Heat and Mass Transfer Enhancement in Unsaturated Porous Packed beds subjected to Electrohydrodynamics ( EHD ) *

Enhancement of heat and mass transfer in a convective drying of singleand double-layer porous packed bed subjected to electric fields is experimentally investigated and analyzed in this paper. In addition, effect of electrical voltage, porosity, and layer structure on drying behaviors in the packed beds composed of glass beads, water and air are examined. Glass beads of 0.125 and 0.38 mm in diameters are employed in the packed beds. Velocity of hot air is about 0.33 m/s and temperature is controlled at 60C. Electric fields are applied in the range of 0 15 kV. Enhancement of heat and mass transfer is revealed through measuring the temperature and weight loss of moisture content of the packed beds. The results show that the convective heat transfer coefficient and drying rate are enhanced considerably with influence of Corona wind on flow above the packed beds. In addition, arrangement of the glass-bead-layered structure greatly affects on the temperature distribution and rate of moisture content in packed bed. An increase in temperature due to the capillary pressure difference in the double-layered packed bed is more pronounced comparing to that in the single layer bed. Moreover, in the double-layered cases, the drying rate of fine-coarse packed bed is much higher than that of coarse-fine packed bed. INTRODUCTION The application of drying techniques for many processes, such as chemical, pharmaceuticals, and agriculture, have been considered for many years. Due to the simultaneous heat and mass transfer taking place during the process, drying is one of most complicated phenomena. In addition, it is not clearly understood. A conventional drying method with hot-air flow is widely used in agricultural industries for removing the moisture content from products. However, its drying period is long, resulting in large energy consumption. In order to improve drying rate, many researchers have paid much attention in development of hot-air drying cooperating with the other methods, e.g., microwave [1-3], infrared [4, 5], and electric fields [6]. In order to increase removing the moisture within material, microwave irradiation penetrates in the bulk of the material, and creates a heat source at a certain location. To heat the surface region, infrared radiation is transmitted through water at a short wavelength, while it is absorbed on the surface at a long wave length [7]. This drying is suitable to dry thin layers of material with large surface exposed to radiation. In order to enhance the removal of water from the porous material surface, Chaktranond et al. [6] applied the electric fields on the hot-air flow. They report that the effect of the corona wind conducted by electric fields enhances the temperature and drying rate of porous packed bed considerably. To get more understanding in the mechanisms of drying in porous materials, this study aim to experimentally investigate and analyze the transfer of heat and mass within singleand double layer porous packed bed subjects to the influence of hot-air flow and electric fields. In addition, effects of porosity and layer structure are reported. CONCEPT AND EQUATIONS Corona wind Schematic of heat-and-mass transfer enhancement with utilizing electrohydrodynamics (EHD) is shown in Fig. 1. When hot-air flow exposed to electric fields, it gives rise to an electrically induced secondary flow, socalled Corona wind. With EHD, flow above packed bed is circulated and is mingled together. This reduces the influence of boundary layer on the packed-bed surface. In addition, it causes moisture on surface to be substantially removed into the air flow, while heat is transfered into the packed bed. Consequently, the rate of moisture removal is enhanced. Fig. 1 Schematic of enhancement of heat and mass transfer with corona wind [6]. Drying equations Water saturation of a porous medium with respect to a particular fluid is defined as water void V Volume of fluid S Total volume of voids V = = (1). Ground + Anode Hot-air flow Vapor diffusion & Capillary pressure Drift of ions 2 O + 2 N + Flow of neutral molecules Heat The 6 Asia-Pacific Drying Conference (ADC2009) October 19-21, 2009, Bangkok, Thailand