flow channelsis observed and the quantity of CO₂ as the products of anodic reactions is also calculated. Consequently, the conclusions are drawn as follows: Firstly, the greater the current density becomes, the larger the gas in the channels and the size of bubbles is. Secondly, with the increasing of the current density, the flux of CO2 becomes larger. Thirdly, as the concentration of methanol becomes higher, the open-circuit voltage and the voltage in the low current density region of FC will become lower. Fourthly, in the high current density region, the performance of FC will be greatly reduced while the concentration of methanol is over-low. Fifthly, the concentration of methanol contributes slightly to the flux of CO2 bubbles. The influence of the increase in the methanol solution in anode channel on the FC performance is slight in the low current density region, yet strong in the high current density region at which time the over-small flux of methanol make FC entering into state of transport loss in advance and the voltage decreasing rapidly. Sixthly, along with the increase in the methanol solution flow rate in the anode channel, the quantity and the average size of the bubbles in the channel reduce greatly while the gas generation changes a little. Seventhly, the performance of FC improves greatly with the rise of its temperature. Eighthly, as the temperature increases, the quantity of bubbles and their average size are larger. At last, the performance of FC will improve when the gas which does not participate in the electrochemistry response is mixed with the methanol solution. However, if this kind of gas accounts too much, the performance of FC will decrease with the unsmooth removal of gas in the channel as a representation.4. A computational model of pressure drop in the parallel anode channels in DMFC is set up, and the total pressure drop of the anodic DMFC is calculated. With the help of the model, the writer of this paper not only make an analysis on the current density, the concentration of methanol solution in the anode channel, methanol solution flow rate in the anode channel and the impact of operating temperature on the pressure drop of anode, but also make an experimental verification on the calculating results. The conclusions are as follows. At first, in the setting condition of this paper, the total pressure drop of DMFC and the current density are inversely proportional. Secondly, the impact of the concentration of methanol solution on the total pressure drop of anode is too slight to be neglected. Thirdly, the total pressure drop of anode will increase as the flux of methanol augmented. Fourthly, on the conditional that the current density is zero, the total pressure drop of anode will also rise with the increase of the methanol solution flow rate. Lastly, the rise of the temperature have a little influence on the pressure drop.