Numerical and experimental flow investigation using ultrasonic PIV for optimizing mechanically agitated lab-scale anaerobic digesters

Abstract

In this article, a non-invasive ultrasonic flow field measurement method is studied to investigate its applicability to resolve fluid flow in bio- and chemical engineering reactors. In detail, the proposed flow field investigation method is applied to mechanically agitated lab-scale reactors for the validation of anaerobic digester mixing simulations. Two reactors with different geometries, which are cold models of real-life anaerobic digesters, are studied. Since conventional non-invasive flow field measurement techniques are not suitable for the investigated reactor geometries, the applicability of ultrasonic measurement is demonstrated. The experimental results are compared to Computational Fluid Dynamics (CFD) simulations using the Finite Volume Method with multiple reference frames to account for the stirrer-induced mixing. It is shown that ultrasonic measurement is an easily applicable non-invasive method for resolving flow fields of opaque fluids found in chemical, mechanical and biological engineering. The study highlights the benefits of the non-invasive measurement approach and also demonstrates the suitability of this flow measurement technique for further applications such as the optimization of the mixing time characteristics of paints and coatings or silicone production. The resolved flow fields are in accordance with numerical reference results with minor deviations of up to 9% for velocity measurements aligned with the main axis of the ultrasound probe. The validated CFD model can be used as a template for modelling similar chemical engineering processes where the mixing time characteristic is critical. It is further shown that the accuracy of this flow measurement method decreases with increased offsets from the main axis. In addition to the axis alignment, the impact of different fluid properties and reactor geometries is also discussed.