| Energy is an essential element to the development and progress of human society.With the fast development of economy,developing and using clean and renewable energies,such as,the windy energy,the tidal energy,the solar energy and the biomass energy,etc.,have become the same objective all over the world.However,due to the intermittent and unstable features of above renewable energies,when they are used to generate electricity,it is hard to supply stable electricity for users.Therefore,it is urgent to develop large scale but cheap electrical energy storage devices to eliminate the oscillations of electricity generated from these renewable energies and to balance the electricity supply and demand well.At present,the liquid metal batteries(LMBs)have become one of the research hotspots as a potential and large scale cheap electrical energy storage technology.LMBs only include three different kinds of immiscible liquid inside,and these three kinds of liquid are self-assembling because of their very different densities.Due to the special composition of LMBs,compared with traditional solid-state batteries,LMBs have some merits,such as,long cycle life,high energy efficiency,and fast charge and discharge speeds,etc.However,until now,one of the main reasons why LMBs have not been widely promoted and commercialized is that,there are different kinds of flow instabilities,which can cause internal short circuit,are prone to occur inside LMBs.Two main parts are includes as follows:1)Generation,development and flow characteristics of electro-vortex flows(EVFs)in the top liquid metal layer of LMBs are investigated numerically,experimentally and theoretically.The reason,why the investigation of the EVFs is separated from the second part,is that the EVFs should not be considered as a kind of magnetohydrodynamic(MHD)instabilities,and moderate EVFs is considered to be beneficial to LMBs since the enhanced mass transfer can suppress the formation of solid intermetallic compounds when LMBs is discharged fast.It is found that there is a 10 μT different between the z components of the measured magnetic field when the applied current is improved from 0 to 80A.And the flow state is turbulent when the applied current is 80A.The developed solver,and those boundary and initial conditions used in cases are verified by comparing the numerical results with the measured data.The influence of Joule heat on flow structure is investigated numerically.And the effect of the external magnetic field on flow structure are quantitatively analysed by comparing distributions of the time-averaged velocities,the helicities of transient velocity fields and azimuthal component of time-averaged velocities for different external magnetic field configuration,respectively.It is found that,the influence of Joule heat on EVFs can be ignored inside the negative electrodes of LMBs,but the z component of the external magnetic field(bz)can change the flow structure and state completely,i.e.,due to the existence of bz,an ordinary jet flow evolves into swirling jet flow,the rest domain around the jet region is stirred by the formed swirling jet flow,the flow state transforms from laminar into turbulence,the jet area starts to swing irregularly around the centerline of the cylindrical container,the tail of the high velocity region showing bifurcations with different shapes and locations,the length of the jet region becomes short and the direction of the jet region deviates from the centerline of the container gradually,etc.2)MHD instabilities in LMBs are investigated numerically and theoretically.Different kinds of flow instabilities can be triggered in LMBs,for example,a kind of kink type Tayler instability and interfacial instability caused by the charging/discharging current and its own magnetic field,the Rayleigh-Benard instability and the Marangoni instability triggered by temperature gradient and different surface tension,respectively.Probability of different kinds of flow instabilities happening is proportional to the volume of LMBs.In other words,the larger the volume of LMBs is,the more easily different kinds of flow instabilities happen.However,the larger the volume of LMBs is,the lower cost per unit of electrical energy is.According to the analysis of the MHD instabilities,a new structure device,called the grid-structure(GS),is proposed.The GS is composed of different numbers of baffles,and each baffle,likes a sandwich,consists of one layer of metal net or metal plate in the middle and two layers of ceramic plates outside.A series of test are conducted on OpenFOAM platform,and the numerical results show that the specially inserted GS can improve the critical applied electric potential which means the proposed GS can suppress the MHD instabilities effectively.In summary,the flow characteristics of the EVFs in negative electrode of LMBs are studied experimentally,numerically and theoretically.The influence of the z component of the external magnetic field on the EVFs is revealed.It is found that the toroidal Lorentz force transforms the ordinary jet into a rotating jet.The rotating characteristics and mechanisms of the flow are analysed qualitatively and quantitatively.By using a new solver developed on OpenFOAM platform,the MHD instabilities inside a three-layer liquid metal battery are studied numerically.The GS is proposed,and the effectiveness of the GS on suppressing MHD instabilities is verified numerically. |
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