Numerical investigation of dense granular flow around horizontal tubes: Qualification of CFD model with validated DEM model

Abstract

Granular material offer many advantages which qualify them for use as heat transfer medium and heat storage material in solar thermal power plants and industrial processes. Moving bed heat exchangers (MBHE) with horizontal tubes are favorable to extract thermal energy from hot granular materials. However, due to the complex flow phenomena and heat transport mechanisms in granular materials precise design tools for MBHE are lacking. In previous studies we introduced a discrete particle model (DEM) to calculate the granular flow in MBHE and validated it by experiments. In this study we use this precise but computationally expensive DEM model to qualify a more efficient continuum model (CFD) which is being introduced in this work. We compare the two models with regard to the granular flow speed along the tube surface and find deviations of |Δu/uref| < 20% in the upper part of the tubes. Both models coherently predict the formation of a static area at the top of the tubes. The formation of the void area below the tubes is less accurately predicted, calling for further adaptations of the CFD model. The thus qualified CFD allows to study large heat exchanger geometries with substantially reduced computational effort.