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This application is a national stage application under 35 U. This invention relates in general to the field of electrohydrodynamic pumps, and more particularly, to an electrohydrodynamic conduction pump, and a particular adaptation for mass transport of isothermal and non-isothermal single phase liquids. The electrohydrodynamic EHD phenomena involve the interaction of electric fields and flow fields in a dielectric fluid medium.
This interaction between electric fields and flow fields can induce the flow motion by electric body force. In general, there are three kinds of EHD pumps operating based on the Coulomb force; induction pumping, ion-drag pumping, and pure conduction pumping. EHD induction pumping relies on the generation of induced charges. This charge induction in the presence of an electric field takes place due to a non-uniformity in the electrical conductivity of the fluid.
Therefore, induction pumping can not be utilized in an isothermal liquid. Ion-drag pumping may be used for generating a space charge in an isothermal liquid. However, ion-drag pumping is not desirable because it can deteriorate the electrical properties of the working fluid.
Accordingly, a need has arisen for an improved EHD pumping mechanism that provides isothermal dielectric liquid pumping as well as isothermal and non-isothermal single phase liquid pumping.
The present invention provides an EHD conduction pump that addresses shortcomings of prior pumping systems. According to one embodiment of the present invention, an EHD conduction pump includes an EHD pumping section and associated connecting tubes. Electrodes are arranged in series in the pumping section. The electrodes are connected to a high voltage low current dc power supply, and a positive polarity dc voltage is applied to the electrodes. The electrodes may be configured with a 3-needle, hollow-tube, or pin-needle design.
One major advantage of the EHD conduction pump is that it is the only EHD pumping mechanism that can be utilized to pump isothermal dielectric liquids without requiring direct electric charge injection. The simple design, non-mechanical, lightweight, the rapid control of performance by varying the applied electric field, and low power consumption are all advantages of the application of the EHD conduction pump.
Potential applications of the present invention include EHD pumping of the working fluid in the liquid-phase in two-phase systems e.
Other technical advantages will be readily apparent to one skilled in the art from the following figures and descriptions. For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:. In an isothermal liquid, only the Coulomb force which is the force acting on the free charges, can contribute to permanent electrohydrodynamic motion. In the absence of a direct charge injection, pumping can be achieved due to the charges associated with the heterocharge layers of finite thickness in the vicinity of the electrodes which are based on the process of dissociation of a neutral electrolytic species and recombination of the generated ions.
This type of pumping is referred to as pure conduction pumping. The conduction pumping mechanism is experimentally investigated here with three different electrode designs. Sufficient pressure heads are generated with very low electric power requirements making the EHD conduction pumping attractive to certain applications such as capillary pumped loops and heat pipes. The electric body force acting on the molecules can be expressed as follows [1]. The first term represents the Coulomb force, which is the force acting on the free charges in an electric field.
The second and third terms represent the polarization force acting on polarized charges. The third term, the electrostriction term, is relevant only for compressible fluids. Thus, EHD pumps require either a free space charge or a gradient in permittivity within the fluid.
There are three kinds of EHD pumps operating based on the Coulomb force: The EHD induction pumping relies on the generation of induced charges. This charge induction in the presence of an electric field takes place due to a nonuniformity in the electrical conductivity of the fluid.
Therefore, this pumping mechanism cannot be utilized in an isothermal liquid. There are two main mechanisms for generating a space charge in an isothermal liquid. However, the ion-drag pumping is not desirable because it can deteriorate the electrical properties of the working fluid.
The second one is associated with heterocharge layers of finite thickness in the vicinity of the electrodes which are based on the process of dissociation of the neutral electrolytic species and recombination of the generated ions [2].
The conduction term here represents a mechanism for electric current flow in which charged carriers are produced not by injection from electrodes, but by dissociation of molecules within the fluid. The visible difference between the ion-drag pumping and pure conduction pumping is in the flow direction. In a typical ion-drag pump, the high voltage is supplied to the emitter electrode e. In this case the flow direction will be from the high voltage emitter electrode to the ground collector electrode.
The opposite applies to the pure conduction pumping with this particular design where the flow direction will be from the ground electrode to the high voltage electrode. However, it should be emphasized that the flow direction is dependant on the electrodes high voltage and ground designs. For the pure conduction pumping, electrodes with relatively large radius of curvature are required.
Furthermore, to reduce the effects of ion injection in the pure conduction pumping, the electrodes, especially the high voltage electrodes, should not contain any sharp points or edges. The present invention illustrates EHD pumping through pure conduction phenomenon.
Very limited work has been conducted in this field and the majority of the published papers in this area have mistakenly assumed that the electrostriction force the third term in Eq. As mentioned above, this issue was addressed and clarified in a recent work by Atten and Seyed-Yagoobi [2].
One application of the present invention is an EHD pump for a two-phase loop e. The pump will be installed in the liquid return passage isothermal liquid from the condenser section to the evaporator section. The simple design, non-mechanical, lightweight, the rapid control of performance by varying the applied electric field, and low power consumption are all advantages of the application of EHD. However, it is to be recognized that the application of this technology is truly applicable to isothermal and non-isothermal single phase liquid.
In general, the I versus V curve for a wide range of the applied voltage is highly non-linear and can be divided into three different regimes. In the low electric field regime, linear behavior is observed which is mainly due to dissolved electrolytic impurities [3]. If the liquid contains a very small number of ions for instance, due to impurities , the electrostatic charges on the solid surface will attract the counterions in the liquid.
The rearrangement of the charges on the solid surface and the balancing charges in the liquid is called the electrical double layer. Because of the electrostatic interaction, the ionic concentration near the solid surface is higher than that in the bulk liquid far away from the solid surface. Immediately next to the solid surface, there is a layer of ions which is strongly attracted to the solid surface and immobile.
This layer is called the compact layer, and is normally about 0. From the compact layer to the uniform bulk liquid, the ionic concentration gradually reduces to that of bulk liquid. Ions in this region are affected less by the electrostatic interaction and are mobile. This layer is called the diffuse layer of the electrical double layer. The thickness of the diffuse layer depends on the bulk ionic concentration and electrical properties of the liquid [4].
Under this low electric field regime, the conduction is mainly due to positive and negative ions generated by dissociated molecules. Thus, away from the diffuse layer, there is a non-equilibrium layer where the dissociation-recombination reactions are not in equilibrium [5].
The conduction mechanism in this regime is mainly caused by the ionic dissociation and the creation of heterocharges, i. The I versus V behavior in this regime is sub-ohmic showing only a slightly increased current with increased voltage.
The thickness of this heterocharge layer is proportional to the corresponding relaxation time of the working fluid and the strength of the local electric field. This regime is directly related to pure conduction pumping mechanism. As shown in FIG. With this electrode configuration, the net axial motion around the ring ground electrode is almost canceled because of the geometrical symmetry.
Thus, only the motion around the high voltage electrode can contribute to the net axial flow. Since the electric field is high near the electrode of small radius of curvature, the thickness of the corresponding heterocharge layer and the pressure across it will be high as well.
This implies that the force or pressure generation by pure conduction pumping has quadratic dependence on the applied voltage and is proportional to the electrical permittivity. This phenomenon is mainly controlled by the electrochemical reactions at the electrode-liquid interface and therefore depends critically on the composition and geometry of the electrodes [6].
Beyond this electric field level, the ion-drag pumping mechanism will be dominant. It includes an EHD pumping section, transparent pressure measurement section, and connecting tubes. Up to five pairs of electrodes are arranged in series in the pumping section. Positive polarity DC voltage is applied to high voltage electrodes. The ring electrodes are electrically grounded. The pressure measurement section is a glass manometer with the inclination angle of 10 to 90 degrees.
Three different designs 3-needle, hollow-tube, and pin-needle were investigated for the high voltage electrodes. The ground electrodes were made of a simple ring flushed against pumping section inner tube wall. These ground electrodes were designed as rings to avoid interference with the net axial flow generated by the high voltage electrodes as discussed in the previous section.
Furthermore, the ring ground electrodes provided a negligible drag coefficient. The 3-needle and pin-needle electrodes are made of steel while the hollow-tube electrodes are made of brass. The ground ring electrodes are made of stainless steel. The high voltage electrodes and the ground electrodes were not coated, however, they were rounded to reduce the ion injection. The working fluid was R refrigerant.
At this temperature, the electric permittivity, conductivity, and density of R are The errors associated with the data presented in the next section were primarily due to the height difference measurements in the pressure measurement section of the apparatus see FIG. Focusing only on 10 to 20 kV applied voltage range, the maximum percent errors in pressure head generation were 3. These maximum errors corresponded to 10 kv due to the smallest pressure heads generated within the above applied voltage range.
The maximum percent uncertainties for the voltage and current values were 2. The pressure heads and the corresponding current levels generated by the pump are presented below as a function of the applied voltage. The results are given for all three electrode designs. The maximum pressure head achieved is approximately Pa at 20 kV with 0.
A higher pressure head can be obtained by increasing the number of electrode pairs.