Improvement of a Nusselt-Based Simulation Model for Heat Transfer in Rotary Heat Exchangers

Round 1

Reviewer 1 Report

This is a really good work. I think, it will be a high cited article.

Author Response

Dear Reviewer,

we truly thank you for your time and review of our work.

Best regards,

Eloy Melian

Reviewer 2 Report

The paper is well written and contains all the necessary information and equations. The conclusions are clearly described.

I recommend the paper for publication in Energies in present form.

Author Response

Dear Reviewer,

We truly thank you for your time and review of our work.

Best regards,

Reviewer 3 Report

1. Introduction: In general, the literature review should be considerable improved. The description of the study should be considerably improved at the last paragraphs. The authors should link the state-of-the-art analysis to their study goals, identifying the knowledge gaps in literature. The authors are also advised to reason both the relevance and novelties introduced by their study (the use of bullets to emphasize the main relevant innovations and contributions is highly recommended). Also, multiple references and reference lumps should be eliminated from the Introduction section for the sake of clarity. Each reference should be cited separately with the corresponding explanation.
2. Results and discussion. Overall, the authors should further develop critical appraisal in their discussion. In addition, it could be more interesting to compare the results obtained in this work with other studies available in the literature.
3. Citations/References: The citations throughout the manuscript and the references section must be carefully checked. In addition, multiple references and reference lumps should be eliminated for the sake of clarity. Each reference should be cited separately with the corresponding explanation.
4. Conclusions: The Conclusions section should be rewritten. The authors do not clearly summarize the actions taken and results. In addition to summarizing the actions taken and main results, the authors should include an explanation about the significance of their results using quantitative reasoning in comparison with suitable benchmarks, mainly those from other studies presented in the literature. Also, the originality and innovations of the study should be clearly summarized and highlighted (the use of bullets is highly advised).

 

 

Author Response

Dear Reviewer,

 

We truly thank you for your time and review, since it was an enrichment for the work. Hereby our answers to your questions:

 

1. Moderate English changes required

We have already submitted this work for a professional, native-speaker correction service. We have again given it two native speakers and corrected several prepositions and wording. These are, among others, in the following lines: 44, 48, 52, 54, 57, 72, 84, 90, 91, 94, 100, 129, 134, 165, 176, 204, 241, 298, 331, 334, 391, 392, 395, 489, 617, 618, 619, 624 and 636.

 

2. Introduction: In general, the literature review should be considerable improved.

Thank you for this comment. In the introduction we have included (in lines 76-79) different simulation approaches by different authors. This clarifies this state-of-the-art and the usual methods used for simulating rotary heat exchangers.

 

3. The description of the study should be considerably improved at the last paragraphs.

Thank you for this suggestion. In lines 59-87 we highlighted and included more details on the work done.

 

4. The authors should link the state-of-the-art analysis to their study goals, identifying the knowledge gaps in literature.

Thank you for your suggestion. We have now :

87. Clearly stated what state-of-the-art simulations have achieved in line 58-60 and the relationship with the goal of this work in line 80-87.
88. Stated in point 4) from introduction (line 73-75), that state-of-the-art simulations often use constant thermophysical properties and we highlighted that in this work locally-calculated thermophysical properties are used for achieving a higher simulation accuracy.
89. Also in point 5) from introduction (line 76-79), we state that that state-of-the-art simulations from literature either use ε-NTU method or computer fluid dynamics, and emphasized that the novelty in this work is the different approach on locally calculated Nusselt numbers.
90. Especially in line 65-67, we address the knowledge gap for locally calculated Nusselt numbers for sine ducts in the developing region, which is in literature non-existent.

 

5. Introduction: The authors are also advised to reason both the relevance and novelties introduced by their study (the use of bullets to emphasize the main relevant innovations and contributions is highly recommended).

We truly thank you for your feedback. We have now highlighted in lines 58-60 that the listed strategies (lines 61-79) are indeed the innovations of this work, compared to other state-of-the-art previous works.

 

6. Introduction: Also, multiple references and reference lumps should be eliminated from the Introduction section for the sake of clarity. Each reference should be cited separately with the corresponding explanation.

We thank you for your point of view. In lines 25, 31, 44, 64, 66, 71, 72, and 77 in the introduction we use merged references following correctly the journal template and “citation style guide from MDPI Journals”. In these cases, we emphasize that consolidated multiple references, authors have used the same strategy or assumptions in their work and juxtapose it to the approach of this study. This use of merged references is in these concrete cases an enrichment of the novelty of this work. To cite each reference separately, would not only make the reading bulky, weakens clarity, and does not bring any added value to the manuscript. For example:

     

“Previous simulations were only validated with one rotary heat exchanger [2,12,14,15,17,20–23] or a single rotor from literature source [13,24–26].” (line 71-72)

 

With this sentence, we emphasize on the amount of previous works, which had been validated by using a single rotary heat exchanger. This highlights also the innovation of this work, by validating the results against four different rotary heat exchangers from different manufacturers.

 

7. Results and discussion: Overall, the authors should further develop critical appraisal in their discussion.

Thank you for the feedback. We have now included a more critical discussion in the discussions section (lines 572 -592).

 

8. Results and discussion: In addition, it could be more interesting to compare the results obtained in this work with other studies available in the literature.

      Thank you for your suggestion. Literature, where different Nusselt models are used and compared to experimental results of rotary heat exchangers is non-existent.  Basically because the vast majority of state-of-the-art simulations in this topics use either the -NTU method [2,12,14–17,20,23,25,26,28,33–36,38] or Computer fluid dynamics [13,18,22,24]. Locally-calculated Nusselt models with entry-region considerations is until now, the road not taken. It is the main innovation of this work and we consider, according to the results, that it precisely what made all the difference. This limits already the comparison of the results to the cases where the authors disclose the thermal effectiveness deviation. We thank you for this observation and therefore for providing more context to the innovation, point 5) was added to the introduction in lines 76-79.

      In this research field, it would also increase the transparency, when authors disclose detailed constructive rotary heat exchanger information in terms of: material thickness, wave height, inclination angle, rotor length, rotational speed and empty tube gas velocity. Consequently, we focused on the Eurovent data where this information is openly disclosed. This helps in transparency and traceability of our work. Therefore, we focused on discussing the results in the literature context in terms of concrete results such as thermal effectiveness deviation, or tendencies in empty tube gas velocity or rotational speed.

 

9. Citations/References: The citations throughout the manuscript and the references section must be carefully checked. In addition, multiple references and reference lumps should be eliminated for the sake of clarity. Each reference should be cited separately with the corresponding explanation.

      We truly thank you suggestion. We use merged references, following correctly the journal template and “citation style guide from MDPI Journals”. In these cases, we highlight that multiple authors have used the same strategy or assumptions in their work and juxtapose it to the approach of this study. To cite each reference separately, would not only make the reading bulky, but weakens clarity, and does not bring any added value to the manuscript.  For example in the discussion:

 

“Previous work had higher deviations: a maximum absolute deviation of 3.5% points for Leong [12] or 3,71% in average for Özdemir [13] or were limited to test only one rotor exchanger [2,12,14,15,17,20–23].”

 

      This sentence emphasizes the limitation in accuracy and in the validation method for other works, and how this is a general weakness found among different literature sources.

 

Still, we found that in line 156 for clarity it was reasonable to do so and was changed.

 

10. Conclusions: The Conclusions section should be rewritten. The authors do not clearly summarize the actions taken and results.

      Thank you for your suggestion. We have added some points to the conclusion in bullets (lines 605-609. The order of the actions taken and results is the following order: First, motivation and innovative actions in this work. Second, quantitative comparison of the results to state-of-the art. And third, recommendations for further studies are presented. Additionally, we renamed our model so it is clearer what we did.

 

11. Conclusions: The authors should include an explanation about the significance of their results using quantitative reasoning in comparison with suitable benchmarks, mainly those from other studies presented in the literature.

      Thank you for this suggestion. This is now clearly presented on lines 605-609. We have included in line the benchmarks of the accuracy of the simulation to the state-of-the-art literature.

 

12. Conclusions: Also, the originality and innovations of the study should be clearly summarized and highlighted (the use of bullets is highly advised).

      Thank you. This has also been included in the conclusions in lines 613-614.

 

We truly thank you for your time and reviews, which helped us to communicate better the importance and relevance of our article.

 

Best regards,

Reviewer 4 Report

This paper is interesting and has a great utility value. I suggest only a minor revision. While reading the article, you may come across the following shortcomings:

1. line 54: The purpose of this work is to develop and validate a simulation (of what ?) for rotary heat exchangers that …;
2. line 58: … of 3.5% points for Leong [12] or 3,71% in average…(dot or comma ?);
3. line 60: 1) to improve the accuracy (of what ?) by using…;
4. line 63: 3) For validating the developed simulation (You start with capital letter ?);
5. line 69: For practical reasons, rotary heat exchangers, which are also known in literature as “thermal 69 wheel”, “heat recovery wheel (either singular or plural ?);
6. line 133, 135: references [30, 31] must be precisely declared;
7. line 142: what width is the tape ?
8. line 614: the specific heat characteristic of aluminum is missing;

due to the scope of the device's operation(from RT up to 150 ^oC), it makes no sense to enter the values of thermophysical parameters in such wide temperature ranges. It would be good to show simulations of device operation with constant values (averaged) of density, thermal conductivity and specific heat of aluminum. In my opinion appendix is not needed.

Author Response

Dear Reviewer,

 

We truly thank you for your time and review, since it was an enrichment for the work. Hereby our answers to your questions:

 

1. line 54: The purpose of this work is to develop and validate a simulation (of what ?) for rotary heat exchangers that …;

      Thank you. In now line 55, it is actually meant that it is a simulation of rotary heat exchangers.

 

2. line 58: … of 3.5% points for Leong [12] or 3,71% in average…(dot or comma ?);

      Thank you. This has been corrected and can be found in line 58. Now both are written with dots.

 

3. line 60: 1) to improve the accuracy (of what ?) by using…;

 

      Thank you. Now we have clearly stated in line 62 that it is about the accuracy of the simulation of rotary heat exchangers.

4. line 63: 3) For validating the developed simulation (You start with capital letter ?);

      Thank you. Since now the formatting changed due to corrections from another reviewer, now the list starts all with new sentences, and capitals included. This is seen in lines 62- 81.

5. line 69: For practical reasons, rotary heat exchangers, which are also known in literature as “thermal 69 wheel”, “heat recovery wheel (either singular or plural ?);

      Thank you. In line 90 it is corrected to the plural form.

6. line 133, 135: references [30, 31] must be precisely declared;

      Thank you. In now line 158, 160we have now clearly stated that it is about the boundary condition assumptions from work from other authors, that we also now use in our model.

7. line 142: what width is the tape ?

      In now line 163 we describe that the width of the foils is typically 20 cm.

 

8. line 614: the specific heat characteristic of aluminum is missing;

      The aluminum specific heat was calculated according to the correlation from Perry and Green [41]. This correlation was taken from this literature source without modifications. Therefore it was not explained in this Appendix.

 

9. Due to the scope of the device's operation(from RT up to 150 ^oC), it makes no sense to enter the values of thermophysical parameters in such wide temperature ranges. It would be good to show simulations of device operation with constant values (averaged) of density, thermal conductivity and specific heat of aluminum.

      We truly thank you for this suggestion. We wanted the developed simulation in this work to be the basis for further research at different temperature levels. This makes it possible to research how the temperature level could influence thermal efficiency, and therefore this work was developed with this aspect in mind. Therefore we use temperature-dependent properties. It is an interesting further research aspect a comparison between both approaches (averaged temperature properties vs. temperature-dependent properties). This would reveal, whether it is truly needed for such accurate results, to include temperature-dependent properties. This, however, out of the scope of this first work.

 

10. In my opinion appendix is not needed.

      Thank you, we also think that the appendix is almost trivial and most of the data can be found in the literature. However, for transparency and traceability, we would like to include it.

 

We truly appreciate your review because it helped us to improve our article.

 

Best regards,

Round 2

Reviewer 3 Report

The revised version of the manuscript has significantly improved its content and language, and the authors have adequately responded to all my comments on the first version. Overall, I am satisfied with the revision and I believe that the paper is suitable for publication in Energies.