Influence of Acoustic Streams on the Efficiency of Ultrasonic Particle Agglomeration

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript mainly reports experimental investigation on the ultrasonic agglomeration of PM2.5 in a thin layer configuration (between a radiating surface and a hard reflector) in uniform and non-uniform ultrasonic fields. The results reported here are original and meaningful, and the manuscript is carefully prepared. However, there are several points that should be corrected or explained before its publication.

 

1. Introduction (Line 60). 

"The first group of effects contributes to the hydrodynamic mechanism (first described by W. Koenig in 1891)".

Please add reference.

 

2. Figure 1. 

The modes of thin plate vibration may be complex (not concentric ripples) and unstable (rapidly changing over time), will it affect the conclusions of the study?

 

3. Line 398

"... carried out at an air temperature of 22 degrees and a relative humidity of 35%. ..."

Ultrasonic radiation may cause an increase in gas temperature. Are there any changes in the inlet and outlet temperatures of the airflow?

 

4. Line 254

"Oscillation frequency, kHz  22.1  22.2 "

Why chooses this frequency? Is there an optimal frequency?

 

5. Please provide the manufacturer and model of laser plane plotter, high-speed camera, etc.  in the revised manuscript. 

 

6. What is the material of the ultrasonic radiation plate?

 

7. How did the authors verify the true vibration mode of the radiation surface?

 

8. How does the author measure the amplitude of radiation surfaces? Please provide instrument information.

 

9. Line 595

"...ensured generation of vortex type acoustic streaming capable of moving particles within the node areas ..."

The author mainly uses acoustic-streaming-induced vortex to explain the improvement of agglomeration effect, but it seems that there is no convincing evidence provided in the paper.

 

10. Figure 7

The flow channel of step-function radiator has many steps, and the flow field is different from that of the flat radiator. The author should first rule out the influence of flow channels without ultrasound, and then conduct subsequent research.

 

11. Figure 8, 11

" 3 – without ultrasonic influence." 

What type of radiator was used in the experiment without ultrasound? flat radiator or step-function radiator?

 

12. Figure 8, 9, 10, 11.

What function does the author use for curve fitting? The far right end of the curve seems bad.

 

13.  Line 6.3

"...an increase in the rate of inertial capture for particles of 2.5 μm size by 6% - from 89% to 95%, for particles of 1.5 μm size by 7%- from 85% to 92%..."

This conclusion is only valid under specific conditions (concentration, sound pressure level, flow rate, frequency....), please provide it in the revised manuscript.

Author Response

My co-authors and I place great value on your comments. We have made detailed changes according to the comments and marked them in green in the revised manuscript.

 

1. Introduction (Line 60). 

"The first group of effects contributes to the hydrodynamic mechanism (first described by W. Koenig in 1891)".

Please add reference.

 

Response:

The link [17] to this work was indicated in the text of the article earlier. We moved it after mentioning W. Koenig. Highlighted in color.

 

2. Figure 1. 

The modes of thin plate vibration may be complex (not concentric ripples) and unstable (rapidly changing over time), will it affect the conclusions of the study?

Response:

Thanks for the interesting question. The plate radiators (flexural-oscillating disks) used in the work are specially designed so that at the required oscillation frequency, excitation is carried out only on one circular (concentric) flexural mode of oscillation. For this purpose, the disk profile is made of a step-variable section. The front side of the disk determines the direction of radiation, and the back side provides the required distribution of vibration amplitudes.

Analysis of the distribution of generated vibrations and adjustment of the dimensions of the radiator are carried out by modeling (using modal analysis in the ANSYS system).

It is known that the excitation of complex uncontrolled vibration modes leads to an increase in mechanical stress and damage to the radiator. To ensure stable operation of the radiator and eliminate changes in the oscillation shape, the frequencies of the nearest oscillation modes are separated from the frequency of the main oscillation mode by at least 200 Hz.

Information about this has been added to the article on page 8

 

3. Line 398

"... carried out at an air temperature of 22 degrees and a relative humidity of 35%. ..."

Ultrasonic radiation may cause an increase in gas temperature. Are there any changes in the inlet and outlet temperatures of the airflow?

Response:

Thanks for the question. Gas heating occurs due to energy losses caused by the work of thorny forces between gas molecules (and, if present, dispersed particles). Such losses in gas are very small, so heating will be minimal. It can be ignored. In turn, heat removal from the ultrasonic radiator was ensured by cooling outside the sintering chamber.

Therefore, no change in gas temperature was noted.

Information about this has been added to the article on page 14

 

4. Line 254

"Oscillation frequency, kHz  22.1  22.2 "

Why chooses this frequency? Is there an optimal frequency?

 

Response:

Thanks for the question, it will help make the article better. The effect of ultrasonic frequency on coagulation efficiency was not studied in this work. This is due, first of all, to the technical complexity (and in some cases impracticability) of creating ultrasonic radiators that provide a high level of sound pressure (more than 170 dB) at high frequencies (more than 25...30 kHz). And studies at lower sound pressure levels are of no practical interest, since they do not provide a coagulation rate sufficient for practical applications. In turn, it is known that the lower the frequency of exposure, the easier it is to create a high sound pressure level.

Therefore, when conducting experimental studies, the frequency of exposure was chosen at the level of the upper limit of audibility of the human ear (at this frequency, exposure with the sound pressure levels indicated in the article is safe)

The rationale for choosing the frequency has been added to the article on page 7

 

5. Please provide the manufacturer and model of laser plane plotter, high-speed camera, etc.  in the revised manuscript. 

Response:

In accordance with your suggestions, we have indicated the names of the devices. Used: Laser level BOSCH GLL 2-20 G.; high-speed camera – “Videosprint” G4/NG; pneumatic atomizer (Patriot HVLP 1.8); noise meter Ecofizika-110A (with microphone - VMK-401 and microphone preamplifier R200-27). Information added to article on page 6

 

6. What is the material of the ultrasonic radiation plate?

Response:

Ultrasonic disk radiators are made of Grade2 titanium. Text added to article on page 8

 

7. How did the authors verify the true vibration mode of the radiation surface?

Response:

Thank you very much for the clarifying question. We have added an appropriate description to make the article better.

The ring mode of vibration is determined visually using indicator powder.

1) To do this, the radiator is excited using a generator (supply voltage 20 volts).

2) Indicator powder is applied to the surface of the radiator (must be installed horizontally).

3) The generator frequency is changed in the range F (calculated) + -10%. When resonance is established on the desired vibration mode, the indicator powder moves to the zeros of vibration due to vibrations.

4) Next, the resulting distribution of vibration zeros is compared with the distribution of zeros obtained using modal analysis in the ANSYS system.

More details about measuring vibration amplitudes and developing radiators can be found in the article

V N Khmelev et al 2020 IOP Conf. Ser.: Mater. Sci. Eng. 862 062079

https://iopscience.iop.org/article/10.1088/1757-899X/862/6/062079

Information has been added to the article on page 8, a link to the article has been added to the bibliography

 

8. How does the author measure the amplitude of radiation surfaces? Please provide instrument information.

Response:

Based on your recommendation, we have added information about the device.

The amplitude of vibrations of the radiator surface was measured using a VM1-5 non-contact vibrometer based on the capacitive method. To measure the amplitude of oscillations on the radiating surface, a three-dimensional positioning system was used, on the carriage of which a meter sensor was installed.

Information added to article on page 6

 

9. Line 595

"...ensured generation of vortex type acoustic streaming capable of moving particles within the node areas ..."

The author mainly uses acoustic-streaming-induced vortex to explain the improvement of agglomeration effect, but it seems that there is no convincing evidence provided in the paper.

           

Response:

Thank you very much for your important note. As indicated in the article, for both cases (without an acoustic vortex and with an acoustic vortex), the hydrodynamic conditions for supplying a gas-dispersed flow to the entrance of the chamber and the parameters of the acoustic impact (sound pressure level, frequency, reflection conditions) were maintained the same.

  This was modeled in Ansys and verified experimentally. Thus, there were no differences in hydrodynamics (with the exception of minor distortions of the flow at the surface of the stepped radiator, which do not affect the efficiency of coagulation) and the acoustics of the process for both cases. Also, when responding to other comments, we pointed out the absence of the influence of humidity and electrification on the efficiency of agglomeration. Thus, all experimental conditions, with the exception of vortex flows, were the same for both cases. Therefore, we believe that the explanations presented in the article for the improvement in the agglomeration effect are valid.

If our arguments are not convincing enough, please write so that we can fix it.

 

10. Figure 7

The flow channel of step-function radiator has many steps, and the flow field is different from that of the flat radiator. The author should first rule out the influence of flow channels without ultrasound, and then conduct subsequent research.

Response:

Thanks for the interesting question. Indeed, this point was not described in the article. During preliminary studies, we found that without ultrasonic exposure, the shape of the radiator surface does not affect the capture efficiency. The obtained efficiency values for both radiators were summarized in Table 2 (added on page 16). As you can see from the table, the difference in efficiency does not exceed the measurement error. The results obtained made it possible to ignore the influence of the disk stages on the gas flow velocity field. Therefore, for the case without ultrasonic exposure, only one graph is shown, averaged for both types of disks. The relevant comments are included in the article on page 16.

 

11. Figure 8, 11

" 3 – without ultrasonic influence." 

What type of radiator was used in the experiment without ultrasound? flat radiator or step-function radiator?

Response:

Thanks for the question. In the experiment without ultrasound, both radiators were used. And the graph showed average values for both radiators. The relevant comments are included in the article on page 16.

 

12. Figure 8, 9, 10, 11.

What function does the author use for curve fitting? The far right end of the curve seems bad.

Response:

Thank you for your comment. A polynomial function was used to obtain the curves. The degree of function depended on the primary data (from 3 to 4 degrees). According to your suggestions, to improve the accuracy of constructing curves, the degree of the polynomial function was increased to 4-6.

In Figure 11a, the amount of available experimental data was not enough to produce a curve with a good end. Therefore, additional measurements were carried out for an initial particle size of 8.5 mm. Figure 11a has been replaced.

 

 

13. Line 6.3

"...an increase in the rate of inertial capture for particles of 2.5 μm size by 6% - from 89% to 95%, for particles of 1.5 μm size by 7%- from 85% to 92%..."

This conclusion is only valid under specific conditions (concentration, sound pressure level, flow rate, frequency....), please provide it in the revised manuscript.

Response:

We have accepted your proposed changes and indicated the conditions under which the specified agglomeration efficiency is achieved (concentration of 2∙10^-2 g/m³ _, sound pressure level of 165 dB, flow rate of 6.2 m³/hour).

Reviewer 2 Report

Comments and Suggestions for Authors

Report on the paper by Khmelev et al.

 

It is an interesting paper because the ultrasonic particle agglomeration in air is experimentally studied using a flat and step-function radiator. However, there are some unclear points.

 

1.     The novelty of the present paper should be more clearly stated in Introduction, compared with Refs. 19 and 29.

 

2.     The effect of electric charging of particles and the effect of humidity on the ultrasonic particle agglomeration should be discussed in the manuscript.

 

 

Comments on the Quality of English Language

The English needs some improvement.

Author Response

My co-authors and I place great value on your comments. We have made detailed changes according to the comments and marked them in green in the revised manuscript.

 

1. The novelty of the present paper should be more clearly stated in Introduction, compared with Refs. 19 and 29.

Response:

Thank you very much for your valuable suggestions regarding our work. We have tried to show the novelty more clearly in the introduction.

In works 19 and 29, it was proposed to increase the efficiency of agglomeration by swirling the flow with dispersed particles at the entrance to the agglomeration chamber. Due to this, a swirling flow is created in the agglomeration chamber and the particles (under the influence of centrifugal forces) are pushed to the outer part of the vortex. As the authors state, an area with increased concentration is created there and this area is affected with maximum efficiency. This approach makes it possible to increase the efficiency of agglomeration, however, due to the small size of the particles and the high degree of their entrainment by the swirling flow, the action of centrifugal forces is not effective enough. Therefore, this approach does not allow efficient implementation of acoustic agglomeration.

Therefore, it is proposed additional impact on small particles in the ultrasonic field through the use of utilization of secondary effects arising under certain conditions of generation of high-intensity ultrasound impact. . One of such secondary intensifying effects is the so-called "ultrasonic wind". This is a second order effect due to momentum transfer, represented by the Reynolds voltage. Theoretical studies of acoustic flows are well developed [23-24] and carefully modeled [25-26].

 

There are known attempts to achieve this by initiating vortex acoustic flows near obstacles or reflectors. Such approaches can improve the efficiency of coagulation. But due to the fact that acoustic flows are not created in the entire volume of the agglomeration chamber, but in the small near-wall region, the maximum increase in efficiency is not ensured [Yuen, W.T., Fu, S.C., Kwan, Joseph K.C., Chao, Christopher Y.H. The use of nonlinear acoustics as an energy-efficient technique for aerosol removal, aerosol science and technology  2014, 48(9), 907-915.].

For this reason, this work proposes a new approach, which consists in creating in the entire volume of the agglomeration chamber a sequence of adjacently located elementary acoustic flows of the vortex type, corresponding in scale to the pressure drop zones in the oscillatory process. This allows the volume of the agglomeration chamber to be filled as much as possible with vortex flows and increases the efficiency of agglomeration.

2. The effect of electric charging of particles and the effect of humidity on the ultrasonic particle agglomeration should be discussed in the manuscript.

 

Response:

Thank you very much for the interesting question. Indeed, humidity and electrification affect the efficiency of agglomeration.

But within the framework of the problem under consideration, we wanted to study the separate influence of acoustic vortices on the efficiency of agglomeration. That is why, to exclude the influence of other factors, which include electrification and humidity, we have taken special measures. Namely, to eliminate (reduce) the influence of electronization, the camera body was made of metal (to drain the electric charge) onto the body. To reduce the influence of humidity, it was kept constant at 35%.

Studying the influence of electrification, humidity and other physical factors on the efficiency of agglomeration together with acoustic flows will be the topic of further research.

 

Comments on the Quality of English Language

The English needs some improvement.

 

Response:

We have tried to edit the English. If the level of English is still not sufficient, we are ready to use the help of the editors.

Reviewer 3 Report

Comments and Suggestions for Authors

Thanks to the authors for a fairly detailed description of their research work. In my opinion, the authors have touched on a fundamental aspect of interaction in nature, namely vortex motion and the interactions associated with it. However, I still have some questions and comments.

1. Table 1 does not specify the unit of vibration amplitude (um).

2. In my opinion, Figure 4 is less informative than, for example, a phase field across different cut planes of the acoustic wave distribution. This would give a more complete representation of the areas of vortex formation for the two systems.

3. Paragraph 3.9, in my opinion, is beyond the scope of this article and could be a separate subject of study for a separate article. Moreover, the authors do not provide its data in their final conclusions. This is a conceptual design. The system shown in Figure 13 can play the role of honeycomb flow straightener or turbulence-reducing screens for air flow. A separate study of the interaction of the flow produced by such a system with the acoustic vortex flow is required.

Author Response

My co-authors and I place great value on your comments. We have made detailed changes according to the comments and marked them in green in the revised manuscript.

 

1. Table 1 does not specify the unit of vibration amplitude (um).

Response:

The table has been corrected.

 

2. In my opinion, Figure 4 is less informative than, for example, a phase field across different cut planes of the acoustic wave distribution. This would give a more complete representation of the areas of vortex formation for the two systems.

Response:

Thank you very much for your valuable suggestions regarding our work. I and my co-authors agree that your suggestions will further improve the quality of our research work. We have changed the pictures with sound pressure level distributions. We also added drawings with phase fields and sound pressure distribution. Brief conclusions on the presented distributions are given on page 10

 

3. Paragraph 3.9, in my opinion, is beyond the scope of this article and could be a separate subject of study for a separate article. Moreover, the authors do not provide its data in their final conclusions. This is a conceptual design. The system shown in Figure 13 can play the role of honeycomb flow straightener or turbulence-reducing screens for air flow. A separate study of the interaction of the flow produced by such a system with the acoustic vortex flow is required.

Response:

Thank you very much for your valuable comments. The information presented is an example of the practical use of the proposed method for increasing the efficiency of agglomeration. Structurally, this conceptual design reflects the parallel installation of several agglomeration chambers installed in one housing. The drawing is redone in color and shows where the reflector is and where the radiator is.

The efficiency of agglomeration when using such a setup should correspond to that obtained in the experimental stand.

Therefore, we think that the information presented on the practical use of the results will increase the relevance of the researched method. We hope that you agree that Clause 3.9 can be retained.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I accept the response of the authors. 

Author Response

Thank you. Happy New Year