Fuel Burner With Improved Ultrasonic Atomizer

Abstract

An apparatus for achieving efficient combustion of fuels employs an improved ultrasonic atomizer. This atomizer includes: an active cylindrical resonator section having an atomizing surface and a passage therethrough for delivering fuel to the atomizing surface; a dummy cylindrical resonator section for providing counteractive forces to the active resonator section and having a flanged portion; a transducer sandwiched therebetween for providing the driving force to the resonator sections including a pair of piezoelectric discs and a high voltage electrode positioned therebetween; clamping means, including internally threaded, electrically conductive cylindrical casing, thrust washer and clamp nut for enclosing the resonator sections and transducer and for holding the resonator sections in compression against the transducer to insure good acoustical coupling while at the same time assuring that losses of acoustical energy are kept to a minimum; an insulating sleeve surrounding the resonator sections and transducer, spacing same from the casing and electrically isolating the transducer from the casing; means mounted on the casing for delivering electrical energy to the transducer from a terminal post; means mounted on the casing for delivering fuel to the fuel passage within the active resonator section; and, sealing means between casing and active resonator section, for protecting the interior portions of the atomizer, especially the transducer, from fuel contamination.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatus for achieving complete and efficient combustion of fuels that employs an improved ultrasonic atomizer.

2. Description of the Prior Art

In a conventional fuel atomization device, the fuel (fuel oil, kerosene, gasoline, etc.) is forced at high pressure through a small orifice breaking the stream into droplets. For a conventional fuel nozzle spray at a flow rate of 1 gal./hr. the mean droplet diameter is usually greater than 125 microns.

In a fuel atomization device using ultrasonic energy, a film of the fuel is injected at low pressure onto a plane surface, and vibrated at frequencies in excess of 20 kHz in a direction perpendicular to the surface. The rapid motion of the plane surface sets up capillary waves in the liquid film. When the amplitude of the capillary wave peaks exceeds that required for stability of the system, the liquid at the peak crests breaks away in the form of droplets. For flow rates of approximately 1 gal./hr. the mean droplet diameter is about 15 microns, an order of magnitude less than for conventional fuel atomization devices, thereby increasing approximately tenfold the effective fuelair interface surface area for a given quantity of fuel. Since burning occurs only at the fuel-air interface, the increase in surface area significantly improves burner efficiency.

The typical prior art ultrasonic atomizer includes a transducer sandwiched between and acoustically coupled to a pair of resonator sections. The transducer includes a pair of piezoelectric crystals with a high voltage electrode positioned therebetween for electrically exciting the crystals and thereby providing the driving force for the resonators. A passage extends through one of the resonator sections for delivery of fuel to its tip where atomization takes place. Because the motion developed by the transducer would be, in itself, insufficient to cause atomization without further amplification the resonator section is provided with a large diameter segment and a small diameter segment so as to form a step in the region of their joining. Amplification at the tip is approximately equal to the ratio of the squares of the larger to the smaller diameters.

At the juncture where the small diameter segment joins the large diameter segment the right angle bend and the reduced cross section of material causes extremely large stress concentration. Since stress levels increase with operating frequency, prior art atomizers have been prone to fracture at this juncture when constructed of conventional materials such as aluminum.

Also in the past the resonator sections -- transducer structure have been clamped together using individual screw type clamping which leads to non-uniform compression of the piezoelectric discs, possible early fracture of the discs, loosening of the screws due to vibrations and consequent loss in performance thereby suffered. The individual screw type units usually employ a soft material, for example, lead, adjacent to the crystals to compensate for non-uniform compression. This "foreign" material causes degradation in performance. Finally, in the prior art individual screw type arrangement, the crystals are completely exposed to contamination by fuel and other contaminants, leading to degradation in performance.

SUMMARY OF THE INVENTION

Accordingly, an object of the invention is an apparatus for achieving efficient combustion of fuels employing an improved ultrasonic atomizer.

Another object is the provision of such an atomizer that resists premature fracture of its resonator sections.

Still another object is the provision of such an atomizer that elminates early fracture of its piezoelectric crystals and fuel contamination of same.

These and other objects are accomplished in accordance with the teachings of the present invention, one illustrative embodiment of which comprises a fuel burner that employs an ultrasonic atomizer that includes: an active cylindrical resonator section having an atomizing surface and a passage therethrough for delivering fuel to the atomizing surface; a dummy cylindrical resonator section for providing counteractive forces to the active resonator section and having a flanged portion; a transducer sandwiched therebetween for providing the driving force to the resonator sections including a pair of piezoelectric discs and a high voltage electrode positioned therebetween; clamping means, including internally threaded electrically conductive cylindrical casing, thrust washer and clamp nut for enclosing the resonator sections and transducer and holding the resonator sections in compression against the transducer to insure good acoustical coupling while at the same time assuring that losses of acoustical energy are kept to a minimum; an insulating sleeve surrounding the resonator sections and transducer, spacing same from the casing and electrically isolating the transducer from the casing; means mounted on the casing for delivering electrical energy to the transducer from a terminal post; means mounted on the casing for delivering fuel to the fuel passage within the active resonator section; and, sealing means between casing and active resonator section, for protecting the interior portions of the atomizer, especially the transducer, from fuel contamination.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawing, wherein:

FIG. 1 is a cross sectional view of a fuel burner employing the novel ultrasonic transducer of the present invention;

FIG. 2 is a sectional view taken along the lines 2--2 of FIG. 1;

FIG. 3 is an enlarged cross sectional view, partially cut away, showing the novel ultrasonic atomizer constructed in accordance with the teachings of the present invention;

FIG. 4 is a sectional view taken along the lines 4-4 of FIG. 3;

FIG. 5 is an enlarged cross sectional view, partially broken away depicting the step region and atomizing surface of the ultrasonc transducer of FIG. 4; and,

FIG. 6 is a view similar to FIG. 5 but of an alternate embodiment depicting a flange type atomizing surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawing, apparatus constructed in accordance with the teachings of the present invention will now be described. While not so limited, the invention finds immediate application in achieving complete and efficient combustion of fuels and will be so described in such an environment.

FIG. 1 illustrates a burner 11 suitable for burning fuel oil for heating a home or other building or for hot water heating. Such a burner is designed to achieve atomization of the fuel oil, ignition of same and to provide a controlled flow of air.

The burner 11 is seen as including atomizer 12, ignition means 13, blower 14, air deflection means 15, flame cone 16, means 17, 18 and 19 for supplying electric power to atomizer 12, ignition means 13 and the motor 33 which drives blower 14 and pump means 20 for supplying fuel oil to the atomizer 12.

Essentially the entire structure is surrounded by and supported on a cylindrical housing 21 with cover plates 22, 23.

The atomizer 12 is of the ultrasonic variety and is positioned axially of housing 21 via support means 24 affixed to and spaced from cover plate 22 by spacers 25, nuts 26 and bolts 27.

Ignition is achieved in the burner 11 through the use of ignition means 13 that includes a pair of ignition electrodes 28, 29 positioned within but electrically isolated from flame cone 16.

Air is forced by means of a blower 14 mounted on the cover plate 23 through the housing 21 over the atomizer 12 through cover plate 22 past the electrodes 28, 29 in flame cone 16 and thoroughly mixed with the fuel.

Before reaching the atomizing tip of atomizer 12 the air is passed through the deflector means 15 that includes a plurality of deflecting vane segments 30 to impart a swirling motion to a portion of the air just before reaching the atomizing tip to achieve better fuel-air mixing. The deflector means 15 is affixed to and supported on the cover plate 22. Flame cone 16 provides confinement and shaping means for the resulting flame.

Electric power is fed to the atomizer 12, ignition means 13 and motor 33 for blower 14, via the power leads 17, 18, 19 extending from terminal post 31, 32. Fuel is supplied to the atomizer 12 at low pressure via means 20 that is also driven by the motor 33 for the blower 14.

Referring in particular to FIG. 3 of the drawing, the atomizer 12 is seen as including: An active cylindrical resonator section 41 having an atomizing surface 42 and a passage 43 therethrough for delivering fuel to the atomizing surface; a dummy cylindrical resonator section 44 for providing counteractive forces to the active resonator section 41 and having a flanged portion 45; a transducer 46 sandwiched therebetween for providing the driving force to the resonator sections including a pair of piezoelectric discs 47, 48 and a high voltage electrode 49 positioned therebetween; clamping means 50 including internally threaded electrically conductive cylindrical casing 51, thrust washer 52 and clamp nut 53 for enclosing the resonator sections and transducer and for holding the resonator sections in compression against the transducer to insure good acoustical coupling while at the same time assuring that losses of acoustical energy are kept to a minimum; an insulating sleeve 54 surrounding the resonator sections 41, 44 and transducer 46, spacing same from the casing 51 and electrically isolating the transducer 46 from the casing 51; means 55 mounted on the casing 51 for delivering electrical energy to the transducer from the terminal post 31; means 56 mounted on the casing 51 for delivering fuel to the fuel passage 43 within the active resonator section 41; and, sealing means 57 between casing 51 and active resonator section 41 for protecting the interior portions of the atomizer 12, especially the transducer 46, from fuel contamination.

The active resonator section 41 comprises a large diameter cylindrical segment 61 and a small diameter cylindrical segment 62, so as to form a step 63 in the region of their joining. The small segment provides an atomizing surface 42 at its terminal end. The fuel to be atomized is fed through the right-angle passage 43 in the active resonator segment terminating at an orifice 64 in the atomizing surface. The cross sectional area of right-angle passage 43 and orifice 64 should be large enough to ensure low pressure flow at the normal fuel delivery rate.

The motion developed by the transducer 46 would be, in itself, insufficient to cause atomization without further amplification of the motion. Amplification is accomplished by the provision of the step 63 at the interface between the large diameter segment 61 and the small diameter segment 62, with the amplification being approximately equal to the ratio of the squares of the larger to the smaller diameters, with the amplitude of motion being maximum at the atomizing surface 42. In a typical embodiment the small diameter segment 62 is one quarter wave length long.

In the geometry thus far depicted which is that of a resonant cavity for longitudinal pressure waves, there will be node locations at planes along the structure corresponding to locations of maximum stress and minimum displacement. These nodal planes are designed to be at the step 63 of the interface between the small diameter segment 62 and large diameter segment 61, at the center of the transducer 46 and at the flange 45 in the dummy resonator section 44. Deviations of the nodal plane positions from their theoretical locations are commonplace in actual devices because of certain inexactness in the analytic expressions, but this circumstance does not cause appreciable degradation in performance.

At the juncture where the small diameter segment 62 joins the large diameter segment 61 which is a nodal region, the right-angle bend and the reduced cross-section of material causes extremely large stress concentrations. Careful machining of the step is required to avoid machining marks or other imperfections which could cause premature failure. Since stress levels increase with operating frequency, atomizers designed for operation at 100 kHz are prone to fracture at this juncture when constructed of conventional materials such as aluminum.

Titanium is selected as the resonator material since it possesses the required strength to withstand the generated stress. Titanium has the additional desirable feature of having a comparable acoustic impedance to the transducer with the result that transmission of motion from the transducer to the resonator sections is efficiently transferred.

The atomizer further includes a dummy resonator section 44 which serves to provide counteractive forces to the active or atomizing resonator section 41. In a typical embodiment the total length of the dummy resonator section 44 is one and a half wave lengths long and includes a flanged portion 45 located one quarter of a wave length from the free end.

The transducer 46 comprises a pair of piezoelectric discs 47, 48 and and an electrode 49 positioned therebetween, excited by high frequency electrical energy fed thereto. Multicrystalline piezoelectric materials such as lead zirconate titanate can be employed. The piezoelectric discs 47, 48 are oriented so that the crystal faces in contact with the high voltage electrode 49 are of the same polarity. The high voltage electrode 49 for supplying power to the conductively plated faces of the piezoelectric discs comprises a titanium tab, whose use is dictated by acoustical compatibility requirements with the other elements of the atomizer.

A novel clamping means is provided that includes an electrically conductive cylindrical casing 51, thrust washer 52 and clamp nut 53 for enclosing the resonator sections and transducer 46 and for holding the resonator sections in compression against the transducers to insure uniform compression and good acoustical coupling. The annular portion 65 of casing 51 has an inwardly extending annular shoulder portion 66 that upon tightening abuts the step 63 of the active resonator section 41. The opposite end of casing 51 is internally threaded at 67.

An insulating sleeve 54 of phenolic resin or other suitable material surrounds the resonator sections 41, 44 and transducer 46 spacing same from the casing and electrically isolating the transducer 46 from the casing 51, as well as providing centering means for the various elements comprising the atomizer along a common axis.

An electrical connector 55 for supplying electrical energy to the titanium electrode 49 is mounted on the casing 51.

Fuel is supplied through the means 20 to the passage 43 within the active resonator section 41 through a leak proof entry area 68 in the casing 51.

An O-ring seal 57 of suitable material, including metal, is positioned between the step 63 and the annular top portion 65 to prevent fuel entry.

It is thus seen that the clamping means serves four functions. It provides the means for maintaining tension between the transducer 46 and the resonator sections 41, 44 necessary to create the proper acoustical coupling and to minimize energy losses. Compressive force is applied between two approximate nodal points, the step 63 in the active resonator section 41 and the flanged portion 45 in the dummy resonator section 44 by the casing 51 working in concert with the clamp nut 53 and thrust washer 52 squeezing transducer 46 and resonator sections 41, 44 together. The clamping cylinder 51 also serves as a common electrical path between the two resonator sections 41, 44 which is necessary in this design since both resonator sections operate at the same electric potential. The casing 51 further serves to provide complete protection from fuel or any other foreign matter from reaching the transducer 46. Finally, the casing 51 acts as a mounting structure.

Since it is tied to two nodal plane, the casing 51 has very little vibratory motion induced and acoustical energy losses are kept to a minimum. Typically, the casing is constructed of a material very different in acoustical properties from the material of the resonator sections in order to further insure a minimum of energy transfer from resonator sections to casing. Stainless steel is one example of preferred material.

In operation, the blower 14 is actuated for circulation of air. Fuel is fed under low pressure from the fuel supply means 20, leak proof entry area 68, passage 43 and orifice 64 to the atomizing surface 42.

High frequency electrical energy is fed to the voltage electrode 49 to actuate transducer 46. Transducer 46 provides the driving force for the resonator sections 41, 44 such that the fuel oil at the atomizing surface 42 breaks away from same in the form of droplets.

The mean fuel droplet diameter varies approximately inversely with the driving frequency to the two-thirds power. Thus, the greater the ultrasonic driving frequency, the smaller will be the droplets and the greater will be burning efficiency as demonstrable by noting that the available air-fuel interface area increases in approximate proportion to the inverse of the droplet diameter. Operation in a frequency region of between 80-100 kHz for my application is preferred since droplet size is optimum for burning in this frequency range. Specifying the frequency fixes the length of the large diameter segment since this must be a multiple of one-half of the wave length of the sound wave corresponding to the chosen frequency. In a typical embodiment, the large diameter segment is one-half wave length in length.

Important other advantages flow from my novel design. In the past the resonator sections-transducer structure have been clamped together using individual screw type clamping which leads to non-uniform compression of the piezoelectric discs, possible early fractures of the discs, loosening of the screws due to vibrations and consequent loss in performance thereby suffered. The individual screw type units usually employ a soft material, for example, lead, adjacent to the crystals to compensate for non-uniform compression. This "foreign" material causes degradation in performance. Finally, in the prior art individual screw type arrangement, the crystals are completely exposed to contamination by fuel and other ambients leading to degradation in performance.

My cylindrical clamping designs assures the imparting of uniform compression to the crystals, and provides complete protection from fuel or any other foreign matter from reaching the crystals.

The atomizing surface 42 (see FIG. 5) can be designed for a specified range of fuel deliveries. For lower delivery rates atomizing surface 42 can be used. Enough surface area (equal to the face area less the cross section of orifice 64) is available for good atomization.

In cases where larger fuel flow rates are required a thin flange 42A is machined on to the tip of the smaller diameter segment 62 as shown in FIG. 6.

While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. In an ultrasonic atomizer that includes a transducer sandwiched between a pair of resonator sections, an atomizing surface at the tip of one of said sections and fuel delivery means to said atomizing surface, the improvement which comprises;

clamping means for holding said resonator sections in compression against said transducer for good acoustical coupling and uniform compression;

said clamping means including a cylindrical casing enclosing said resonator sections and said transducer and engaging said resonator sections at approximate nodal planes; and,

sealing means positioned between said casing and said one of said resonator sections for preventing fuel contamination of said transducer.

2. The invention defined by claim 1 wherein said one of said resonator sections has a large diameter segment and a small diameter segment terminating in said atomizing surface, the interface between said large diameter and small diameter segments constituting a step for amplification of vibratory motion at said atomizing surface, and said resonator sections being of titanium.

3. The invention defined by claim 1 wherein said transducer includes a pair of piezoelectric discs having electrically similar essentially parallel contact faces and a high voltage electrode positioned therebetween, said electrode comprising a titanium tab.

4. The invention defined by claim 1 wherein said casing is of a material having different acoustical properties from the material of said resonator sections.

5. The invention defined by claim 2 wherein the small diameter segment of said one of said resonator sections terminates in a flanged portion acting as said atomizing surface.

6. A fuel burner comprising the combination of:

the ultrasonic fuel atomizer of claim 1;

means for delivering air for combustion of said fuel;

means for imparting rotary motion to a portion of said combustion air;

means for delivering fuel to said atomizer;

ignition means operable adjacent the atomizing surface of said atomizer; and,

means for confining flame in a cone of proper dimensions.

7. An ultrasonic fuel atomizer comprising: an active resonator section having a large diameter segment, a small diameter segment extending therefrom having an atomizing surface at its terminating end, and a fuel passage therethrough to an orifice in said terminating end for fuel delivery to said atomizing surface, the interface between said large diameter and small diameter segments constituting a step for amplification of vibratory motion at said atomizing surface;

a dummy resonator section for providing counteractive forces to said active resonator section having a flanged portion;

a transducer for delivering mechanical energy to said resonator sections sandwiched between the large diameter end of said active resonator section and said dummy resonator section, said transducer including a pair of piezoelectric discs having electrically similar essentially parallel contact faces and a high voltage electrode positioned therebetween;

clamping means for holding said resonator sections in compression against said transducer for good acoustical coupling and uniform compression of said piezoelectric discs over their entire contact faces;

said clamping means including an electrically conductive cylindrical casing enclosing said resonator sections and said transducer and engaging said resonator sections at approximate nodal planes;

an insulating sleeve surrounding said resonator sections and said transducer, spacing same from said casing and electrically isolating said casing from said piezoelectric discs and centering said transducer and said resonator sections along a common axis;

means mounted on said casing for delivering electrical energy to one of said electrically similar contact faces of said piezoelectric discs through said high voltage electrode and to the other of said faces of said piezoelectric discs through said casing and said resonator sections;

means mounted on said casing for delivering fuel to said fuel passage within said resonator section;

and,

sealing means positioned between said casing and active resonator section for preventing fuel contamination of said transducer.

8. The invention defined by claim 7 wherein said clamping means bears against said active and said dummy resonator section flanged portion.

9. The invention defined by claim 7 wherein said resonator sections are of titanium.

10. The invention defined by claim 7 wherein said high voltage transducer electrode is a titanium tab.

11. The invention defined by claim 7 wherein said casing is of a material having different acoustical properties from the material of said resonator sections.

12. The invention defined by claim 7 hwerein the small diameter segment of said active resonator section cerminates in a flanged portion acting as said atomizing surface.

13. A fuel burner comprising the combination of:

the ultrasonic fuel atomizer of claim 7;

means for delivering air for combustion of said fuel;

means for imparting rotary motion to a portion of said combustion air;

means for delivering fuel to said atomizer;

ignition means operable adjacent the atomizing surface of said atomizer; and,

means for confining flame in a cone of proper dimensions.