Airflow improving foam berry pulp microwave drying uniformity and energy efficiency

In order to improve the drying quality and the utilization of microwave energy for the raspberry drying processing, a combination method including theoretical analysis, numerical simulation and bench test is employed to study the effects of the synergic action of ventilation and microwave drying on the drying uniformity and the utilization rate of microwave energy for foamed pulp. The results showed that the heat transfer and mass transfer process of foamed pulp were depended on the synergistic drying process of ventilation and microwave, which were apparent thermal conductivity, the coefficient of gas percolation, the thermal conductivity of gas phase and the thermal conductivity of liquid phase, respectively. The heat conduction and the moisture transfer of foamed pulp were related to the changes of the coefficient of heat transfer and mass transfer of foamed pulp under microwave drying. In addition, the temperature and moisture content of foamed pulp directly influence the dielectric properties of foamed pulp, which determine the absorption of microwave energy (AME) of the material. The ratio of heat convection amount at the boundary of the material layer to the heat conduction amount inside the material layer (RVD) were introduced to characterize the synergistic effect of ventilation and microwave for the drying of raspberry pulp. The results showed that the synergistic effect of ventilation and microwave has a positive effect at RVD less than 27.79 to promote the utilization rate of microwave energy (URME), other than the synergistic effect reducing the utilization rate of microwave energy. The ventilation flow air could take away the steam from the foam pulp, and reduce the pressure of wet air on the surface of the material, which accelerated the evaporation of water and the emergence and rupture of the bubble of the foamed pulp. The phenomenon of bubble boiling inside raspberry pulp enhances the heat and mass transfer process to improve the distribution uniformity of temperature (DUT) and moisture content (DUM). Ventilation velocity less than 1.5 m/s had a significant positive influence on the drying uniformity inside material layer. However, no significant effect was found the ventilation velocity higher than 1.5 m/s on the drying uniformity. Ventilation velocity of 1.5 m/s was developed as the reasonable level with highest drying efficiency and the utilization rate of microwave energy. Compared with no ventilation, ventilation velocity of 1.5 m/s improved the URME, AME, DUT and DUM to 17.57%, 20%, 19% and by 27%, respectively. The research results meet the requirements of drying production of berries with low consumption of energy and high quality, which also provide a theoretical basis for the optimization of microwave foam in drying technology for berries.