Modeling the physical processes in natural fiber composites during batch and continuous pressing.

摘要

A three-dimensional model based on fundamental principles to simulate heat and mass transfer and mat densification during hot pressing of wood-based composites is presented. Mechanisms included in the analysis are water vapor and air transfer by gas convection and molecular gas diffusion, conductive and convective heat transfer, and sorptive effects, as well as densification of the material and development of internal stresses. The implementation of the model enables one to simulate the local evolution of temperature, moisture content, vapor and air pressure, density and internal stresses, as well as overall heat and gas flow patterns.; The model consists of a set of constitutive flux equations that are coupled by local energy and mass balances, combined with a one-dimensional rheological model. Nonlinear dependencies of the material and gas properties are accounted for. A particular modeling approach is developed and implemented to simulate the pressing process not only in batch but also in continuous presses.; The numerical results of the model are in good agreement with those of experiments performed in a large-size laboratory press and an industrial continuous press. A small selection of topics important during hot pressing is given particular attention by means of model predictions. Among these are considerations of the importance of different heat transfer mechanisms and how they vary over space and time, and changes in gas composition inside the mat throughout the process.; The model may be used in the future to optimize the manufacture of existing natural fiber composites and to aid in the development of new products and technology for their production.