Why Carbon Nanotubes Improve Aqueous Nanofluid Thermal Conductivity: A Qualitative Model Critical Review

Round 1

Reviewer 1 Report

This is an interesting paper on CNTs and their influence on aqueous nanofluid thermal conductivity. The paper presents itself as a Review, but the Abstract seems to suggest it’s an original research paper. Section 1.2 clarifies that it is indeed a review, but the authors should emphasize it in the Abstract as well. The same goes for the article’s title, it should emphasize the fact that the reader is looking at a review paper. 

Although one of the seminal works of Iijima on CNTs is included in the references, I suggest the authors to include the following citation, which is also of great importance in the field. https://www.nature.com/articles/354056a0

Also, I suggest the inclusion of books on general theory of CNT, like the following one (but not limited to it):

http://link.springer.com/10.1007/3-540-39947-X

This is particularly needed when the authors introduce general concepts or properties of CNTs, for instance in section 4.1, where no references are included for the first 3 paragraphs of said section. However, the very nature of this paper being a review, makes it imperative to include the suggested references —as well as references that give support to different pieces of data given in that same section (e.g., thermal conductivities of MWCNT compared with those of SWCNT, by themselves and on suspensions. 

Overall, this is a valuable review paper and the qualitative model is also interesting and of potential use for researchers. I recommend the publication of this after the authors address the aforementioned comments. Thank you. 

Author Response

Comments and Suggestions for Authors

This is an interesting paper on CNTs and their influence on aqueous nanofluid thermal conductivity. The paper presents itself as a Review, but the Abstract seems to suggest it’s an original research paper.

Response: The Abstract has now been revised as shown in yellow highlight.

Section 1.2 clarifies that it is indeed a review, but the authors should emphasize it in the Abstract as well. The same goes for the article’s title, it should emphasize the fact that the reader is looking at a review paper. 

Response: Please see the amendments in the Title and Abstract all in color highlight.

Although one of the seminal works of Iijima on CNTs is included in the references, I suggest the authors to include the following citation, which is also of great importance in the field. https://www.nature.com/articles/354056a0

Response: Done. Other references have also been added.

Also, I suggest the inclusion of books on general theory of CNT, like the following one (but not limited to it):

http://link.springer.com/10.1007/3-540-39947-X

Response: Done. More relevant references have been added.

This is particularly needed when the authors introduce general concepts or properties of CNTs, for instance in section 4.1, where no references are included for the first 3 paragraphs of said section. However, the very nature of this paper being a review, makes it imperative to include the suggested references —as well as references that give support to different pieces of data given in that same section (e.g., thermal conductivities of MWCNT compared with those of SWCNT, by themselves and on suspensions. 

Response:

We thank our Reviewer. This is a critical point and we have added all needed references. Other references have also been added.

As per the last point: (e.g., thermal conductivities of MWCNT compared with those of SWCNT, by themselves and on suspensions). Please see the yellow highlighted discussion above Table 1 and below Table 1 .   For suspensions: please see:  the yellow highlights above Figure 5.

Overall, this is a valuable review paper and the qualitative model is also interesting and of potential use for researchers. I recommend the publication of this after the authors address the aforementioned comments. Thank you. 

Response: Thanks to our reviewer.

Author Response File: Author Response.pdf

Reviewer 2 Report

The article makes a significant contribution to the field of nanofluid research by providing a comprehensive qualitative model for understanding the enhancement of thermal conductivity in CNT aqueous nanofluids. Addressing the following points could further strengthen the article and its appeal to the research community.

1. The article provides a thorough review of existing literature on carbon nanotubes (CNTs) in aqueous nanofluids and their impact on thermal conductivity. However, it could benefit from a more detailed comparison of its qualitative model against quantitative models in existing literature to highlight the unique contributions and potential applications of this qualitative approach.

2. The proposed qualitative model and its postulates offer an insightful framework for understanding the mechanisms behind thermal conductivity enhancement in CNT aqueous nanofluids. It would be beneficial to explicitly address how each postulate is supported by existing empirical evidence or theoretical arguments, thereby strengthening the model's foundation.

3. The article is primarily focused on a qualitative model. Incorporating experimental validation or case studies where the model's predictions align with observed data could significantly enhance the article's impact. This addition would provide a tangible bridge between the theoretical model and practical applications.

4. While the model provides a broad overview of factors influencing thermal conductivity in CNT nanofluids, a more explicit discussion on the model's limitations and the conditions under which it is most applicable would add depth. This includes potential limitations in the model's ability to predict thermal conductivity under varying experimental conditions.

5. The article touches upon the impact of CNT functionalization on thermal conductivity. A deeper exploration of how different functionalization techniques and the nature of functional groups affect the CNT-water interactions and, consequently, the thermal conductivity could provide valuable insights for researchers aiming to tailor nanofluid properties for specific applications.

6. The article effectively identifies gaps in the current understanding and areas needing further research. Expanding this section to include potential future research directions, such as the exploration of hybrid nanofluids or the impact of dynamic flow conditions on thermal conductivity, would be beneficial. This could guide subsequent studies in this promising field.

Author Response

Comments and Suggestions for Authors

The article makes a significant contribution to the field of nanofluid research by providing a comprehensive qualitative model for understanding the enhancement of thermal conductivity in CNT aqueous nanofluids. Addressing the following points could further strengthen the article and its appeal to the research community.

1. The article provides a thorough review of existing literature on carbon nanotubes (CNTs) in aqueous nanofluids and their impact on thermal conductivity. However, it could benefit from a more detailed comparison of its qualitative model against quantitative models in existing literature to highlight the unique contributions and potential applications of this qualitative approach.

Response: Please See Sections 3 and 4 with the added references.

2. The proposed qualitative model and its postulates offer an insightful framework for understanding the mechanisms behind thermal conductivity enhancement in CNT aqueous nanofluids. It would be beneficial to explicitly address how each postulate is supported by existing empirical evidence or theoretical arguments, thereby strengthening the model's foundation.

Response: Please see the revisions on the postulates in green highlight together with added references

3. The article is primarily focused on a qualitative model. Incorporating experimental validation or case studies where the model's predictions align with observed data could significantly enhance the article's impac This addition would provide a tangible bridge between the theoretical model and practical applications.

Response: At the beginning of Section 4 (Results & discussion), we have clarified how the published results validate the qualitative model and its assumptions.

4. While the model provides a broad overview of factors influencing thermal conductivity in CNT nanofluids, a more explicit discussion on the model's limitations and the conditions under which it is most applicable would add depth. This includes potential limitations in the model's ability to predict thermal conductivity under varying experimental conditions.

Response: Model limitations have now been added. Please see the new Section 5 Future outlooks. Limitations for the presented model are listed, together with future recommendations.

5. The article touches upon the impact of CNT functionalization on thermal conductivity. A deeper exploration of how different functionalization techniques and the nature of functional groups affect the CNT-water interactions and, consequently, the thermal conductivity could provide valuable insights for researchers aiming to tailor nanofluid properties for specific applications.

Response: Please see revisions on Section 4.4.1 based on the reviewer’s suggestion. All are shown in green highlight.

6. The article effectively identifies gaps in the current understanding and areas needing further research. Expanding this section to include potential future research directions, such as the exploration of hybrid nanofluids or the impact of dynamic flow conditions on thermal conductivity, would be beneficial. This could guide subsequent studies in this promising field.

Response: Please see the new Section 5 Future outlook.

We thank our Reviewer and we respected all comments.

Author Response File: Author Response.pdf

Reviewer 3 Report

The paper addresses a qualitative model for carbon nanotubes to improve aqueous nanofluid thermal conductivity.

The research gap is not clearly justified. What are the applications for this model?

Methodology: How was the validation carried out?

Figure 7a. How many simulations were done for the tendency? The trend is unclear in the plot.

Abstract is too short and no main findings and specific values have been included.

Conclusions are too general with just future recommendations. 

English should be improved.

Author Response

Comments and Suggestions for Authors

The paper addresses a qualitative model for carbon nanotubes to improve aqueous nanofluid thermal conductivity. Response: That is correct

The research gap is not clearly justified. What are the applications for this model?

Response:   The color highlights in Section 1.2 (Scope and relevance) show Novelty and relevance of the present review. No similar earlier reports were published.

As per applications: This is now highlighted in many places. The philosophy is to tailor the CNT aqueous nanofluid (in terms of composition, CNT type, surfactant, concentration, functionalization etc.) baed on needed applications.

Main applications of the study are in cooling and heating processes. This is now highlighted in Section 1.1 and in the Conclusion in blue color.

Methodology: How was the validation carried out?

Response: Please see the blue highlights added to Section 2 Methodology. I hope this is what outr Reviewer means by the comment.

Figure 7a. How many simulations were done for the tendency? The trend is unclear in the plot.

Response: As per number of simulations, I am sorry as I cannot get what our respectable reviewer means.

The revised Figure caption clarifies the trends in Figure 7(a) and (b) I hope. Please see blue highlights.

If any thing else is needed, please advise me.

Abstract is too short and no main findings and specific values have been included.

Response:  That is correct. Please see the revised Abstract in color highlights.

Conclusions are too general with just future recommendations. 

Response:  Please see the revised conclusion section in blue highlight.

Comments on the Quality of English Language

English should be improved.

Response: We have made careful revision for English language, as can be observed in the color highlights.

We wish to thank our Reviewer. All comments made by the reviewer have been respected and met.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Paper can be accepted.

English must be improved