Search

In this study, the thermal deformation of a machine tool structure due to the heat generated during operation was analyzed, and embedded cooling channels were applied to exchange the heat generated during the operation to achieve thermal error suppression. Then, the finite volume method was used to simulate the effect of cooling oil temperature on thermal deformation, and the effect of thermal suppression was experimentally studied using a feed system combined with a cooler to improve the positioning accuracy of the machine tool. In this study, the supply oil temperature in the structural cooling channels was found to significantly affect the position accuracy of the moving table and moving carrier. If the supply oil temperature in the cooling channels is consistent with the operational ambient temperature, the position accuracy of the moving table in the Y direction and the moving carrier in the X and Z directions has the best performance under different feed rates. From the thermal suppression experiments of the embedded cooling channels, the positioning accuracy of the feed system can be improved by approximately 25.5 % during the dynamic feeding process. Furthermore, when the hydrostatic guideway is cooled and dynamic feeding is conducted, positioning accuracy can be improved by up to 47.8 %. The machining accuracy can be improved by approximately 60 % on average by using the embedded cooling channels in this study. Therefore, thermal suppression by the cooling channels in this study can not only effectively improve the positioning accuracy but also enhance machining accuracy, proving that the method is effective for enhancing machine tool accuracy.

This study investigated a composite polymer desiccant material&rsquo, s performance, which is prepared by impregnating solid desiccant such as sodium polyacrylate (SAP) on to hygroscopic salts such as lithium chloride (LiCl). Dehumidification performance of the proposed composite polymer desiccant (SAP-LiCl) was analyzed by coating the suitable weight percentage (wt %) of the desiccant onto a single fin-tube heat exchanger (FTHE) system and testing the desiccant-coated heat exchanger (DCHE) in a testing tunnel under various operating conditions. Net dehumidification efficacy of DCHE in terms of sorption and desorption amount and thermal performance (COPth) were analyzed. For instance, with processed air inflow temperature, relative humidity and regeneration temperature setting of 30 &deg, C, 80% RH and 70 &deg, C, DCHE&rsquo, s sorption, desorption amount and COPth were recorded as high as 945.1 g, 1115.1 g, and 0.39, respectively. It was further realized that the performance of the DCHE could be enhanced by modulating the cyclic switching time for dehumidification and regeneration processes. For instance, with the aforementioned processed airflow conditions, when the cyclic switching time tuned as 60 min instead of 10 min for a total time period of 120 min, there is a net 58% improvement to the COPth of the system. It was further observed that, under the same time period corresponding to the increase in cyclic switching time, the overall COPth can be enhanced, however, the water vapor sorption and desorption amounts of desiccant were decreased.

In this study, a multifunctional air and water source heat pump system was developed with a parallel refrigerant piping arrangement, which possessed six operational functions: space cooling (SC), space heating (SH), water heating (WH), water cooling (WC) and two composite operational modes. The two composite operational modes were the SC/WH mode and the SH/WH mode. The performance of the multifuctional heat pump system under different ambient conditions was investigated based on the testing standards of CNS 14464 and CNS 15466. In this study, the effect of the direct water heating (DWH) and circulating water heating (CWH) methods on the performance was investigated. It was found that the water heating performance of the system by the DWH method is better than that of the system by the CWH method. The water heating capacity and COPw,h of the DWH method can be improvement by 2.6% to 22.1% and 2.9% to 50.8%, respectively. Moreover, this study developed a refrigerant pressure balance method to achieve an effective steady state of the refrigerant pressure after operational mode switching. By the refrigerant pressure balance method, the required time to attain the steady state could be greatly reduced&mdash, by 50%. However, the deviation of the refrigerant mass flow rate between the refrigerant pressure balance method and the refrigerant pump down method after operational mode switching ranged from 0.15% to 7.6%.

This article develops a direct methanol fuel cell (DMFC) with a magnet-actuated bubble removal mechanism. A micro-DC motor is used to control the bubble removal mechanism. The lower magnetic device...

A uniform magnetic field distribution is a critical aspect in the transmitter array coil design process for achieving a homogenous vertical magnetic field distribution. The free position and orientation features can thus be implemented in the wireless power charging system. This paper presents vertical magnetic field distribution generated by a single-layer circular flat spiral air core transmitter array coil model analysis and evaluation using a numerical analysis method. This method is developed based on the off-symmetry axis magnetic field distribution due to a circular current loop derived from the Biot-Savart law. The proposed evaluation criteria are used to obtain the vertical magnetic field distribution characteristic of the evaluated array coil model. The vertical magnetic field distribution of several circular flat spiral air core coils, in both single and array coil models with different coil geometries were investigated to obtain the relation between the coil parameters and the distance between the adjoining coil centers to generate uniform vertical magnetic field distribution. A case study was also conducted to analyze and evaluate several array coil model patterns (1 &times, 2 array coil, 1 &times, 3 array coil, 2 &times, 2 array coil, 2 &times, 3 array coil) to meet uniform vertical magnetic field distribution. The array coil model is composed of an identical single circular flat spiral air-core coil. Every single coil has inner coil diameter (Di), outer coil diameter (Do), wire diameter (W), pitch (P) and a number of turns (N) at 25 mm, 47.8 mm, 0.643 mm, 0.03 mm, 17 respectively. The study and evaluation of several array coil pattern models show that the distance between the adjoining coil centers should be defined close to the half of coil outer diameter (1/2Do) to generate close to uniform vertical magnetic field distribution. The vertical magnetic field distribution average and magnetic field effective transmitting areas array coil model with the given coil parameters changing as the effect in variation in distances between the adjoining coil centers.

This study investigated the dehumidification effect of silica gel and sodium polyacrylate desiccants coated on fin and tube heat exchangers under various humidity and temperature conditions. In the system, the two desiccant-coated heat exchangers are periodically operated in mass equilibrium states between dehumidification and regeneration processes. The corresponding cyclic switching times between the two processes are investigated to determine suitable switching times that will further improve the operational efficiency of the system. The experimental results show that while water at a temperature of 50°C was supplied during the regeneration process, the sorption ability was greater and the vapor was discharged in a shorter period of the regeneration process for the sodium polyacrylate desiccant than for silica gel. Thus, the moisture sorption ability of sodium polyacrylate in the system is superior to that of silica gel under the same operational conditions. Suitable cyclic switching times for both des...

In this study, silica gel and sodium polyacrylate desiccants are coated onto a finned tube heat exchanger (Desiccant Coating Heat Exchanger, DCHE), which can absorb the vapor in the process air for dehumidification. In the experiments, the desiccant is coated on fins using the dense coating method, which causes the fixed fin area to be coated with greater amounts of desiccants for a better dehumidification performance. This study discusses the dehumidification performances of a single stage DCHE and two-stage DCHEs in series under different relative humidity conditions of the inlet process air and different regeneration water temperatures. The results show that the thermal coefficient of performance (COPth) of the DCHEs for the two desiccants prepared by the dense coating method is better than that of DCHEs with the general immersing coating method by a factor of 2&ndash, 2.4. The two-stage DCHEs in series have a lower supply humidity ratio than a single stage DCHE at different inlet humidity levels, and they can be used in the industry when a special low humidity manufacturing process is required. The overall dehumidifying capacities of two-stage series-connected DCHEs at regeneration temperatures of 50 &deg, C and 70 &deg, C are approximately twice as high as those of a single stage DCHE. The COPth value of a single stage or two stages increases with an increase in the inlet humidity of the process air. The COPth values of the sodium polyacrylate single stage and two-stage DCHEs are 1&ndash, 1.3 times greater than those of the silica gel single stage and two-stage DCHEs at a high inlet air humidity. Finally, the effects of different regeneration water temperatures on the performance of DCHEs are investigated. With an increase in the regeneration water temperature, the COPth value, dehumidifying capacity and regeneration capacity of single stage or two-stage DCHEs increase as well.

This study presents a multiphysics simulation analysis that was performed for the cooling channel of a built-in spindle. The design of experiments (DOE) method was employed to optimize the dimension of the cooling channel, and a practical machining experiment was performed to validate the effect of the design. In terms of the temperature, pressure drop, thermal deformation, manufacturing cost, and initial cost considerations, the paralleling type cooling channel of the front bearing and the helical type cooling channel of the motor were adopted in the study. After the optimal design of the cooling channel was applied, the bearing temperature was reduced by a maximum decrease of 6.7 &deg, C, the spindle deformation decreased from 53.8 &mu, m to 30.9 &mu, m, and the required operational time for attaining the steady state of the machine tool was shortened from 185.3 min to 132.6 min. For the machining validation, the spindle with the optimal cooling channel design was employed for vehicle part machining, the flatness of the finished workpiece was increased by 61.3%, and the surface roughness (Ra) was increased by 52%. According to the findings for the optimal cooling channel, when the spindle cooling efficiency is increased by the optimal cooling channel design, the thermal deformation and warm-up period can be reduced effectively, and the machining precision can be enhanced. This method is an efficient way to increase the accuracy of a machine tool.

This study proposed an improved design for the typical sludge drying system cooperating with three air-source heat pumps. Performance improvement of the system was conducted by consideration of the internal configuration of the drying chamber and the heat and mass transfer effect of the air in the cabin. Since the heat pump drying system in this study adopts the design of continuous sludge feed, the efficiency of heat and mass transfer between the sludge on the conveyor belt and thet air streams from the condenser in the drying chamber becomes very important. Simulation analysis of the computational fluid dynamics (CFD) software was used to understand the flow field and temperature field distributions in the drying chamber with air deflectors in order to find the appropriate internal configuration design in the drying chamber and system operating parameters. The SMER value (ratio of dehumidification amount and energy consumption) were measured to verify the system improvement effect. The purposed system can enhance the dehumidification SMER value by 32.44%. The simulation temperature differences and the actual measured results are less than 10%, the water content percentage of the discharged dry sludge decrease by 30% after redesign the cabin.

Thermal error is the most important source of error in the machine tool, and the thermal deformation caused by the temperature rise during the cutting process often leads to lower machining accuracy and poor quality. About 40%–70% machining error of machine tool is caused by thermal deformation. Considering that it is difficult to avoid the generation of heat during the operation or the cutting process of the machine tool, how to predict the temperature change of the machine tool and then design effective thermal management measures becomes an important task in the development of ultra-high precision machine tools. In this study, the thermal deformation of the machine tool structure due to the heat generated during operation is analyzed, and the embedded cooling channels is applied to exchange the heat generated during the operation to achieve the purpose of thermal error suppression. Then, the finite volume method is used to simulate the effect of cooling oil temperature on the thermal deformation and the thermal suppression is conducted on the feed system in combination of the cooler in order to improve the positioning accuracy of the machine tool. From the simulation results, it is observed that when the temperature of cooling oil is lower in the embedded cooling channels, more heat generated is taken away by the hydrostatic guideway and the average temperature of the hydrostatic guideway is reduced, thus decreasing the thermal deformation. Through the thermal suppression experiment of the embedded cooling channels, the positioning accuracy of the feed system can be improved by about 39.1% during the dynamic feeding process.

Study of negative differential switching behavior is reported in metal-insulator-p-n thyristor structure with a GaAs native thin oxide, where the thin insulator is grown at low temperature by a chemical liquid phase oxide (CLPO) with a thickness at 10nm range. A significant S-shaped negative differential resistance (NDR) is shown to occur that originates from the regenerative feedback in a tunnel metal/insulator/semiconductor (MIS) interface and p-n junction. The influence of oxide thickness on the switching characteristics is investigated. The switching voltages are found to be increased when increasing the oxide thickness, while the holding voltages are almost kept constant even the thyristor exposes under the light.

A traditional III-V GaAs/AlGaAs based semiconductor solar cell heterojunction structure, i.e. n-GaAs/n-AlGaAs/n-GaAs/i-GaAs/p-GaAs is used in this study to investigate the surface polymer passivation on this kind of solar cell. For GaAs/AlGaAs solar cell, the top n-GaAs/n-AlGaAs structure is responsible for the selective etching stop layer. The selective etched surface barriers associated with polymer gratings with different aspect ratios, i.e. grating period (gp) to depth (d), are produced on solar cell surface by using the photolithography and Micro Electro Mechanical Systems (MEMS) techniques. A reflective-type diffraction optical grating is fabricated on the surface of the solar cell to redirect the incident light reflected from the solar cell back onto the solar cell surface. The experimental results show that the addition of the surface optical gratings reduce the light reflectivity value as high as 25%. From inspection of surface reflectivity , the average reflectivity is found to be decreased from 12.1% down to 7.9 %.

Effects of a metal oxide structure on the oxygen sensing properties are investigated. In the existence of the etched V-groove gallium oxide geometry, its electrical and sensing properties, i.e., high sensitivity and response time areinvestigated. Grain sizes of the material are dependent on different sputtering conditions and investigated by AFM. Under the partial-voltage circuit measurement, it can be confirmed that the increase of temperature will result in the increase of resistance for the Ga2O3 oxygen sensor. From experiment, good stability and repeatability of the oxide sensor are demonstrated when tested under oxygen concentration. These properties show that the oxide structure has a good potential for high sensitivity oxygen sensor.

In this study, external electric potentials with high (5Hz) and low (0.5Hz) switching frequencies are applied to induce electro-osmotic flow in micro-mixers to achieve sequential segment injection. A micro-mixer arranged in asterisk-patterns combined with an expansion chamber is designed to deliver a certain quantity of solution into the micro-mixer and achieve rapid proportional mixing. The main purpose of the expansion chamber is to achieve highly efficient mixing by increasing the contact area that produces the diffusion effect. The induced electro-osmitic flows in the microchannels were investigated by numerical simulations in the study. The asterisk-pattern micro-mixer can reduce the examination time by allowing the sequential segment injection of three solutions into the micro-channel. In this study, the micro-mixers are subjected to low and high switching frequencies to investigate the mixing efficiencies of the mixers. The simulation results show that suitable restraint electric field intensities are required to form the sequential segment solutions into approximate rectangular shapes in the micro-channel. Furthermore, the high switching frequency allows both patterns to achieve better mixing efficiencies than with the low switching frequency.

In order to enhance the performance of the direct methanol fuel cell (DMFC), the product of CO2 bubble has to be efficiently removed from the anode channel during the electrochemical reaction. In this study, the materials of Polymethyl Methacrylate (PMMA) with hydrophilic property and polydimethylsiloxane (PDMS) with hydrophobic property are used to form the anode cannel. The channel is fabricated through a microelectromechanical system (MEMS) manufacture process of the DMFCs. In addition, some particles with high hydrophobic properties are added into the PDMS materials in order to further reduce the hydro-resistance in the anode channel. The performance of the DMFCs is investigated under the influence of operation conditions, including operation temperature, flow rate, and methanol concentration. It is found that the performance of the DMFC, which is made of PDMS with high hydrophobic particles, can be greatly enhanced and the hydrophobic property of the particles can be unaffected by different operation conditions.

This study investigated the performance of a single-stage scroll compressed air source heat pump coupled with a flash tank indirect vapor injection. In the refrigerant circulating piping of the heat pump, an indirect vapor injection piping connecting a flash tank at the refrigerant outlet of a condenser and the suction of a scroll compressor was designed. By the indirect refrigerant vapor injection from the flash tank, the saturation pressure and temperature of the refrigerant (R134a) in the evaporator can be raised and the sub-cooling of the refrigerant at the inlet of expansion valve can be enlarged as well. Thus, energy consumption of the compressor can be reduced and cooling capacity of the evaporator can be boosted. It was found that the suitable amount of indirect vapor refrigerant strongly depends on the operational environmental temperatures. By adjusting the suitable indirect vapor injection volume into the compressor, the performance of the heat pump was enhanced. In this study, a suitable indirect vapor injection volume was found and 5~15% performance increments can be obtained while the heat pump operated under ambient temperatures in a range from 5°C to 35°C.

The electrokinetic instability (EKI) phenomenon occurs when microfluidic flows with an electrical conductivity gradient are driven by a high-intensity external electrical field. Although EKI limits the robust performance of complex electrokinetic bioanalytical systems, it can be actively exploited to achieve the rapid mixing of micro- and nanoliter volume solutions in microscale devices. This paper investigates the EKI phenomenon in a double T-shaped microchannel, in which two aqueous electrolyte solutions with a 3.5:1 conductivity ratio are driven electrokinetically into the mixing channel via the application of a DC electrical field. A stratified flow condition is formed when the intensity of the applied DC electrical field is below a certain threshold value. However, as the intensity is increased, a series of flow circulations forms at the interfaces of neighboring solutions flows, and then propagates in the downstream direction when the intensity of the electrical field is increased beyond a certain critical threshold value. Electrical field intensity perturbations aligned in the direction of the conductivity gradient are then added to the DC electrical field at the upper inlet of the double T-shaped microchannel near the main mixing channel. It is found that these perturbations can stir the microfluidic instability and the induced flow instability conditions can enhance the mixing efficiency.

This study investigates the problem of non-Newtonian liquid evaporation from an impermeable vertical plate in a porous medium under mixed convection conditions. The analysis examines the effects of non-Darcian flow phenomena on the evaporation process and investigates the heat and mass transfer characteristics under a range of flow conditions in the aiding and opposing flow types. The parameters of interest include Rak,non, Pex,non, Pr and Sc. The data required for the calculations include a fixed wall temperature of Tw = 45°C, a free flow temperature of T∞ = 20°C and an air free flow rate of u∞ = 4m/sec and the corresponding humidity of 50% flows over fixed temperature and moist wall.The results indicate that an increasing buoyancy force enhances the mixing effect and therefore improves the rate of heat transfer, the latent heat flux is the primary mode of heat transfer in evaporation processes and the velocity gradient at the wall increases with an increasing buoyancy effect. Furthermore, the evaporation rate increases with a reducing flow index and an increasing buoyancy effect.

This paper presents an experimental investigation into electrokinetically focused flow injection for bio-analytical applications. A novel microfluidic device for microfluidic sample handling is presented. The microfluidic chip is fabricated on glass substrates using conventional photolithographic and chemical etching processes and is bonded using a high-temperature fusion method. The proposed valve-less device is capable not only of directing a single sample flow to a specified output port, but also of driving multiple samples to separate outlet channels or even to a single outlet to facilitate sample mixing. The experimental results confirm that the sample flow can be electrokinetically pre-focused into a narrow stream and guided to the desired outlet port by means of a simple control voltage model. The microchip presented within this paper has considerable potential for use in a variety of applications, including high-throughput chemical analysis, cell fusion, fraction collection, sample mixing, and many other applications within the micro-total-analysis systems field.

Investigation of optical anisotropy spectra in the R-plane (i. e., the [1012]-oriented layer plane) of GaN/Al0.2Ga0.8N quantum wells with different widths is studied. The optical matrix elements in the wurtzite quantum wells are calculated using the k·p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in [1012]-oriented GaN/Al0.2Ga0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x′-polarization to y′-polarization as the well width increases. DOI: 10.2529/PIERS060801054904

In order to enhance the performance of the Direct Methanol Fuel Cell (DMFC), the product of CO 2 bubble has to be efficiently removed from the anode channel during the electrochemical reaction. In this study, the materials of Polymethyl Methacrylate (PMMA) with hydrophilic property and polydimethylsiloxane (PDMS) with hydrophobic property are used to form the anode cannel. The channel of the DMFC is fabricated through a microelectromechanical system (MEMS) manufacture process. Two DMFC are stacked together with a common cathode channel for reducing the volume of the cells, and form a twin stack DMFC. The performance of the twin stack DMFC is investigated under the influence of operation conditions, including operation temperature and flow rate. The twin stack DMFC, whose anode channel is made of PDMS possess the best performance among the twin stack DMFC under any of the specific operating conditions tested. However, the increase in the operation temperature and fuel flow rate resulted in decreased the hydro-resistance reduction effect of PDMS, and the performance enhancement of the PDMS DMFC is not obvious in the operating condition of high temperature and high flow rate.

In this paper, a series of numerical simulations was performed to investigate the pumping performance of electro-osmotic micropumps containing electrode arrays patterned on the upper and lower sides of a microchannel. The simulations have been analyzed with a linear electro-osmotic model based upon the Debye-Huckel theory of the double layer. The potential drop across the diffuse layer is assumed to be less than 25 mV (k(B)T/e), and there is a linear response between the surface charge and the voltage drop across the double layer. The double layer is not resolved but is lumped into effective parameters that are imported from the Debye-Huckel and Stern layers. We examined the effects of the relative positioning of the electrodes in the opposing arrays (i.e., symmetrical or staggered), and the phase lag and the angular frequency of the alternating current (ac) signals applied to the electrodes within the two arrays. A critical height of the microchannel was observed, below which the interactions of the applied electrical potentials on the walls became significant. The optimum pumping effect was obtained when the electrode arrays were symmetrical to one another around the centerline of the channel and were activated by ac potentials with a 0° phase shift. The corresponding angular frequency of the maximum pumping velocity for different phase shifts of the applied ac signals was also determined. Overall, the simulation results presented in this paper provide a useful insight into the optimal design parameters and operating conditions for micropumps containing two arrays of microelectrodes on the microchannel walls.

This work presents the experimental investigation with electrokinetic flow mixing for bio-analytical chip applications. By DC electrokinetic instability induced technique, we design a 5x5 microfluidic device which possesses microfluidic sample handling in flow multi-switching. The device not only control single sample flows into different outlet considered but also the multi-sample injection into specific outlet ports. Experimental results indicate that the sample flow could be electrokintcally pre-focusing to a narrow stream and then guided into a desired outlet port and successfully control devices of the voltage in the microfluidic chip.

This paper investigates the EKI phenomenon in a double T-shaped microchannel, in which two aqueous electrolyte solutions with a 3.5:1 conductivity ratio are driven electrokinetically into the mixing channel via the application of a DC electrical field. A stratified flow condition is formed when the intensity of the applied DC electrical field is below a certain threshold value. However, as the intensity is increased the lower high-conductivity stream in the entrance region of the main mixing channel fluctuates alternately in the upward and downward direction resulting in a series of flow circulations forms at the interfaces of neighboring solutions flows, and then propagate in the downstream direction. It is found that the electric perturbations added at upper inlet of the microchannel near the main mixing channel can stir the microfluidic instability and the induced flow instability conditions can enhance the mixing efficiency.

This study investigates the performace of electroosmotic micropumps with arrays of microelectrodes for pumping electrolyte in a microchannel. Traveling-wave potentials are applied to the microelectrode arrays. Ions accumulate in the double layer in response to the applied signal. The electric field acts on the charge pulling the fluid in the direction of the traveling wave. In this study, the effects of the widths and heights of the electrodes, the gap size between the electrodes and the frequency of the applied traveling-wave potential on the pumping velocity of the micropump are investigated in order to obtain the optimum design of the micropump. The optimum operating frequency of the traveling-wave potential is about 1 kHz for the micropumps with different electrode widths and gap sizes. The pumping velocities increase with the decrease of the electrode widths and gap sizes for the micropumps. For the micropumps with different electrode widths and gap sizes, it is found the optimum electrode heights are about 5.5 μm when the gap sizes are less than the electrode widths, and the optimum electrode heights are about 10.4 μm when the gap sizes are greater than the electrode widths.

This paper investigates the electrokinetic instability (EKI) phenomenon in a T-type microchannel, in which two aqueous electrolyte solutions with a 10:1 conductivity ratio are driven electrokinetically into the mixing channel via the application of a DC electrical field. It is found that fluctuating, unstable waveforms are formed in the entrance region of the mixing channel and then propagate in the downstream direction when the intensity of the electrical field is increased beyond a certain critical threshold value. Electrical field intensity perturbations aligned in the direction of the conductivity gradient are then added to the DC electrical field at the upper inlet of the T-type microchannel. It is found that the application of the electrical field intensity perturbations induces flow instability conditions, and hence improves the mixing efficiency. Furthermore, these perturbations stir the microfluidic instability and can reduce the required critical threshold value of the applied DC electric field intensity.

This paper investigates two-dimensional, time-dependent electroosmotic flow driven by an AC electric field via patchwise surface heterogeneities distributed along the micro-channel walls. The time-dependent flow fields through the micro-channel are simulated for various patchwise heterogeneous surface patterns using the backwards-Euler time stepping numerical method. Different heterogeneous surface patterns are found to create significantly different electrokinetic transport phenomena. The transient behavior characteristics of the generated electroosmotic flow are then discussed in terms of the influence of the patchwise surface heterogeneities, the direction of the applied AC electric field, and the velocity of the bulk flow. It is shown that the presence of oppositely charged surface heterogeneities on the micro-channel walls results in the formation of localized flow circulations within the bulk flow. These circulation regions grow and decay periodically in phase with the applied periodic AC electric field intensity. The location and rotational direction of the induced circulations are determined by the directions of the bulk flow velocity and the applied electric field.