Coupling and stability of interfacial waves in liquid metal batteries


Coupling and stability of interfacial waves in liquid metal batteries

Horstmann, G. M.; Weber, N.; Weier, T.

Liquid metal batteries (LMBs) are discussed today as a cheap grid scale energy storage, as required for the deployment of fluctuating renewable energies. LMBs incorporate stratified three-layer fluid systems consisting of two liquid metal electrodes separated by a thin molten salt electrolyte (see sketch below). Due to the large electrical conductivities of the liquid metals, LMBs are highly susceptible to become unstable by interactions of induced or external magnetic fields with internal cell currents. Several different types of instabilities have been identified as to be crucial for the LMB operation. Besides the Tayler instability and electrovortex flows, the metal pad roll (MPR) instability, originally known from aluminium reductions cells (ARCs), emerged as a key instability mechanism capable to cause short-circuits by exciting interfacial gravity-capillary waves. The MPR instability can be induced only by the interaction of a homogeneous vertical magnetic field with horizontal compensation currents arising due to small perturbations of the interfaces. While this mechanism is well understood in the case of ARCs, in LMBs an additional interface is present that may strongly influence the global stability depending on several parameters. Both interfaces can be closely coupled for thin salt-layers such that they both may excite each other and may be connected by different oscillating modes. The analysis of the coupling behavior is the main target of this study.
As the main part of this talk I will present an analytical analysis using linear wave theory describing coupled gravity-capillary waves enclosed in cylindrical containers. We have derived a fourth-order dispersion relation containing two different coupling modes. Further, we found that the global coupling behavior can be completely described by only two dimensionless parameters. On this basis, we suggest a coupling criterion predicting for which parameter regimes both interfaces can be considered as to be fully decoupled such that two-layer stability analysis becomes sufficient. Our study is further accompanied by both numerical simulations and experiments. For highly coupled cases we discovered different kinds of interface displacements not known from ARCs.
Some of the found states cannot be explained by the MPR instability mechanism alone and probably involve some new physical aspects.

Keywords: Liquid Metal Battery; MHD; Metal Pad Roll Instability; Wave Coupling

  • Lecture (Conference)
    International workshop on liquid metal battery fluid dynamics (LMBFD2017), 16.-17.05.2017, Dresden, Deutschland

Permalink: https://www.hzdr.de/publications/Publ-25608
Publ.-Id: 25608