Experiments and modeling of the flow and heat transfer in the solids conveying zone of single-screw food extrusion

Experimental and numerical investigations of the powder flow and the heat transfer associated with the solids conveying process inside a single-screw extruder have been carried out. Solids conveying is the least understood operation for extruders, yet it can be the source of many problems such as flow surging, low rates, high discharge temperatures, and material degradation.; A specially constructed single-screw extruder has been developed to provide more realistic barrel temperature distribution and better temperature and pressure measurements. The effects of the initial pressure, the screw speed, and the barrel heating/cooling mode on the behavior of the solids conveying and transition were obtained for the yellow corn meal with 25% moisture (on wet basis). To keep track of the particle movement, tablets (diameter 3 mm, height 1.5 mm) made from a mixture of corn meal (with the same moisture level) and green food color were dropped into the hopper at intervals as tracers. It was found that the flow in the rear part of the solids conveying (compacted powder zone) was actually helical. The flow in the cross channel direction cannot be neglected.; But so far, all the theories on solids conveying in single-screw extruders considered the solids to move as a plug flow in the down channel direction only (i.e., there is no relative movement in the granular solids, the velocity of the granules is constant in time and uniform in space). These theories cannot explain the phenomena observed in our experiments.; The fluid-like continuous motion models both in incompressible and compressible forms were developed. In powder flow, the stresses could be generated from viscous force, internal friction force or the combination of both, which is much more complicated than that of the fluid flow. The detailed stress generating mechanism under various conditions was studied and the suitable constitutive equation for the solids conveying in extruders was sought.; The governing equations incorporated with the specific constitutive equation of powder flow and the slip boundary conditions, which usually exist in solids conveying of extrusion, were numerically solved under the assumption that the screw was held stationary and the barrel rotated in the opposite direction to that of the screw. The flow in the screw channel was simplified as the flow through a duct with the moving barrel on the top. Characteristic curves for various thermal boundary conditions and different screw speeds were obtained which gave reasonable results about the pressure development, the mass flow rate and the velocity and temperature fields.; The relationship between the mass flow rate and the discharge pressure was calculated and compared with the experimental data by Spalding and Hyun [87]. Using the incompressible model, a constant density has to be chosen to obtain the non-dimensional flow rate. Thus the calculated discharge pressure varies with the density chosen. This problem does not exist in the compressible model. It was found that the discharge pressure calculated from the compressible model lies somewhere between the discharge pressures calculated from the incompressible model, specifying the density as the inlet density and then the outlet density. The calculated discharge pressure decreased with increasing mass flow rate, as did the experiments. But there are some differences between the simulated and the experimental data. The reasons were analyzed. The applicability of the parabolic controlling equations were also discussed.