Modelling radio frequency heating of randomly stacked walnut kernels with different particle size

Radio frequency (RF) heating is a promising physical processing method for disinfestation of cereals, fruits, nuts, and other foodstuffs but the influence of the shape and size of granular agricultural products on heating uniformity of RF treatment is not fully under-stood. A coupled electromagnetic field and heat transfer model for RF heating of randomly stacked walnut kernels with different particle size in container was established to obtain their heating pattern based on three-dimensional scanning technology and the discrete element method. The established model was validated using temperature-time profile and top surface temperature distributions of samples in the container. Results showed that the simulated results of established model matched the experimental data due to the low relative percent error (<5%) of temperature-time profile and average temperature of treated kernels. With the electrode gap of 98 mm, the heating rates of samples were 5.1, 6.5, and 7.0 degrees C min -1 for quarter, half, and whole kernels, respectively. The heating uniformity index increased with increasing particle size and reduced with increasing bulk density during RF heating. To obtain the same heating rate, the anode current and top voltage of RF system increased from 0.33 A to 5.8 kV to 0.35 A and 6.5 kV when the particle size decreased from whole kernel to quarter one, respectively. Furthermore, the quarter kernels have better heating uniformity than whole and half kernels when were heated with same electrode gap or same heating rate. The over-heating of RF treatment was found at corners and edges of the container and contact points between particles. The model may provide effective method to evaluate the RF heating patterns on randomly stacked particle mate-rials with irregular shape and useful information to improve the RF heating uniformity for large-scale applications. (c) 2023 IAgrE. Published by Elsevier Ltd. All rights reserved.