The Influence of Forced Convective Heat Transfer on Hybrid Nanofluid Flow in a Heat Exchanger with Elliptical Corrugated Tubes: Numerical Analyses and Optimization

Round 1

Reviewer 1 Report

Dear Authors:

It was my pleasure to review and learn from your article. I have following comments and suggestions to improve and clarify your manuscript:

• In the abstract, please one sentence to determine the research gap that this study want to fill or the major problem that want to solve.
• In the introduction, please add the factors that are critical to the performance of the heat exchanger.  Please read and cite following articles that geometry of tubes (twisted and straight), flow direction (countercurrent and concurrent), and flow rates that impact the Re numbers are critical to the performance of the heat heat exchanger [1]. It was also found that thermo-physical properties of nano-fluids such as specific heat, viscosity, thermal conductivity and its heat transfer coefficient are very important for heat transfer application in heat exchangers [2]. In addition, one follow up study found that flow rate in the tube side and shell side are both critical to the performance of the heat exchanger [3].
  • [1] Qian, X.; Yang, Y.; Lee, S.W. Design and Evaluation of the Lab-Scale Shell and Tube Heat Exchanger (STHE) for Poultry Litter to Energy Production. Processes 2020, 8, 500.
  • [2]Ibrahim, H., Sazali, N., Shah, A. S. M., Karim, M. S. A., Aziz, F., & Salleh, W. N. W. (2019). A review on factors affecting heat transfer efficiency of nanofluids for application in plate heat exchanger. Journal of Advanced Research in Fluid Mechanics and Thermal Sciences, 60(1), 144-154.
  • [3]Qian, X.; Lee, S.W.; Yang, Y. Heat Transfer Coefficient Estimation and Performance Evaluation of Shell and Tube Heat Exchanger Using Flue Gas. Processes 2021, 9, 939.

• In Table 1, length is 500m? why this is so long?
• In Table 2, why circular tube were included?
• In Table 3 and 4, what are weight concentrations? Please add explanation.
• In Figure 2, please add 3 different diagrams for Grid I, Grid II and Grid III that easy to visual the difference.
• In Table 8, any particular to use EG, fraction of 0.1%, 0.2% and 0.4%?
• In line 397, 0.2%?
• In Figure 5, please add a little more discussion that why the gravity has a noteworthy effect? How do we determine it?
• In the conclusion, the content should be more concise.  More focus on the major results of this study instead of all the findings. Please rewrite it. In addition, please add the future study. 

I am looking forward to review the revised manuscript with improvement.

All the best, 

Reviewer

Author Response

applsci-1382269," The influence of Forced Convective Heat Transfer on EG/MgO-MWCNT Hybrid Nanofluid as Two-phase Flow in Heat Exchanger with Elliptical Corrugated Tubes: Numerically Validated Analyses and Optimizations"

 

RESPONSE TO REVIEWERS’ COMMENTS & REVISION

 

Reviewer #1:

 

The authors would like to express their gratitude to the reviewers for their efforts and useful comments that allowed us to enhance the quality of the paper. We tried to incorporate all the suggestions into the revised manuscript and we sincerely thank the editor and reviewers’ comments. The parts added to address your comments are indicated by the green highlight in the changes marked version of the manuscript. Please find below our responses to all the comments by the reviewers.

 

Comment 1.1:

Dear Authors:

It was my pleasure to review and learn from your article. I have following comments and suggestions to improve and clarify your manuscript:

 

Response 1.1:

We thank the reviewer for his/her kind words and we make sure to address all the comments to the best of our abilities.

 

Comment 1.2:

In the abstract, please one sentence to determine the research gap that this study want to fill or the major problem that want to solve.

 

Response 1.2:

The increasing need of different industries for heat exchangers with higher efficiency and smaller dimensions has always been considered by various researchers and is one of the focus topics in the present study. Many thanks.

 

Comment 1.3:

In the introduction, please add the factors that are critical to the performance of the heat exchanger.

 

Response 1.3:

Thank you very much for your critical insightful comments. According to the opinion of the esteemed reviewer, the necessary correction was made in the text of the article. Changes have been highlighted.

 

Comment 1.4:

Please read and cite following articles that geometry of tubes (twisted and straight), flow direction (countercurrent and concurrent), and flow rates that impact the Re numbers are critical to the performance of the heat heat exchanger [1]. It was also found that thermo-physical properties of nano-fluids such as specific heat, viscosity, thermal conductivity and its heat transfer coefficient are very important for heat transfer application in heat exchangers [2]. In addition, one follow up study found that flow rate in the tube side and shell side are both critical to the performance of the heat exchanger [3].

• [1] Qian, X.; Yang, Y.; Lee, S.W. Design and Evaluation of the Lab-Scale Shell and Tube Heat Exchanger (STHE) for Poultry Litter to Energy Production. Processes 2020, 8, 500.
• [2]Ibrahim, H., Sazali, N., Shah, A. S. M., Karim, M. S. A., Aziz, F., & Salleh, W. N. W. (2019). A review on factors affecting heat transfer efficiency of nanofluids for application in plate heat exchanger. Journal of Advanced Research in Fluid Mechanics and Thermal Sciences, 60(1), 144-154.
• [3]Qian, X.; Lee, S.W.; Yang, Y. Heat Transfer Coefficient Estimation and Performance Evaluation of Shell and Tube Heat Exchanger Using Flue Gas. Processes 2021, 9, 939.

 

Response 1.4:

The articles mentioned by the esteemed reviewer were used in the text.

 

Comment 1.5:

In Table 1, length is 500m? why this is so long?

 

Response 1.5:

Thanks to the attention of the esteemed reviewer, the unit of this quantity is mm, which was written incorrectly m. Necessary correction was made.

 

Comment 1.6:

In Table 2, why circular tube were included?

 

Response 1.6:

In order to provide a better comparison and understanding of the results, the circular tube is also included in Table 2.

 

Comment 1.7:

In Table 3 and 4, what are weight concentrations? Please add explanation.

 

Response 1.7:

The coefficients presented in these tables are a series of coefficients related to fitting and are in percentage.

 

Comment 1.8:

In Figure 2, please add 3 different diagrams for Grid I, Grid II and Grid III that easy to visual the difference.

 

Response 1.8:

According to the studied geometry, one type of mesh has been used, but this mesh has had a number of different grids. For this reason, a figure is provided for the mesh.

 

Comment 1.9:

In Table 8, any particular to use EG, fraction of 0.1%, 0.2% and 0.4%?

In line 397, 0.2%?

 

Response 1.9:

The mentioned volume fractions have been used. Necessary corrections were made in the text of the article and changes were highlighted.

 

Comment 1.10:

In Figure 5, please add a little more discussion that why the gravity has a noteworthy effect? How do we determine it?

 

Response 1.10:

The opinion of the honorable reviewer is correct. This statement was not correct in this way. The necessary correction was made in the text.

 

Comment 1.11:

In the conclusion, the content should be more concise.  More focus on the major results of this study instead of all the findings. Please rewrite it. In addition, please add the future study. 

I am looking forward to review the revised manuscript with improvement.

All the best, 

Reviewer

 

Response 1.11:

According to the esteemed reviewer, the conclusion section was modified and future study was added.

 

 

In overall, we thank the reviewer for his/her precious time in helping us with their suggestions and comments.

 

Author Response File: Author Response.docx

Reviewer 2 Report

This article presents a numerical analysis of the thermal-hydraulic performance of a corrugated tube-and-fin heat exchanger. Overall, the research subject is interesting but the article suffers from several major problems. The first is the oversimplified computational domain, which actually observes the heat transfer and pressure drop in a single tube section, completely neglecting the conjugate heat transfer and the tube-to-tube effects encountered in heat exchangers. Second, the flow is actually modeled as single phase by adjusting the physical properties of the base fluid to imitate the desired volume fraction of the nanofluid. The nanofluid discrete particle motion and their interaction with the base fluid are not modeled. Third, the results and discussion section must be improved, the discussion basically explains that the heat transfer and the pressure drop are increased with the nanofluid volume fraction, but no deeper insights are given.

1. The title of the article is too long and it does not reflect the content of the article: there is no analysis of two-phase flow in a heat exchanger. The heat exchanger is represented by a single corrugated tube section and the nanofluid flow is not modeled as two-phase flow.
2. Line 108: should be "Poiseuille flow"
3. I suggest using the full terms instead of an excessive number of
   abbreviations: HT = heat transfer, NP = nanoparticles, NF = nanofluids, PD = pressure drop, VF = volume fraction, HE = heat exchanger. This will make the text easier to read and understand.
4. In the Introduction, between lines 60-116, you give a detailed
   literature overview about the effects of nanoparticles on the thermal and hydraulic performance of heat exchangers. Also, it would be good to report how much is the relative increase of heat transfer and pressure drop (in %) with respect to heat exchangers with single phase flow and without nanoparticles.
5. Figure 1b shows the computational domain, which is a tube section in
   the heat exchanger. Further, a constant temperature / heat transfer
   coefficient is applied to the tube wall boundary condition, which is a very crude assumption. Generally, numerical investigations of heat exchangers employ conjugate heat transfer in flow passages with several tube rows, including fins, tubes, the air-side fluid and the tube-side fluid, to capture the entirety of the thermal and hydraulic effects in heat exchangers. A single tube section can hardly represent the fin-and-tube heat exchanger shown in figure 1-a). The over-simplified computational domain and the crude boundary conditions could lead to unrealistic results and conclusions.
6. Table 1: the tube length is 500 m?
7. What are the value for weight concentration in Table 4? Tables 3 and 4
   report coefficients for solving equations in Table 2, not Table 1.
8. The correlations presented in Tables 2-7 are based on two textbooks
   of older date (1980 and 1995). These equations should be checked against newer research articles.
9. The dynamic viscosity should be reported in Pa·s (SI system) instead of in cP.
10. The equations in Table 2 are used for validating the numerical model?
11. Section 2 presents the computational approach. I would say that the
    fluid flow is observed as single-phase since the effects of the carbon
    nanotubes and MgO nanoparticles are taken into account by adjusting the physical properties of the base fluid (ethylene glycol) to imitate the nanofluid. For a two-phase flow simulation, the nanotubes and the nanoparticles should be modelled as discrete solid particles interacting with the continuous phase.
12. The grid sensitivity should be checked in results of Nusselt number
    and pressure drop, not only velocity and temperature profiles at the
    outlet. Further, 3 grid resolutions were selected: 3,243,983 (Grid I),
    3,728,623 (Grid II) and 3,954,131 (Grid III), but the relative grid size
    between the coarsest grid and the densest grid is only 22%, which could be too small to see any significant change in the results. For example, why not selecting the three grid sizes as 1,000,000; 3,000,000 and 5,000,000?
13. Section 2.2.2 shows the validation for fluid friction. However, there
    should be also the validation for heat transfer coefficients or Nusselt
    numbers. Besides, the heat transfer equations are given in Tables 2-3.
14. Figure 5: it is unclear which contours are for velocity distributions
    and which are for temperature distributions?
15. Lines 232-233: at Re=1000 the maximum heat transfer coefficient (HTC)
    is 2700 W/m2K (figure 6) not 2210 W/m2K, for a 0.4% nanofluid fraction. Explain why the base fluid achieves higher HTC than the 0.1% nanofluid fraction?
16. Line 251: should be "Reynolds number", line 257: should be "volume
    fractions of 0.2% and 0.4%". Line 276: should be: "volume fraction of 0.4%". Line 298: should be "volume fraction of 0.4%". Lines 302-303: check the volume fractions. Lines 330, 348, 350: again the volume fractions are incorrect.
17. Figure 11 shows the Nusselt ratio, however, the corresponding text
    (lines 283-288) describes the pressure drop (PD) ratio.
18. The heat exchanger thermal hydraulic performance was assessed using the entropy generation approach. Why not using the area goodness (j/f) and the volume goodness (j/f^1/3) factors, which are standard in the field of heat exchangers?
19. The present analysis is carried out for nanofluid volume fractions of 0.1%, 0.2% and 0.4%. You mention that the optimum nanofluid volume fraction is 0.4%. What would have happened to the heat transfer coefficient and pressure drop at higher nanofluid volume fractions (e.g. 2%, 5% and 10%)? How much nanofluid becomes too much and the heat exchanger performance is worsened? Is there an optimum nanofluid volume fraction that yields the best heat exchanger thermal-hydraulic performance (j/f factor, j/f^1/3 factor)?
20. What is the pressure reduction (PR) in section 3.6 and how it is calculated? Figure 15 shows the effect of nanofluid volume fraction on PR. However, lines 321-331 also discuss the impact on entropy generation, which is shown later in figure 16.
21. Line 337 introduces the thermal entropy generation and the frictional entropy generation. However, figure 16 shows only the thermal entropy generation.
22. Figure 18 shows that the fluid inlet temperature influences the shape of the dimensionless entropy generation curve, especially for Re > 200 - why is that?

Author Response

applsci-1382269," The influence of Forced Convective Heat Transfer on EG/MgO-MWCNT Hybrid Nanofluid as Two-phase Flow in Heat Exchanger with Elliptical Corrugated Tubes: Numerically Validated Analyses and Optimizations"

 

RESPONSE TO REVIEWERS’ COMMENTS & REVISION

 

Reviewer #2:

 

The authors would like to express their gratitude to the reviewers for their efforts and useful comments that allowed us to enhance the quality of the paper. We tried to incorporate all the suggestions into the revised manuscript and we sincerely thank the editor and reviewers’ comments. The parts added to address your comments are indicated by the green highlight in the changes marked version of the manuscript. Please find below our responses to all the comments by the reviewers.

 

Comment 2.1:

This article presents a numerical analysis of the thermal-hydraulic performance of a corrugated tube-and-fin heat exchanger. Overall, the research subject is interesting but the article suffers from several major problems.

 

Response 2.1: We sincerely thank reviewer’s precious time and appreciation of the current research, and we make sure to address all the comments to the best of our abilities. Many thanks.

 

Comment 2.2:

The first is the oversimplified computational domain, which actually observes the heat transfer and pressure drop in a single tube section, completely neglecting the conjugate heat transfer and the tube-to-tube effects encountered in heat exchangers.

Response 2.2:

Thank you very much from the point of view of the esteemed reviewer, it is tried to be considered in the continuation of studies in this case.

 

Comment 2.3:

Second, the flow is actually modeled as single phase by adjusting the physical properties of the base fluid to imitate the desired volume fraction of the nanofluid. The nanofluid discrete particle motion and their interaction with the base fluid are not modeled.

 

Response 2.3:

In this paper, the nanofluid properties depend on the volume fraction of nanoparticles and temperature are considered. When the effect of temperature is considered in the studies, the discussion of different motions of nanoparticles, including Brownian motions, is also indirectly considered. The honorable reviewer rightly points out that there are other models that have considered effects such as Brownian motion, etc. in more detail. We try to use these ideas to improve other studies.

 

Comment 2.4:

Third, the results and discussion section must be improved, the discussion basically explains that the heat transfer and the pressure drop are increased with the nanofluid volume fraction, but no deeper insights are given.

 

Response 2.4:

Changes were made to improve the results section. Attempts were made to improve this section.

 

 

Comment 2.5:

1. The title of the article is too long and it does not reflect the content of the article: there is no analysis of two-phase flow in a heat exchanger. The heat exchanger is represented by a single corrugated tube section and the nanofluid flow is not modeled as two-phase flow.

 

Response 2.5:

According to the esteemed reviewer, the title of the article was corrected.

 

Comment 2.6:

1. Line 108: should be "Poiseuille flow"

 

Response 2.6:

Correction was made and changes were highlighted.

 

Comment 2.7:

1. I suggest using the full terms instead of an excessive number of
   abbreviations: HT = heat transfer, NP = nanoparticles, NF = nanofluids, PD = pressure drop, VF = volume fraction, HE = heat exchanger. This will make the text easier to read and understand.

 

Response 2.7:

Thanks to the point of view of the esteemed reviewer, this action has been taken in order to facilitate the writing and to avoid overlapping.

 

Comment 2.8:

1. In the Introduction, between lines 60-116, you give a detailed literature overview about the effects of nanoparticles on the thermal and hydraulic performance of heat exchangers. Also, it would be good to report how much is the relative increase of heat transfer and pressure drop (in %) with respect to heat exchangers with single phase flow and without nanoparticles.

 

Response 2.8:

In order to provide the opinion of the honorable reviewer, a paragraph was added to the end of this section in the text of the article.

 

Comment 2.9:

1. Figure 1b shows the computational domain, which is a tube section in the heat exchanger. Further, a constant temperature / heat transfer coefficient is applied to the tube wall boundary condition, which is a very crude assumption. Generally, numerical investigations of heat exchangers employ conjugate heat transfer in flow passages with several tube rows, including fins, tubes, the air-side fluid and the tube-side fluid, to capture the entirety of the thermal and hydraulic effects in heat exchangers. A single tube section can hardly represent the fin-and-tube heat exchanger shown in figure 1-a). The over-simplified computational domain and the crude boundary conditions could lead to unrealistic results and conclusions.

 

Response 2.9:

Thank you very much from the point of view of the esteemed reviewer, it is tried to be considered in the continuation of studies in this case.

 

Comment 2.10:

1. Table 1: the tube length is 500 m?

 

Response 2.10:

Thanks to the attention of the esteemed reviewer, the unit of this quantity is mm, which was written incorrectly m. Necessary correction was made.

 

Comment 2.11:

1. What are the value for weight concentration in Table 4? Tables 3 and 4
   report coefficients for solving equations in Table 2, not Table 1.

Response 2.11:

The coefficients presented in these tables are a series of coefficients related to fitting and are in percentage. The opinion of the honorable reviewer is completely correct. The table titles have been modified.

 

Comment 2.12:

1. The correlations presented in Tables 2-7 are based on two textbooks
   of older date (1980 and 1995). These equations should be checked against newer research articles.

Response 2.12:

The opinion of the honorable reviewer is correct. It was better to use newer references. The useful point of view of the dear reviewer will be used to improve other researches.

 

 

Comment 2.13:

1. The dynamic viscosity should be reported in Pa·s (SI system) instead of in cP.

 

Response 2.13:

All units were modified in the text of the article. Centipoise became Pa.s. Changes were highlighted.

 

Comment 2.14:

1. The equations in Table 2 are used for validating the numerical model?

 

Response 2.14:

References 53 and 54 have been used for validation.

 

Comment 2.15:

1. Section 2 presents the computational approach. I would say that the fluid flow is observed as single-phase since the effects of the carbon nanotubes and MgO nanoparticles are taken into account by adjusting the physical properties of the base fluid (ethylene glycol) to imitate the nanofluid. For a two-phase flow simulation, the nanotubes and the nanoparticles should be modelled as discrete solid particles interacting with the continuous phase.

 

Response 2.15:

In order to provide the opinion of the esteemed reviewer, the necessary correction was made in the whole text about the words single-phase and a two-phase. The hypotheses were also corrected.

 

Comment 2.16:

1. The grid sensitivity should be checked in results of Nusselt number and pressure drop, not only velocity and temperature profiles at the outlet. Further, 3 grid resolutions were selected: 3,243,983 (Grid I), 3,728,623 (Grid II) and 3,954,131 (Grid III), but the relative grid size between the coarsest grid and the densest grid is only 22%, which could be too small to see any significant change in the results. For example, why not selecting the three grid sizes as 1,000,000; 3,000,000 and 5,000,000?

 

Response 2.16:

Because Nusselt and pressure drop values are integral and average values, they are not good options for checking mesh. For this reason, velocity and temperature profiles have been used. A mesh with a higher number of points can also be used, but due to the unavailability of suitable computers for simulation with a higher number of meshes, the values presented in the article have been used. Certainly finer meshes can lead to accurate results. But the present results also have acceptable accuracy.

 

Comment 2.17:

1. Section 2.2.2 shows the validation for fluid friction. However, there
   should be also the validation for heat transfer coefficients or Nusselt
   numbers. Besides, the heat transfer equations are given in Tables 2-3.

 

Response 2.17:

The purpose of validation is to check the correct performance of computing software. For this purpose, it is enough to examine one of the quantities. In this study, pressure drop is considered. Not all quantities studied in the study (such as Nusselt, entropy, etc.) need to be validated separately. However, the opinion of the esteemed reviewer is correct and more validations can lead to improved article quality.

 

Comment 2.18:

1. Figure 5: it is unclear which contours are for velocity distributions
   and which are for temperature distributions?

 

Response 2.19:

The opinion of the honorable reviewer is completely correct. Figure 5 was modified.

 

Comment 2.19:

1. Lines 232-233: at Re=1000 the maximum heat transfer coefficient (HTC)
   is 2700 W/m2K (figure 6) not 2210 W/m2K, for a 0.4% nanofluid fraction. Explain why the base fluid achieves higher HTC than the 0.1% nanofluid fraction?

 

Response 2.19:

The value 2700 was modified in the text. In the volume fraction of 0.1%, the increase in viscosity can overcome the increase in thermal conductivity. As a result, due to the greater viscosity of the nanofluid in this volume fraction, the possibility of heat exchange will be less and the heat transfer coefficient will be less. In other volume fractions, increasing the thermal conductivity prevails over increasing the viscosity.

 

Comment 2.20:

1. Line 251: should be "Reynolds number", line 257: should be "volume
   fractions of 0.2% and 0.4%". Line 276: should be: "volume fraction of 0.4%". Line 298: should be "volume fraction of 0.4%". Lines 302-303: check the volume fractions. Lines 330, 348, 350: again the volume fractions are incorrect.

 

Response 2.20:

Thanks to the attention of the esteemed reviewer, the necessary corrections were made in the whole text. Changes have been highlighted.

 

Comment 2.21:

1. Figure 11 shows the Nusselt ratio, however, the corresponding text
   (lines 283-288) describes the pressure drop (PD) ratio.

 

Response 2.21:

Correction was made and changes were highlighted.

 

Comment 2.22:

1. The heat exchanger thermal hydraulic performance was assessed using the entropy generation approach. Why not using the area goodness (j/f) and the volume goodness (j/f^1/3) factors, which are standard in the field of heat exchangers?

 

Response 2.22:

Thanks to the opinion of the esteemed reviewer in this study, the purpose was not to study and use these quantities. It has not been used due to lack of familiarity with these quantities. This point will definitely be used in future studies.

 

Comment 2.23:

1. The present analysis is carried out for nanofluid volume fractions of 0.1%, 0.2% and 0.4%. You mention that the optimum nanofluid volume fraction is 0.4%. What would have happened to the heat transfer coefficient and pressure drop at higher nanofluid volume fractions (e.g. 2%, 5% and 10%)? How much nanofluid becomes too much and the heat exchanger performance is worsened? Is there an optimum nanofluid volume fraction that yields the best heat exchanger thermal-hydraulic performance (j/f factor, j/f^1/3 factor)?

Response 2.23:

Optimal conditions meant achieving the highest heat transfer coefficient and at the same time having the lowest pressure drop, which we have achieved in the reported conditions. The use of the quantities mentioned by the reviewer could also be done as an effective change. As mentioned in the previous comment, the use of these quantities was not considered in the present study.

 

Comment 2.24:

1. What is the pressure reduction (PR) in section 3.6 and how it is calculated? Figure 15 shows the effect of nanofluid volume fraction on PR. However, lines 321-331 also discuss the impact on entropy generation, which is shown later in figure 16.

 

Response 2.24:

The meaning of pressure difference was the pressure difference at the inlet and outlet of the studied geometry. Inlet and outlet pressure values are calculated with the help of software. Description of this section and new sentences were added.

 

Comment 2.25:

1. Line 337 introduces the thermal entropy generation and the frictional entropy generation. However, figure 16 shows only the thermal entropy generation.

 

Response 2.25:

Correction was made and changes were highlighted.

 

Comment 2.26:

1. Figure 18 shows that the fluid inlet temperature influences the shape of the dimensionless entropy generation curve, especially for Re > 200 - why is that?

 

Response 2.26:

The effect of fluid inlet temperature on thermophysical properties as well as values of heat transfer and pressure drop has caused entropy values to be affected and different curves to be seen.

 

 

In overall, we thank the reviewer for his/her precious time in helping us with their suggestions and comments.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper proposes a numerical analyses of the performance of a given nanofluid flowing in an elliptical corrugated tube. No experimental data are measured nor data available in the literature have been considered (if any). The main weakness of the paper is actually the absence of experimental data to test the results obtained, even if the validation of the numerical procedure has been obtained for a similar case (smooth elliptical tube). One serious drawback is related to the use of equations in table 2 with coefficients valid for smooth tubes (and derived form reference [57]), not for corrugated ones. I think this point must be clearly explained by the authors and a robust motivation is necessary for the paper to be accepted. 

A part this main weak point, the paper requires some other revisions, as here below suggested:

1. A nomenclature is necessary, reporting all the symbols used in the paper
2. page 3: rows 131-132: please cite the references where the figures 17% and 28% were reported. While the pressure drops is actually generally increased by the presence of nanoparticles, the effects on the heat transfer rate depend, among other factors, on the flow regime. Frequently, negative effects are observed for turbulent flow, while more promising results are obtained for laminar flow. This point should be discussed in the introduction.
3. Eq. 3): k must be lower case
4. Table 2: each equation reported in table must be identified by a number
5. Tables 1, 2: use different symbols to define the axes in Table 1 and the coefficients of equation the coefficients in table 2.
6. Tables 2 and 3: use the same case for the symbols of the coefficients (you used lower case in table 2 and capitals in table 3)
7. Table 4: I guess "Friction coefficients" instead of "weight concentrations (%)" should be used
8. Page 8, row 208: what do you mean by "extreme error"? I guess it is rather a "maximum deviation"
9. Page 9, row 225: here "maximum error" is actually "maximum deviation"
10. Page 10, row 235: "...using single-phase model...": I guess it is a two-phase model
11. Page 10, rows 252-253: I suggest to write: "For a given Re, the HT coefficient is increasing at increasing nanoparticles volume fraction, while the pure base fluid HT is between those at 0.1" and 0.2% vf". Use similar sentences int he rest of the paper.
12. Page 11, row 2698: what do you mean by "Dynamic Volume Fraction"?
13. Pages 13-19: please revise carefully the table numbers and their citations in the text: the tables order is confused.
14. Page 13, row 295: "4%" is actually "0.4%". Check this figure also ahead in the text.

Author Response

applsci-1382269," The influence of Forced Convective Heat Transfer on EG/MgO-MWCNT Hybrid Nanofluid as Two-phase Flow in Heat Exchanger with Elliptical Corrugated Tubes: Numerically Validated Analyses and Optimizations"

 

2^nd RESPONSE TO REVIEWERS’ COMMENTS & 2^ND REVISION

 

Reviewer #3: Round 1

 

The authors would like to express their gratitude to the reviewers for their efforts and useful comments that allowed us to enhance the quality of the paper. We tried to incorporate all the suggestions into the 2^nd revised manuscript and we sincerely thank the editor and reviewers’ comments. The parts added to address your comments are indicated by the highlight in the changes marked version of the manuscript. Please find below our responses to all the comments by the reviewers.

 

Comment 3.1:

 

The paper proposes a numerical analyses of the performance of a given nanofluid flowing in an elliptical corrugated tube. No experimental data are measured nor data available in the literature have been considered (if any). The main weakness of the paper is actually the absence of experimental data to test the results obtained, even if the validation of the numerical procedure has been obtained for a similar case (smooth elliptical tube). One serious drawback is related to the use of equations in table 2 with coefficients valid for smooth tubes (and derived form reference [57]), not for corrugated ones. I think this point must be clearly explained by the authors and a robust motivation is necessary for the paper to be accepted. 

A part this main weak point, the paper requires some other revisions, as here below suggested:

Response 3.1:

Thanks to the esteemed reviewer, the coefficients of table 2 have not been used for the corrugated elliptical tube. Calculations of h, Nu, etc. for corrugated elliptical tubes have been performed with the help of numerical simulation with the help of software.

 

Comment 3.2:

1. A nomenclature is necessary, reporting all the symbols used in the paper

 

 

Response 3.2:

 

Thank you for your precious time and help in suggesting this. The Nomenclature is now written within the manuscript highlighted in Yellow colour.

 

Comment 3.3:

 

 

1. page 3: rows 131-132: please cite the references where the figures 17% and 28% were reported. While the pressure drops is actually generally increased by the presence of nanoparticles, the effects on the heat transfer rate depend, among other factors, on the flow regime. Frequently, negative effects are observed for turbulent flow, while more promising results are obtained for laminar flow. This point should be discussed in the introduction.

 

Response 3.3:

 

Thanks to the reviewer, references are now cited. The point mentioned by the esteemed reviewer is also mentioned in the text of the manuscript.

 

Comment 3.4:

 

1. Eq. 3): k must be lower case

 

Response 3.4:

Many thanks, it is now corrected in the 2^nd revised manuscript.

 

Comment 3.5:

 

 

1. Table 2: each equation reported in table must be identified by a number

 

Response 3.5:

Many thanks, it is now corrected in the 2^nd revised manuscript.

 

 

Comment 3.6:

 

1. Tables 1, 2: use different symbols to define the axes in Table 1 and the coefficients of equation the coefficients in table 2.

 

Response 3.6:

The letters used for the coefficients mentioned in Tables 1 and 2 are now corrected and the changes are highlighted in the 2^nd revised manuscript.

 

Comment 3.7:

 

 

1. Tables 2 and 3: use the same case for the symbols of the coefficients (you used lower case in table 2 and capitals in table 3)

 

Response 3.7:

Thanks to the esteemed referee, in these tables, capital letters have been used consciously for coefficients, and the difference in the use of these symbols is clear.

 

Comment 3.8:

 

 

1. Table 4: I guess "Friction coefficients" instead of "weight concentrations (%)" should be used

 

Response 3.8:

Thank you very much to the esteemed reviewer for pointing this out, Friction coefficients are replaced by weight concentrations.

 

Comment 3.9:

 

 

1. Page 8, row 208: what do you mean by "extreme error"? I guess it is rather a "maximum deviation"

 

Response 3.9:

 

Thanks to the esteemed reviewer for your precious time, maximum deviation is now replaced by extreme error.

 

Comment 3.10:

 

 

1. Page 9, row 225: here "maximum error" is actually "maximum deviation"

 

Response 3.10:

 

We, sincerely, thank the reviewer to help us enhance the quality off the manuscript, we have now corrected this in the 2^nd revised manuscript.

 

 

Comment 3.11:

 

 

1. Page 10, row 235: "...using single-phase model...": I guess it is a two-phase model

 

Response 3.11:

We have now corrected this in the 2^nd revised manuscript. Many thanks,

 

 

Comment 3.12:

 

 

1. Page 10, rows 252-253: I suggest to write: "For a given Re, the HT coefficient is increasing at increasing nanoparticles volume fraction, while the pure base fluid HT is between those at 0.1" and 0.2% vf". Use similar sentences int he rest of the paper.

 

Response 3.12:

The sentence of the esteemed reviewer was added to the text of the article.

 

 

Comment 3.13:

 

 

1. Page 11, row 2698: what do you mean by "Dynamic Volume Fraction"?

 

Response 3.13:

The meaning was the same as Volume Fraction. This was corrected in all texts throughout the manuscript in the 2^nd revised version.

 

Comment 3.14:

 

 

1. Pages 13-19: please revise carefully the table numbers and their citations in the text: the tables order is confused.

 

Response 3.14:

Thanks. All table numbers in the whole text are checked and corrected.

 

Comment 3.15:

 

 

1. Page 13, row 295: "4%" is actually "0.4%". Check this figure also ahead in the text.

 

Response 3.15:

Thanks, these values are now incorporated and corrected throughout the text of the manuscript.

In overall, we thank the reviewer for his/her precious time in helping us with their suggestions and comments that enabled us to address the 2^nd round of corrections.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors:

You made a significant revisions to address the comments and suggestions.

Please check again grammar, spelling, and references.

In conclusion, it looks like have 6 characteristics in line 412.

Looking forward to read more papers from your group.

Best regards,
Reviewers

 

Author Response

applsci-1382269," The influence of Forced Convective Heat Transfer on EG/MgO-MWCNT Hybrid Nanofluid as Two-phase Flow in Heat Exchanger with Elliptical Corrugated Tubes: Numerically Validated Analyses and Optimizations"

 

2^nd RESPONSE TO REVIEWERS’ COMMENTS & 2^ND REVISION

 

Reviewer #1: Round 2

 

The authors would like to express their gratitude to the reviewers for their efforts and useful comments that allowed us to enhance the quality of the paper. We tried to incorporate all the suggestions into the 2^nd revised manuscript and we sincerely thank the editor and reviewers’ 2^nd comments. The parts added to address your comments are indicated by the highlight in the changes marked version of the manuscript. Please find below our responses to all the comments by the reviewers.

 

Comment 1.1:

Dear Authors:

You made a significant revisions to address the comments and suggestions.

Please check again grammar, spelling, and references.

In conclusion, it looks like have 6 characteristics in line 412.

Looking forward to read more papers from your group.

Best regards,
Reviewers

Response 1.1:

 

We thank the reviewer for his/her kind words and we make sure to double check the grammar, spelling and references and made sure to address all the comments to the best of our abilities.

 

Thanks to the esteemed reviewer, the correction has been made and the highlight changes have become green.

 

Author Response File: Author Response.docx

Reviewer 2 Report

The authors have addressed some of the easier comments and suggestions. However, the major problems still remain in the manuscript: the oversimplified compational domain and nanofluid flow physics, the weak validation and the arbitrary optimization. I cannot recommend this article for publication.

Author Response

applsci-1382269," The influence of Forced Convective Heat Transfer on EG/MgO-MWCNT Hybrid Nanofluid as Two-phase Flow in Heat Exchanger with Elliptical Corrugated Tubes: Numerically Validated Analyses and Optimizations"

 

2^nd RESPONSE TO REVIEWERS’ COMMENTS & 2^ND REVISION

 

Reviewer #2: Round 2

 

The authors would like to express their gratitude to the reviewers for their efforts and useful comments that allowed us to enhance the quality of the paper. We tried to incorporate all the suggestions into the 2^nd revised manuscript and we sincerely thank the editor and reviewers’ 2^nd comments. The parts added to address your comments are indicated by the highlight in the changes marked version of the manuscript. Please find below our responses to all the comments by the reviewers.

 

Comment 2.1:

 

The authors have addressed some of the easier comments and suggestions. However, the major problems still remain in the manuscript: the oversimplified compational domain and nanofluid flow physics, the weak validation and the arbitrary optimization. I cannot recommend this article for publication.

 

 

Response 2.1:

Thanks to the esteemed reviewer, in the previous review, all 26 comments of the esteemed reviewer were answered. Also, the opinions of the other two reviewers were fully considered and all cases were answered. The other two esteemed reviewers accepted the amendments and considered them acceptable. While thanking the esteemed reviewer again, we tried to respond to all comments as accurately as possible.

Author Response File: Author Response.docx

Reviewer 3 Report

The paper has been amended by the authors following the suggestions from the previous revision. I suggest the publication after minor revisions only:

1. Table 1: 'The minor axis': delete 'The'
2. Table 4: 'Friction coefficients(%)': delete (%)
3. page 11 row 253: substitute 0.1" with 0.1%

Author Response

Dear Reviewer,

Thank you very much for your corrections, we have implemented all of the latest comments in our 3rd revised manuscript submitted.

We sincerely thank you for your approval of publications.

With best regards,

Saim Memon

Round 3

Reviewer 2 Report

After two rounds of revisions, my opinion has not changed. The manuscript has not been significantly improved to recommend publication. The major problems were not addressed by the authors: 1) the oversimplified numerical approach (single phase analysis in tube section, no conjugate heat transfer), 2) lacking validation (friction factor only, no validation of heat transfer coefficients), 3) shallow discussion without physical explanations to support the results. All this negatively affects the credibility of the results and conclusions.

Author Response

Dear Reviewer,

With all due respects, we have implemented all of the comments raised by all reviewers. Two reviewers approved it and we also addressed all of your comments.