EHD-ENHANCED DRYING WITH WIRE ELECTRODE

The enhancement of drying rate by electrostatic field (corona wind) has been experimentally evaluated in this study. The samples used are 3 mm and 6 mm glass beads saturated with water. Two sets of experiments were conducted, with and without cross-flow, at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of drying enhancement using electric field (with or without cross-flow). Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. For EHD drying without cross-flow, the enhancement in drying rate increases with the applied voltage. With the presence of cross-flow, the enhancement in drying rate is nearly independent of the applied voltage and is close to the result obtained by drying with cross-flow alone.     

EHD-ENHANCED DRYING WITH WIRE ELECTRODE

ABSTRACT

The enhancement of drying rate by electrostatic field (corona wind) has been experimentally evaluated in this study. The samples used are 3 mm and 6 mm glass beads saturated with water. Two sets of experiments were conducted, with and without cross-flow, at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of drying enhancement using electric field (with or without cross-flow). Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. For EHD drying without cross-flow, the enhancement in drying rate increases with the applied voltage. With the presence of cross-flow, the enhancement in drying rate is nearly independent of the applied voltage and is close to the result obtained by drying with cross-flow alone.     

EHD-Enhanced Drying with Needle Electrode

Abstract

The enhancement of drying rate by corona discharge from a needle electrode has been experimentally evaluated in this study. The electrode produces corona wind, which resembles a round jet, impinges and removes moisture from the surface. Corona discharges with positive and negative polarities have been considered. The samples used are 3 and 6 mm glass beads saturated with water. Two sets of experiments have been conducted (one with the cross-flow of air and the other one without), at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case, a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a reference in the evaluation of drying enhancement using electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a wire electrode to determine the optimal configuration to be used in an EHD drying system.     

EHD-Enhanced Drying with Needle Electrode

The enhancement of drying rate by corona discharge from a needle electrode has been experimentally evaluated in this study. The electrode produces corona wind, which resembles a round jet, impinges and removes moisture from the surface. Corona discharges with positive and negative polarities have been considered. The samples used are 3 and 6 mm glass beads saturated with water. Two sets of experiments have been conducted (one with the cross-flow of air and the other one without), at 1 kV increment from the corona threshold voltage until the occurrence of sparkover. For each case, a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a reference in the evaluation of drying enhancement using electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a wire electrode to determine the optimal configuration to be used in an EHD drying system.     

EHD-Enhanced Drying with Multiple-Wire Electrode

The enhancement of drying rate by corona discharge from multiple wire electrodes has been experimentally evaluated in this study. The corona wind produced by an array of three wire electrodes, which resembles a series of slot jets, impinges, and removes moisture from the surface of 6 mm glass beads saturated with water. Corona discharges with positive and negative polarities have been considered. Two sets of experiments have been conducted (one with the cross-flow of air and the other without), at 1 kV increment from the corona threshold voltage to a maximum voltage of 25 kV. To evaluate the drying enhancement, a companion experiment was carried out simultaneously for each case under the same ambient conditions but without the application of electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a single wire electrode to evaluate the effectiveness of a multiple-wire system.     

EHD-Enhanced Drying with Multiple-Wire Electrode

Abstract

The enhancement of drying rate by corona discharge from multiple wire electrodes has been experimentally evaluated in this study. The corona wind produced by an array of three wire electrodes, which resembles a series of slot jets, impinges, and removes moisture from the surface of 6 mm glass beads saturated with water. Corona discharges with positive and negative polarities have been considered. Two sets of experiments have been conducted (one with the cross-flow of air and the other without), at 1 kV increment from the corona threshold voltage to a maximum voltage of 25 kV. To evaluate the drying enhancement, a companion experiment was carried out simultaneously for each case under the same ambient conditions but without the application of electric field. Each experiment lasted for at least 5 h. The weight loss of water as well as the ambient temperature and humidity were measured. The results are compared with those obtained with a single wire electrode to evaluate the effectiveness of a multiple-wire system.     

EHD-Enhanced Water Evaporation

The enhancement of water evaporation by electrostatic field (corona wind) has been experimentally evaluated in this study. Two sets of experiments were conducted, with and without cross-flow, at an increment of 1 kV from the corona threshold voltage until the occurrence of sparkover. Two types of electrodes (wire and needle) were used. In addition, both positive and negative corona discharges were applied. The weight loss of water due to evaporation as well as the ambient temperature and humidity were measured. For each case a companion experiment was carried out simultaneously under the same ambient conditions but without the application of electric field. The result of which is used as a basis in the evaluation of the evaporation enhancement using electric field. Each experiment lasted for at least 5 h. With the application of electric field alone, the enhancement in the water evaporation rate increases with the applied voltage. With the introduction of cross-flow, the enhancement in the evaporation rate becomes nearly independent of the applied voltage and is close to the result obtained with cross-flow alone.     

SIMPLIFIED MODELLING FOR THE DRYING OF A NON HYGROSCOPIC CAPILLARY POROUS BODY USING A COMBINATION OF DIELECTRIC AND CONVECTIVE HEATING

The drying of non-hygroscopic, capillary porous materials with dielectric and convective heating is considered. Simplified models are used to examine the effect of a partially wetted surface or receding evaporation front on the temperature of the wet material. Physical mechanisms which can enhance the internal moisture flow, as compared with convective heating alone, are examined ualitatively.     

SIMPLIFIED MODELLING FOR THE DRYING OF A NON HYGROSCOPIC CAPILLARY POROUS BODY USING A COMBINATION OF DIELECTRIC AND CONVECTIVE HEATING

The drying of non-hygroscopic, capillary porous materials with dielectric and convective heating is considered. Simplified models are used to examine the effect of a partially wetted surface or receding evaporation front on the temperature of the wet material. Physical mechanisms which can enhance the internal moisture flow, as compared with convective heating alone, are examined ualitatively.     

The Effect of Electric Field on Heat and Mass Transfer

The process of evaporation from flat and cylindrical surfaces into an air stream flowing tangentially to them has been experimentally investigated. An electric field has been induced in the system from an emitting electrode in the form of needles arranged in a plate placed above the grounded water surface. A decisive effect of the ion-drag force on the evaporation process has been confirmed in the experiments proving that the ionic wind had intensified the process. About 8-fold increase in the average values of the heat and mass transfer coeficients caused by an electric field of an intensity 5 kV/cm in comparison with those obtained without the field has been observed. General correlations relating the evaporation rate to the process parameters have been suggested.

The system under consideration can be treated as a model one for a solid dried at a constant rate period. The drying experiments with a kaolin plate have been carried out under conditions corresponding to those applied in water evaporation from a flat surface to provide the same values of the heat and mass transfer coefficients. Intensification of the drying process by the electric field, similar to that of water evaporation, has been recorded.     

STEAM DRYING OF SINTERED SPHERES OF GLASS BEADS

Superheated steam drying of sintered spheres of glass beads with different diameters is investigated to reveal the effects of the gravitational force on drying rates. In a previous study, the drying rate curves of small samples with coarse glass beads in which the frictional resistance to flow of water and the effect of the gravitational force are negligible, were predicted by an evaporation zone model.In the present investigation, the drying rate curves of sintered spheres of glass beads with diameters ranging from 1.53×10^-2m to 4.99×lO^-2m are experimentally and theoretically obtained, and also the capillary pressure curves are measured by use of Haines' apparatus. The drying rate curves of large samples in which the effect of the gravitational force can not be negligible, are compared with those for small samples and the difference among these curves is discussed in terms of the moisture distributions which are estimated from the evaporation zone model with the observed capillary pressure curves.     

TEE EFFECIS OF EXTENDED THIN FILM EVAPORATION AND EXPWNAL DIFFUSION RESISTANCE DURING THE CONSTANT DRYING RATE PERIOD

Extended thin film evaporation with external diffusion resistance is analyzed for the constant rate period of the drying process, in which a polar liquid evaporates from porous bodies made of glass. The extended thin film is defined as the Liquid film in which the disjoining pressure dominates the fluid flar field and works as the driving force replenishing the evaporating Liquid. The results of the analysis shows that due to the existence of the evaporating thin Liquid film, the evaporation fran the extended thin film can compensate the reduction of evaporation rate caused by the increase of the dry spots and keep the drying rate the same as or even greater than that of the completely wetted surface. The external diffusion resistance makes the vapor concentration near the porous solid surface remain constant and therefore keeps     

STEAM DRYING OF SINTERED SPHERES OF GLASS BEADS

ABSTRACT

Superheated steam drying of sintered spheres of glass beads with different diameters is investigated to reveal the effects of the gravitational force on drying rates. In a previous study, the drying rate curves of small samples with coarse glass beads in which the frictional resistance to flow of water and the effect of the gravitational force are negligible, were predicted by an evaporation zone model.In the present investigation, the drying rate curves of sintered spheres of glass beads with diameters ranging from 1.53×10^-2m to 4.99×lO^-2m are experimentally and theoretically obtained, and also the capillary pressure curves are measured by use of Haines' apparatus. The drying rate curves of large samples in which the effect of the gravitational force can not be negligible, are compared with those for small samples and the difference among these curves is discussed in terms of the moisture distributions which are estimated from the evaporation zone model with the observed capillary pressure curves.     

Solidification by Convective Drying of Particle Layer Wetted with Dilute Agar Gel

Abstract:

Solidified porous slab is formed through convective drying of glass particle layer wetted with aqueous dilute agar gel. Measured critical mean moisture content increases with increasing initial moisture or agar content. The agar gel moves in viscous flow caused by capillary pressure during drying. A new drying model based on the receding evaporation plane model is proposed. Drying period is divided into surface and internal evaporation periods. Wet slab consists of dried and wet zones during the internal evaporation period, while the wet slab consists of wet zone only during the surface evaporation period. In the new model, the evaporation rate from the wet zone in the internal drying period is estimated with the linear driving force (LDF) approximation in the field of adsorption engineering. Critical moisture content, that is, mean moisture content between the surface and internal periods, is estimated with a mass balance on the surface. Simulated results by the new drying model with reasonable fitting parameters agree very well with measured drying data.     

Solidification by Convective Drying of Particle Layer Wetted with Dilute Agar Gel

Solidified porous slab is formed through convective drying of glass particle layer wetted with aqueous dilute agar gel. Measured critical mean moisture content increases with increasing initial moisture or agar content. The agar gel moves in viscous flow caused by capillary pressure during drying. A new drying model based on the receding evaporation plane model is proposed. Drying period is divided into surface and internal evaporation periods. Wet slab consists of dried and wet zones during the internal evaporation period, while the wet slab consists of wet zone only during the surface evaporation period. In the new model, the evaporation rate from the wet zone in the internal drying period is estimated with the linear driving force (LDF) approximation in the field of adsorption engineering. Critical moisture content, that is, mean moisture content between the surface and internal periods, is estimated with a mass balance on the surface. Simulated results by the new drying model with reasonable fitting parameters agree very well with measured drying data.     

TEE EFFECIS OF EXTENDED THIN FILM EVAPORATION AND EXPWNAL DIFFUSION RESISTANCE DURING THE CONSTANT DRYING RATE PERIOD

ABSTRACT

Extended thin film evaporation with external diffusion resistance is analyzed for the constant rate period of the drying process, in which a polar liquid evaporates from porous bodies made of glass. The extended thin film is defined as the Liquid film in which the disjoining pressure dominates the fluid flar field and works as the driving force replenishing the evaporating Liquid. The results of the analysis shows that due to the existence of the evaporating thin Liquid film, the evaporation fran the extended thin film can compensate the reduction of evaporation rate caused by the increase of the dry spots and keep the drying rate the same as or even greater than that of the completely wetted surface. The external diffusion resistance makes the vapor concentration near the porous solid surface remain constant and therefore keeps     

Theoretical analysis of water droplet drying on different substrates on liquid crystal display panels heated with infrared radiation

To analyze the drying process of water droplets on glass panels in liquid crystal displays (LCDs), we calculated the evaporation time of water droplets under infrared radiation heating. Assuming that the water droplets mainly sit on either a glass substrate or copper circuit line in LCD panels, we theoretically analyzed the heat transfer mechanisms during the evaporation process of the water droplets on the different substrates by including substrate reflection. From the calculations, we found that heat conduction between the water droplets and the copper substrate plays an important role in the beginning of the evaporation, resulting in longer evaporation times due to heat loss. To confirm the mechanism, we calculated the drying time of 150 µm droplets in the substrates, comparing the conduction and radiation energy. Finally, we calculated the process parameters of water droplet drying in the LCD industry.     

Sensitivity analysis pertaining to effective parameters on electrohydrodynamic drying of porous shrinkable materials

Abstract

A phenomenological computational fluid dynamics model was developed to simulate drying process of a porous body using electric field corona discharge. The set of coupled nonlinear partial differential equations were solved simultaneously and compared with the experimental findings in the literature. The relative error of the corona wind velocity compared to the experiments was less than 1%. The main gradients of the EHD volume force and corona wind were close to the discharge electrode. Moreover, for no inlet air, the corona wind velocity and field distribution indicated the existence of vortices as the main factor for enhancing mass transfer during the drying process. At a constant air velocity, increase in the voltage caused increasing the corona velocity. In addition, by increasing the air velocity to some extent, the corona velocity first increased and then started to drop. As a result, for any voltage and electrode distance from the surface, an optimum air velocity could be determined. Due to the sweep impact of the primary air flow and moving the ionized molecules to the outside, the drying rates at air velocity of 1?m s^?1 were higher than those for air velocity of 1.5?m s^?1. Applying an intake air flow also altered the optimal electrode velocity from the surface due to the occurred change in the corona discharge. Therefore, is concluded that the severity of mass humidity changes is affected by the applied voltage, electrode distances from the surface, temperature, and the intake air velocity.     

MICROWAVE VACUUM DRYING OF POROUS MEDIA: VERIFICATION OF A SEMI-EMPIRICAL FORMULATION OF THE TOTAL ABSORBED POWER

Vacuum drying experiments were performed on a laboratory scale dryer with two porous materials: a packing of initially saturated glass beads and of unsaturated pharmaceutical granules. Several incident powers and two vacuum pressure levels were tested in order to demonstrate different drying mechanisms. The drying kinetics, temperature of the product and the absorbed power are presented and the coupling between the drying rate and the absorbed power is shown. A semi-empirical formulation of the total absorbed power is proposed taking into account the water content, the temperature and the dielectric properties of each phase. This formulation is based on a quasi-static assumption which allows the local electric field inside the material to be expressed with an analytical equation.     

MICROWAVE VACUUM DRYING OF POROUS MEDIA: VERIFICATION OF A SEMI-EMPIRICAL FORMULATION OF THE TOTAL ABSORBED POWER

Vacuum drying experiments were performed on a laboratory scale dryer with two porous materials: a packing of initially saturated glass beads and of unsaturated pharmaceutical granules. Several incident powers and two vacuum pressure levels were tested in order to demonstrate different drying mechanisms. The drying kinetics, temperature of the product and the absorbed power are presented and the coupling between the drying rate and the absorbed power is shown. A semi-empirical formulation of the total absorbed power is proposed taking into account the water content, the temperature and the dielectric properties of each phase. This formulation is based on a quasi-static assumption which allows the local electric field inside the material to be expressed with an analytical equation.