A dynamically varying technique for phase heat and mass transfer in porous media and fluid/porous domains has been developed and incorporated into a conjugate finite volume CFD framework. The local interstitial phase mass exchange within porous zones is estimated by comparison of the resistances to moisture transport within the solid and void constituents of the porous material; the higher resistance expression being used as rate-determining. Based on the mass transfer expression, a local estimate of the Biot number is used to physically apportion the withdrawal of vaporization energy from the solid and fluid constituents of the porous medium. A similar approach is used for coupling of the clear fluid and porous CFD cells at the clear fluid/porous interface. A phase ratio concept is introduced at these interfaces to be able to couple the different phases in heat and mass transfer. The model is generic, involving details that allow application to a wide range of cases and is one of the least empirically-adjusted models. The microscopic coupling approach has been validated for Coal packed bed drying. Results show good agreement with experimental data in terms of the matching trends and the small margins for temporal moisture and temperature variation. The macroscopic coupling technique has been tested using the cases of drying of an apple slice and the dehydration of mineral plaster. Investigating the two cases showed a clear difference in behavior as the apple slice case is material-side-resistance dominant-i.e. diffusively dominant-and the plaster case is air-side-resistance dominant-i.e. convectively dominant. The results are physically reasonable and compare well to available experiments and reported results from the literature. The comparisons indicate the capability of the present approach to model the dynamic coupling accurately in a computationally time-efficient manner.
A produce gas respiration model and fruit-stack geometric digital generation approach is used with commercial CFD software (ANSYS CFXTM) to conduct shape-level simulations of the fluid flow, heat and respiration processes that occur during the storage of produce, with the ultimate purpose of providing detailed information that can be used to develop closure coefficients for volume-averaged simulations. A digital generation procedure is used to develop an accurate representation of the shapes of the different produce. The produce shapes are then implemented into a discrete element modelling tool to generate a randomly-distributed stack of produce in a generic container, which is then utilized as a representative elementary volume (REV) for simulations of airflow and respiration. Simulations are first conducted on single pieces of produce and compared to a recently published experimental data for tomatoes and avocadoes to generate coefficients for the respiration model required for the shape-level simulations on the REV. The results of the shape-level simulation are then processed to produce coefficients that can be used for volume-averaged (porous-continuum-level) calculations, which are much more practical for simulations of large areas of storage comprised of hundreds or thousands of boxes of different commodities. The results show that the multi-level approach is a viable means for developing the simulation parameters required to study refrigeration, ripening and storage/transport of produce.
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