## 5 CFD in Extrusion Processes
### Coupling CFD with Reaction Kinetics and Product Texture Modelling
2021 demonstrated how screw speed and die channel geometry influence shear-induced structuring, providing valuable guidelines for designing fibrous analogues with targeted mechanical properties [77; 106]. Key mechanisms include shear- and extensional-flow-induced molecular alignment, thermal softening and subsequent setting of biopolymer matrices, and water evaporation leading to expansion and porous structure development. To simulate these effects, CFD can be integrated with viscoelastic and thermomechanical models describing the deformation behavior of complex biopolymer systems. Additionally, porosity evolution and bubble nucleation models are employed to predict structure formation during flash evaporation, while crust formation models may be used to describe setting phenomena at the die exit, particularly in high-temperature or high-shear regimes. A practical example is the modeling of fiber alignment in plant-based meat analogues, where the combined analysis of velocity gradients and shear stress fields within the die enables prediction of anisotropy and structural orientation that directly influence consumer perception and sensory acceptance [2]. The coupling of CFD with kinetic and texture models can be implemented through two main strategies. The most common is sequential coupling, in which CFD simulations are first performed to obtain temperature, pressure, shear, and residence time data. These outputs are then used as inputs for standalone kinetic or structural models implemented in platforms such as MATLAB, COMSOL, or via custom scripts. Alternatively, a fully coupled modeling approach involves solving the governing equations for fluid flow, heat and mass transfer, and reaction kinetics simultaneously within a unified numerical framework. Although more accurate and detailed, this method is computationally demanding and requires robust solver capabilities. Some commercial packages, such as COMSOL Multiphysics and ANSYS Fluent with user-defined functions (UDFs), support such multiphysics coupling, while others rely on co-simulation strategies or modular integration [78]. Integrated CFD-kinetic-texture models have been successfully applied to optimize die and screw configurations, predict expansion, crispness, and porosity in extruded snacks, control the degree of protein denaturation in high-protein products, and design plant-based analogues with defined fiber orientation and mechanical behavior. These models also facilitate identification of thermal hotspots that may compromise nutrient stability, supporting targeted process adjustments to minimize degradation. In industrial contexts, such tools are increasingly incorporated into digital twin systems, supporting real-time virtual prototyping and reducing the dependency on costly physical trials [107]. In the production of plant-based meat analogues via high-moisture extrusion, CFD simulations--when coupled with rheological and thermal data--enable prediction of fibrous alignment within soy or pea protein matrices, thus allowing texture tuning and consumer acceptability optimization [108; 109]. Despite their potential, coupled simulations face several challenges. Accurate parameter estimation--such as activation energies, pre-exponential factors, or viscoelastic moduli--requires experimental measurements under process-relevant conditions. Computational cost remains a barrier, especially for three-dimensional, transient, or multiphase simulations. Furthermore, the validation of predicted conversion rates, structural features, or textural attributes demands reliable in situ measurements, which may require advanced techniques such as near-infrared spectroscopy (NIR), MRI, or mechanical texture analysis [110].