Evolution of cavitation activity incarbonate dioxide aqueous solution in the process of ultrasonic treatment
The ultrasonic cavitation activity in aqueous solution of carbon dioxide was studied. The ICA-3M cavitometer was used for cavitation activity estimation. Erosion rate was estimated in a number of regimes. The spectra of cavitation noise were recorded.
| Рубрика | Химия |
| Вид | статья |
| Язык | английский |
| Дата добавления | 19.02.2021 |
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EVOLUTION OF CAVITATION ACTIVITY INCARBONATE DIOXIDE AQUEOUS SOLUTION IN THE PROCESS OF ULTRASONIC TREATMENT
ultrasonic activity carbon dioxide
Kotukhov Alexei,
Zharko Natalia, Minchuck Viacheslav,
Krasouski Andrei, Dezhkunov Nikolai,
Research Laboratory of Ultrasonic Technologies and Equipment, Belarusian State University of Informatics and Radioelectronics, Minsk, Belarus
Abstract. The ultrasoniccavitationactivity in aqueous solution of carbon dioxide was studied. The ICA-3M cavitometer which principle is based on recording the intensity of the high-frequency part of the cavitation noise(CN) spectrum was used for cavitation activity estimation. Erosion rate was also estimated in a number of regimes. The spectra of cavitation noise were recorded as well.
It is shown that in a supersaturated solution of carbon dioxide the cavitation activity is about zero. During the degassing process cavitation activity grows and tends to the limit of the activity level in distilled water.
Based on the cavitation noise spectra analysis it was concluded that at the first stage of degassing, intense shock waves are not generated by bubbles and cavitation in this mode cannot have an intensive destructive effect on solid surfaces and biological tissues and cells. At the second stage the situation changes and the concentration of rapidly collapsing bubbles increases significantly.
Keywords: acousticcavitation, carbon dioxide solution, cavitation activity, bubble collapse.
INTRODUCTION
Currently, ultrasound is widely used for intensification of physicochemical processes in liquids. In this case, the key role is usually played by cavitation - the phenomenon of formation, pulsation and collapse of gas microbubbles in a liquid under the action of alternating pressure [1].During the collapse of cavitation bubbles, shock waves and micro-jets of liquid are generated, and the vapor-gas mixture inside the bubble is heated to several thousand degrees [2, 3, 4].These factors determine the cavitationactivity, i.e. the intensity of its impact on various processes and objects in the liq- uid.The activity of cavitation depends on many factors, among which the gas content of the liquid, i.e. gas concentration is one of the most important.
Dissolved gases can affect cavitation activity in various ways. A gas with a lower coefficient of thermal conductivity, other things being equal, will provide a higher temperature inside the bubbles by reducing heat transfer from the bubble to the liquid [5].The presence of a gas with a higher solubility in a liquid reduces the cavitation threshold and increases the concentration of bubbles in the cavitation area [1, 3, 5].This should lead to increasing of cavitation activity. But if the concentration is too high due to the interactions of the bubbles with each other because of the Bjerkness forces and the shock waves, they can break up into fragments in the early stage of collapse.As a result, the energy conversion efficiency stored by the bubble in the rarefaction phase will decrease, which may lead to decreasing of cavitation activity [6, 7, 8].With a large concentration of bubbles, an important factor is the fast absorption of ultrasound energy in the cavitation area and the screening of the internal volume of the cavitation area by external bubbles [2, 7, 8], which can also reduce the cavitation activity.With a large concentration of gas in the liquid, the vapor-gas mixture pressure inside the bubbles will also be significant, which should lead to damping of the collapse process, lowering the temperatures reached in the bubble and the intensity of the shock waves generated in the cavitation area.This factor can contribute to a more “soft” effect of cavitation on the surfaces of solids and biological structures.
The results of [9] are inconsistent with [5, 6], namely: it is concluded that the cavitation activity increases with increasing CO2 concentration in water. The same conclusion was made by the authors [10, 11] based on the results of high-speed photo-registration of the cavitation area. Thus, the results of studies of the influence of the gas content of a liquid on the activity of ultrasonic cavitation presented in the literature are conflicting.
Carbon dioxide has a high degree of solubility in water and therefore provides the possibility of varying the gas content in a wide range. Setting the task of this study, the authors proceeded from the suggestion that under the action of ultrasound in a supersaturated carbon dioxide solution, its concentration will quickly change due to ultrasonic degassing. Note that experimental studies with carbon dioxide solutions in this area almost not been carried out.
EXPERIMENTAL
The scheme of the used setupis shown in Figure 1
Figure 1. Diagram of measurements in a cylindrical reactor
1-positioning device, 2 - pump, 3 - tank for draining the liquid,
4 - sensor, 5 - piezoceramic radiator, 6 - ultrasonic bath, 7 - frequency meter, 8 - ultrasonic generator, 9 - spectrum analyzer, 10 - cavitometer, 11 - electronic thermometer
The sonochemical cell used in this work is made in the form of a cylindrical tank made of stainless steel.The inner diameter of the tank is 78 mm, the height is 100 mm. A 50 mm diameter piezoceramic radiator with a resonant frequency of 34.6 kHz is glued to the bottom of the tank with epoxy glue.
The cavitometer ICA-3M (BSUIR, Minsk) [12] was used to measure cavitation activity. The physical principle of operation of this device is based on the spectral analysis of cavitation noise, i.e. acoustic signal generated by the cavitation area. The sensor of the device is a cylindrical waveguide with a diameter of 3 mm, on one end of which a piezo element is mounted. The waveguide acoustic signal from the cavitation area is transmitted to the piezoelectric element, where it is converted into an electrical one. The diameter of the receiving element of the hydrophone is 3 mm, height - 1 mm. The cavitometer allows you to measure (in relative units) the total activity of cavitation and the contribution of collapsing bubbles, i.e. non-stationary cavitation activity.
The device is equipped with a program that allows you to record the change over time of the full output signal of the sensor and the activity of cavitation and simultaneously display this change in the form of a graph on a computer monitor. Figure 2a presents the measurement results characterizing the distribution of the total sound pressure (dependence 1) and cavitation activity (dependence 2) obtained by moving the sensor along the axis of the working capacitance from the liquid-gas interface to the radiator. The measurements were performed in tap water at a temperature of 23 ± 1 °C and a liquid level above the radiator of 80 mm, which was settled for two days.
It can be seen that in places corresponding to the maxima of cavitation activity (according to the device readings), the maximum of the total output signal is also observed. The distance between adjacent maximum and minimum values is approximately 1 / 4, where 1 is the sound wavelength (~ 41 mm, for the given conditions). The presence of maxima and minima is connected to the presence of a standing component of the sound field, which is formed by the interaction of the waves incident on the liquid-gas interface and reflected from this interface.
Figure 2. Distribution of the full hydrophone output signal and cavitation activity along the emitter axis in distilled water
a) The measured distribution of the full hydrophone output signal (1) and cavitation activity (2) along the emitter axis; b) Scheme ofpressure distribution in the standing wave field for the frequency used.
The liquid level in the working tank above the radiator is 80 mm, L is the distance from the sensor to the radiator, 1 is the full output signal, 2 is cavitation activity, fluid temperature = 22 ± 1.5°C.
In experiments to study the effect of degassing on cavitation, the sensor was placed on the axis of the radiator so that its receiving element was at a distance of 77 mm from the radiator, i.e. at the point of the first maximum of the standing component of the ultrasound field.
Two versions of the procedure of the exposure of supersaturated carbon dioxide solution to ultrasound were used. In the first embodiment, the tank was filled with liquid for 30 seconds, 30seconds more were kept without ultrasound (i.e. in silence conditions), then ultrasound and cavitometer were turned on and readings were recorded using the data processing program described above for 1 minute. Then the ultrasound was turned off for 10 minutes, after which the procedure was repeated.
According to the second version, ultrasound and digital cavitometer were switched on immediately after filling the working capacity with liquid. Processing occurred within 1 min. Then the generator and digital cavitometer were turned off, the analog cavitometer was connected and the ultrasound was turned on again for 2 minutes, and during this time the output signal was recorded in different frequency ranges. After that, the generator was turned off for 10 minutes. After a 10-minute break, the cycle was repeated. With this technique, an additional 2-minute exposure to ultrasound occurred before every 10 minute break. As it turned out, the latter mode of treatment compared with continuous sounding provides more intensive degassing with the same full time ultrasound exposure to the solution.
RESULTS AND DISCUSSION
Figure 3 shows the results of simultaneous recording of the full output signal of the hydrophone and the signal of the high-frequency component of the cavitation noise with continuous sound recording, i.e. according to the first version of the used procedure, and figure 4 - according to the second version. The dependencies in Figures 3 and 4 are obtained by “stitching” separate 1-minute dependencies.The connection points of the dependencies are highlighted with dashed lines.
For the conditions of this experiment a good correlation between the cavitation activity and the full output signal of the hydrophone was observed, namely: as the intensity of the full signal increases, so does the activity of cavitation.
At the beginning of the experiment cavitation activity is close to zero.Within 10-15 seconds after turning on the ultrasound the cavitation activity increases. Under the action of ultrasound occurs intensive degassing with the release of a large number of visually observed bubbles with sizes up to several millimeters. Smaller bubbles are held in the standing wave field and gradually increase in size due to the rectified diffusion of gas into the bubble. Then growth of cavitation activity slows down and practically stops. And by the 90th second ofsonication, there is even a slight decrease in cavitation activity.
Synchronous quasi-periodic jumps of the output signals of the cavitometer are recorded at the first stage of degassing.This pattern is probably associated with the formation of cavitation bubbles clusters, which periodically float to the surface of the liquid.At the moment of ascent, the excess bubbles are removed from the ultrasonic field and the absorption of ultrasound in the cavitation area decreases for a short time.The damping effect of large bubbles also decreases, which leads to the corresponding nearly periodic bursts of cavitation activity. Then begins a new cycle of bub- blesgrowth and clusters formation, the activity of cavitation is somewhat reduced, and so on.
Figure 3. Dynamics of the changing in time of the full output signal of the sensor and cavitation activity during the degassing of the supersaturated carbon dioxide solution
1 -- full output signal; 2 - cavitation activity, liquid temperature = 22 ± 1.5°C. Degassing according to the first technique.
After reaching a certain degree of degassing (in Fig. 3 - approximately at the 100th second), a rapid, often abrupt increasing of the cavitation activity is ob- served.It can be assumed that at this moment there is a qualitative change in the state of the cavitation area, probably due to the interaction of the bubbles. To clarify the mechanism of this jump requires additional research. Then a slight decrease and after that slow increase of the cavitation activity were observed.
At the beginning of the experiment the solution was full of big bubbles with sizes much bigger than the bubbles resonance size. Such bubbles are stable and they do not make input in cavitation activity [1,2]. In sucha liquid no significant tensile stresses can be achieved that can cause pulsations and collapse of cavitation bubbles with dimensions of resonance size or less (approximately 0.05 mm for the conditions of these experiments). This explains low level of cavitation activity at the beginning of the experiment.
As a result of the degassing process the concentration of large bubbles that greatly weaken the strength of the liquid decreases. The negative pressure in the stretching phase of the sound wave gradually increases, small bubbles are activated and the activity of cavitation increases.
Figure 4. Dynamics of the changing in time of the full output signal of the sensor and cavitation activity during the degassing of the supersaturated carbon dioxide solutionaccording to the second technique.
Figure 4 shows the similar dependencies obtained by the second version of the technique, i.e. with an additional 2-minute sonication system and 10-minute breaks after each stage of sonication.
The resulting graphs in Figure 4 are obtained by stitching individual dependencies. The stitching points are marked with dashed vertical lines. At the beginning of the experiment, cavitation as in the first case activity is close to zero. At the initial stage, cavitation activity increases slowly and, within the first minute, reaches a plateau with a tendency to decrease. As in the first case (Fig. 3), there is a section with rapid growth (approximately at the 70th second) that is not associated with switching off or switching on the ultrasonic vibrations. A shorter period of time from the beginning of the experiment to the moment of this jump in comparison with the first version of the experimental procedure is connected with the fact that additional sonication was carried out for 1 minute after turning off the digital cavitometer.
The difference between the results of this experiment and the results obtained in the first version of the procedure (Figure 3) is that when ultrasound is turned on after a ten-minute break, the recorded signalshave significantly higher values compared to the values at the moments of ultrasound shutdown at the end of the previous stage.Due to this reason the resulting dependencies have a stepped form. This feature is caused by the fact that in this case, after each 60-second registration cycle of the studied parameters, additional processing was carried out for 1 minute (during which signal intensities in different frequency ranges were measured using an analog ICA-3M cavitometer).During this time, additional degassing occurred, the total concentration of bubbles decreased even more. As a result, the concentration of bubbles in the cavitation area at the beginning of the subsequent cycle turned out to be significantly lower than at the end of the previous one, which provided an increase in the activity of cavitation and in the full output signal of the sensor.
Figure 5 shows the evolution of the cavitation noise spectra as degassing under the action of ultrasound. The marker indicates the main frequency fo = 34.6 kHz.At the beginning of the experiment (Fig. 5a), the signal intensity at the main frequency is lower than the intensities of higher harmonics. This may be caused by the fact that at the initial stage of degassing, when the volume concentration of cavities in the path of the sound wave is high, the emitted wave is strongly absorbed in the cavitation area. Should be noted, that the spectrum of this type was recorded for the first time: normally main harmonic is the highest one [1, 13, 14, 15].
At the first stage of degassing, the intensity of fogrows faster than the other components of the spectrum (Fig. 5a and 5b). As the gas content decreases (due to degassing), the signal intensity at the main frequency increases, as well as the harmonics intensity and the intensity of the continuous component of the cavitation noise spectrum.
Figure 5. Evolution of the cavitation noise spectra during degassing The spectra were taken after degassing for 1 min (a), 2 min (b), 3 min (c), 4.5 min (d); and a spectrum of distilled water (e). Liquid temperature - 21 ± 2.5 °C.
The intensity of the main harmonic in the first stage of degassing grows faster than the high-frequency components and the continuous component. If we compare the spectra in Figures 5a and 5b, we see that the intensity fo increased over the entire degassing time by about 40 dBm and the intensity at frequencies from 3fo to 5fo and the continuous component by about 7 ... 12 dBm. about 7dBm,
At the first stage of degassing (Fig. 5a), the cavitation noise spectrum is characterized by a relatively low intensity of the continuous component and a low intensity of high-frequency components. This result, in accordance with [15], indicates that no intense shock waves are generated during bubble pulsations, therefore, cavitation in this mode cannot have an intense destructive effect on solid surfaces and biological tissues. In this case, a relatively “soft” cavitational effect can be realized. At the second stage (Fig. 5b, 5c), the increase in the intensity of the low-frequency components slows down, and the high-frequency part, on the contrary, grows faster. At the final stage of degassing (Fig. 5d), the cavitation noise spectrum of a carbon dioxide solution approaches the spectrum of distilled water (Fig.5e).
Thus, as the carbon dioxide solution is degassed in the cavitation area, the proportion of large cavities that are ineffective in terms of generating shock waves decreases, and increases the proportion of bubbles, with the collapse of which intense shock waves and other cavitation effects are generated.
CONCLUSIONS
It is shown that in a supersaturated carbon dioxide solution, cavitation activity is close to zero. When ultrasound is turned on, intensive degassing begins with the release of a large number of visually detectable bubbles with sizes up to several millimeters, while the cavitation activity slowly increases. Quasi-periodic jumps in the output signal of the hydrophone are fixed, which is probably caused by the formation of clusters of cavitation bubbles which periodically float up to the surface of the liquid.
Two stages of the cavitation area evolution were identified during the degassing of a supersaturated carbon dioxide solution: at the first stage, the cavitation activity and the full output signal increase slowly, the transition from the first to the second stage is characterized by the fast, often abrupt, increasing of cavitation activity.
The spectra of cavitation noise for the first and second stages of the cavitation area differ significantly, which indicates the possibility of identifying stages by the spectrum of cavitation noise. Based on the analysis of the spectra was concluded that at the first stage, during the pulsation of bubbles, intense shock waves are not generated and cavitation in this mode cannot have an intense destructive effect on solid surfaces and biological tissues. In the second stage, the situation changes and the concentration of rapidly collapsing bubbles increases significantly.
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