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This research aims to analyze the influences of electric-driven swirling flow on the enhancement of the hot-air drying efficiencies. Two ground wires are placed in the flow direction, while electrode wires are placed in the normal direction of flow. In addition, electrode positions respected to the surface of porous material are varied both in the normal direction with Ly = 2 and 4 cm and in the flow direction with Lx = 30 and 33.75 cm. High electrical voltage is tested at V = 5, 10, and 20 kV. The temperature and bulk mean velocity of hot air entering test section are controlled at 60 ºC and 0.33 m/s, respectively. In this study, a packed bed used as a porous material composes of water, air and 0.125-mm-diameter glass beads. The initial saturation of packed bed is Si = 0.5. It is found from experimental results when electric fields are exposed to hot-air flow, swirling flow is created in the plane normal to flow above the packed bed surface. When the electrical voltage is applied at V = 20 kV, surface temperature and the rate of moisture removal are rapidly increased, especially in the beginning period of drying. This is due to high moisture content in material. Higher drying efficiency is obtained when the electrodes are placed near the center of packed-bed surface (Lx = 33.75 cm). It is found from the flow visualization that pattern and location of the swirling depend on electrode positions. With the electrically-driven swirling flow, the drying rate is enhanced approximately 1.7 - 2 times, and the convective heat transfer coefficient is increased around 2 – 2.8 times.
Keywords: spiral flow; electric field; hot-air drying; enhancement of heat and mass transfer
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