Fluid sprinkling – spraying – and diffusing – Electrostatic type
Reexamination Certificate
1999-10-29
2001-05-08
Morris, Lesley D. (Department: 3752)
Fluid sprinkling, spraying, and diffusing
Electrostatic type
Reexamination Certificate
active
06227465
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrostatic atomizers and to devices in which atomization of liquid is used, including fuel atomizers and combustion devices.
BACKGROUND OF THE INVENTION
Electrostatic atomizers disperse liquid by applying a net electrical charge to the liquid, typically as a stream of the liquid passes through an orifice. The negative charges developed within the liquid tend to repel one another, dispersing the liquid into droplets. The injection of the net charge into the liquid may be accomplished utilizing a pair of opposed electrodes arranged adjacent to the stream of liquid and electrically connected to a high voltage power source. Such an electrostatic atomizer, called the SPRAY TRIODE™ atomizer, is disclosed in certain embodiments of U.S. Pat. No. 4,255,777, the disclosure of which is hereby incorporated by reference herein. Another electrostatic atomizer utilizes an electron beam to apply a net negative charge to the liquid. Certain embodiments of U.S. Pat. Nos. 5,093,602 and 5,378,957, the disclosures of which are hereby incorporated by reference herein, disclose apparatus and methods for electrostatic atomization utilizing an electron beam.
Electrostatic atomization of Newtonian fluids adheres to the following equation: D=75/&rgr;
e
. D is the mean droplet size in microns and &rgr;
e
is the charge density of the fluid, in coulombs per meter cubed. Thus, the same size droplets will be produced whenever a particular charge density is achieved.
The greater the charge density injected into the liquid, the greater the droplet dispersion, the smaller the droplet size and the narrower the droplet distribution. A limit on the charge density which can be injected into the liquid is the phenomenon of corona-induced breakdown, which interrupts dispersion of the liquid. When a critical level of charge is reached, the spray plume collapses.
FIG. 6A
shows a spray plume during uninterrupted operation and
FIG. 6B
shows a spray plume during operation interrupted by corona-induced breakdown. For a combustion device, this means interruption of the flame operating on the electrostatically atomized fuel.
For example, a combustion device has been run on fuel atomized by the SPRAY TRIODE™ electrostatic atomizer. It was found that sustained operation close, i.e.,, within 50V, to the critical level for corona-induced breakdown, which was about 5 kV or more, was required for blue flame operation. However, when the net charge reached the critical level, operation of the combustion device was dramatically interrupted. Furthermore, the critical level of net charge at which corona-induced breakdown occurs depends upon the properties and flow rate of the fuel, which vary during operation of the combustion system. Changes in ambient pressure and temperature also affect the operation of the electrostatic atomizer.
It would be desirable to develop an electrostatic atomizer with improvements in sustained operation and the maximum charge density provided to a liquid.
SUMMARY OF THE INVENTION
The present invention addresses these needs.
An electrostatic atomizer in accordance with the invention comprises a charge injection device for injecting a net charge into a fluent material to thereby atomize the fluent material, and a power source powering the charge injection device. The power source is arranged to vary the net charge injected by the charge injection device cyclically in accordance with a pattern of variation so that the net charge repeatedly increases to a higher value at or above a long-term breakdown value and repeatedly decreases to a lower value below the long-term breakdown value whereby corona-induced breakdown of the atomizer is reduced. The occurrence of corona-induced breakdown in an electrostatic atomizer depends upon the net charge injected into the stream of liquid and the time for which that net charge is applied to the liquid. Accordingly, by “pulsing” the net charge injected into the stream of liquid, so that the net charge is increased above the long-term breakdown value for a relatively short period of time, corona-induced breakdown can be avoided.
The electrostatic atomizer, in preferred embodiments, has a power source arranged to vary the net charged injected so that the higher value of the net charge is injected for a first interval of time and the lower value of the net charge is injected for a second interval of time during each cycle of variation. Accordingly, the net charge injected into the stream of liquid can be decreased before the onset of corona-induced breakdown. The first interval of time is less than about 15 milliseconds in certain applications.
In certain preferred embodiments, the power source of the electrostatic atomizer is arranged to vary the net charge injected so that the higher value of the net charge is injected for a time period, the net charge is decreased to the lower value, and then immediately increased to the higher value.
In certain preferred embodiments, the electrostatic atomizer includes a body defining an orifice so that the fluent material is atomized as it passes out of the orifice. The fluent material may comprise a liquid. The body may define a flow passage extending to the orifice and the charge injection device may include a first electrode and a second electrode disposed adjacent the flow passage. The first electrode and the second electrode are preferably electrically connected to the power source in the preferred embodiments.
In certain preferred embodiments, the electrostatic atomizer includes a conically-shaped electrode having a pointed end facing the orifice of the electrostatic atomizer, as well as electrodes having a number of other shapes. The second electrode may comprise a disc having at least one aperture formed in the disc. In these preferred embodiments, the first and second electrodes are disposed in the vicinity of the orifice so that the stream of liquid is injected with a net charge and is thereby atomized. However, in other preferred embodiments, the charge injection device may comprise an electron gun. Any charge injection device for injecting a fluent material with a net charge may be used.
In certain preferred embodiments, the net charge is repeatedly increased from a base level of net charge by a predetermined incremental amount of net charge to a higher level of net charge and then decreased to the base level. Preferably, the base level is injected for a first time period and the higher level is injected for a second time period. The second time period is less than the time required for the corona-induced breakdown to occur at the value for the higher level of net charge. The first time period may be about twice as long as the second time period. In other preferred embodiments, the higher level of net charge is injected for a time period, the net charge is decreased to the base level and immediately increased to the higher level.
The net charge injected into the fluent material is related to the operating voltage applied to the charge injection device. Accordingly, in preferred embodiments, the power source of the electrostatic atomizer is arranged to apply an operating voltage to the charge injection device and to vary the operating voltage so that the operating voltage repeatedly increases to a higher value at or above a long-term breakdown value and repeatedly decreases to a lower value below the long-term breakdown value whereby corona-induced breakdown is reduced. There is a particular operating voltage for a charge injection device for which, if the operating voltage is maintained constant at that value, corona-induced breakdown occurs. Accordingly, one strategy for reducing corona-induced breakdown is to “pulse” the operating voltage of the charge injection device from a base voltage, below the critical voltage at which corona-induced breakdown will occur, to a higher voltage above the critical voltage.
In certain preferred embodiments, the fluent material comprises a liquid and the electrostatic atomizer includes a source of liquid for providing a s
Charged Injection Corporation
Lerner David Littenberg Krumholz & Mentlik LLP
Morris Lesley D.
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