Screw heat exchangers can be used for the heating of loose soil with the purpose of disinfecting it. This allows the residual soil on crops to be returned to the field after being taken along during harvest. The primary mode of heat transfer inside a screw heat exchanger is direct contact of soil particles with the heated walls. Rigorous mixing of the soil, induced by the screw's motion, should increase the number of particles that come into contact with the wall and is essential for effective heating. As a consequence, accurate prediction of the heat transfer relies on a correct representation of the soil's motion and mixing.
In this work, the mass and heat transfer of soil are simulated with Computational Fluid Dynamics (CFD). This study uses a novel three-phase Eulerian model to correctly represent the soil material. The model combines three phases that each include a physical property of soil: yield stress from the Herschel-Bulkley phase, particulate flow behaviour from the granular phase, and porosity originating from the air fraction. The model captures the most important soil physics, such as avalanching.
This soil model is used for the simulation of a screw heat exchanger. Analysis of the heat flux exposes the underperformance of the screw flight compared to the shaft and jacket of the screw. This is because the flight is not directly heated but acts as a fin to the shaft. Still, a clear temperature increase of the soil is obtained through the screw conveyor. Furthermore, flow phenomena such as backflow and soil spilling over the shaft were found to improve heat transfer. Finally, a prediction is made for the outlet temperature of a longer prototype screw, based on a simulation of the first five pitches. The soil temperature increases from 25 °C to 74.32 °C at the outlet, which is above the required temperature for disinfection.