EP0021194B1 - Ultrasonic atomiser for liquid fuels - Google Patents

Abstract

1. Ultrasonic atomizer for producing a finely atomized stream of extremely fine liquid particles, comprising a driving means whose output plane provides a longitudinal vibratory displacement at a predetermined ultrasonic operating frequency, comprising a vibration amplifyng means in the form of a stepped ultrasonic horn with a first cylindrical portion (34) whose input end plane is coincident with the output plane of the driving means (33) and whose length is equal to a quarter wavelength at the operating frequency, further comprising a second cylindrical portion (35) adjoining the other end of the first cylindrical portion, with a diameter substantially smaller than that of the first cylindrical portion (34) and with a flanged tip (36) adjoining the outer end of the second cylindrical portion, the diameter of the flange being substantially greater than the diameter of the second, but less than the diameter of the first cylindrical portion, and the outer face of the flanged tip forming an atomizing surface, further comprising means for delivering a liquid flowing radially outwardly at the atomizing surface for atomization by the vibrations produced by the driving means, characterized in that the atomizing surface (29) has a convexly conical surface extending in accordance with the edge of the flanged tip and therefore producing a substantially cone-shaped spray dispersion of finely distributed droplets flowing over this surface when the atomizer is excited by the operating frequency, with the axis of this cone-shaped flow extending parallel to the direction of the longitudinal vibration, and the apex angle of the convexly conical surface forming the supplementary angle for the conical flow angle of the atomized liquid ; in that, furthermore, the flanged tip comprises a short cylindrical portion (38) contiguous to the atomizing surface, with the same diameter as the base of the conical atomizing surface, and therefore ensures that the atomizing surface effects only longitudinal vibrations ; and in that the dimensions of the stepped ultrasonic horn correspond to the dimensions resulting from the solving of the time-invariant differential equation for the propagation of longitudinal vibrations in a solid medium operated at the preselected ultrasonic frequency.

Description

The invention relates to an ultrasonic atomizer for generating a finely sprayed jet of very fine liquid particles with a driver, the output plane of which undergoes a longitudinal oscillating displacement at a predetermined frequency in the ultrasonic range, with a vibration amplifier in the form of a stepped ultrasonic horn with a first cylindrical section, the input-side plane coincides with the output plane of the driver, and whose length corresponds to a quarter wavelength at the operating frequency, and with a second cylindrical section, which adjoins the other end of the first cylindrical section and has a much smaller diameter than the first cylindrical section, and one at the outer end of the second cylindrical section adjoining, provided with a flange, the diameter of the flange being substantially larger than the diameter of the second, but k leiner than the diameter of the first cylindrical portion, and the end face of the flanged tip forms an atomizing surface, and with means for supplying a liquid flowing radially outward on the atomizing surface for atomization by means of the vibrations generated by the driver.

Such an ultrasonic atomizer is known from US-A-4 153 201 and DE-A-2 749 859. As stated therein, the atomization efficiency of a probe-provided ultrasonic electro-mechanical transducer can be improved by giving the probe an enlarged diameter tip in the form of a rigid flange, and by the shape of the atomized liquid fuel and the density of the atomized liquid fuel the geometrical outlines of the flanged, atomizing surface can be influenced.

For example, a flat surface perpendicular to the probe axis creates a very specific pattern and density of the atomized liquid. If the surface is convexly curved, the spray of the atomized liquid is wider and fewer atomized particles are found per unit area of the cross-sectional area than with a flat surface. A concave curved surface narrows the shape of the beam and the density of the particles in the beam is greater than that of a flat surface.

In addition to the prior art, US-A-3 317 139 should also be mentioned, from which an ultrasonic atomizer is also known, the atomizer surface of which is designed as a conical tip which atomizes the liquid in all directions to the outside.

In applications where an ultrasonic transducer of this type is used as an atomizer in a burner operated with liquid fuel, it is often desirable to generate a conical beam with a wide opening angle of approximately 60 °. However, atomizers with a conically convex atomizing surface have not been found to be entirely satisfactory for the generation of such a jet. Test results showed that only an angle of about half the predicted angle could be generated in the beam. It has also been found that a transducer tip provided with a rigid flange with a cone-shaped convex, atomizing surface was very difficult to control, and very strong power pulses were required to atomize the fuel. However, such an unstable mode of operation is not acceptable for fuel atomizers, such as those used in oil burners in households or in industrial plants. On the other hand, transducers with a rigid flange tip and with planar sputtering surfaces have worked stably and with good efficiency, however the jet generated by the flat sputtering surface is not fanned out far enough for a good mixing with the incoming air and a good flame in the usual way To produce high pressure nozzles from oil burners.

The object of the invention is therefore to provide an ultrasonic atomizer with such an atomizing surface, which delivers a stable, semi-liquid, conical jet with a predetermined apex angle and a uniform distribution of the atomized particles from practically the entire atomizing surface.

This object of the invention is achieved in that the atomizing surface has a convex conical surface which extends to the edge of the flanged tip and thus, when the atomizer is excited with the operating frequency, a substantially conical beam distribution of over this surface flowing, finely divided droplets produces, the axis of this conical flow parallel to the direction of the longitudinal vibration and the apex angle of the convex conical surface forms the supplementary angle for the conical flow angle of the atomized liquid, that further the tip provided with the flange converges which has a short cylindrical section adjoining the atomizing surface and has the same diameter as the base of the conical atomizing surface and thus ensures that the atomizing surface only carries out longitudinal vibrations, and that the dimensions of the graduated ultrasonic horn correspond to the dimensions that result from the solution of the time-invariant differential equation for the propagation of longitudinal vibrations in a solid medium that is operated at the predetermined ultrasonic frequency.

Further refinements of the invention can be found in the further claims.

The invention will now be described in more detail with reference to a preferred embodiment of the invention in conjunction with the accompanying drawings.

• Fig. 1 eine Seitenansicht, teilweise im Schnitt, eines Zerstäuberwandlers gemäß der Erfindung,
• Fig. 2 eine vergrößerte Teilansicht der Sonde mit der mit einem Flansch versehenen Spitze gemäß Fig. 1, und
• Fig. 3 ein Diagramm der Longitudinal-Schwingungsamplitude, aufgetragen über dem Abstand längs der verstärkenden Sonde gemäß der Erfindung.

In the drawings shows

• 1 is a side view, partly in section, of an atomizer converter according to the invention,
• Fig. 2 is an enlarged partial view of the probe with the flanged tip of FIG. 1, and
• 3 is a diagram of the longitudinal vibration amplitude plotted against the distance along the amplifying probe according to the invention.

1 shows an electromechanical ultrasound transducer 11 which consists of a disk-shaped electrode 12 which is arranged between a pair of piezoelectric disks 13 and 14 which in turn lie between a front atomizer part 15 and a rear compensation section 16. The front and rear sections are provided with the screwed flanges 17 and 18, respectively, and the whole is held together by cap screws 19 which are inserted through aligned bores in the flanges 17 and 18 in annular seals 20 and 21 and in the disc-shaped electrode 12 , before they are screwed into threaded holes in a mounting plate 22.

To avoid a short circuit, the screws 19 are surrounded by insulating sleeves 23 which penetrate the bores in the disk-shaped electrode. A connector 24 on the top of the disc-shaped electrode. is used to connect a cable 25 which is connected to an ultrasonic frequency generator 26 of conventional design. Since the mounting plate is usually part of an electrically grounded device such as. B. an oil burner, or is attached to it, so that all other parts with the exception of the disc-shaped electrode are grounded, so that thereby a closed earth connection is created via the earth connection of the ultrasonic frequency generator. An AC voltage with a predetermined ultrasound frequency thus builds up over the two piezoelectric disks 13 and 14 between the disk-shaped electrode 12 and the front and rear sections of the transducer.

The front atomizer portion 15 of the transducer includes a radially extending bore 27 in the flange 17 which meets an axially extending bore 28 which extends through the front portion to an opening in the center of the atomizer surface 29. A feed line 30 connects a fuel reservoir 31 via a short pipe section 32 which is inserted into the bore 27 or via an otherwise customary coupling to the bore 27.

From a functional point of view, the transducer 11 comprises a symmetrical, double-balanced ultrasound driver and a vibration amplifier 11. The driver consists of the disk-shaped electrode 12, the two piezoelectric disks 13 and 14, the rear compensation section 16 and a part 33 of the front atomizer part 15 with dimensions which are identical to the rear compensation section 16. The section 33 of the front atomizing part 15 thus forms a front compensation section which is essentially adapted to the rear compensation section 16.

The remaining part of the front atomizer part 15 forms the vibration amplifier 11, which consists of a first cylindrical section 34 with the same diameter as the compensating part 33 with a length A and a second cylindrical section 35 in the form of a probe of substantially smaller diameter than the cylindrical section 34 having a length B and a third section 36 which is in the form of a flanged tip, the length C and the diameter of which is greater than that of the probe, but considerably smaller than that of section 34. Preferably the interior is of the bore 28, at least in the outlet part corresponding to the amplifier section 11, preferably lined with a decoupling sleeve 37, which consists of a material which has a very high attenuation at ultrasonic frequencies. Polytetrafluoroethylene is preferably used for this purpose because it is also unaffected by hydrocarbon fuels and most other liquids that need to be atomized.

Although the vibration amplifier 11 is an inseparable part of the front atomizer section, it is desirable to design the transducer in two stages for best performance. In the first stage, a test setup of a converter is produced, which is connected to driver part I of the final converter design, i. H. is identical to the longitudinally balanced converter with double compensation.

wobei c die Schallgeschwindigkeit in dem für die Vorder- und rückwärtigen Abschnitte gewählten Material ist. Dieses Material sollte den Schall sehr gut leiten. Aluminium, Titan, Magnesium und deren Legierungen sind gute Beispiele geeigneter Materialien, jedoch können auch andere Materialien verwendet werden.The length of this experimental transducer structure is then calculated equal to half the wavelength A. at an operating frequency f selected on a trial basis from the equation

where c is the speed of sound in the material chosen for the front and rear sections. This material should conduct the sound very well. Aluminum, titanium, magnesium and their alloys are good examples of suitable materials, but other materials can also be used.

The transducer test setup is then used to determine the actual resonance frequency looking for. Since the calculated length is based on a pure longitudinal vibration in a homogeneous cylinder with a constant diameter, which consists of the transducer material of the front and rear section, the influence of the flanges, the mounting plate, the retaining screws, the various materials of the disk-shaped electrode and the piezoelectric one is neglected Washers, the sealing rings, the imperfect adaptation of the surfaces between the elements and an attachment outside a node, the coupling of the fuel line and the bores and other deviations from the theoretical model. These effects are difficult and in most cases cannot be recorded analytically at all, but cumulatively they shift the actual resonance frequency of the double-balanced converter by a very substantial amount from the theoretically calculated resonance frequency. If you use the experimentally determined resonance frequency as the operating frequency of the atomizer, you get a balanced driver that can be operated with optimum efficiency.

If greater efforts are warranted by the intended application, a more accurate prediction of the driver's actual resonant frequency can be obtained by considering that each quarter-wavelength of the front and rear sections is composed of three cylindrical elements of different diameters, densities, and sound transmission rates, corresponding to that Piezoelectric element, the flange or the section with a smaller diameter, is composed. With given dimensions of the piezoelectric element and the flange, the length of the section with a smaller diameter can be determined by solving the known wave differential equation by the condition in which the electrode-side end of the section lies in a node plane (zero deflection), while the other end of the compensation section lies on an antinode (zero stress).

In the second section, a new front atomizer section is produced which contains the stepped reinforcement section with the length A, the length B and C being calculated as a quarter length of the operating frequency determined empirically in the first step. Since the amplifier section is made in one piece from homogeneous material with simple dimensions, the length dimensions A, B and C, which were determined by solving the wave equation, can be used to construct a section whose resonance frequency is very close to the operating frequency used for the calculations. In other words, if you mentally disassemble the transducer into a balanced driver part 1, the resonance frequency of which can only be determined by experiment and an amplifier section whose resonance frequency can be theoretically predicted without too unusual difficulties, a complete atomization transducer can be constructed, for which an operation with optimal efficiency driver part and amplifier part are adapted to each other.

This design method for an ultrasonic nebulizer transducer is described in detail in the above-referenced U.S. Patent No. 4,153,201, which also indicates that it is very desirable to use a rigid flange as the nebulizer tip at the end of the amplifying probe, indicating that for best results the entire length of the probe and flanged tip, d. H. B + C, should be smaller than the length A of the amplifier part with a larger diameter. The reason for this is that the tip of the probe, which is provided with a rigid flange, results in a mass load at the end of the probe, whereby the location of the plane with the highest vibration amplitude is shifted by a noticeable amount compared to a smooth probe without an enlarged tip.

According to the above-mentioned patent, a flat atomizer surface perpendicular to the axis of the probe was preferred because all areas of such a surface vibrate with the same amplitude when the tip is rigid at the operating frequency of the transducer. At the same time, it had been suggested that a convexly curved atomizing surface could be used in cases where a further distribution of the atomized particles was desired. However, as already explained, subsequent investigations have shown that such convex atomizing surfaces did not work particularly satisfactorily.

Close observation of the convex atomizer surface under operating conditions indicated that the atomization of the liquid was limited to a narrow, annular area in the immediate vicinity of the outlet opening of the supply line, where the atomizer surface was essentially perpendicular to the probe axis. In the more distant areas, where the convex atomizer surface forms an increasingly larger angle with this vertical plane, only very small amounts of the liquid have been atomized. From these results it could be assumed that an angled surface is ineffective for atomizing a liquid into a wide-angle jet would be.

Surprisingly, however, it was found that a conical or frustoconical atomizer surface according to the invention gave exceptionally good results in tests. The observations during the experiments show that the liquid is atomized over the entire cone surface and that the direction of atomization is approximately perpendicular to the conical surface.From this it follows that a desired opening angle for a jet can only be achieved by using a conical or frustoconical atomizer surface chooses one has a supplementary opening angle. For example, a conical atomizing surface with an opening angle of 120 ° will provide a substantially conical jet with an opening angle of 60 ".

FIG. 2 shows an enlarged partial side view of the outer end of the amplifier section of the converter shown in FIG. 1 with a tip provided with a flange with a frustoconical surface.

As in the case of the flat atomizer surface, a flanged tip gives better results because of the increased atomizing area. It is also very important that the flange is rigid. Thus, the outer edge of the frustoconical surface 29 should be surrounded by a short cylindrical base part 38. The length of this base part 38 should be sufficient for the necessary rigidity and ensure that the atomizer surface vibrates uniformly and does not bend at the operating frequency of the transducer, since it is desirable to minimize the mass of the flanged tip for a given diameter and cone angle to keep.

wobei

• k = 2πf/c
• S, = Querschnittsfläche der Sonde
• S_{2 =} Querschnittsfläche des Flansches

ist.Since the total length of the probe and tip has a critical influence on the vibration amplitude of the atomizing surface, it is extremely important that the lengths B of the probe 35 and C of the tip 36 are determined as accurately as possible. In the case of a probe provided with a flange with a flat surface at its tip, the boundary conditions for the differentiate wave equation are simple, so that an analytical solution is relatively easy to obtain. For a flanged probe with a cylindrical flange and a flat atomizer surface, the following relationship between lengths B and C was determined analytically:

in which

• k = 2πf / c
• S, = cross-sectional area of the probe
• S _{2 =} cross-sectional area of the flange

is.

The analytical solution for a conical tip is much more difficult and complex than for a cylindrical tip because the diameter of the tip is not constant over the length. However, an attempt to construct a useful conical tip for an atomizer probe by taking the equation for the cylindrical tip and mentally replacing the conical tip with an "equivalent" cylinder was unsuccessful.

The reason for this approximation was that the relative masses of the probe and the tip are arguably the most important factors influencing the respective length dimensions. As a result, a conical tip with the same mass as an "equivalent" cylindrical tip should have the same vibration amplitude. Nevertheless, an atomizer with a conical tip with dimensions determined on the basis of this simplified assumption has not provided satisfactory atomization. This result, when considered together with the equally unsatisfactory result of the previously mentioned experiments with a transducer with a spherically convex tip of the probe, suggests that an angled surface is not suitable for satisfactory atomization.

Surprisingly, however, it has been found that good atomization can be obtained with an atomizer which has a conical tip, the dimensions of which have been determined exactly by strict analytical solution. This clearly shows the critical influence that even slight deviations in dimensions can exert on the operating behavior of an atomizer in the case of a conical atomizer surface.

The analytical method for determining the correct dimensions of a quarter-wavelength section of a reinforcing probe with a flanged tip with a frusto-conical atomizing surface will now be described.

In Fig. 3 the small diameter probe and the frustoconical tip of the amplifier section of Fig. 2 are shown approximately to scale in a diagram in which the normalized vibration amplitude is plotted against the axial distance. The x coordinate thus denotes the position in the axial direction and the r coordinate denotes the radial direction. The separating surfaces between the three individual parts of the probe are designated x _i , _X2 and x ₃ , the graded transition from the part of the probe with a reduced diameter to the remaining part of the transducer is 0, and the projected angle of the frustoconical tip is included _X4 .

wobei ηi die Verschiebung gegenüber dem Gleichgewicht (äquivalent zur Schwingungsamplitude) in dem i-ten Bereich (i = 0, 1, 2) als Funktion der Position x ist. A_; (x) ist die Querschnittsfläche in jedem Bereich, wiederum als Funktion von x, und k ist die Wellenzahl, bezogen auf die Frequenz der Welle f und die Ausbreitungsgeschwindigkeit c des Schalls im Medium mit der Formel k = 2πf/c.The time-independent equation for the propagation of longitudinal waves in a solid medium at a single frequency f is

where ηi is the shift from equilibrium (equivalent to the vibration amplitude) in the i-th range (i = 0, 1, 2) as a function of the position x. A _; (x) is the cross-sectional area in each area, again as a function of x, and k is the wave number, based on the frequency of the wave f and the propagation speed c of the sound in the medium with the formula k = 2πf / c.

• a) es ist nur eine einzige Frequenz einer sinusförmigen Schwingung vorhanden;
• b) die Querabmessungen sind für die ausgewählte Frequenz kleiner als eine Viertel-Wellenlänge; und
• c) elastische Linearität.

Equation 1 applies under the conditions of

• a) there is only a single frequency of a sinusoidal oscillation;
• b) the transverse dimensions are less than a quarter wavelength for the selected frequency; and
• c) elastic linearity.

These conditions are met in the present case.

The cross-sectional areas A _i (x) were determined for each of the three zones

The wave equations assigned to these three zones are given by

In zones 0 and 1, where the cross-sectional areas are not a function of x, the area expression can be deleted from the wave equation. The cross-sectional area is variable in zone 2, and thus the wave equation takes on a significantly different shape. Although the cone angle does not appear expressly in the printout, the choice of the value for x ₄ applies to this parameter unmistakably.

Analytical solutions for all second order differential equations of equations (3) are possible. Equations (3a) and (3b) both have simple harmonic solutions. Equation (3c) is a normal form of a zero-order spherical Bessel function, whose two solutions J and Y, known as the spherical Bessel function, are given for the zero-order by

wobei die sechs Konstanten A₀, A₁, A₂, B₀, B₁ und B₂ bis jetzt unbekannt sind, wobei deren Werte von der Art der Randbedingungen an den Trennflächen zwischen den Zonen und an den Abschnittsenden abhängt.The forms of the three solutions are as follows:

the six constants A ₀ , A ₁ , A ₂ , B ₀ , B ₁ and B _{2 are} hitherto unknown, the values of which depend on the type of boundary conditions at the interfaces between the zones and at the end of the section.

• i) an jeder Trennfläche zwischen den Zonen (x = x₁, x₂) muß die Amplitude der Welle über der Trennfläche kontinuierlich sein und die durch die Bewegung der longitudinalen Welle erzeugten Spannungen müssen ebenfalls kontinuierlich sein.
• ii) Bei der Amplitude x = 0 muß die Schwingungsamplitude Null sein, da dies eine Knotenpunktsebene ist.
• iii) Am Ende der Spitze (x = x₃) muß die Spannung verschwinden, da die Ebene von x₃ ein Schwingungsbauch ist.

The boundary conditions can easily be specified as follows:

• i) at each interface between the zones (x = x ₁ , x ₂ ) the amplitude of the wave above the interface must be continuous and the stresses generated by the movement of the longitudinal wave must also be continuous.
• ii) With the amplitude x = 0, the vibration amplitude must be zero, since this is a node level.
• iii) At the end of the peak (x = x ₃ ) the tension must disappear, since the plane of x _{3 is} an antinode.

These boundary conditions can be represented by six simple equations:

From these six equations and the solutions of the differential equations (Equation 3) it is possible to find the six unknown constants (the A's and B's). There is still something arbitrary about this type of calculation since it is only possible to determine the ratios of each of these constants to one of them. It is therefore necessary to arbitrarily set a value for one of the constants in order to calculate the others. However, this does not cause any practical difficulties, since in the present case only the relative amplitudes are of interest.

Before calculating these constants, the values of xi, x ₂ , x ₃ and _X4 (also S _o and S _i ) must be defined. However, a basic remark must be made with regard to this analysis, namely that the length coordinates are not independent of one another. Rather, they are connected by the requirement that the total length should be equal to a quarter wavelength.

wobei

ist.If you solve the six equations for the boundary conditions (Equation 5) by substituting the corresponding form of the solutions of the wave equations (Equation 3) in each of these equations, you get a 6 x 6 determinant that is set to zero. If you solve this determinant, you get a long algebraic expression between the four coordinates. The form of this relationship, called the characteristic equation, is as follows:

in which

is.

If you arbitrarily choose any three of the four coordinates, then a single value for the fourth coordinate can be calculated by solving the characteristic equation. As can now be seen, it follows that logically, x _{1 is} the coordinate to be calculated after values for x ₂ -x ₁ , x ₃ -x ₂ and x ₄ -x ₃ and the cylinder cross-sectional areas have been assumed. It is pointed out that the actual quantities x ₂ -x ₁ etc. are given here and not the coordinates themselves. These quantities are functionally equivalent in the evaluation of the characteristic equilibrium and lead to a considerable simplification.

• a) die Masse der mit einem Flansch versehenen Spitze muß klein genug sein, um eine übermäßige Belastung des gesamten Zerstäubers zu vermeiden;
• b) die kegelförmige Fläche muß groß genug sein, damit für die beabsichtigten Strömungsgeschwin digkeiten eine ausreichende Zerstäuberfläche vorhanden ist;
• c) der Kegelwinkel muß entsprechend dem gewünschten Öffnungswinkel des zerstäubten Strahls gewählt werden;
• d) am Fuß des Kegels sollte ein zylindrischer Abschnitt vorgesehen und sollte so dick gewählt sein, daß sichergestellt ist, daß die gesamte Spitze als starrer Körper vibriert, und
• e) die Spitze muß notwendigerweise kegelstumpfförmig sein, damit eine kleine ebene Fläche rund um die Austrittsöffnung der Bohrung gebildet ist.

The following requirements must be taken into account when choosing suitable values for the above dimensions:

• a) the mass of the flanged tip must be small enough to avoid excessive stress on the entire atomizer;
• b) the conical surface must be large enough so that a sufficient atomizing surface is available for the intended flow speeds;
• c) the cone angle must be chosen according to the desired opening angle of the atomized jet;
• d) a cylindrical section should be provided at the foot of the cone and should be chosen so thick that it is ensured that the entire tip vibrates as a rigid body, and
• e) The tip must necessarily be frustoconical so that a small flat surface is formed around the outlet opening of the bore.

wobei g der algebraische Ausdruck mit den trigonometrischen Funktionen der Parameter ist.The contradicting requirements of rigidity and low mass determine the optimal length of the cylindrical base of the cone x ₂ -x ₁ . The desired opening angle defines the cone opening angle and the size of the hole determines the diameter x ₃ . The diameter of x ₂ is then determined so that the required atomizing surface is created. The opening angle and the diameters x ₂ , x ₃ then define the distances x ₃ -x ₂ and x ₄ -x ₃ . This leaves the length x of section 0 with a reduced diameter as the only unknown dimension. The value of x _{1 is} calculated from the characteristic equation described above, which now takes the form

where g is the algebraic expression with the trigonometric functions of the parameters.

example

An ultrasonic atomizer was designed for an operating frequency of .85 kHz, with the front and rear sections made of aluminum, the piezoelectric disks made of lead zirconium titanate and the disk electrode made of hard copper. Since the longitudinal velocity of sound waves in aluminum is approximately 5.13 x 10 ⁵ cm / sec, a quarter wavelength at the operating frequency is approximately 1.51 cm.

To ensure that the transducer only works as a longitudinal oscillator, the cross dimensions of the elements should be less than a quarter wavelength. Since the gain factor of the probe is equal to the ratio of the cross-sectional areas of the transducer body and the probe, the diameter of the probe should be as small as possible so that a sufficiently high vibration amplitude is achieved which exceeds the threshold value required for the atomization of the liquid to be atomized. On the other hand, the smallest diameter of the probe is limited by the fact that a hole must be provided for the fuel supply, and that the probe must still be sufficiently rigid and rigid to carry a rigid flange with the required atomizing surface and nevertheless avoids vibration in the manner of a spring clamped on one side.

Taking these considerations into account, the following dimensions were selected for a gain ratio of approximately 8:

For the desired opening angle for the atomized fuel of 60 ", the corresponding opening angle for the conical atomizing surface was chosen to be 120 °. The length of the cylindrical base of the conical flange ( _X2 - _X1 ) should be about 0.05 cm to ensure that the flange vibrates with the rigid body, so from simple geometric considerations the total axial length of the conical surface for the tip of the probe (x ₄ -x ₂ ) is approximately 0.20 cm. The actual surface is frustoconical with a diameter of End face of about 0.21 cm, thus x ₄ -x ₃ = 0.06 cm, thereby reducing the axial length of the frustoconical surface (x ₃ -x ₂ ) to about 0.14 cm.

Recall that the predetermined values for the parameters of the characteristic equation were:

Experiments carried out on an atomizer constructed with these dimensions of the above-mentioned example gave a jet with useful stability, in which the liquid indicated almost from the entire surface at an angle of approximately 30 ° with respect to the transducer axis (ie 60 ° opening angle of the jet) was atomized by the arrows X and Y in Fig. 2). In addition to creating the desired jet opening angle, the truncated cone-shaped atomizer surface significantly reduced the degree to which the atomized droplets then flow again compared to the jet emitted by a flat atomizing surface, resulting in an exceptionally uniform droplet distribution. When this test atomizer was installed in a commercially available oil burner in exchange for a normal atomizer working with a high pressure nozzle, it produced a very good self-supporting flame with an appearance that largely resembled the flame of the nozzle originally used.

The results obtained with a nebulizer calculated according to the rigorous analytical solution given above differed significantly from the previously described experimental results with an atomizer whose reinforcing tip was designed with the simplifying assumption required to use the equation for a cylindrical flange tip. This difference in results is extremely surprising since the difference in the total length of the probe plus tip between the approximated and the theoretically exact solution was only about 10 percent. This shows that the longitudinal dimensions of the amplifier part of the ultrasonic atomizer provided with a conical tip are extremely critical according to the invention.

To complete the analysis, it is desirable to use the coefficients A; and B to calculate the solutions according to equations (3). These are not required to determine further dimensional information, but are useful for determining the efficiency of the construction of the entire amplifier section. As already explained, absolute values for these coefficients can only be obtained if one of these coefficients is given an arbitrarily chosen value. This is normal in a system of equations like the one here, where the solutions are those that do not correspond to forced vibrations, i.e. H. where no external stimulating force acts in the tip section.

It is only natural to set an arbitrary value for one of the coefficients of the solution for zone 0 (equation 4a), since this zone of the amplifier section couples the double compensation section to the nozzle. For A _o = 0 as the result of the boundary condition of equation (5a), B _o = 1 was chosen as an arbitrary value. The four remaining coefficients were then calculated by using equation (4) for the relationships for the boundary conditions according to equation (5) and solving the resulting simultaneous equations. For the given system, the results are

FIG. 3 shows a diagram of the relative displacement with respect to the position along the amplifier section. The relative amplitude is defined as the ratio of the actual amplitude to the amplitude that would occur at any point if the amplifier section were a uniform cylinder with a cross-sectional area of r _o ² with the length of a quarter wavelength. It should be noted that the peak causes an amplitude reduction of only about 3 percent.

Claims (5)

1. Ultrasonic atomizer for producing a finely atomized stream of extremely fine liquid particles, comprising a driving means whose output plane provides a longitudinal vibratory displacement at a predetermined ultrasonic operating frequency, comprising a vibration amplifyng means in the form of a stepped ultrasonic horn with a first cylindrical portion (34) whose input end plane is coincident with the output plane of the driving means (33) and whose length is equal to a quarter wavelength at the operating frequency, further comprising a second cylindrical portion (35) adjoining the other end of the first cylindrical portion, with a diameter substantially smaller than that of the first cylindrical portion (34) and with a flanged tip (36) adjoining the outer end of the second cylindrical portion, the diameter of the flange being substantially greater than the diameter of the second, but less than the diameter of the first cylindrical portion, and the outer face of the flanged tip forming an atomizing surface, further comprising means for delivering a liquid flowing radially outwardly at the atomizing surface for atomization by the vibrations produced by the driving means, characterized in that the atomizing surface (29) has a convexly conical surface extending in accordance with the edge of the flanged tip and therefore producing a substantially cone-shaped spray dispersion of finely distributed droplets flowing over this surface when the atomizer is excited by the operating frequency, with the axis of this cone-shaped flow extending parallel to the direction of the longitudinal vibration, and the apex angle of the convexly conical surface forming the supplementary angle for the conical flow angle of the atomized liquid; in that, furthermore, the flanged tip comprises a short cylindrical portion (38) contiguous to the atomizing surface, with the same diameter as the base of the conical atomizing surface, and therefore ensures that the atomizing surface effects only longitudinal vibrations; and in that the dimensions of the stepped ultrasonic horn correspond to the dimensions resulting from the solving of the time-invariant differential equation for the propagation of longitudinal vibrations in a solid medium operated at the preselected ultrasonic frequency.

2. Ultrasonic atomizer according to claim 1, characterized in that there ir provided for delivery of the liquid to be atomized to the atomizing surface, a bore (28) extending axially through the cylindrical portions and the flanged tip and comprising an opening at the center of the atomizing surface.

3. Ultrasonic atomizer according to claim 2, characterized in that the flanged tip comprises a thin, annular, planar surface surrounding the opening of the exiting passage so that the atomizing surface is frusto-conical shaped.

4. Ultrasonic atomizer according to claim 1, characterized in that the first cylindrical portion (34) of the vibration amplifying means has the length A, the second portion (35) has the length B and the flanged tip (36) has the axial length C, and in that the sum of B and C is less than A.

5. Ultrasonic atomizer according to claim 4, characterized in that the axial lenghts of both cylindrical portions (34, 35) and of the frusto-conical portion of the flanged tip (36) are related in accordance with the following equations:

where

and x, is the axial length of the second cylindrical portion (35), x₂-x₁ is the length of the cylindrical portion (38), _X2-_X3 is the length of the frusto-conical portion of the flanged tip, _X3-_X4 is the axial distance from the outer end of the frusto-conical portion to the apex of the cone containing the frusto-conical surface of the tip, So is the cross-sectional area of the second cylindrical portion, and S, is the cross-sectional area of the cylindrical portion of the flanged tip.

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