Method of and apparatus for electrostatic fluid acceleration control of a fluid flow

[0002] The instant application is related to U.S. patent application Ser. No. 09/419,720 filed Oct. 14, 1999 now U.S. Pat. No. 6,504,308 and incorporated herein in its entirety by reference.

[0003] 1. Field of the Invention

[0004] The invention relates to electrical corona discharge devices and in particular to methods of and devices for fluid acceleration to provide velocity and momentum to a fluid, especially to air, through the use of ions and electrical fields.

[0005] 2. Description of the Related Art

[0006] The prior art as described in a number of patents (see, e.g., U.S. Pat. Nos. 4,210,847 of Spurgin and 4,231,766 of Shannon, et al.) has recognized that the corona discharge device may be used to generate ions and accelerate fluids. Such methods are widely used in electrostatic precipitators and electric wind machines as described in Applied Electrostatic Precipitationpublished by Chapman & Hall (1997). The corona discharge device may be generated by application of a high voltage to pairs of electrodes, e.g., a corona discharge electrode and an attractor electrode. The electrodes should be configured and arranged to produce a non-uniform electric field generation, the corona electrodes typically having sharp edges or otherwise being small in size.

[0007] To start and sustain the corona discharge device, high voltage should be applied between the pair of electrodes, e.g., the corona discharge electrode and a nearby attractor (also termed collector) electrode. At least one electrode, i.e., the corona discharge electrode, should be physically small or include sharp points or edges to provide a suitable electric field gradient in the vicinity of the electrode. There are several known configurations used to apply voltage between the electrodes to efficiently generate the requisite electric field for ion production. U.S. Pat. No. 4,789,801 of Lee and U.S. Pat. Nos. 6,152,146 and 6,176,977 of Taylor, et al., describe applying a pulsed voltage waveform across pairs of the electrodes, the waveform having a duty cycle between 10% and 100%. These patents describe that such voltage generation decreases ozone generation by the resultant corona discharge device in comparison to application of a steady-state, D.C. power. Regardless of actual benefit of such voltage generation for reducing ozone production, air flow generation is substantially decreased by using a duty cycle less than 100%, while the resultant pulsating air flow is considered unpleasant.

[0008] U.S. Pat. No. 6,200,539 of Sherman, et al. describes use of a high frequency high voltage power supply to generate an alternating voltage with a frequency of about 20 kHz. Such high frequency high voltage generation requires a bulky, relatively expensive power supply typically incurring high energy losses. U.S. Pat. No. 5,814,135 of Weinberg describes a high voltage power supply that generates very narrow (i.e., steep, short duration) voltage pulses. Such voltage generation can generate only relatively low volume and rate air flow and is not suitable for the acceleration or movement of high air flows.

[0009] All of the above technical solutions focus on specific voltage waveform generation. Accordingly, a need exists for a system for and method of optimizing ion induced fluid acceleration taking into consideration all components and acceleration steps.

[0010] The prior art fails to recognize or appreciate the fact that the ion generation process is more complicated than merely applying a voltage to two electrodes. Instead, the systems and methods of the prior art are generally incapable of producing substantial airflow and, at the same-time, limiting ozone production.

[0011] Corona related processes have three common aspects. A first aspect is the generation of ions in a fluid media. A second aspect is the charging of fluid molecules and foreign particles by the emitted ions. A third aspect is the acceleration of the charged particles toward an opposite (collector) electrode (i.e., along the electric field lines).

[0012] Air or other fluid acceleration that is caused by ions, depends both on quantity (i.e., number) of ions and their ability to induce a charge on nearby fluid particles and therefore propel the fluid particles toward an opposing electrode. At the same time, ozone generation is substantially proportional to the power applied to the electrodes. When ions are introduced into the fluid they tend to attach themselves to the particles and to neutrally-charged fluid molecules. Each particle may accept only a limited amount of charge depending on the size of a particular particle. According to the following formula, the maximum amount of charge (so called saturation charge) may be expressed as:

[0013] Q_p={[(1+2λ/d_p)]²+[1/(1+2λ/d_p)]*[(∈_r−1)/(∈_r+2)]*π∈₀d_p²E,

[0014] where d_p=particle size, ∈r is the dielectric constant of the dielectric material between electrode pairs and E₀is the dielectric constant in vacuum.

[0015] From this equation, it follows that a certain number of ions introduced into the fluid will charge the nearby molecules and ambient particles to some maximum level. This number of ions represents a number of charges flowing from one electrode to another and determines the corona current flowing between the two electrodes.

[0016] Once charged, the fluid molecules are attracted to the opposite collector electrode in the direction of the electric field. This directed space over which a force F is exerted, moves molecules having a charge Q which is dependent on the electric field strength E, that is, in turn proportional to the voltage applied to the electrodes:

[0017] F=−Q*E.

[0018] If a maximum number of ions are introduced into the fluid by the corona current and the resulting charges are accelerated by the applied voltage alone, a substantial airflow is generated while average power consumption is substantially decreased. This may be implemented by controlling how the corona current changes in value from some minimum value to some maximum value while the voltage between the electrodes is substantially constant. In other words, it has been found to be beneficial to minimize a high voltage ripple (or alternating component) of the power voltage applied to the electrodes (as a proportion of the average high voltage applied) while keeping the current ripples substantially high and ideally comparable to the total mean or RMS amplitude of the current. (Unless otherwise noted or implied by usage, as used herein, the term “ripples” and phrase “alternating component” refer to a time varying component of a signal including all time varying signals waveforms such as sinusoidal, square, sawtooth, irregular, compound, etc., and further including both bi-directional waveforms otherwise known as “alternating current” or “a.c.” and unidirectional waveforms such as pulsed direct current or “pulsed d.c.”. Further, unless otherwise indicated by context, adjectives such as “small”, “large”, etc. used in conjunction with such terms including, but not limited to, “ripple”, “a.c. component,”, “alternating component” etc., describe the relative or absolute amplitude of a particular parameter such as signal potential (or “voltage”) and signal rate-of-flow (or “current”).) Such distinction between the voltage and current waveforms is possible in the corona related technologies and devices because of the reactive (capacitive) component of the corona generation array of corona and attractor electrodes. The capacitive component results in a relatively low amplitude voltage alternating component producing a relatively large corresponding current alternating component. For example, it is possible in corona discharge devices to use a power supply that generates high voltage with small ripples. These ripples should be of comparatively high frequency “f”(i.e., greater than 1 kHz). The electrodes (i.e., corona electrode and collector electrode) are designed such that their mutual capacitance C is sufficiently high to present a comparatively small impedance X_cwhen high frequency voltage is applied, as follows: $X_c=\frac{1}{2\pi \mathrm{fC}}$

[0019] The electrodes represent or may be viewed as a parallel connection of the non-reactive reactive d.c. resistance and reactive a.c. capacitive impedance. Ohmic resistance causes the corona current to flow from one electrode to another. This current amplitude is approximately proportional to the applied voltage amplitude and is substantially constant (d.c.). The capacitive impedance is responsible for the a.c. portion of the current between the electrodes. This portion is proportional to the amplitude of the a.c. component of the applied voltage (the “ripples”) and inversely proportional to frequency of the voltage alternating component. Depending on the amplitude of the ripple voltage and its frequency, the amplitude of the a.c. component of the current between the electrodes may be less or greater than the d.c. component of the current.

[0020] It has been found that a power supply that is able to generate high voltage with small amplitude ripples (i.e., a filtered d.c. voltage) but provides a current with a relatively large a.c. component (i.e., large amplitude current ripples) across the electrodes provides enhanced ions generation and fluid acceleration while, in case of air, substantially reducing or minimizing ozone production. Thus, the current ripples, expressed as a ratio or fraction defined as the amplitude of an a.c. component of the corona current divided by the amplitude of a d.c. component of the corona current (i.e., I_a.c./I_d.c.) should be considerably greater (i.e., at least 2 times) than, and preferably at least 10, 100 and, even more preferably, 1000 times as large as the voltage ripples, the latter similarly defined as the amplitude of the time-varying or a.c. component of the voltage applied to the corona discharge electrode divided by the amplitude of the d.c. component (i.e., V_a.c.V_d.c.).

[0021] It has been additionally found that optimal corona discharge device performance is achieved when the output voltage has small amplitude voltage alternating component relative to the average voltage amplitude and the current through the electrodes and intervening dielectric (i.e., fluid to be accelerated) is at least 2, and more preferably 10 times, larger (relative to a d.c. current component) than the voltage alternating component (relative to d.c. voltage) i.e., the a.c./d.c. ratio of the current is much greater by a factor of 2, 10 or even more than a.c./d.c. ratio of the applied voltage. That is, it is preferable to generate a voltage across the corona discharge electrodes such that a resultant current satisfies the following relationships:

[0022] V_a.c.<<V_d.c.and I_a.c.˜Id.c.

[0023] or

[0024] V_a.c./V_d.c.<<I_a.c./I_d.c.

[0025] or

[0026] V_a.c.<V_d.c.and I_{a.c. >Id.c.}

[0027] or

[0028] VRMS˜VMEAN and IRMS >^IMEAN

[0029] If any of the above requirements are satisfied, then the resultant corona discharge device consumes less power per cubic foot of fluid moved and produces less ozone (in the case of air) compared to a power supply wherein the a.c./d.c. ratios of current and voltage are approximately equal.

[0030] To satisfy these requirements, the power supply and the corona generating device should be appropriately designed and configured. In particular, the power supply should generate a high voltage output with only minimal and, at the same time, relatively high frequency ripples. The corona generating device itself should have a predetermined value of designed, stray or parasitic capacitance that provides a substantial high frequency current flow through the electrodes, i.e., from one electrode to another. Should the power supply generate low frequency ripples, then X_cwill be relatively large and the amplitude of the alternating component current will not be comparable to the amplitude of the direct current component of the current. Should the power supply generate very small or no ripple, then alternating current will not be comparable to the direct current. Should the corona generating device (i.e., the electrode array) have a low capacitance (including parasitic and/or stray capacitance between the electrodes), then the alternating current again will not be comparable in amplitude to the direct current. If a large resistance is installed between the power supply and the electrode array (see, for example, U.S. Pat. No. 4,789,801 of Lee, FIGS. 1 and 2), then the amplitude of the a.c. current ripples will be dampened (i.e., decreased) and will not be comparable in amplitude to that of the d.c. (i.e., constant) component of the current. Thus, only if certain conditions are satisfied, such that predetermined voltage and current relationships exist, will the corona generating device optimally function to provide sufficient air flow, enhanced operating efficiency, and desirable ozone levels. The resultant power supply is also less costly.

[0031] In particular, a power supply that generates ripples does not require substantial output filtering otherwise provided by a relatively expensive and physically large high voltage capacitor connected at the power supply output. This alone makes the power supply less expensive. In addition, such a power supply has less “inertia” i.e., less stored energy tending to dampen amplitude variations in the output and is therefore capable of rapidly changing output voltage than is a high inertia power supply with no or negligible ripples.

[0032] FIG. 1A is a schematic diagram of a power supply that produces a d.c. voltage and d.c.+a.c. current;

[0033] FIG. 1B is a waveform of a power supply output separately depicting voltage and current amplitudes over time;

[0034] FIG. 2A is a schematic diagram of a corona discharge device having insufficient interelectrode capacitance to (i) optimize air flow, (ii) reduce power consumption and/or (iii) minimize ozone production;

[0035] FIG. 2B is a schematic diagram of a corona discharge device optimized to benefit from and cooperate with a power supply such as that depicted in FIG. 3;

[0036] FIG. 3 is a schematic diagram of a power supply that produces a high amplitude d.c. voltage having low amplitude high frequency voltage ripples; and

[0037] FIG. 4 is an oscilloscope trace of a high voltage applied to a corona discharge device and resultant corona current.

[0038] FIG. 1A is a block diagram of a power supply suitable to power a corona discharge device consistent with an embodiment of the invention. High voltage power supply (HVPS) 105 generates a power supply voltage 101 (FIG. 1B) of varying amplitude Vac_ac+dc. Voltage 101 has superimposed on an average d.c. voltage of V_dcan a.c. or alternating component of amplitude V_achaving an instantaneous value represented by the distance 103 (i.e., an alternating component of the voltage). A typical average d.c. component of the voltage 101 (Vdc) is in the range of 10 kV to 25 kV and more preferably equal to 18 kV. The ripple frequency “f” is typically around 100 kHz. It should be noted that low frequency harmonics, such as multiples of the 60 Hz commercial power line frequency including 120Hz may be present in the voltage wave-form. The following calculation considers only the most significant harmonic, that is the highest harmonic, in this case 100kHz. The ripples′ peak-to-peak amplitude 103 (V_acbeing the a.c. component of the voltage 101) may be in the range of 0 to 2000 volts peak-to-peak and, more preferably, less than or equal to 900V, with an RMS value of approximately 640V. Voltage 101 is applied to the pair of electrodes (i.e., the corona discharge electrode and the attractor electrode). Resistor 106 represents the internal resistance of HVPS 105 and the resistance of the wires that connect HVPS 105 to the electrodes, this resistance typically having a relatively small value. Capacitor 107 represents the parasitic capacitance between the two electrodes. Note that the value of capacitor 107 is not constant, but may be roughly estimated at the level of about 10 pF.

[0039] Resistor 108 represents the non-reactive d.c. ohmic load resistance R characteristic of the air gap between the corona discharge and attractor electrodes. This resistance R depends on the voltage applied, typically having a typical value of 10 mega-Ohms.

[0040] The d.c. component from the HVPS 105 flows through resistor 108 while the a.c. component primarily flows through the capacitance 107 representing a substantially lower impedance at the 100 kHz operating range than does resistor 108. In particular, the impedance X_cof capacitor 107 is a function of the ripple frequency. In this case it is approximately equal to:

[0041] X_c=1/(2 πf C)=1/(2*3.14*100,000*10*10⁻¹²)=160 kΩ

[0042] The a.c. component I_a.c.of the current flowing through capacitance 107 is equal to

[0043] I_a.c.=V_a.c./X_c=640/160,000=0.004 A=4 mA.

[0044] The d.c. component Idc of the current flowing through the resistor 108 is equal to

[0045] I_dc=V_dc/R=18 kV/10MΩ=1.8 mA.

[0046] Therefore the a.c. component I_acof the resulting current between the electrodes is about 2.2 times greater than the d.c. component I_dcof the resulting current.

[0047] The operation of device 100 may be described with reference to the timing diagram of FIG. 1B. When the ionization current reaches some maximum amplitude (I_max), ions are emitted from the corona discharge electrode so as to charge ambient molecules and particles of the fluid (i.e., air molecules). At this time maximum power is generated and maximum ozone production (in air or oxygen) occurs. When the current decreases to I_min, less power is generated and virtually no ozone is produced.

[0048] At the same time, charged molecules and particles are accelerated toward the opposite electrode (the attractor electrode) with the same force (since the voltage remains essentially constant) as in the maximum current condition. Thus, the fluid acceleration rate is not substantially affected and not to the same degree as the ozone production is reduced.

[0049] Acceleration of the ambient fluid results from the moment of ions forming the corona discharge electrodes to the attractor electrode. This is because under the influence of voltage 101, ions are emitted from the corona discharge electrode and create an “ion cloud” surrounding the corona discharge electrode. This ion cloud moves toward the opposite attractor electrode in response to the electric field strength, the intensity of which is proportional to the value of the applied voltage 101. The power supplied by power supply 105 is approximately proportional to the output current 102 (assuming voltage 101 is maintained substantially constant). Thus, the pulsated nature of current 102 results in less energy consumption than a pure d.c. current of the same amplitude. Such current waveform and relationship between a.c. and d.c. components of the current is ensured by having a low internal resistance 106 and small amplitude alternating component 103 of the output voltage. It has been experimentally determined that most efficient electrostatic fluid acceleration is achieved when relative amplitude of the current 102 alternating component (i.e., I_ac/I_dc) is greater than the relative amplitude of voltage 101 alternating component (i.e., V_ac/V_dc). Further, as these ratios diverge, additional improvement is realized. Thus, if V_ac/V_dcis considerably less than (i.e., no more than half) and, preferably, no more than {fraction (1/10)}, {fraction (1/100)}, or, even more preferably, {fraction (1/1000)} that of I_ac/I_dc, (wherein V_acand I_acare similarly measured, e.g., both are RMS, peak-to-peak, or similar values) additional efficiency of fluid acceleration is achieved. Mathematically stated a different way, the product of the constant component of the corona current and the time-varying component of the applied voltage divided by the product of the time-varying component of the corona current and the constant component of the applied voltage should be minimized, each discrete step in magnitude for some initial steps providing significant improvements: $\frac{I_{dc}\times V_{ac}}{I_{ac}\times V_{dc}}\le 1;\mathrm{.01};\mathrm{.001};\mathrm{.0001};\cdots$

[0050] FIG. 2A shows the corona discharge device that does not satisfy the above equations. It includes corona discharge electrode 200 in the shape of a needle, the sharp geometry of which provides the necessary electric field to produce a corona discharge in the vicinity of the pointed end of the needle. The opposing collector electrode 201 is much larger, in the form of a smooth bar. High voltage power supply 202 is connected to both of the electrodes through high voltage supply wires 203 and 204. However, because of the relative orientation of discharge electrode 200 perpendicular to a central axis of collector electrode 201, this arrangement does not create any significant capacitance between the electrodes 200 and 201. Generally, any capacitance is directly proportional to the effective area facing between the electrodes. This area is very small in the device shown in the FIG. 2A since one of the electrodes is in the shape of a needle point having minimal cross-sectional area. Therefore, current flowing from the electrode 200 to the electrode 201 will not have a significant a.c. component. Corona discharge devices arrangements similar to that depicted in FIG. 2A demonstrate very low air accelerating capacity and comparatively substantial amount of ozone production.

[0051] FIG. 2B shows an alternative corona discharge device. A plurality of corona discharge electrodes are in the shape of long thin corona discharge wires 205 with opposing collector electrodes 206 in the shape of much thicker bars that are parallel to corona wires 205. High voltage power supply 207 is connected to corona discharge wires 205 and collector electrode 206 by respective high voltage supply wires 209 and 210. This arrangement provides much greater area between the electrodes and, therefore creates much greater capacitance therebetween. Therefore, the current flowing from corona wires 205 to collector electrodes 206 will have a significant a.c. component, providing that high voltage power supply 207 has sufficient current supplying capacity. Corona discharge devices arrangements like shown in the FIG. 2B provide greater air accelerating capacity and comparatively small ozone production when powered by a high voltage power supply with substantial high frequency current ripples but small voltage ripples (i.e., alternating components).

[0052] FIG. 3 is a schematic diagram of a high voltage power supply circuit 300 capable of generating a high voltage having small high frequency ripples. Power supply 300 includes high voltage dual-winding transformer 306 with primary winding 307 and secondary winding 308. Primary winding 307 is connected to a d.c. voltage source 301 through a half-bridge inverter (power transistors 304, 313 and capacitors 305, 314). Gate signal controller 311 produces control pulses at the gates of the transistors 304, 313 through resistors 303 and 317. An operating frequency of these pulses is determined by values selected for resistor 310 and capacitor 316. Secondary winding 308 of transformer 306 is connected to bridge voltage rectifier 309 including four high voltage high frequency power diodes. Power supply 300 generates a high voltage output between the terminal 320 and ground which is connected to the electrodes of corona discharge device.

[0053] FIG. 4 depicts oscilloscope traces of the output current and voltage waveform, high voltage 401 at the corona discharge device and together with the resultant current 402 produced and flowing through the array of electrode. It can be seen that voltage 401 has a relatively constant amplitude of about 15,300 V with little or no alternating component. Current 402, on the other hand, has a relatively large alternating current component (ripples) in excess of 2mA, far exceeding the current mean value (1.189mA).

[0054] In summary, the present invention includes embodiments in which a low inertia power supply is combined with an array of corona discharge elements presenting a highly reactive load to the power supply. That is, the capacitive loading of the array greatly exceeds any reactive component in the output of the power supply. This relationship provides a constant, low ripple voltage and a high ripple current. The result is on a highly efficient electrostatic fluid accelerator with reduced ozone production.

[0055] It should be noted and understood that all publications, patents and patent applications mentioned in this specification are indicative of the level of skill in the art to which the invention pertains. All publications, patents and patent applications are herein incorporated by reference to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety.