Drying characteristics and microstructures of pumpkin slices with ultrasound combined far-infrared radiation

Far-infrared radiation drying presents several advantages such as strong thermal effect and high energy efficiency comparing with traditional hot air drying, and ultrasound technology has significant reinforcement influence on mass transfer during drying process. Hence, the combination of far-infrared radiation technology and ultrasound technology could strengthen both heat transfer and mass transfer theoretically. However, the literature about the drying of ultrasound combined with infrared radiation is scarce until now. In this research, a drying equipment including a contact ultrasound system and a far-infrared radiation heating system was fabricated. Drying characteristics of pumpkin slices under the contact ultrasound combined with far-infrared radiation were studied, and the effects of contact ultrasound and far-infrared radiation heating on the microstructure of pumpkin slices were analyzed with scanning electric microscope technology. The results showed that the drying times at far-infrared radiation heater''s temperatures of 160, 200, 240 and 280 ℃ were 510, 390, 270 and 180 min, respectively, and the rise of far-infrared radiation heater''s temperature could significantly increase thermal energy as well as drying rate. Moreover, the temperature inside pumpkin slices which were heated by far-infrared radiation was higher than the surface temperature during drying process, which indicated that the energy of far-infrared radiation could impinge on and then penetrate into pumpkin slices before the energy was converted to heat. Contact ultrasound could improve drying rate significantly and the increase of ultrasound power from 0 to 60 W could lead to about 26.7% reduction of drying time at the radiation temperature of 200 ℃, which indicated the cavitation effect and mechanical effect of ultrasound could improve the internal energy of moisture and weaken the acting force of organization structure on water molecules, and in result enhance internal mass transfer. However, the difference of drying rates was reduced as drying process went on, which indicated that the strengthening effect of ultrasound on drying rate decreased with the reduction of moisture content. The values of effective moisture diffusivity ranged from 0.98×10^-9 to 2.85×10^-9 m2/s, and increased with the increase of ultrasound power and far-infrared radiation temperature. The rise of ultrasound power could improve the effect of far-infrared radiation heating on moisture diffusion to some degree, yet the increase of far-infrared radiation heating could slightly weaken the strengthening effect of ultrasound on mass transfer. The improvement of ultrasonic power could enhance the cavitation effect and mechanical effect and enlarge the affected region, and in result, augment the size and the number of micro-tunnels inside organization structure of material. The rising of far-infrared radiation temperature could provide more thermal energy and achieve higher moisture evaporation rate, and produce more micro capillaries as well as cause larger micro tunnels. The application of contact ultrasound technology during far-infrared radiation heating could reduce drying energy consumption by 6.67%-20.21%. The application of far-infrared radiation heating technology combined with contact ultrasound technology could affect internal microstructure of material and intensify mass and heat transfer significantly and synergistically, and achieve higher drying rate, shorter drying time and lower operation cost subsequently. The finding of this work can provide theoretical basis and technical reference for the research and application of drying technology of contact ultrasound combined with far-infrared radiation.