Effect of mixer geometry and operating conditions on mixing efficiency of a non-Newtonian fluid in a twin screw mixer

Wheat Flour Quality Assessment by Fundamental Non-Linear Rheological Methods: A Critical Review

Wheat quality assessment involves physical, physicochemical, chemical, and sensory characterization of wheat kernels and the resulting wheat flour, dough, and bread. The physical tests conducted on wheat flour dough are mostly based on empirical methods. Empirical methods have been useful in industry and research to relate wheat flour quality to baking performance. However, these methods have the disadvantage of providing data in arbitrary units, which makes the fundamental interpretation of results difficult. Therefore, this review focuses on the use of fundamental rheological methods to determine wheat flour quality in terms of processing performance. During the transition from wheat flour to bread, wheat flour dough is mostly exposed to large deformations, and the quality of wheat flour determines its response to these large deformations and its baking quality. For this reason, this review only focuses on the application of fundamental rheological tests that are conducted in the non-linear viscoelastic region where wheat flour dough experiences large deformations.

Non-Isothermal Simulation and Safety Analysis of Twin-Screw Extrusion Process for Synthetizing Glycidyl Azide Polymer-Based Energetic Thermoplastic Elastomer

In order to study the temperature variation and flow characteristics in the twin-screw reactive extrusion process of synthetizing glycidyl azide polymer-based energetic thermoplastic elastomer (GAP-ETPE), a non-isothermal simulation and a safety analysis were carried out. Firstly, based on the synthesis principle of GAP-ETPE, a mechanical sensitivity test, viscosity test and differential scanning calorimetry (DSC) of GAP-ETPE were carried out. Secondly, a three-dimensional physical model of the intermeshing co-rotating conveying element was established by Gambit. A three-dimensional non-isothermal numerical simulation of the conveying and kneading elements was carried out using FLUENT 19.0 software. The temperature, pressure and shear stress field of conveying and kneading elements with different staggered angles were analyzed and compared. The results show that the maximum temperature of the kneading element is always slightly higher than that of the conveying element at the same rotational speed, but the average temperature in the flow channel is always slightly higher than that of the kneading element. The inlet and outlet pressure difference of the kneading elements with a 90° offset angle is the smallest and the safety is the highest. The shear stress in the flow channel of the conveying element is higher than that of the kneading element as a whole, but the shear stress near the outlet of the 90° kneading element is higher than that in the flow channel of the conveying element. Among the kneading elements, the 90° kneading element has the strongest dispersing and mixing ability, followed by the 60° and 45° kneading elements. According to the thermal and physical parameters of the material, the ignition response time is approximately 6 s, which provides a theoretical guide for the safety design of the GAP-ETPE twin-screw extruder.

Hydrodynamics and mixing performance in a continuous miniature conical counter-rotating twin-screw extruder

The mixing performance in a miniature conical counter-rotating twin-screw extruder was investigated by means of experimental technique and computational fluid dynamics (CFD) simulation. An on-line experimental device based on the fluorescence detection was built to study the residence time distribution (RTD). Polypropylene was selected as the flow material and the blend of Polypropylene and anthracene was used as the tracer. This on-line detection device has good reproducibility. The hydrodynamics in the extruder was simulated by the finite element method (FEM) combined with the mesh superposition technique (MST). The particle tracking technique was carried out to evaluate the RTD and mixing efficiency. The effect of screw speed and feed rate on the hydrodynamics and mixing performance was investigated. The velocity magnitude and local shear rate tend to become smaller from inlet to outlet due to the decreasing diameters of screws. The mean length of stretch increases exponentially from inlet to outlet and the mean value of time averaged mixing efficiency remains positive along the axial direction.

Theoretical Studies of the Interaction between Screw Surface and Material in the Mixer

Mixing is one of the most commonly used processes in food, animal feed, chemical, cosmetic, etc., industries. It is supposed to provide high-quality homogenous, nutritious mixtures. To provide appropriate mixing of materials while maintaining the process high efficiency and low energy consumption it is crucial to explore and describe the material flow caused by the movement of mixing elements and the contact between particles. The process of mixing is also affected by structural features of the machine components and the mixing chamber, speed of mixing, and properties of the mixed materials, such as the size of particles, moisture, friction coefficients. Thus, modeling of the phenomena that accompany the process of mixing using the above-listed parameters is indispensable for appropriate implementation of the process. The paper provides theoretical power calculations that take into account the material speed change, the impact of the material friction coefficient on the screw steel surface and the impact of the friction coefficient on the material, taking into account the loading height of the mixing chamber and the chamber loading value. Dependencies between the mixer power and the product degree of fineness, rotational speed of screw friction coefficients, the number of windings per length unit, and width of the screw tape have been presented on the basis of a developed model. It has been found that power increases along with an increase in the value of these parameters. Verification of the theoretical model indicated consistence of the predicted power demand with the power demand determined in tests performed on a real object for values of the assumed, effective loading, which was 65–75%.

Numerical Optimization Simulation of Synchronous Four-Wing Rotor

The mixer is the most widely used batch mixing equipment in the rubber industry. The rotor is a core component and has a great impact on the mixing effect of the equipment. The current rotor structure design is done empirically, being tightly dependent on practical experience. This paper proposes a method for optimizing the rotor structure by using optimization algorithms combined with numerical simulation technology. Using MATLAB software, a parametric design program for synchronous rotors and a set of optimization programs for the particle swarm optimization (PSO) algorithm were written. The global distribution index was used as the fitness function to optimize the synchronous rotor configuration. A comparative analysis of the rotors before and after optimization shows that the optimization process is feasible, and the results are reliable. This provides new ideas for the design and development of mixer rotors.

Twin-Shaft Mixers’ Mechanical Behavior Numerical Simulations of the Mix and Phases

In this paper, the mechanical behavior of concrete twin-shaft mixers is analyzed in terms of power consumption and exchanged forces between the mixture and the mixing organs during the mixing cycle. The mixing cycle is divided into two macro phases, named transient and regime phase, where the behavior of the mixture is modeled in two different ways. A force estimation and power consumption prediction model are presented for both the studied phases and they are validated by experimental campaigns. From the application of this model to different machines and by varying different design parameters, the optimization of the power consumption of the concrete twin-shaft mixers is analyzed. Results of this work can be used to increase productivity and profitability of concrete mixers and reduce energy waste in the industries which involve mixing processes.

Fill Factor Effects in Highly-Viscous Non-Isothermal Rubber Mixing Simulations

A finite volume technique in a commercial computational fluid dynamics (CFD) code is employed in this study to simulate transient, incompressible, non-Newtonian and non-isothermal rubber mixing. The simulation processes are conducted in a two-dimensional(2D) domain, where a mixing chamber partially-filled with rubber is equipped with a pair of two-wing non-intermeshing counter-rotating rotors. The main objective is to assess the effect of different fill factors of rubber on dispersive and distributive mixing characteristics by simulating 15 revolutions of the rotors rotating at 20 min−1. 50%, 60%, 70%, 75%, 80% and 90% are the six different fill factors chosen for the study. An Eulerian multiphase method has been applied to solve for the two different phases, rubber and air. The non-Newtonian property of rubber is handled using the shear rate dependent Carreau-Yasuda model, along with an Arrhenius function to include the temperature dependency. In addition to the governing equations related to the conservation of mass, momentum and energy, the volume of fluid (VOF) method is chosen to track the interface between air and rubber. With regard to the results, flow patterns, thermal distributions, viscosity behavior and volume fraction are analyzed for the different fill factors. In addition, dispersive and distributive mixing behavior is also assessed in detail using Lagrangian statistics, such as mixing index, cumulative distribution of maximum shear stress, cluster distribution index (CDI), scale of segregation (SOS) and length of stretch (LOS), calculated from massless particles. Both the Eulerian and Lagrangian results showed that fill factors between 70% and 80% presented the most reasonable and efficient mixing scenario, and also exhibited the best dispersive and distributive mixing characteristics combined.

Three-Dimensional, Non-Isothermal Simulations of the Effect of Speed Ratio in Partially-Filled Rubber Mixing

Three-dimensional, transient, non-isothermal calculations have been carried out using a commercial computational fluid dynamics (CFD) software in a two-wing rotor-equipped chamber partially-filled (75% fill factor) with rubber, to analyze the mixing efficiency for three different rotor speed ratios of 1, 1.125 and 1.5. The moving mesh technique has been used to incorporate the motion of the rotors. The Eulerian based volume of fluid (VOF) method has been used to track the interface between the two fluids, which are rubber and air. To assign the highly viscous and non-Newtonian properties of rubber, the Carreau-Yasuda model along with an exact Arrhenius formulation that accounts for the shear and temperature dependent viscosity, has been used here. Governing equations including the continuity, momentum and energy equations have been solved to characterize the flow field and various mixing parameters. Eulerian-based fields such as velocity magnitude, viscous heat generation, and average temperature and viscosity are compared between cases with different speed ratios. Dispersive and distributive mixing behaviour are assessed through a Lagrangian approach that tracks the paths of a set of massless particles. Statistical quantities such as cumulative distribution of maximum shear stress, cluster distribution index, and axial and inter-chamber particle transfer rates are calculated and presented as well. Results showed that the speed ratio of 1.5 displayed the best dispersive and distributive mixing characteristics in comparison to the other cases.

Numerical Investigation of Effect of Rotor Phase Angle in Partially-Filled Rubber Mixing

Among several operational parameters such as rotor speed, fill factor and ram pressure, the orientation of the mixing rotors with respect to each other plays a significant role in the mixing performance. An understanding of the flow field and mixing characteristics associated with the orientations of the rotors will help in obtaining a final product with a better quality. For that purpose three phase angle orientations: 45°, 90° and 180° are investigated here in a 75% filled chamber with two rotors counter-rotating at an even speed of 20 min–1. Two dimensional, transient, isothermal, incompressible simulations are carried out using a CFD code. While an Eulerian multiphase method was used to solve for the transport variables in the two phases: rubber and air, the volume of fluid (VOF) method was used to solve for the interface between the two phases. A non-Newtonian Carreau-Yasuda model was used to characterize rubber. Massless particles were injected in the domain to calculate statistical quantities in order to assess dispersive and distributive mixing characteristics associated with rotor orientations. The flow field is analyzed via pressure and velocity contours. Dispersive mixing was analyzed through histograms of mixing index and cumulative probability distribution functions of maximum shear stress experienced by the particles. Distributive mixing was quantified statistically using cluster distribution index and interchamber material transfer. The phase angle of 180° was found to perform the best in terms of both dispersive and distributive mixing characteristics.

Mechanistic modeling of modular co-rotating twin-screw extruders

In this study, we present a one-dimensional (1D) model of the metering zone of a modular, co-rotating twin-screw extruder for pharmaceutical hot melt extrusion (HME). The model accounts for filling ratio, pressure, melt temperature in screw channels and gaps, driving power, torque and the residence time distribution (RTD). It requires two empirical parameters for each screw element to be determined experimentally or numerically using computational fluid dynamics (CFD). The required Nusselt correlation for the heat transfer to the barrel was determined from experimental data. We present results for a fluid with a constant viscosity in comparison to literature data obtained from CFD simulations. Moreover, we show how to incorporate the rheology of a typical, non-Newtonian polymer melt, and present results in comparison to measurements. For both cases, we achieved excellent agreement. Furthermore, we present results for the RTD, based on experimental data from the literature, and found good agreement with simulations, in which the entire HME process was approximated with the metering model, assuming a constant viscosity for the polymer melt.