RU2547508C1 - Method for ultrasonic cavitational processing of liquid media and objects located in medium - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method includes placing liquid media and objects located in a medium inside a mechanical oscillatory channel system, having a natural oscillation frequency, in which parametric resonances are excited or parametric excitation of self-oscillation is performed, setting as the cavitational processing efficiency criterion the oscillation amplitude of the channel system, determining optimum frequency of oscillation frequency of power exciters with experimentally predetermined natural and parametric oscillation frequencies.

EFFECT: shorter processing time, higher power of acoustic waves and higher efficiency of cavitational processing of liquid media and objects located in the media.

12 dwg

Description

The invention relates to the field of cavitation treatment of liquid media as well as environments where the specific content of water or another liquid phase exceeds ~ 40-50% of the total mass, the processing of objects in a processed liquid medium. Acoustic ultrasonic cavitation is effectively used in a number of areas of the economy, where the following processes are implemented / 1-6 /:

- dispersion;

- homogenization and emulsification;

- mixing;

- disintegration;

- deagglomeration.

In practice, this covers the processes of obtaining multicomponent media (emulsions, suspensions, aqueous solutions and systems), ultrasonic sterilization (disinfection) of water, milk, other products, cleaning tools and medical supplies, etc.

The method of processing liquid media, which is implemented in the scheme of an ultrasonic reactor, can be taken as a prototype / 1 /. It consists in the fact that an ultrasonic wave is created in the volume of a liquid using a rod emitter, at the end of which there is a source of oscillations, as a rule, a piezoelectric emitter.

There are many options for calculating the shape of the rod emitter and the possibility of attaching several piezoelectric emitters at its end, but all of them are aimed at increasing the amplitude of the oscillations of the rod at the lower end and on the side walls / 8 /.

This is due to the fact that the developed cavitation zone in practice is measured by the dimensions of several centimeters from the surface of the oscillations. Therefore, the bottom of the rod is considered the most effective zone, since a standing wave is formed in the processed fluid between the flat end of the emitter and the flat bottom. It is noted that the diameter of the end face is difficult to make in sizes larger than 50-70 mm. Radiation from the cylindrical surface of the rod has a significantly lower amplitude of oscillations and a cylindrical divergence. Taking into account the reflected acoustic waves from the walls of the outer cylinder-glass, it can be estimated that it is practically impossible to obtain the optimal regime of a stable standing plane coherent ultrasonic wave in the processed liquid medium, by analogy with the insignificant region between the end of the emitter and the bottom of the cylinder-glass.

The complex picture of transmitted and reflected ultrasonic waves in the medium, the absence of wave coherence and energy concentration at one frequency leads to the fact that it is practically impossible to obtain emulsions with a dispersed phase size of less than 1.0 μm, the homogeneity level does not exceed 15% - 20% in the main mode . Moreover, the volume of the processed fluid is limited.

Another alternative method of ultrasonic cavitation treatment of liquid media is implemented in rotary-pulsating homogenizers / 2 /.

In the sounding chamber, due to periodically occurring alternating motions of the fluid from the rotating stator-rotor system, an ultrasonic wave with cavitation effects occurs. This is an intermediate option between acoustic and hydrodynamic cavitation. Such homogenizers are currently most widely used. They are quite simple, allow you to process large volumes of liquid, much cheaper than ultrasonic counterparts. Good high-speed homogenizers allow emulsions with a dispersed phase size of ~ 1.5 μm in the main mode, the level of homogeneity does not exceed 10-15%. Nevertheless, this method also has a number of fundamental limitations.

This is due to the low efficiency of electromechanical systems (up to 10%), which limits the power of the ultrasonic wave to 1.5-2 W / cm ² , does not allow working with viscous media, with the processing of static volumes of liquid (in the stator-rotor volume), suspensions containing solid inclusions and a number of other fundamental limitations.

The closest in essence is a method of obtaining an emulsion cosmetic product according to patent No. 2422129 of 05/27/2009.

The principal difference of this method from others is the use of the resonant properties of a mechanical oscillatory system-channel with its own oscillation frequency to excite an acoustic wave in liquid media. The most widely used are vibration channels of a rectangular profile, for which natural vibration frequencies can be determined. These solutions are known / 7-10 / and depend on many parameters: the wall thickness of the channel system (deflection modes — plates, flexible plates, membranes), amplitudes, vibration conditions (damping), boundary fixing conditions, material characteristics, and a number of other parameters. The amplitude of the deflections are essentially nonlinear in nature from the force of impact / 9, 10, 13 /. This limits the amplitude of the forced oscillations, which, as a rule, is chosen close to or equal to the first natural frequency of the oscillations. In FIG. 1. presents a typical resonant characteristic of a membrane having a natural frequency of ~ 23.5 kHz. Forced oscillations are created at the same frequency using a piezoelectric pathogen. It can be seen that the resonance characteristic is asymmetric to the right side (the side of high frequencies). This is a direct consequence of the nonlinearity of the forced vibrations / 9, p. 143 /, when the skeletal line has a hard character, that is, with an increase in the deflection, the frequency moves away from the resonance towards higher frequencies. A further increase in the power of power exciters at the resonant frequency is extremely inefficient and energy-intensive.

This is a fundamental restriction on the increase in the amplitude of oscillations of a mechanical oscillatory system-channel, which limits the amplitude of an acoustic wave in a liquid medium / 3, 4 / and, accordingly, the efficiency of cavitation processing of liquid media and objects located in the medium.

The aim of the invention is to increase the efficiency (decrease in processing time and increase the power of the acoustic wave) of cavitation effects on the processed liquid medium and objects located in the medium without increasing the power of power ultrasonic pathogens.

This goal is achieved by the fact that in a mechanical oscillatory system-channel with a liquid medium and objects, either a mode with additional parametric resonances is realized, or parametric excitation of self-oscillations is realized / 11, 12 /. In systems with an infinite number of degrees of freedom, which include plates and membranes, at certain frequencies and magnitude of the dynamic deflection amplitude, the modes of forced oscillations (linear and nonlinear), forced oscillations with additional parametric resonances, or self-oscillations / 9, 12 / can be realized. In the latter case, the external force exciter of ultrasonic vibrations / 9, p. 175 / delivers energy to maintain self-oscillations. The ideal case is that the self-oscillation frequency is close to or equal to the frequency of the forced vibrations, which in turn are close to the first harmonic of the membrane’s own vibrations (plates). First of all, membranes should be considered, because due to the low bending stiffness, their first frequency of natural vibrations (at practically significant sizes) lies in the range of ultrasonic vibrations, that is, above ~ 20 kHz. The first harmonic of the natural oscillations of the stiffer bending plates lies in a much lower frequency range (up to ~ 1 kHz).

The basis of this method, to increase the amplitude of the oscillations of the channel system, is the principle of obtaining the maximum restoring force (elastic force) of the oscillatory system at resonant frequencies. The resonance condition minimizes the energy supplied to the oscillatory system. It is known / 9, 13 / that parametric resonances generate itself, in our case, a mechanical oscillatory system-channel due to periodic changes in its certain parameters. Of all the parameters of an elastic vibrational system (mass and inertial forces, damping, elasticity), two can be essentially nonlinear - this is damping and elasticity. The mass for the considered channel structures is a practically constant value. Non-linear characteristics of the system lead to the effect of pumping energy between harmonics with different frequencies.

To implement this method, it is necessary to determine the frequencies of the parametric resonance and the resonance frequency of the self-oscillations of the channel system and, based on these values, select the vibration frequencies of the power pathogens.

The determination of the frequency of parametric resonance and the frequency of self-oscillations is possible using theoretical calculations and numerical methods. However, to perform the calculations, adequate dependences of nonlinear liquid damping and nonlinear elasticity are needed. Obtaining such dependencies in itself is a very complex and time-consuming task.

The implementation algorithm of this method includes 2 stages.

At the first stage, a preliminary assessment of the vibration frequencies is carried out. As noted above, the maximum vibration amplitude should be close to the first harmonic of the membrane vibrations.

The determination of the frequency of the first harmonic of the membrane vibrations can be performed by the ratio / 7 /:

where c is the propagation velocity of elastic waves;

k _x , k _y are wave numbers whose values are determined by the boundary conditions;

L _x , is the length of the side of the plate directed along the axis Ox;

L _y is the length of the side of the plate directed along the axis Oy;

j _x , j _y is an integer equal to 1 for the first harmonic of the oscillations.

The above dependence contains, including linear dimensions of the surface of a rectangular membrane. Simple calculations show that for the case L _y >> L _x (an infinitely long membrane), we obtain a frequency equal to the ratio of the elastic velocity C to half the wavelength on the membrane surface. That is, a standing wave is formed with a frequency determined by the size L _x (membrane width). When the ratio L _y / L _x = 2-5 (the membrane is a finite length), an increase in the influence of the boundary of the second side and an increase in frequency will be observed. In FIG. Figure 2 shows a typical vibration spectrum of a channel system of a rectangular section, in which the side has a size in the plan of 0.30-0.10 m. The frequency of the forced oscillations is 35.17 kHz, which is higher than the first harmonic of vibrations for a membrane with the indicated dimensions (in plan 0 , 30-0.10 m), which is equal to 30.2 kHz (difference ~ 15%).

The channels are made of stainless steel AISI 316 (analog 12X10HT), with a longitudinal wave velocity of ~ 5800 m / s, liquid medium-water with the addition of oil (~ 20%).

An analysis of a typical spectrum shows that in addition to the frequency of forced oscillations, there are harmonics of ~ 18 kHz and ~ 53 kHz, that is, a series of multiple frequencies with a step of ~ 17-18 kHz and the first harmonic are 2 times lower than the frequency of the forced oscillations are obtained.

This allows us to conclude that the regime of parametric oscillations is realized.

The vibrational energy at all harmonics is close to each other.

After comparing the identified parametric frequencies with characteristic sizes corresponding to these frequencies (dividing the propagation speed of elastic waves by frequency), we find that the first parametric harmonic of 18 kHz corresponds to a characteristic size of 0.32 m, and the harmonic 53 kHz to a characteristic size of 0.11 m.

These values are close to the dimensions in the plan (0.30-0.10 m) of the channel side, on the surface of which ultrasonic waves are excited. That is, the parametric frequencies are associated with the linear dimensions of the membrane so that standing waves with a wavelength of 1 (or an even number of half-waves equal to 2) are formed along the length and width of the membrane.

In FIG. 3 shows an industrial installation on which 20 such flow channels are installed. Various emulsions and suspensions act as the processed media, and a similar installation is also used to restore milk powder.

The interconnection of parametric oscillations with the dimensions of the channel sides is possible if nonlinear elastic forces at the edges of the membrane are decisive.

In our case, a thin neutral layer acts as an elastic bond, which occurs during the elastoplastic bending of the metal / 14, 15 p. 95 / and retains its elastic properties.

In FIG. Figure 4 shows the experimentally obtained dependence of the amplitude of parametric oscillations of a rectangular membrane on its thickness, confirming the influence of the boundary conditions forming the amount of elastic restoring forces on the amplitude of the oscillations. The dependence is obtained under the condition of one-angle elastoplastic bending of an AISI 316 steel sheet at an angle of 90 degrees, in the presence of liquid damping (water) in the channel system at a frequency of ~ 5 kHz.

The maximum vibration amplitude of 3.5-4.0 μm (micrometer) is observed at a bent sheet thickness of ~ 1.5 mm. It is with such a thickness of the metal that the dynamic nonlinear elasticity of the neutral layer is optimal. The presence of a thin elastic nonlinear bond along the perimeter of the side of the channel system is one of the reasons for the behavior of a sufficiently thick plate as a membrane (the ratio of deflection to thickness ~ 0.5%), which is experimentally fixed. The second main reason is the level of force exposure during the generation of ultrasonic vibrations. This level is easily estimated by multiplying the weight force exciters (to piezo resonator is 1-2 kg) on acceleration that for typical conditions of 5 * 10 ^ 4-10 ^ 5 m / s ^2/3 /. The level of force impact is estimated at approximately ~ 10 kN. Under the action of such loads in static conditions, the channel system would undergo irreversible deformations, but since the duration of the power load is short-term (the half-life is ~ 10 μs), the stability of the system is preserved and is nonlinearly elastic.

Thus, with the linear dimensions of the rectangular membrane L _y / L _x = 2-5 (membrane of finite length), the frequency of oscillations of the power pathogen can be determined from the linear dimensions L _x and L _y based on the above dependence and the condition for the formation of a standing wave along the length and the width of the membrane. The frequency of the lower harmonic will be determined as C / L _y , where C is the propagation velocity of elastic waves, the frequency of the upper harmonic will be determined as C / L _x , respectively, the frequency of the power pathogen is determined as the average of the condition of the frequency step multiplicity ((C / L _y ) + (C / L _x )) / 2.

With an increase in the linear dimensions (length) of the channel system L _y / L _x > 5, a transition to the case of a ″ membrane of infinite length ″ is observed. By analogy with the foregoing, a low-frequency harmonic is formed (associated with an increased length L _y ), the harmonics with the frequency corresponding to the first harmonic of the membrane oscillations (calculated from the above dependence) and the harmonic with the frequency associated with the width size L _x remain the determining ones.

In FIG. Figure 5 shows a typical spectrogram for this case of weak membrane vibrations (~ 1 μm). The size of the side in the plan is 0.9-0.105 m, that is, L _y / L _x = 9.

A low-frequency harmonic is generated, the determining ones are the harmonic with a frequency corresponding to the first harmonic of the membrane oscillations ~ 28 kHz (calculated according to the above dependence), as well as the third harmonic with a frequency of ~ 40 kHz corresponding to a size of 0.145 m, which is close to the channel width L _x .

With an increase in the amplitude of oscillations (> 2-3 μm) and the power supplied to ultrasonic power exciters, parametric resonances arise with frequencies that are multiples of ~ 14 kHz. A typical spectrogram for this case is shown in FIG. 6. A mode with a frequency of 28 kHz is present in the vibrations - this is the frequency of the first harmonic of the membrane vibrations with dimensions of 0.9-0.105 m. A cosmetic emulsion with an oil phase content of ~ 20% acted as a processed liquid.

In FIG. 7 shows an industrial installation for processing cosmetic emulsions with channels of this type.

Thus, for the case of linear dimensions (length) of the channel system L _y / L _x > 5, the frequency of oscillations of the power pathogen and the maximum amplitude of oscillations can be directly determined based on the above dependence.

In this case, parametric oscillations will form with a number of frequencies 1 / 2ω, ω, 3 / 2ω, and so on.

When processing more viscous liquids (more than ~ 100 centipoise), the forces of nonlinear liquid damping increase with respect to the forces of nonlinear elasticity.

There are technological processes when, during cavitation processing, the viscosity characteristics of liquid media change dynamically (cosmetic emulsions, milk recovery, preparation of paints and varnishes, etc.)

In this case, the frequency of the power pathogen obtained earlier can differ significantly from the resonant frequency of the channel system and the amplitude of the oscillations will be less than the maximum.

To take into account the real structural parameters of the oscillatory system-channel and obtain changes in nonlinear frequency characteristics in dynamics, stage 2 of the algorithm is performed.

The second (2) stage of the algorithm additionally allows you to determine the frequency of self-oscillations.

The self-oscillation mode is convenient in that it is an intrinsic characteristic of the nonlinear oscillatory system / 9, 11, 12 / and weakly depends on the amplitude of the input action, in contrast to parametric oscillations, where the main ones are elastic nonlinear restoring forces and there is a dependence of the oscillation frequency on the oscillation amplitude (deflection ) membranes.

To obtain the frequency characteristics of parametric oscillations and self-oscillations, a simple and reliable experimental method is proposed.

The experimental setup is shown in FIG. 8.

It consists of: 1 - oscillatory channel system; 2 - power causative agents of ultrasonic vibrations of any type (piezoceramic, magnetostrictive, etc.); 3 - accelerometer; 4 - accelerometer charge amplifier; 5 - digital oscilloscope; 6 - registrar (computer); 7 - divider with galvanic isolation; 8 - digital oscilloscope frequency counter; 9 - generator of ultrasonic vibrations.

In this case, power pathogens of the piezoceramic type are used. The measuring path (3-6) consisted of a 4344 type piezo-accelerometer, type AP-12, type AP-33 of a 2635 amplifier from Bruel & Kjaer, a digital Velleman oscilloscope with registration of signals to a personal computer.

As generator 9, typical generators G3-109 with a high-voltage output for supplying piezoceramic exciters and generators UZG2-22, commercially available by a number of enterprises, were used. The UZG2-22 generator is convenient in that it allows you to manually set the required frequency, or to scan with the time delay necessary to record the results, while it has indicators of frequency, power consumption and a number of other parameters. The divider 7 and the AKTAKOM digital oscilloscope frequency counter 8 are auxiliary measuring devices that allow you to control the signal directly on the power exciters 2, taking into account the mechanical feedback from the channel system to piezoceramics.

The experimental technique uses a working analogue (or one of the elements) of the oscillatory channel system and an analogue of the processed liquid medium. The frequency of power pathogens is determined in step 1, which is described above.

By changing the frequency of the generator 9, an analysis is made of the amplitude of oscillations and the spectrum of oscillations of the channel system for a number of fixed frequencies in the frequency range from ω to ω + (10-20%) ω. The choice of movement toward higher frequencies is associated with the rigid nature of the skeletal line, FIG. one.

In FIG. 9 successively shows the oscillation amplitudes, and FIG. 10 - vibration spectra of a channel system with a first harmonic frequency of 37.5 kHz.

Amplitudes and spectra were recorded at frequencies of 37.5 kHz, 38 kHz, 39 kHz, 41.8 kHz. The influence of various factors unaccounted for is manifested in the fact that the maximum oscillation amplitude is observed not at a frequency of 37.5 kHz, but at a different frequency, namely 39 kHz, the difference is 4%. With a slight difference in frequency, the difference in the amplitude of the oscillations is significant ~ 2 times.

According to the spectrum of oscillations, a transformation (decrease) of parametric harmonics and the appearance of a dominant harmonic are observed. These are signs of a transition to a regime of self-oscillations, at the frequency of which the effects of a change in frequency from the amplitude of oscillations are weakly manifested.

Thus, the experimental method of refinement (stage 2) of the nonlinear frequency characteristics of the channel system and the oscillation frequency of the power pathogen can give a significant increase in the oscillation amplitude (estimate 2-2.5 times) with respect to the case of a simple application of the calculated dependence.

This dependence can be used for special cases - parametric oscillations and liquid inviscid media (up to ~ 100 centipoise). In this case, the calculated dependence allows you to determine the frequency of oscillations of power pathogens with acceptable accuracy in practice.

For general cases, a refinement of the frequency characteristics is required.

It was shown / 3, p. 120 / that an increase in the amplitude of oscillations at a frequency of 20 kHz in water from 0.92 μm to 2.92 μm gives an increase in acoustic power from 1 W / cm ² to 10 W / cm ² . These amplitudes correspond to the oscillation amplitudes that are used in practice and analyzed (Fig. 4).

Known cavitation thresholds for various liquid media / 4, 6 /, which for water are 1-2 W / cm ² , vegetable oils - 8-10 W / cm ² , for viscous liquids like glycerin, the cavitation threshold is ~ 25 W / cm ² .

The industrial applicability of the proposed method is confirmed by its implementation in 3 industrial plants and a number of pilot plants for processing various liquid media (emulsions and suspensions). There are pilot plants for placing large-sized objects in them, FIG. 12 Application of the parametric oscillation mode allows to reduce the processing time of liquid media and objects through the use of acoustic waves with different multiple frequencies.

Application of such a mode to the installation shown in FIG. 3, it was possible to reduce the preparation time of 600-700 kg of cosmetic emulsion by 1.5 times (from 90-100 minutes to 50-60 minutes) with a limited volume of the emulsion, which is in the ultrasonic processing mode in the channels. The results confirm the conclusions of work / 5, p. 60 / that ″ with the simultaneous action of ultrasonic waves of two different frequencies (22-44 kHz), a significant increase in the cavitation efficiency is observed, much larger than with a linear summation of the action of each of the fields of different frequencies ″.

In the mode of parametric oscillations and self-oscillations, in addition to the maximum amplitude of the oscillations, a decrease in the minimum power consumption is achieved. When this channel system was connected to a working generator with a frequency of 39 kHz set, the maximum oscillation amplitudes (~ 2-2.5 μm) were obtained, while the dominant harmonic in the oscillation spectrum shown in FIG. 11, the minimum power consumed by the generator was recorded. With the detuning of this frequency by 1-2 kHz, an increase in power consumption by 20-40% was observed.

Thus, the objective of the invention is achieved, namely, increasing the efficiency (reducing processing time and increasing the power of the acoustic wave) of cavitation effects on the processed liquid medium and objects located in the medium without increasing the power of power ultrasonic pathogens.

LITERATURE

1. Bronin F.A. Investigation of cavitation destruction and dispersion of solids in a high-intensity ultrasonic field. Abstract for the degree of candidate of technical sciences, MISIS, 1967.

2. Chervyakov V.M., Only V.G. The use of hydrodynamic and cavitation phenomena in rotary devices. - M.: Publishing House Engineering, 2008.

3. Bergman L. Ultrasound and its application in science and technology. - M .: Foreign literature, 1956.

4. Sirotyuk M.G. Experimental studies of ultrasonic cavitation. In the book. Powerful Ultrasonic Fields, ed. Rosenberg L.D., 1968.

5. Margulis M.A. The basics of sound chemistry. Chemical reactions in acoustic fields. - M.: Higher School, 1984.

6. Khmelev V.N., Popova O.V. Multifunctional ultrasonic devices and their application in small industrial, agricultural and household conditions: scientific monograph, Alt. Gos. Tech. University of I.I. Polzunova. - Barnaul: Publishing house of Altai State Technical University.

7. Koshlyakov N.S., Gliner E.B., Smirnov M.M. Partial differential equations of mathematical physics. M., Higher School Publishing House, 1970.

8. Vibration in technology. Handbook in 6 volumes, ed. Chelomeya V.N., Moscow: Engineering, 1979.

9. Volmir A.S. Nonlinear dynamics of plates and shells. M .: Nauka, 1972.

10. Timoshenko SP, Voinovsky-Krieger S. Plates and shells, M., series Physico-Mathematical Heritage, Publishing House URSS, 2009.

11. Obukhov A.N. Parametric excitation of self-oscillations in vibrating machines, Abstract for the degree of candidate of technical sciences, Institute of Mechanical Engineering, M., 2007.

12. Izrailovich M.Ya., Obukhov A.N. Parametric self-oscillation control. M., Publishing House URSS, 2010.

13. Ogibalov P. M., Koltunov M. A. Shells and plates. M., Publishing House of Moscow State University, 1969.

14. Romanovsky V.P. Handbook of cold stamping, M., Publishing House Engineering, 1965.

15. Popov EA Fundamentals of the theory of sheet stamping. M., Publishing House Engineering, 1977.

Claims (1)

The method of ultrasonic cavitation treatment of liquid media and objects located in the medium by placing them inside a mechanical vibrational channel system having a natural frequency of oscillation, characterized in that they excite parametric resonances or parametric excitation of self-oscillations, set the maximum oscillation amplitude of the system as a criterion for the efficiency of cavitation processing -channels, determine the optimal frequency or frequencies of oscillations of power pathogens by preliminary exp The experimental determination of natural and parametric frequencies of oscillations using the relation:

where c is the propagation velocity of elastic waves;
k _x , k _y are wave numbers whose values are determined by the boundary conditions;
L _x , is the length of the side of the plate directed along the axis Ox;
L _y is the length of the side of the plate directed along the axis Oy;
j _x , j _y is an integer equal to 1 for the first harmonic of the oscillations.

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