Experimental and Numerical Study of the Moisture Loss Rate in Conditioned Alfalfa Stem using Computational Fluid Dynamics

Today, the development of the livestock industry and feed supply is a vital issue due to the growing world population, the importance of animal protein supply, and the growing requirement for livestock products.A porous medium refers to a solid-void (pore) space that is occupied by a fluid (gas or liquid). Generally, many of these pores are interconnected which makes the transportation of mass and heat possible through the pores and this contributes to a faster transportation process through the solid matrix. Porosity is the fraction of void space to total volume.While the pores are large enough, water vapor and air in the porous media can be transported by molecular diffusion. Molecular diffusion of a gas species (e.g., vapor) in a gas mixture (e.g., vapor and air) is described by Fick’s law.

In this study, the samples were classified into four categories, including control, 3-impacts (low conditioning), 8-impacts (average conditioning), and 13-impacts (high conditioning). Each category included six samples (50-grams) that were used to measure different characteristics at different stages. All samples were weighed every two hours using a digital scale (0.001 gr precision). The leaf-stem separation force then was extracted using a texture analyzer. All experiments were repeated three times, and finally, the mean of these three repetitions was reported as the final value for the intended parameter.The geometry of the alfalfa stem was drawn in Gambit software and after meshing and applying boundary conditions; it was transferred to ANSYS Fluent software. Then, while the solver was selected, adjusted under relaxation factors were applied. In the following, mesh independency was checked and the results were reported.

To ensure numerical accuracy, the experimental data should be validated with the simulation results. For this purpose, experimental moisture losses were compared to the software results and showed a good agreement. Then, the moisture ratio curves (kinetics of drying) and force-time chart were presented.The impact of the moisture content of the tissue was evaluated on the value of force per time. Therefore, three samples of alfalfa with different relative humidity in terms of leaf-stem separation force were reported.The results of the numerical simulations were presented as two main contours: the velocity magnitude and moisture (water vapor) mass fraction. The simulation results were provided for all different modes and compared to the experimental data. Finally, errors between both results were presented in a table.

Regarding the quality and losses of the final product and comparisons between four different modes (control, 3 impacts, 8 impacts, and 13 impacts), the mode with 8 impacts was selected as the best mode. The Force-time chart illustrated two peaks due to the special multi-layer texture of the alfalfa. Regarding reducing the moisture ratio of the alfalfa as compared to the optimal, the force required to separate the leaves from the alfalfa stem was significantly decreased. Also, a significant increase in the losses was observed for impacts modes higher than 8.