Near-wall measurements of the bubble- and Lorentz-force-driven convection at gas-evolving electrodes
Near-wall measurements of the bubble- and Lorentz-force-driven convection at gas-evolving electrodes
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DOI：
10.1007/s00348-015-2029-0
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发表时间：
2015-08-01
影响因子：
2.4
通讯作者：
Cierpka, Christian
中科院分区：
文献类型：
--
作者：
Baczyzmalski, Dominik;Weier, Tom;Cierpka, Christian
化学能量存储系统，例如，以氢气或甲醇的形式存在的燃料，由于其能量密度大，因此具有建立挥发性可再生能源的巨大潜力。然而，通过水电解的氢气生产的效率受到在电极表面处形成的气泡的限制，并且可以通过加速气泡分离来增强。为了表征电极附近复杂的多相流，在水电解槽中同时测量流体速度和氢气泡的尺寸和轨迹。采用PIV/PTV测量液相速度，采用阴影法测定气泡运动轨迹。由于测量不确定性受到靠近壁面的高空隙率的强烈影响，因此必须采用特殊的测量和评估技术。特别是，先进的PTV方案的应用允许更精确的流体速度测量更接近电极。基于这些数据，近壁流的稳定性特性进行了评估和比较的壁射流。PTV也被用来研究洛仑兹力对近壁流体速度的影响。结果表明，随着洛仑兹力的增加，平行于壁面的液相速度显著增加。据推测，这增强了氢气泡从电极表面的脱离，并因此降低了气泡覆盖率并提高了效率。此外，还研究了大气泡上升时的路径振荡对近壁流动的影响。这些气泡会对电极附近的传质产生强烈影响，从而影响工艺的性能。
Chemical energy storage systems, e.g., in the form of hydrogen or methanol, have a great potential for the establishment of volatile renewable energy sources due to the large energy density. The efficiency of hydrogen production through water electrolysis is, however, limited by gas bubbles evolving at the electrode's surface and can be enhanced by an accelerated bubble detachment. In order to characterize the complex multi-phase flow near the electrode, simultaneous measurements of the fluid velocities and the size and trajectories of hydrogen bubbles were performed in a water electrolyzer. The liquid phase velocity was measured by PIV/PTV, while shadowgraphy was used to determine the bubble trajectories. Special measurement and evaluation techniques had to be applied as the measurement uncertainty is strongly affected by the high void fraction close to the wall. In particular, the application of an advanced PTV scheme allowed for more precise fluid velocity measurements closer to electrode. Based on these data, stability characteristics of the near-wall flow were evaluated and compared to that of a wall jet. PTV was used as well to investigate the effect of Lorentz forces on the near-wall fluid velocities. The results show a significantly increased wall parallel liquid phase velocity with increasing Lorentz forces. It is presumed that this enhances the detachment of hydrogen bubbles from the electrode surface and, consequently, decreases the fractional bubble coverage and improves the efficiency. In addition, the effect of large rising bubbles with path oscillations on the near-wall flow was investigated. These bubbles can have a strong impact on the mass transfer near the electrode and thus affect the performance of the process.