Vector Flow Imaging of a Highly Laden Suspension in a Zinc-Air Flow Battery Model
Flow batteries using suspension electrodes, e.g., zinc-air flow batteries (ZABs), have recently gained renewed interest as potential candidates for grid energy storage or mobile applications. The performance of ZABs depends on the local flow conditions of the suspension in the electrochemical cell, which acts as an electrode. Hence, it is crucial to measure and understand the complex flow characteristics of such solid-liquid suspensions. The investigated suspension electrode is an opaque slurry that consists of microscopic zinc particles and an aqueous potassium hydroxide electrolyte. Commonly, ultrasound Doppler velocimetry is used for flow imaging in opaque fluids. However, due to the high particle concentration in the suspension electrode, strong scattering and wavefront distortions of the ultrasound are introduced. In this paper, we show that this results in an increased measurement uncertainty for Doppler-based velocity estimation. Instead, ultrasound image velocimetry is applied to measure the 2-D and two-component flow field in the zinc-electrolyte suspension. This is possible by adapting the measurement system to the suspension with a calibration setup. The total measurement uncertainties of 4.1% and 2.5% for the axial and lateral flow components are derived from the calibration measurements. For the first time, the flow field of such a suspension could be measured in a scaled fluidic model of a ZAB. The comparison of the estimated flow rates from the velocity profiles showed good agreement to a gravimetric reference. A significant difference in the flow characteristics of a macroscopically homogeneous electrolyte and the same electrolyte loaded with 8 vol.-% zinc particles, i.e., the suspension electrode, was found. Along with the demonstration of the measurement technique for opaque, concentrated suspensions, the measurement data will be used to calibrate and validate numerical models for comparable multiphase fluids.