Effect Of MHD Convection On Bubble Behavior And Two-Phase Flow Characteristics In Water Electrolysis

With the rapid development of fuel cell and other emerging energy industries,the production method of ultra-high purity hydrogen has attracted more and more attention.Hydrogen production by water elecyrolysis technology is the best method to produce high-purity hydrogen at present.It has the advantages of low-carbon,environment friendly,flexible production,high purity of gas products and high value of by-products.However,its high energy consumption and low efficiency are the bottleneck restricting its wide application.At present,the output of water electrolytic hydrogen production accounts for only about 4%of the world’s total.In addition to producing high-purity hydrogen,water electrolysis technology is also a good way to store energy.It can be combined with renewable energy and smart grid to store waste electricity as hydrogen chemical energy.We can also use it to form a distributed energy supply system,which meets the strategic requirements of building a"clean and low-carbon,safe and efficient"energy system,and promote the overall structural reform of energy.Currently the efficiency of commercial alkaline water electrolyzer is generally lower than 61.5%.The electric energy consumed to produce 1 Nm~3 hydrogen is between 4.5 and 5.0 k Wh.If commercial power is used for production,the cost of hydrogen production can reach30～40 RMB/kg.Therefore,how to improve the energy efficiency and reduce the cost of hydrogen production is almost the most important research topic.Previous studies have shown that under the condition of external magnetic field,the Lorentz force generated by the coupling of magnetic field and electric field can drive the electrolyte to flow,which is called MHD convection.This convection can accelerate the separation of bubbles on the electrode surface,so as to reduce the energy loss caused by gas phase generation.Compared with the most commonly used method of pumping electrolyte in industry,the advantage of imposing external magnetic field method is that the Lorentz force can be generated as long as the electric field and magnetic field exist at the same time.In other words,the electrolyte can be driven without additional energy input,and the cheap Nd Fe B permanent magnets can meet the requirements.However,the influence and mechanism of the external magnetic field on the process of hydrogen production by water electrolysis are still not clear,especially the bubble dynamic behavior on the electrode surface and the gas-liquid two-phase flow characteristics in the electrolyzer under the action of MHD convection.Thus,it needs to be deeply studied by experimental and numerical methods.The research on this paper mainly includes the following parts:1.Explore the influence mechanism of magnetohydrodynamic(MHD)convection on gas-liquid two-phase flow in electrolytic cell.Different forms of electrodes and electrolyzers are used to carry out water electrolysis experiments in a magnetic field.Different forms of MHD convection are formed by changing the relative direction of magnetic field and electric field,which means changing the direction and action range of Lorentz force.By comparing the gas-liquid two-phase flow in the electrolytic cell with and without magnetic field,some scientific questions have been clarified.The questions include the gas-liquid two-phase distribution,flow feature,two-phase mass transfer characteristics in the electrolytic cell and the change of cell voltage under corresponding conditions.The characteristics and action mechanism of MHD convection are thereby explored.The results show that MHD convection plays an absolutely dominant role in the electrolytic cell,which can disturb the electrolyte and reduce the overpotential,so as to improve the electrolytic efficiency by 2%～3.4%.In addition,it is of great significance to study how to use MHD convection for bubble management,so a special electrolyzer is designed to further reduce the energy consumption of water electrolysis,and the experimental results matched expectations.2.Study the influence mechanism of micro-MHD convection on the dynamic behavior of bubbles on the electrode surface.When the electric field is distorted around the bubble,it will be coupled with the external magnetic field to produce Lorentz force,which drives the electrolyte to produce micro-MHD convection on the bubble scale.In this paper,the microelectrode is used as the cathode to carry out the hydrogen bubble growth experiment,which provides a reference for an unsolved scientific problem:when using the conventional flat electrode to produce hydrogen by water electrolysis,regardless of the direction of the applied magnetic field,the cell voltage can be reduced and the electrolysis efficiency can be improved.But when microelectrode is used for electrolysis,the separation of bubbles will be delayed to a certain extent when the magnetic field is parallel to the electric field.It is generally believed that this problem is related to micro-MHD convection and needs further research.By obtaining the visual images of the dynamic behavior of bubble nucleation,growth and detachment under the action of magnetic field in the experiment,and simulating the flow field and pressure distribution around the bubble based on COMSOL Multiphysics simulation platform,it is expected to reveal the physical mechanism of the influence of micro-MHD convection on bubble behavior from the perspective of bubble dynamics.The results show that under the action of micro-MHD convection,a low pressure zone will be generated at the bottom of the single bubble generated on the microelectrode,resulting in a top-down pressure and hindering the detachment of the bubble.But for the bubbles on the plate electrode,the direction of pressure is outward,which can promote the separation of bubbles.3.Investigate the mechanism and influence of micro-magneto convection(MMC)effect in water electrolysis.When paramagnetic particles are added in electrolyte,MMC effect will occur due to the action of non-uniform magnetic field or uneven concentration distribution of paramagnetic particles.If the MMC effect can be introduced into the electrolytic cell and amplified to cooperate with MHD convection,the gas phase components on the electrode surface will be strongly disturbed and effectively removed,so that the electrolytic efficiency can be improved within the limited magnetic field strength range.This research content is mainly to explore the magnetic gradient force and paramagnetic gradient force of magnetic particles and the mechanism of their impact from a theoretical perspective.By analyzing the possibility of synergistic effect between MMC effect and MHD convection,the efficiency of water electrolysis may be improved.The influence range of MMC effect is only within the micron diffusion layer,which can not have a substantive effect on the bubbles growing on the electrode.MMC effect tends to bind paramagnetic particles in the boundary layer,which will have an adverse effect on the electrochemical reaction near the wall in the process of water electrolysis.