Multi-Dimensional Numerical Analysis of Flow Instabilities in 3D-Shaped Honeycomb Absorbers

Abstract

Considered a valid alternative to other receiver types, open volumetric receivers for central tower power plants have been developed and researched for multiple decades. 3D-Shaped absorber geometries, made possible by modern manufacturing technologies, particularly offer the promise of high thermal efficiencies. Yet, volumetric absorbers carry the inherent risk of flow instabilities, which can lead to a total failure of the receiver due to local overheating. Therefore, a numerical, multi-dimensional methodology for the flow stability analysis of volumetric absorbers is proposed. The first stage of the methodology applies an 1D LTNE continuum model to quantify the general pressure loss characteristic of an absorber and identifies critical pressure drop levels with potential for flow instability. These levels are tested as part of the second analysis stage using a discrete 3D CFD model against different irradiation and flow perturbations. If the absorber is able to compensate the disturbances, its flow behavior is deemed stable. Otherwise, the absorber geometry is considered susceptible to local overheating due to flow instability. The new methodology has been validated against two reference absorbers with known stable resp. unstable flow behavior. Further, the new approach was applied to analyze the flow stability of two 3D-shaped honeycomb absorber geometries. It was shown that the geometric features of 3D-shaped absorbers can directly improve the flow stability, but do not prevent instabilities automatically. Therefore, a flow stability analysis should be included in design/optimization processes of new volumetric absorbers alongside a maximization of the thermal efficiency.