Design, Heat Transfer, and Visualization of the Milli-Reactor by CFD and ANN

Round 1

Reviewer 1 Report

Comments to the Author(s):

In this manuscript, the authors proposed a millimeter-scale reactor design method and analysed the effect of geometric parameters like the number of blades, angle of inclination, and blade pitch. The authors conducted simulation research using CFD, proposed a surrogate model of the analysed phenomenon, and analyse the influence of different factors on heat transfer and pressure drop.

The authors have done a lot of work to analyse all these cases and it brings us to a better understanding of the heat transfer in this system. The manuscript is well organized, fits the scope of the journal and the topic is important, but the novelty should be better expressed. The conclusions are justified by the results of the presented calculations. On the basis of this and the below-detailed comments, I can recommend this manuscript for minor revision.

 Detail comments:

 1.      In the introduction section, I recommend adding a paragraph about what is original and new in your work compared to the other research. The sentences from lines 72 to 82 do not clearly show this.

2.      Please, explain what is the limitation of the studies conducted by other researchers.

3.      I request the authors to briefly explain why the specific assumptions (mentioned in Lines 102 to Line 106 in Page 3 e.g. value of heat flux) were made in this study. This will be useful for the readers to understand the simulation clearer.

4.      I request the authors to briefly explain what kind of average fluid temperature was calculated. (line 127)

5.      I strongly recommend showing the mesh for the selected case (e.g. after the grid independence test).

6.      There is a reference error in Line 272 Page 9.

7.      It should be "left" instead of "lift" (Figure 8 caption - Line 321).

8.      There is a typo error in Line 354 ("75o") and in Line 420.

Author Response

We really appreciate you for your carefulness and conscientiousness. Your suggestions are really valuable and helpful for revising and improving our paper. I apologize for any misunderstanding caused by my carelessness and poor use of English, which is not my native language. According to your suggestions, we have made the following revisions on this manuscript:

1. In the introduction section, I recommend adding a paragraph about what is original and new in your work compared to the other research. The sentences from lines 72 to 82 do not clearly show this.
2. Please, explain what is the limitation of the studies conducted by other researchers.

Response:

Thank you for your constructive comments. I am sorry that our literature review did not present well the shortcomings of the current study and our innovative points. The shortcoming of the current study is the lack of a reaction-specific millimetre reactor design method, thus resulting in over-design. Therefore, our work relies on reactor stability criteria to ensure a design method that allows for a small pressure drop under conditions of stable reactor operation. And a simple structure with easily adjustable internal components has been designed.

In our new manuscript, the following has been added.

“Although there have been many studies on how to develop various reaction processes in milli-reactors, there are very few reports on how to design milli-reactors. Kockmann et al.^{1, 2} used classical correlations in microreactor design to determine the minimum channel size to meet mixing, heat transfer and reaction requirements. Kucherov et al.³ made customized flow reactors by 3D printing and copper plating on the reactor surface to achieve the reaction requirements. However, in the current study, there is a lack of methods to design and adapt reactor structures to specific reaction requirements. It is well known that enhanced mixing and heat transfer come at the cost of energy dissipation (pressure drop) and that a one-sided pursuit of reactor transfer performance inevitably leads to large pressure drops. The currently available milli-reactors have to use complex structural designs in order to ensure versatility and are therefore exposed to high-pressure drops. Flow chemistry system development is often limited by the inability of suitable pumps. However, different reaction systems require different mixing and heat transfer capabilities; for example, a moderately exothermic class B reaction requires not only moderate heat transfer but also enough residence time. At this point, unacceptable pressure drops may occur if the microreactors are simply connected in series. It is therefore very attractive to keep the reactor running steadily and prevent runaway while keeping the pressure drop as low as possible and preventing over-design through structural design in the reactor design step, taking into account the reaction characteristics. Unfortunately, however, there is very little research on the design of milli-reactors for reaction requirements. Furthermore, considering the convenience and practicality, the structure of such a reactor should be simple, easy to manufacture and the reactor performance can be adjusted by simple parameter changes. Therefore, this paper proposes a millimeter-scale reactor design method that incorporates runaway reactor criteria and designs a structurally inserts.”

 

3. I request the authors to briefly explain why the specific assumptions (mentioned in Lines 102 to Line 106 in Page 3 e.g. value of heat flux) were made in this study. This will be useful for the readers to understand the simulation clearer.

Response:

Thank you for your constructive comments. An explanatory element to the hypothesis has been included in our new manuscript:

“In order to study the heat transfer capacity of the reaction millimetre reactor, a constant heat flux of 2000 W/m2 was used at the reactor wall. the tiny heat flux ensures that the temperature changes at the inlet and outlet of the reactor are minimal, while the fluid properties remain constant. This value is widely accepted in intensive heat transfer studies.4-6”

 

4. I request the authors to briefly explain what kind of average fluid temperature was calculated. (line 127)

Response:

Thank you for your constructive comments. In the new manuscript we indicate the averaging method adopted.

“Among them, the area-weighted average temperature and the mass-weighted average fluid temperature in the full development stage are ?? and ?? respectively.”

 

5. I strongly recommend showing the mesh for the selected case (e.g. after the grid independence test).

Response:

Thank you for your constructive comments. In the new manuscript we have given a detailed view of the grid.

 

6. There is a reference error in Line 272 Page 9.

Response:

Thank you for your care. Sorry for our carelessness, it has been revised.

 

7. It should be "left" instead of "lift" (Figure 8 caption - Line 321).

Response:

Thank you for your care. Sorry for our carelessness, it has been revised.

 

8. There is a typo error in Line 354 ("75o") and in Line 420.

Response:

Thank you for your care. Sorry for our carelessness, it has been revised.

Reviewer 2 Report

 

In this study, a millimeter-scale reactor was studied, the reactor's flow and heat transfer characteristics are studied by CFD simulations. A neural network is trained based on the acquired data.

Comments:

1.     A lot of CFD results are present in this work. The tittle of this manuscript may be a combined CFD and ANN study.

2.     A figure of mesh is recommended. The 12 million mesh may be displayed as an example.

3.     For validation, the friction factor 64/Re is used for laminar flow, maybe the SST K-W model is not suitable. The figure 2 might be the simulation by the laminar model. By using SST model, a good agreements were obtained as the max deviations between the simulative and theoretical values were 2.7% for the Nusselt number and 5.0% for the friction factor. However, the difference between laminar and SST K-W model is less than 6.5%. The description is not convincing.

4.     PIV measurement might be set in section 2.1 or just after CFD models. PIV results are strong validations for CFD results. Figure 17 and related results may be set in section 3.1.1 as a start.

5.     When talk about ANN, the number of datasets should be given.

6.     Should r in nomenclature be radius in the tube.

Technically:

Should it be SIMPLE or PISO algorithm for solving the pressure and velocity equations? Maybe not Couple algorithm in line 119.

Cite literature in line 296.

 

Writing:

The equation 4 and 5 are not explained in the manuscript. Are they the velocity and temperature distribution in the tube inlet?

 

Details:

In line 251, it should be Figure 4 not Figure 1. Also in line 279, figure 5.

In line 272, Error…

The citing of references should be [1]. Same for the others in the manuscript.

 

Font errors:

The font of the manuscript may be improved. For example, line 89 to 91, the ( ) should be the same as that in line 88. In line 102, the coma “,” should be identical to the others. Also in line 302.

In line 111, the letter “Re” in Reynolds stress should not be italic.

In line 171, the Chinese might be revised.

 

 

Author Response

We really appreciate you for your carefulness and conscientiousness. Your suggestions are really valuable and helpful for revising and improving our paper. I apologize for any misunderstanding caused by my carelessness and poor use of English, which is not my native language. According to your suggestions, we have made the following revisions on this manuscript:

1. A lot of CFD results are present in this work. The tittle of this manuscript may be a combined CFD and ANN study.

Response:

Thank you for your hypothetical comments. We have changed the title to: Design, heat transfer, and visualization of the milli-reactor by CFD and ANN

 

2. A figure of mesh is recommended. The 12 million mesh may be displayed as an example.

Response:

Thank you for your constructive comments. In the new manuscript we have given a detailed view of the grid.

 

3. For validation, the friction factor 64/Re is used for laminar flow, maybe the SST K-W model is not suitable. The figure 2 might be the simulation by the laminar model. By using SST model, a good agreements were obtained as the max deviations between the simulative and theoretical values were 2.7% for the Nusselt number and 5.0% for the friction factor. However, the difference between laminar and SST K-W model is less than 6.5%. The description is not convincing.

Response:

Thanks for your advice. We apologise for the misunderstanding that our writing has generated. In fact, to verify the reliability of the SST K-W model, we used the SST K-W model to calculate the heat transfer and pressure drop in the plain tube and the results were in good agreement with the theoretical values. To further verify the reliability of the SST K-W model at low Reynolds numbers, we calculated the flow and heat transfer with the inclusion of internal components using the SST K-W model and the laminar flow model and found that the difference was less than 6.5%. In the new manuscript, we revised it as：

“Furthermore, we used the laminar model to calculate the heat transfer and pressure drop in the millireactor at the same Re up to Re ≤800, and the difference in the results between the laminar model and the k−ω SST is less than 6.5%.”

 

4. PIV measurement might be set in section 2.1 or just after CFD models. PIV results are strong validations for CFD results. Figure 17 and related results may be set in section 3.1.1 as a start.

Response:

Thank you for your suggestion, we have repositioned the relevant sections.

5. When talk about ANN, the number of datasets should be given.

Response:

Thank you for your suggestion. In the new manuscript we have given, the number of data sets is 960.

 

6. Should r in nomenclature be radius in the tube.

Response:

Thank you for your advice. We have amended it.

 

7. Should it be SIMPLE or PISO algorithm for solving the pressure and velocity equations? Maybe not Couple algorithm in line 119.

Response:

Thank you for your advice. We apologise for the writing error here, we are in fact using the Coupled algorithm.

 

8. Cite literature in line 296.

Response:

Thank you for your suggestion, references have been added.

 

9. The equation 4 and 5 are not explained in the manuscript. Are they the velocity and temperature distribution in the tube inlet?

Response:

Thank you for your advice. In the new manuscript we have indicated that they represent the fully developed speed and temperature distribution at the entrance. 

 

10. In line 251, it should be Figure 4 not Figure 1. Also in line 279, figure 5.

In line 272, Error…

The citing of references should be [1]. Same for the others in the manuscript.

Response:

Thank you for your advice. Apologies for our carelessness, we have amended it.

 

11. The font of the manuscript may be improved. For example, line 89 to 91, the ( ) should be the same as that in line 88. In line 102, the coma “,” should be identical to the others. Also in line 302.

In line 111, the letter “Re” in Reynolds stress should not be italic.

In line 171, the Chinese might be revised.

Response:

Thank you for your advice. Apologies for our carelessness, we have amended it.

Reviewer 3 Report

Processes-1976047

The authors have applied computational fluid dynamics (CFD), artificial neural networks (ANN), and particle image velocimetry (PIV) to simulate the Nusselt number and dimensionless friction coefficient in a millimeter scale reactor. The effect of different parameters (Reynolds number, the number of blades, ...) has also been investigated. As a referee, I have the following comments that must be addressed by the authors during a possible revision stage.

 

GENERAL COMMENTS:

·         Line 8: Please remove "Based on."

·         Define all abbreviations/notations in their first appearance in the text.

·         The last part of the abstract should present your key NUMERICAL findings.

·         Keywords need to be modified.

·         The introduction should clearly state the drawback/limitation of the millimeter-scale reactor.

·         I found no expression about the novelty of your study. Would you please present the novelty of your work? What progress against the most recent state-of-the-art similar studies was made?

·         Please use the same notation for the parameters throughout the manuscript. For instance, you used both u and U for showing the velocity in Eqs. (1) to (3).

·         Line 111: Reynolds stress or shear stress?!!

·         Please check the number of equations. The numbers are suddenly changed to 1 on Page 5 and 25 on Page 7.

·         Please recheck all equations and provide valid references for them. It seems some of them have a typo.

·         Line 204: The statement "Because the data sample has two dependent variables to be predicted, two ANNs 204 need to be trained" is not correct. The ANN can be easily developed to handle multi-output problems. For example, see this article “Auto-detection interpretation model for horizontal oil wells using pressure transient responses.” This feature of the ANN must be confessed in the manuscript.

·         Modify Figure 3 so that it completely shows your followed procedure in this study.

·         Correct PLs, Line 383.

·         Change the y-title of figure 15 to the relevancy factor.

 

SCIENTIFIC COMMENTS:

·         The genetic algorithm has only been used in the abstract, and the main manuscript provides no clue in this regard.

·         Line 9 claims that you optimized the reactor structure. Unfortunately, the main manuscript provides no results in this regard.

·         What is your criteria for selecting the geometrical model of insert (Figure 1) and its parameters (Table 1)?

·         I think the "tube's inlet is a fully developed velocity and temperature boundary layer" expression needs to be justified appropriately (Line 113).

·         I did not understand the errors reported in Table 2. How did these errors calculate? Do you have experimental data for the error calculation? If so, please explain.

 

·         The distribution of the training/validation/testing groups of the ANN development is not known.

Author Response

We really appreciate you for your carefulness and conscientiousness. Your suggestions are really valuable and helpful for revising and improving our paper. I apologize for any misunderstanding caused by my carelessness and poor use of English, which is not my native language. According to your suggestions, we have made the following revisions on this manuscript:

1. Line 8: Please remove "Based on."

Response:

Thank you for your advice. Apologies for our carelessness, we have amended it.

 

2. Define all abbreviations/notations in their first appearance in the text.

Response:

Thank you for your advice. Apologies for our carelessness, abbreviations are defined in new manuscripts.

 

3. The last part of the abstract should present your key NUMERICAL findings.

Response:

Thank you for your suggestion. In the new manuscript, we added the following：

“The inserts make the heat transfer performance up to 21 times, and the pressure drop up to 16 times. The inclined angle of blade is recommended to be 45°, which can effectively improve heat transfer without generating excessive pressure drop.”

 

4. Keywords need to be modified.

Response:

Thanks for the suggestion and we added "PLS" to the keywords.

 

5. The introduction should clearly state the drawback/limitation of the millimeter-scale reactor.

Response:

Thank you for your advice. In the new manuscript we mention that the main disadvantages of the millimetre reactor are the high pressure drop and the high price.

 

6. I found no expression about the novelty of your study. Would you please present the novelty of your work? What progress against the most recent state-of-the-art similar studies was made?

Response:

Thank you for your constructive comments. I am sorry that our literature review did not present well the shortcomings of the current study and our innovative points. The shortcoming of the current study is the lack of a reaction-specific millimetre reactor design method, thus resulting in over-design. Therefore, our work relies on reactor stability criteria to ensure a design method that allows for a small pressure drop under conditions of stable reactor operation. And a simple structure with easily adjustable internal components has been designed.

In our new manuscript, the following has been added.

“Although there have been many studies on how to develop various reaction processes in milli-reactors, there are very few reports on how to design milli-reactors. Kockmann et al.^{1, 2} used classical correlations in microreactor design to determine the minimum channel size to meet mixing, heat transfer and reaction requirements. Kucherov et al.³ made customized flow reactors by 3D printing and copper plating on the reactor surface to achieve the reaction requirements. However, in the current study, there is a lack of methods to design and adapt reactor structures to specific reaction requirements. It is well known that enhanced mixing and heat transfer come at the cost of energy dissipation (pressure drop) and that a one-sided pursuit of reactor transfer performance inevitably leads to large pressure drops. The currently available milli-reactors have to use complex structural designs in order to ensure versatility and are therefore exposed to high-pressure drops. Flow chemistry system development is often limited by the inability of suitable pumps. However, different reaction systems require different mixing and heat transfer capabilities; for example, a moderately exothermic class B reaction requires not only moderate heat transfer but also enough residence time. At this point, unacceptable pressure drops may occur if the microreactors are simply connected in series. It is therefore very attractive to keep the reactor running steadily and prevent runaway while keeping the pressure drop as low as possible and preventing over-design through structural design in the reactor design step, taking into account the reaction characteristics. Unfortunately, however, there is very little research on the design of milli-reactors for reaction requirements. Furthermore, considering the convenience and practicality, the structure of such a reactor should be simple, easy to manufacture and the reactor performance can be adjusted by simple parameter changes. Therefore, this paper proposes a millimeter-scale reactor design method that incorporates runaway reactor criteria and designs a structurally inserts.”

 

7. Please use the same notation for the parameters throughout the manuscript. For instance, you used both u and U for showing the velocity in Eqs. (1) to (3).

Response:

Thank you for your suggestion, it has been amended.

 

8. Reynolds stress or shear stress?!!

Response:

We are sure it is Reynolds stress. This is consistent with the relevant literature.

 

9. Please check the number of equations. The numbers are suddenly changed to 1 on Page 5 and 25 on Page 7.

Please recheck all equations and provide valid references for them. It seems some of them have a typo.

Response:

Thank you for your comments. Due to an error in the numbering of the equations in the manuscript upload, we have corrected it.

 

10. Line 204: The statement "Because the data sample has two dependent variables to be predicted, two ANNs 204 need to be trained" is not correct. The ANN can be easily developed to handle multi-output problems. For example, see this article “Auto-detection interpretation model for horizontal oil wells using pressure transient responses.” This feature of the ANN must be confessed in the manuscript.

Response:

Thank you for your advice. In the new manuscript we have revised it to: "Although the ANN can be easily developed to handle multi-output problems, we trained two ANNs for the sake of research simplicity"

 

11. Modify Figure 3 so that it completely shows your followed procedure in this study.

Response:

Thank you for your advice. This process is specific to the reactor design approach and is in a slightly different order to the research in this paper. For example, the section on PIV is not included in the design process.

 

12. Correct PLs, Line 383.

Response:

Thank you for your suggestion, it has been amended.

 

13. Change the y-title of figure 15 to the relevancy factor.

Response:

Thank you for your suggestion, it has been amended.

 

14. The genetic algorithm has only been used in the abstract, and the main manuscript provides no clue in this regard.

Response:

Thank you for your advice. I apologize for the writing error here and have removed the genetic algorithm.

 

15. Line 9 claims that you optimized the reactor structure. Unfortunately, the main manuscript provides no results in this regard.

Response:

Thank you for your advice. By optimisation we mean choosing the most suitable reactor structure for a particular reaction. This results in a smaller pressure drop while still meeting the heat transfer requirements. The whole process of reactor design is in fact a process of optimising the reactor structure for a specific reaction.

 

16. What is your criteria for selecting the geometrical model of insert (Figure 1) and its parameters (Table 1)?

Response:

Thank you for your question. The blades construction is an effective solution for increasing heat transfer capacity with a low pressure drop. It is based on the enhanced effect of longitudinal vortices on heat and mass transfer. The parameters have been chosen in order to study its performance over as large a range as possible.

 

17. I think the "tube's inlet is a fully developed velocity and temperature boundary layer" expression needs to be justified appropriately (Line 113).

Response:

Thank you for your advice. I am sorry for any misunderstandings that may have arisen as a result of our writing. In fact, what we mean here is: setting the inlet conditions of the reactor to a fully developed rate and temperature. This is a conventional method of inlet setting and can be found in many articles.^{5, 7, 8} In the new manuscript, we have made it clear that this refers to the entrance condition.

 

18. I did not understand the errors reported in Table 2. How did these errors calculate? Do you have experimental data for the error calculation? If so, please explain.

Response:

Thank you for your question. The error here refers to the difference between the results of the coarser mesh and the results of the finest mesh. This is a conventional expression that can be found in many articles.⁹

 

19. The distribution of the training/validation/testing groups of the ANN development is not known.

Response:

Thank you for your question. In the new manuscript we state that "There are 960 data sets, 90% of which are used for training and 10% for backtesting"

Round 2

Reviewer 2 Report

This manuscript is carefully revised and suitable to be published in Processes.

Reviewer 3 Report

Accept.