Heat and moisture transport in okra cylinders with shrinkage effects under solar drying: a multiphysics-based simulation approach

The study of the heat and moisture transport of plant-based materials is of considerable value to the agri-food sector as the in-depth insight provided facilitates the development of better-performing, sustainable, and quality-driven drying techniques and optimized process conditions. This work reports the findings of the experimental tests conducted on a passive mixed-mode solar dryer with okra cylinders of varying thicknesses of 5, 10, and 15 mm with a uniform diameter of 12 ± 0.25 mm. The convective heat and mass transport coefficients were analysed with shrinkage and no-shrinkage impact integrated into the models to accurately predict the drying behaviour, and enhance drying efficiency by considering the geometrical and structural alterations. The results obtained reveal that shrinkage incorporation magnifies the mean values of the convective heat transfer coefficient in the range of 72.29 ≤ ≤ 78.45%, whereas without accounting for shrinkage in the mass transfer, the effective diffusion and mass transfer coefficients range between 74.86 ≤ ≤ 83.14% and 52.68 ≤ hm ≤ 58.83%, respectively for the range of the studied sample thickness. The cylinder thickness remarkably impacted the heat and moisture transport coefficients. Empirical correlations of hc-values with Nusselt and Reynolds numbers were developed for each sample thickness. The COMSOL Multiphysics finite element technique was used to numerically model the structural behaviour of the okra cylinder in terms of transient heat and moisture distribution during the drying operation. The predicted cylinder temperature with shrinkage effect and moisture ratio results exhibited a strong correlation with the experimental data with very low error values.