3D Thermo-Fluid Dynamic Simulations of High-Speed-Extruded Starch Based Products

Alessandro Cubeddu, Cornelia Rauh, Antonio Delgado

Department of Food Biotechnology and Food Process Engineering, Faculty III—Process Science and Engineering, Berlin Institute of Technology, Berlin, Germany.
Institute of Fluid Mechanics, Technical Faculty, University of Erlangen-Nuremberg, Erlangen, Germany.

DOI: 10.4236/ojfd.2014.41008                

Abstract

This paper aims to investigate a method to perform non-isothermal flow simulations in a complex geometry for generalised Newtonian fluids. For this purpose, 3D numerical simulations of starch based products are performed. The geometry of a co-rotating twin-screw extruder is considered. Process conditions concern high rotational speed (up to 1800 rpm), different flow rates (30, 40 and 60 kg/h) and water contents (22% and 36%), for a total of 54 simulations. To cope with the geometry complexity a Mesh Superposition Technique (MST) was adopted. The pseudoplastic behaviour of the fluid is taken into account by considering viscosity as function of shear rate (Ostwaldde Waele relationship) and temperature (Arrhenius law). Simulated temperature variations are compared with measurements at same process conditions for validation. Qualitative behaviour of temperature T and shear stress  along the screw are analysed and comparisons of different process conditions are presented. By these simulations a database is formed to develop a process control strategy for novel extruder operating points in food technology.

Keywords

High Speed Extrusion; Non-Isothermal Numerical Simulation; Ostwald-De Waele Model

1. Introduction

Since their massive development in the first half of 20th century, extruders have been experiencing a non-stop growth in all industrial fields, not only in plastic industry but also in food processing. The need to develop new products and increase the existent production encourages manufacturers to develop new geometrical screw configurations and to promote research to better understand the complex nature of extrusion processes. Especially in the area of breakfast cereals and convenience food many works have been carried out in order to explain the rheological properties of starch based extrudates. Senouci and Smith [1] as well as Xie et al. [2] characterised shear stress and melt viscosity as function of product temperature, moisture content and melt amylose/ amylopectin content and investigated the regression power law parameters. The influence of the specific mechanical energy (SME) on cornmeal viscosity was illustrated by Chang et al. [3] while product degradation and expansion mechanisms were widely studied in many other works [4] [5] [6] [7] . An exhaustive review of the state of the art about the rheology of starch polymers and related rheometric techniques can be found in [8] . Engineering aspects of extruder performances from an energy point of view have been also investigated [9] [10] and assessment of twin-screw extrusion in producing snack food [11] as well as effects on product characteristics [12] have been experimentally studied. Some of the most important independent variables in the extrusion process are: preconditioning, screw speed N, water content x_w, flow rate, barrel temperature T_W, die geometry and extruder configuration [13] . All manipulable variables influence in a non-linear way the mechanical and thermal stresses and are responsible for the quality of the final product. Due to the complexity of the extrusion process and high number of design variables, most research has been historically carried out on an experimental basis and only recently, with the growth in computer science, the empirical approach has been supported by numerical contributions. Since the begin of the 1990s theoretical models have been used to simulate the transformation of starchy twin-screw extrudates [14] , but only with the advent of the new millennium first attempts have been done to solve the governing equations of fluid dynamics in 3D twin-screw geometries by means of Finite Element Method (FEM). Ishikawa et al. [15] modelled six discrete angular positions of the screw geometry and performed a non-isothermal quasi-steady-state simulation of a polypropylene material. Avalosse et al. [16] reproduced the results of Ishikawa using a Mesh Superposition Technique (MST). The MST as well as the Mesh Partitioning Technique [17] allows simulating the extrusion process without regenerating the finite element mesh as the screws rotate. Use of MST to perform isothermal fluid dynamic simulations of extruded starch based matrix can be found in [18] .

The present work is intended to provide a method to set non-isothermal flow simulations in a complex geometry for generalised Newtonian fluids. The new contribution is the analysis and discussion of the flow behaviour of the studied extrudate comparing a wide range of process conditions, which cover new and extremely high operating points, up to 1800 rpm. For this aim the MST implemented in the POLYFLOW^® software is applied to a twin-screw co-rotating geometry and non-isothermal 3D numerical simulations of starch based products are preformed. The analysis of the extrudate flow behaviour is carried out from a thermal and kinematic point of view for screw speeds ranging from 400 rpm to 1800 rpm. Three different feed rates (30, 40 and 60 kg/h) and two water contents (22% and 36%) are also considered for a total of 54 simulations. The modelled zone is the melt-conveying zone of the extruder, before the die, where the mixture flows as molten monophasic dough. Counter pressure is taken into account. Material parameters were provided by the Department of Food Process Engineering and Department of Applied Mechanics of Karlsruhe Institute of Technology, that performed the experiments and measurements. Validation is made with experimental data for the temperature across the simulated area. Profiles of temperature and shear stress along the simulated area are qualitatively analysed. Quantitative comparisons of all process conditions are also discussed. The performed simulations investigate novel extrusion process conditions in food technology and offer a base of numerical data which can be used to develop a control strategy for optimisation and control of the extrusion process.

2. Materials and Methods

2.1. Geometry and Simulated Area

In Figure 1 the extruder cross section is schematically represented. The modelled area is found at the end of the extruder (melt-conveying zone). The advantages of simulating the flow just before the die are a filling degree of 100%, the possibility to take into account the counter pressure and to compare the calculated temperature variations with the measurements. In fact the temperature transducer plugs are situated across the considered elements. The simulated area consists of three screw elements in a row. Frontal and lateral view with dimensions (in mm) of a single screw and the lateral view of the whole simulated domain are illustrated in Figure 2.

Figure 1. Schematic draw of the extruder and location of the modeled area.

Conflicts of Interest

The authors declare no conflicts of interest.

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