Transducer assembly, ultrasonic atomizer and fuel burner

A transducer assembly includes a first half wavelength double-dummy section having a pair of quarter wavelength ultrasonic horns and a driving element sandwiched therebetween. A second half wavelength stepped amplifying section extends from one end of the first section and has a theoretical resonant frequency equal to the actual resonant frequency of the first section. When used as a liquid atomizer, the small diameter portion of the stepped amplifying section has a flanged tip to provide an atomizing surface of increased area. To maintain efficiency, the length of the small diameter portion of the second section with a flange should be less than its length without a flange. A decoupling sleeve within an axial liquid passageway eliminates premature atomization of the liquid before reaching the atomizing surface. In a fuel burner incorporating the atomizer, ignition electrode life is increased by locating the electrodes outside the normal flame envelope. During the ignition phase, drive power to the atomizer is increased to widen the spray envelope to the location of the electrodes. A variable orifice controls combustion air flow in accordance with fuel rate while maintaining constant lower speed. Either three-step or continuous fuel rate modulation saves fuel and reduces pollution.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to transducer assemblies and to apparatus 
employing same for achieving efficient combustion of fuels. An example of 
same is found in the U.S. Pat. to H. L. Berger, No. 3,861,852, issued Jan. 
21, 1975. 
(2) Description of the Prior Art 
When designing ultrasonic transducer assemblies such as those employed in 
apparatus for achieving combustion of fuels, a theoretical model for the 
ultrasonic horn is used in the developmental stage. The theoretical model 
is that of a one dimensional transmission line. 
In the actual operating environment, however, deviations from the 
theoretical model are introduced. The deviations are due to, among other 
things: the finite dimensions of the sections of the horn setting up modes 
other than longitudinal, e.g. expansion in a transverse direction; 
clamping means; sealing means; physical mismatch between component parts 
(planarity); etc. 
The introduction of the deviation into the theoretical model normally 
produces internal losses in the transducer assembly and thus reduces Q, 
the mechanical merit factor. 
The approach used in designing such prior art transducer assemblies so as 
to achieve maximum Q has been to: treat the entire assembly as a 
theoretical structure; choose the vibration frequency at which the 
structure is in resonance; provide an ultrasonic horn, according to a 
theoretical model whose size is such as to provide the resonance 
condition; and, utilize materials and associated hardware such as fuel 
supply means, clamp means, seals, etc., of such type and so positioned as 
to minimize losses inherent in the deviation from the theoretical model. 
The prior art design approaches have failed to achieve maximum Q for a 
number of reasons: inappropriate design (deviations from the theoretical 
model); and, poor acoustical coupling between the center electrode and the 
piezoelectric crystals of the driving element and between the driving 
element crystals and adjacent ultrasonic horn sections caused either by 
imperfect machining of the crystals or by the presence of contaminants 
between the mating surfaces. 
A second problem associated with transducer assemblies of the type used in 
apparatus for achieving combustion of fuels is the non-uniform delivery of 
fuel to the atomizing surface with consequent non-uniform distribution of 
fuel from same. It has been discovered that with such prior art 
assemblies, fuels which have low surface tension as, for example, 
hydrocarbon fuels, begin to atomize within the fuel passage leading to the 
atomizing surface. This premature atomization creates bubbles within the 
fuel passage. The bubbles eventually work their way to the atomizing 
surface, but their arrival at the atomizing surface results in a temporary 
interruption in fuel flow to portions of the surface and, as a result, 
non-uniform distribution of fuel over the surface. The bubble remains 
intact for a short period of time on the atomizing surface and thus the 
surface area beneath the bubble during the interval is not wet with fuel. 
A third problem associated with transducer assemblies of the type used in 
apparatus for achieving combustion of fuels is that the fuel, once 
delivered to the atomizing surface, even if delivered uniformly, is not 
distributed or atomized from same uniformly. It has been discovered that 
one of the reasons for non-uniform distribution is the flexing action of 
the atomizing surface itself, characteristic of the prior art structure. 
A fourth problem associated with prior art transducer assemblies is lack of 
efficiency. Briefly stated, in an ultrasonic fuel atomizer a film of fuel 
is injected at low pressure onto an atomizing surface and vibrated at 
frequencies in excess of 20 kHz in a direction perpendicular to the 
atomizing surface. The rapid motion of the plane surface sets up capillary 
waves in the liquid film. When the amplitude of wave peaks exceeds that 
required for stability of the system, the liquid at the peak crests breaks 
away in the form of droplets. 
The smaller the droplet size the greater the fuel-air interface for a given 
volume of fuel. The increased fuel-air interface allows better utilization 
of primary combustion air resulting in low-excess air combustion, a 
desirable feature from an efficiency standpoint. 
Going one step further, for a given fixed volume flow rate of fuel reaching 
the atomizing surface, the thinner the film, the more surface area will be 
involved in the atomizing process. This allows for greater atomizing 
capacity. It has been discovered that prior art transducer assemblies have 
been limited in this respect, however, due to the fact that the fuel fed 
to the atomizing surface does not cover the entire surface before 
atomization occurs. Additionally the surface tension associated with 
smooth metallic atomizing surfaces give rise to a tendency for not wetting 
the entire surface. 
SUMMARY OF THE INVENTION 
An object of the invention is the provision of an improved, reliable, high 
power, high Q transducer assembly of the type used in apparatus for 
achieving efficient combustion of fuels. 
Another object is an improved method for designing such assemblies. 
Still another object is the elimination of premature atomization of fuel in 
the fuel passage leading to the atomizing surface of an ultrasonic fuel 
atomizer. 
A further object is uniform atomization of fuel from the entire atomizing 
surface of an ultrasonic fuel atomizer. 
A still further object is uniform distribution of fuel over the entire 
atomizing surface in a thin film. 
Another object is an improved fuel burner with increased ignition electrode 
lifetime. 
Still another object is air flow control means within the fuel burner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1-3, in accordance with one aspect of the invention the 
design of a transducer assembly is optimized, for, among other things, 
maximum Q, by designing for a predetermined theoretical natural frequency 
a first half wavelength transducer assembly section comprising a driving 
element and two identical horn sections (FIG. 1) such that the resulting 
structure forms a symmetric geometry with respect to the longitudinal 
axis. This first assembly section is referred to as a double-dummy 
ultrasonic horn. In the next operation step, an actual double-dummy horn 
is constructed according to the design of the first assembly section, and 
the resonant frequency of the first section is measured A second half 
wavelength section (FIG. 2) that includes an amplification step and an 
atomizing surface is next designed to have a theoretical resonant 
frequency that matches the empirically measured resonant frequency of the 
actual first section. A liquid atomizing transducer assembly that combines 
the first and second sections is then constructed (FIG. 3), the final 
transducer assembly being designed for maximum Q and for achieving 
efficient combustion of fuels. 
Referring first to FIG. 1 the first section 11 of the novel transducer 
assembly is seen as including front 12A and rear 13 ultrasonic horn 
sections and a driving element 14 comprising a pair of piezoelectric discs 
15, 16 and an electrode 18 positioned therebetween, excited by high 
frequency electrical energy fed thereto through a terminal 18a. 
Driving element 14 is sandwiched between flanged portions 19, 20 of horn 
sections 12A, 13 and securely clamped therein by means of a clamping 
assembly that includes a mounting ring 21 (for securing the assembly to 
other apparatus) and a plurality of assembly bolts 22 which pass through 
holes in electrode 18, flange sections 19 and 20, and into threaded 
openings in mounting ring 21. The assembly bolts 22 are electrically 
isolated from the electrode 18 by means of insulators 23. 
The first section 11 further includes a fuel tube 24 for introducing fuel 
into a channel within the transducer assembly and a pair of sealing 
gaskets 26, 27 compressed between horn flange sections 19, 20. 
In a typical embodiment: the horn sections 12A, 13 and flange sections 19, 
20 are preferably of good acoustic conducting material such as aluminum, 
titanium or magnesium; or alloys thereof such as Ti--6Al--4V 
titanium-aluminum alloy, 6061-T6 aluminum alloy, 7075 high strength 
aluminum alloy, AZ 61 magnesium alloy and the like; the discs 15, 16 are 
of lead-zirconate-titanate such as those manufactured by Vernitron 
Corporation or of lithium niobate such as those manufactured by Valtec 
Corporation; the electrode 18 is of copper; the terminal 18a, mounting 
ring 21, and assembly bolts 22 are of steel; the insulators 23 are of 
nylon, tetrafluoroethylene or some other plastic with good electrical 
insulating properties; and, the sealing gaskets 26, 27 are of silicone 
rubber. 
The double-dummy design of the first section 11 has symmetric 
half-wavelength geometry, yet the actual first section assembly contains 
anomalous features, i.e. clamping at non-nodal planes, copper electrode, 
clamping bolts and mounting bracket, that will cause the actual resonant 
frequency of this section to deviate from the theoretical design 
frequency. The characteristic frequency, for maximum Q, of this first 
section is measured. A typical frequency for effective atomization is 
85KHZ. This completes the first step in the design of the transducer 
assembly. 
Referring to FIG. 2, another half-wave section 29 is added to the first 
section 11. The section 29 includes a large diameter segment 12B, a small 
diameter segment 30 so as to form an amplification step 31, a flanged tip 
32 with atomizing surface 33, a central passage 34 for delivering fuel to 
the atomizing surface 33 and an internally mounted decoupling sleeve 35. 
The decoupling sleeve is a substance such as tetrafluoroethylene which 
provides acoustic isolation from the surface of passage 34. 
It will be observed by those skilled in the art that section 29 contains 
few anomalies compared with a purely theoretical model. Its theoretical 
resonant frequency is selected to match the actual resonant frequency of 
the first section 11. 
In order to complete the design, the two sections 11 and 29 are formed 
integrally so as to yield a transducer assembly (FIG. 3) optimized for 
maximum Q and for use in achieving efficient combustion of fuels. 
Prior art transducer assemblies used for ultrasonic atomization of fuel 
have typically employed a flanged tip 32 with atomization surface 33. The 
flanged tip increases atomization capabilities due to increased area of 
atomizing surface 33. 
The addition of such flange has been at the expense of atomizer efficiency. 
Referring to FIG. 2, let A=length of horn front section 12B, B=length of 
small diameter segment 30 and C=thickness of flanged tip section 32. 
In prior art assemblies that do not use a flange, A/B=1 since they are both 
quarter wavelength sections. 
In prior art assemblies utilizing a flange A/B+C=1. 
It has been determined that maintaining the ratio at 1, even after addition 
of the flange, is inefficient and reduces power transfer, but by 
maintaining the ratio A/B+C&gt;1 efficiency levels can be maintained at 
pre-flange addition levels. Thus, for example, if 
D.sub.3 =diameter of flange section 32 
D.sub.2 =diameter of small diameter segment 30 for D.sub.3 /D.sub.2 =1.53 
A/B+C (without flange)=A/B=1 and A/B+C (with flange)=1.12 
and the efficiency levels achieved with the flange match those of the 
assembly without the flange. 
The foregoing example applies to assemblies of aluminum, titanium, 
magnesium and previously mentioned alloys, and assumes that for all these 
materials the velocity of sound is approximately the same. For other 
materials with different velocities of sound the ratio A/B+C will differ 
but always will be greater than 1. 
The long-term reliability of the device is dramatically enhanced by sealing 
the discs 15 since fuel contamination is no longer possible. The space 
between the clamping flange sections 19, 20 is filled with a silicone 
rubber compound as by sealing gaskets 26, 27. In the past, fuel creepage 
onto the faces of the discs 15, 16 has caused degradation of same and has 
resulted in poor long-term atomizer performance. The phenomenon causes a 
loss in mechanical coupling between elements of the horn. The gaskets 26, 
27 solve the problem and atomizer performance is not affected by the added 
mass as has been confirmed by before and after measurement of impedance, 
operating frequency and flange displacement. The slightly higher internal 
heating caused by sealing the discs 15 does not reduce the atomizer's 
useful life since internal temperatures are still well below the maximum 
operating temperature for piezoelectric crystals. The gaskets 26, 27 are 
of a compressible material and have an inner periphery conforming to but 
initially slightly greater than the outer circumference of the discs 15, 
16. Upon clamping, the inner periphery of gaskets 26, 27 come into light 
contact with the outer circumference of the discs 15, 16. 
Another aspect of the present invention is the elimination of premature 
atomization of fuel in the fuel passage leading to the atomizing surface. 
As noted previously, in prior art structures the fuel can begin to atomize 
within the fuel passage leading to the atomizing surface. This premature 
atomization creates voids within the fuel passage at the fuel-wall 
interface which leads to the formation of bubbles within the fuel passage. 
The bubbles eventually work their way to the atomizing surface, but their 
arrival at the atomizing surface results in a temporary interruption in 
fuel flow to a portion of the surface and as a result, non-uniform 
distribution of fuel over the surface. The bubble remains intact for a 
short period of time on the atomizing surface and thus the surface area 
beneath the bubble during that interval is not wet with fuel. The net 
effect of this non-uniform and constantly varying distribution of fuel on 
the surface is a spatially unstable spray of fuel, a condition which leads 
to unstable combustion. 
The foregoing problem is eliminated by the provision of a decoupling sleeve 
35 within the fuel passage 34 that extends up to, say within 1/32 of an 
inch of the atomizing surface 33. The sleeve is typically made of plastic 
and press fit into passage 34 extending inwardly to large diameter segment 
12B. The difference in acoustical transmitting properties between the 
material of the sleeve 35 and the horn section 29 is such that the 
vibrating motion of section 29 is not imparted to the fuel within the fuel 
passage 34 encompassed by the sleeve 35. 
Still another object of the present invention is achieving uniform 
atomization from the atomizing surface of an ultrasonic fuel atomizer. 
It has been discovered that the non-uniform distribution or atomization is 
due in part to the fact that the atomizer tip flexes during vibration and 
that the non-uniform distribution is decreased when the flange face or 
atomizing surface 33 moves as a rigid plane. The atomizing surface will 
move as a rigid plane by increasing the thickness of the flanged tip 32 
such that the tip 32 and surface 33 remain rigid during vibration. In a 
typical embodiment tip 32 is 0.050" thick. 
A further aspect of the present invention is achieving greater atomizing 
capacity. As noted above, it has been discovered that prior art transducer 
assemblies have been limited in this respect due to the fact that the fuel 
fed to the atomizing surface does not cover the entire surface before 
atomization occurs. Additionally the surface tension normally associated 
with smooth metallic atomizing surfaces gives rise to a tendency for not 
wetting the entire surface. 
The aforementioned prior art difficulties are overcome in accordance with 
the teachings of the present invention by reducing surface tension at the 
fuel-atomizing surface interface thereby permitting the fuel when fed to 
the atomizing surface to flow more readily over the atomizing surface and 
by the provision of means for more evenly distributing fuel over the 
atomizing surface. 
In accordance with one embodiment and referring to FIG. 4, surface tension 
at the fuel-atomizing surface is reduced by coating the atomizing surface 
with a substance that reduces surface tension. FIG. 4 depicts the flanged 
tip 32 as having an atomizing surface 33 with a thin coating 41 thereon. 
Examples of such materials are tetrafluoroethylene, polyvinyl chloride, 
polyesters and polycarbonates. 
In accordance with another embodiment and referring to FIG. 5, the ability 
of fuel to reach the outer edges is increased by the provision of 
preferred paths or channels 42 in the atomizing surface 33. The inclusion 
of channels in the atomizing surface which extend to the periphery of the 
flanged tip promotes flow of fuel over the entire atomizing surface. Thus 
for a given quantity of fuel, the result is a thin film over substantially 
the entire atomizing surface instead of a somewhat thicker film centered 
about the central fuel passage. 
In accordance with another embodiment and with reference to FIG. 6 heating 
means 43 are provided to heat the atomizing surface during operation to 
temperatures on the order of up to 150.degree. F. The heat reduces the 
viscosity of the fuel and promotes easier wetting of the surface. 
In accordance with another embodiment and with reference to FIG. 7, the 
atomizing surface is etched as at 44, by sand-blasting, thereby greatly 
increasing surface area and reducing film thickness for a given quantity 
of fuel. 
The geometrical contour of the flanged atomizing surface influences the 
spray pattern and density of particles developed by atomization. Thus, for 
example, a planar face atomizing surface 33 such as depicted in FIGS. 2-7 
will generate a particular pattern and density. If the surface is made to 
be convex, as shown at 33' in FIG. 8, the spray pattern is wider and there 
are fewer particles per unit of cross-sectional area than with a planar 
surface. A concave surface 33" such as that depicted in FIG. 9 narrows the 
spray pattern and density of particles is greater than with a planar 
surface. Different spray patterns may be required depending on the 
application. 
Turning attention now from the transducer assembly per se to a fuel burner, 
a recurring problem is the short life of the ignition electrodes. These 
electrodes provide the spark for initiating the ignition of the fuel/air 
mixture within the flame cone. Once ignition occurs, however, the 
electrodes extend into the flame envelope resulting from ignition and this 
constant exposure to high intensity heat during the firing cycles leads to 
rapid deterioration of the electrodes and frequent replacement of same. 
In accordance with another aspect of the present invention, the 
aforementioned prior art difficulty has been greatly diminished by 
locating the ignition electrodes outside the normal flame envelope, but 
increasing the drive power to the atomizer electrodes during the ignition 
phase. This has the effect of increasing the angle of the spray envelope 
considerably, bringing the ignition electrodes within the space occupied 
by the fuel/air mixture and resulting flame envelope. As soon as ignition 
is accomplished the angle of the spray envelope is returned to its normal 
running mode by decreasing drive power to the atomizer electrodes such 
that the ignition electrodes are located outside the normal flame 
envelope. 
Referring now to FIG. 10, the fuel burner 50 is seen as including blast 
tube 51, a transducer assembly 52, ignition means including ignition 
electrodes 53, blower 54 for supplying air for combustion and for cooling 
the transducer assembly 52, air deflection means 55, flame cone 56, 
variable means 57 for supplying electric power, flame sensor 58, and pump 
means 59 for supplying fuel from a fuel tank 60 to the transducer 
assembly. The ignition electrodes 53 are located between blast tube 51 and 
flame cone 56 and held by ceramic or porcelain insulators surrounded by 
high temperature asbestos material and near the atomizing surface but at a 
sufficient distance, typically 1/2 inch, to prevent arcing of the ignition 
spark to the atomizer structure. During the ignition phase additional 
electrical power is supplied by the power supply 57 to the input leads of 
the transducer assembly (greater voltage and current than during normal 
operation). Optionally, this can be accomplished automatically by 
programming the power supply electronics such that prior to ignition the 
circuit supplies an excessive amount of power to the input leads of the 
transducer assembly apparatus. During the ignition phase the ignition 
electrodes are located within the flame envelope generated within the 
flame cone (FIG. 10A). Once ignition has been established the flame sensor 
58 sends a signal back to the power supply electronics switching the 
atomizer drive power to its normal operating mode, reducing the envelope 
of the flame and thus the ignition electrodes 53 found to be located 
outside the normal flame envelope (FIG. 10B). This promotes longer 
ignition electrode life by virtue of the electrodes being kept at a cooler 
temperature during the normal operating cycle. The ignition electrodes 
will not foul nor will they be oxidized by continuous heating. 
An advantage to the use of an ultrasonic fuel atomizer is that one can vary 
the flow rate of fuel over a wide range. However, in order to implement a 
variable flow rate burner it is advantageous to have means to change the 
flow rate of combustion air through the burner combustion tube 51. This 
can be done either by electrically controlling the blower motor speed or 
by providing a variable sized orifice for air flow located in the air 
stream while maintaining a constant motor speed. With reference to FIGS. 
11-13 the latter method is preferred because only by this means can the 
static pressure head of air within the burner be maintained in order to 
develop turbulence necessary for proper combustion. This is implemented by 
an iris-type diaphragm 61 located within the combustion tube (FIGS. 11 and 
12) that is controlled electrically as shown in FIG. 13. 
The control of the iris diaphragm 61 is done electrically. For each fuel 
flow rate the amount of air is automatically adjusted by opening or 
closing the diaphragm until optimum burning conditions are sensed. The 
optimum burning conditions are sensed by monitoring the CO.sub.2 level in 
the flue gas as at 62 from the furnace and feeding back data from that 
sensor to air control circuitry 63 for iris diaphragm 61 until a 
predetermined CO.sub.2 level, say 12.5-13% CO.sub.2, is achieved. 
In the prior art an oil burner will operate in a two stage mode, "off" and 
"on" and at a fixed fuel flow rate. It has been determined that such two 
stage operation suffers from a number of disadvantages. Firstly, it is 
uneconomical in the sense that it consumes more fuel than is necessary 
and, secondly, it contributes to pollution. In the two stage operation 
when the system is turned from the off position to the on position or 
vice-versa, the firing is accompanied by generation of high volumes of 
unburned hydrocarbons and carbon monoxide. 
It has been determined that the aforementioned prior art difficulties may 
be eliminated and in accordance with the teachings of the present 
invention by going to a "three stage" modulated mode of operation. 
The three stage mode, and with reference to FIG. 14, refers to a system in 
which there are three different firing rates--high, low and off. For 
example, the three rates could typically be 
High--0.60 gal./hr. 
Low--0.20 gal./hr. 
Off--0.00 gal./hr. 
The high rate is called for by a duct or stack thermostat 71 in response to 
sensing a heat deficiency, just as is done in conventional heating systems 
with conventional thermostats. When the heat demand has been satisfied (as 
determined by the thermostat setting) the system returns to the "low" 
firing rate via control valve 72 to furnace control assembly 73 in order 
to maintain system ductwork and heat exchanger at an elevated temperature 
and to eliminate the draft losses occurring if the system were turned off 
completely as is the case in conventional heating systems. 
The operating cycle is between a high flow rate and a low flow rate, for 
example, 10 minutes at high firing rate, then 20 minutes at low, then 10 
minutes more at high, etc. The time at high and low firing rates will vary 
with demand for heat. This cycle allows for more efficient utilization of 
the furnace since the system is already warm when the high part of the 
heating cycle begins. Moreover, the firing rate for the high mode need not 
be as great as needed for a conventional cycle since the modulated system 
will respond to the heat demand more quickly given the already warm 
conditions created during the low period. 
The off part of the three stage system would be used only during times of 
zero heat demand such as on days when outside temperatures equal or exceed 
the inside temperatures. This condition could be sensed by an external 
temperature sensor 74 fed into the system or could be manually controlled 
by the user. 
In accordance with another aspect of the present invention, the transducer 
assembly of the present invention can be used in an oil burner furnace 
system that employs continuous modulation. 
With reference to FIG. 15 the firing rate of a system is allowed to vary 
continuously between some fixed upper and lower limits in response to an 
external control signal supplied to the burner electronics as, for 
example, in the solar panel supplementary heating system depicted. When 
the temperature of the hot water tank 81 is to be maintained above a 
minimum temperature T.sub.0, the variable nature of the solar derived 
energy via pump 82 and solar panel 83 requires that any solar energy 
deficit be made up by the appropriate flux of heat from the oil burner 
assembly 84. This deficit, being variable, is sensed as at 85 and demands 
that the oil burner 84 be able to fire at any possible rate within the 
design limits of the system such that the sum of the solar and oil burning 
heat delivered remains fixed at the required level. 
It should be obvious to those skilled in the art that while my invention 
has been illustrated for use in a burner suitable for burning fuel oil for 
heating a home it may be used elsewhere to great advantage. It may be 
used, for example, in a burner for a mobil home where its low flow rate, 
typically less than one-half gallon per hour, and variable flow feature 
have obvious economic advantage. The invention may also be used for 
feeding fuel into internal combustion or jet engines. The invention may 
also be used for atomization of other liquids such as water. 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 and omission may be made 
without departing from the spirit and scope of the invention.