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

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 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 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 from Partially Wetted Glass Beads with Auxiliary Heating from Below

Earlier studies have shown that an electric field in the form of corona wind can significantly enhance drying rate. It is particularly effective in the early stage of drying when the material is fully wetted. However, the effectiveness of corona wind in drying a partially wetted material, which is commonly believed decreasing, has not been critically examined. For the present study, enhancement of water evaporation from partially wetted glass beads by corona wind is experimentally evaluated. In addition, auxiliary heating provided by a thermofoil heater attached to the bottom surface of the sample container has been used as a possible means to maintain the effectiveness of corona wind. For the present study, a fine copper wire with a diameter of 0.5 mm is used as the emitting electrode. It is charged by direct current with negative polarity from the corona threshold voltage until the occurrence of sparkover. The water level inside the glass beads is below the external surface and is maintained constant in each set of experiments. For each case, a companion experiment is carried out simultaneously under the same ambient conditions but without the application of electric field or auxiliary heating, the result of which is used as a basis in the evaluation of the evaporation enhancement. The weight loss of water as well as the ambient temperature and humidity are continuously monitored. Each experiment lasted for at least 5 h. The results show that the electric field is effective in the enhancement of water evaporation from partially wetted glass beads, but its effectiveness diminishes when the water level in the glass beads recedes. By applying auxiliary heating, this shortcoming of EHD-enhanced drying can be overcome.     

ELECTROHYDRODYNAMIC ENHANCEMENT OF NUCLEATE BOILING HEAT TRANSFER IN HEAT EXCHANGERS

The effect of a radial d.c. electric field on nucleate boiling heat transfer was investigated experimentally using a single-tube shell/tube heat exchanger. A dielectric liquid (Freon R. 114) was used in the shell and the tube was heated by circulating water through it.

It was found that the application of a sufficiently intense electric field to the boiling heat transfer surface resulted both in the elimination of boiling hysteresis and the enhancement of heat transfer for the range of superheat studied. An application of approximately 20 kV was more than sufficient to eliminate the hysteresis.

It was also observed that there appeared to be a transition between two situations. At lower superheat there is an appreciable improvement in the heat transfer coefficient due to both initial (0 to 10 kV) and incremental (10 to 20 kV) voltage increases. At higher superheat, however, the greater improvement (about four times) is obtained with the initial voltage application     

Filter Press for Electrodewatering of Waterworks Sludge

An electrodewatering filter press was designed and its performance was evaluated for basic operating parameters such as the electric field strength, time of electric field application, and changes in pressure. Dewatering efficiency improves with the increase of the electric field strength, application time, and pressure. Considering the operating costs due to energy consumption and electrode erosion, the optimal conditions were found to be 70 V/cm of electric field strength, 30 min of application time, and 588 kPa of pressure. The rate of electrodewatering doubled compared with that of mechanical dewatering (MDW) while the water content of the dewatered cake decreased by 25%. When an electric field is applied to the cake, clogging of the filter cloth becomes minimized due to electrophoretic mobility. The discharge of water from the cake porous matrix is facilitated owing to electroosmosis and thermal effect due to joule heating. As a result, the dewatering capacity of electrodewatering improves compared with the mechanical dewatering. The energy consumption of electrodewatering was about 370-450 kWh/t (dry solid), which accounts for only one tenth of the existing sludge treatment costs.     

Filter Press for Electrodewatering of Waterworks Sludge

An electrodewatering filter press was designed and its performance was evaluated for basic operating parameters such as the electric field strength, time of electric field application, and changes in pressure. Dewatering efficiency improves with the increase of the electric field strength, application time, and pressure. Considering the operating costs due to energy consumption and electrode erosion, the optimal conditions were found to be: 70 V/cm of electric field strength, 30 min of application time, and 588 kPa of pressure. The rate of electrodewatering doubled compared with that of mechanical dewatering (MDW) while the water content of the dewatered cake decreased by 25%. When an electric field is applied to the cake, clogging of the filter cloth becomes minimized due to electrophoretic mobility. The discharge of water from the cake porous matrix is facilitated owing to electroosmosis and the thermal effect due to joule heating. As a result, the dewatering capacity of electrodewatering improves compared with the mechanical dewatering. The energy consumption of electrodewatering was about 370-450 kWh/t (dry solid), which accounts for only one tenth of the existing sludge treatment costs.     

Effect of Electro-Osmotic Dewatering on the Quality of Tofu Sheet

As a new dewatering technology with high efficiency and energy saving, electro-osmotic dewatering can be applied to food processing. The objective of this work was to study the effect of combined field dewatering (CFD) involving electro-osmosis and mechanical pressure on the quality of tofu sheet. Combined field dewatering was performed on the dewatering process of tofu sheet for 10 to 30 min at a voltage ranging from 30 to 50 V under various pulse-time ratio (t₊ = 120 s, t_- = 5 s) electric field and 0.047 MPa pressure field. The texture and structure of tofu sheet after treatments were examined by a rheometer and scanning electron microscope (SEM). The result showed that the three quality indexes, i.e., hardness, tensile strength, and ductility of tofu sheet near the anode were 45%, 30%, and 12% higher respectively than those dewatered by common mechanical pressure method. However, those near the cathode were 15%, 25%, and 10% lower respectively. When the electric field was applied for 10 min at 30 V, these indexes reached their maximum values. Meanwhile, the tofu sheet structure near the anode was more compact and homogeneous than that near the cathode. The result indicated that CFD could enhance the quality of tofu sheet.     

Control of Sludge-to-Wall Adhesion by Applying a Polarized Electric Field

An application of a polarized electric field to reduce adhesion of a biological sludge to the dryer wall was studied experimentally by contact drying of a sludge cake deposited on a heated metal plate serving as the cathode. It was found that the adhesion intensity is greatly reduced due to a water layer formed at the heated plate, when a fraction of water in the sludge migrates to the cathode by electro-osmosis, as well as by gas bubbles, which are generated by the electrochemical reaction. The voltage gradient from 4 to 5 V/cm was found optimal for drying 10-mm-thick sludge samples. Under these conditions, the electric energy consumption amounted to 3.87 kWh/t. An increase in cathode temperature reduced the sludge-adhering intensity and shortened drying time. The beneficial effect of an electric field on reducing sludge adherence decreases with increasing wall temperature.     

Constant Rate Expressing of Juice from Biological Tissue Enhanced by Pulsed Electric Field

Pulsed electric field (PEF) is an innovative technology, which can be successfully combined with solid/liquid expressing of juice from biological materials. The fresh juicy plants contain the juice in closed cells. The application of PEF ensures the electropermeabilization of cellular membranes, which facilitates the expulsion of liquid from the interior of cells. This article describes the influence of PEF on the kinetics of juice extraction from the layer of sugar beet particles expressed at a constant rate. Experimental study was carried out using a laboratory filter-press cell connected to a PEF treatment system. The PEF was applied by two different modes: as a pretreatment operation before pressing and as a treatment during pressing. The application of PEF to nonpressurized cake leads to increase of energy consumption and higher applied voltage. The PEF treatment of excessively pressurized cakes enhances the juice yield; however, the expressing of juice is significantly delayed. The best result is obtained when the sugar beet tissue is treated with PEF at 1.5-5 bars. The influence of PEF parameters on the juice yield was also studied. The intensity of PEF was varied from 0 to 1000 V/cm and the number of pulses was varied between 0 and 1000. It was established that the optimal parameters of PEF after the cake pressurization at 5 bar were in the next range: intensity of PEF E = 500 V/cm and duration of PEF application 0.03-0.05 s.     

电场强化微槽道结构毛细芯蒸发器的传热特性

该实验以R141b作为工质,采用圆柱形电极,在不同运行压力条件下,实验研究了电场强度对微槽道结构换热表面蒸发/沸腾传热特性的影响。实验结果表明：随着电场强度的增加,传热强化效果越明显;运行压力对强化换热的效果具有重要的影响,在施加相同电场强度的条件下,不同运行压力的强化传热效果明显不同,传热系数最大可强化为未施加电场条件下传热系数的1.28倍。     

高压静电场对海水蒸发和沸点的影响

研究了不同条件下高压静电场对海水蒸发速率和沸点的影响。实验结果表明:高压静电场下海水蒸发的速率是常态下的1.5~2.1倍;海水沸点随高压静电场强度的增大而降低。由实验现象和数据分析得出:水分子是极性分子,在高压静电场下易形成电偶极矩,电偶极矩使水分子之间的氢键断裂,从而水分更易蒸发;氢键断裂的能量来源于海水的内能,而不是电场能转化而来的能量。     

亲水改性碳钢极板用于PM2.5脱除

火电厂大气污染物排放标准日趋严格,湿式静电除尘器作为终端治理设备逐渐得到广泛应用。以亲水改性刚性极板为研究对象,建立了卧式湿式静电除尘器中试实验台,开展了PM_2.5脱除特性的实验研究,研究了改性极板表面水膜增强颗粒物脱除效率的机制,考察了气体温度、停留时间、工作电压、初始浓度、冲洗水流量等主要运行参数对颗粒物脱除效率的影响规律。结果表明：改性刚性极板表面的纤维层可以减少反冲气流,减少颗粒的电迁移阻力;表面在小水量情况下亦可维持均匀稳定的水膜,水膜的存在抑制了反电晕和二次扬尘的发生,使得电晕电流高且水膜蒸发使烟气湿度提高,颗粒荷电量和电迁移速度提高,这两方面均提高了颗粒脱除效率。停留时间延长、工作电压提高均会引起颗粒脱除效率的增加,但颗粒物入口浓度、冲洗水流量对颗粒脱除效率影响不大。使用改性刚性极板的湿式静电除尘器可减少阳极冲洗水量,对粒径0.04～0.48 μm的颗粒有较高脱除效率,可在低电压下达到较高的颗粒物总脱除效率,具有较好的应用前景。     

The effect of electric field on mass transfer from drops dispersed in a viscous liquid

Mass transfer of benzoic acid from drops dispersed into a continuous phase of mineral oil with high viscosity has been investigated both with and without an imposed non-uniform d.c. electric field. Total mass transfer efficiency was significantly enhanced in the presence of the high voltage electric field. The results indicate that the application of the electric field to the drop formation process does not change the fundamental mass transfer mechanisms and the mass transfer during drop free fall can also be described by existing mass transfer models. The results also suggest that the high viscosity of the continuous phase inhibits the drops oscillation as well as circulation when the drop sizes become smaller under the applied high voltage electric field. Therefore the significant enhancement of mass transfer efficiency in the present work is mainly due to the increased specific interfacial area.     

电场强化臭氧向水中传质的试验研究

首次提出了利用电场强化臭氧向水中传质过程的方法,结果表明:电场能够强化臭氧向水中的传质过程,而且效果和电场的强弱有关,试验中所加电压越高,对于传质的加强效果越明显。当电压为8kV时,和没有电场的情况相比,总传质系数提高41%。该方法为研究臭氧向水中的传质提供了新的途径。     

Separation Characteristics of Submicron Particles in an Electrostatic Precipitator with Alternating Electric Field Corona Charger

Experiments were carried out to investigate filtration characteristics of an electrostatic precipitator (ESP) with alternating and direct electric field corona charger. Potassium Sodium Tartrate Tetrahydrate as the material was used to generate polydisperse submicron aerosols by using Constant Output Atomizer. The results indicated that the aerosol penetration through the ESP decreased as applied voltage increased. The maximum collection efficiency of either alternating or direct electric field was more than 98%, though the applied voltage of alternating electric field was higher than that of direct electric field. However, from the viewpoint of power consumption, the direct electric field had higher power consumption rate than alternating electric field at similar collection efficiency. For example, when the 0.2 μm particle penetration was about 4%, the power consumptions of alternating and direct electric field were 16 watt and 18 watt, respectively. Hence the separation quality of alternating electric field was better than that of direct electric field. Moreover, considerably lower ozone concentration was found in alternating electric field ESP, compared to direct electric field.     

新型静电聚结器中水滴聚结特性研究

传统电脱水器采用裸电极,含水率较高时在高强电场作用下容易发生击穿现象,今设计了包覆绝缘层的高压电极并加工了新型静电聚结器,可有效避免击穿现象的发生。采用水/原油乳状液为实验介质,并利用显微高速摄像系统结合图像处理技术对水滴的聚结规律进行了观察和分析,探索了电场强度、流量、含水率等因素对水滴聚结特性的影响。结果表明:包覆绝缘层的高压电极可有效防止电击穿现象的发生,增加电场强度有助于油水分离,但高于临界场强后容易导致液滴破碎;含水率为10%、20%、30%时,最优电场强度不同,分别是372、320和204 kV m 1;含水率10%和30%乳化物液滴粒径增大倍数明显大于含水率20%的工况;电场作用时间影响液滴聚结效果,高强电场在低流量下具有很显著的作用;随着流量的增加电场作用降低,但高强电场在高流量下依然使液滴粒径明显增大。