Review Reports

Round 1

Reviewer 1 Report

General comments:
The paper presents studies of the thermal performance of a heated pipe. The authors have conducted a series of numerical simulations to evaluate the thermal performance in the presence of metal foam and twisted tape inserts. CFD method was applied via Comsol software.
The contents of this paper lays well within the aims and scopes of Journal. Nevertheless, the following remarks should be addressed before publication.

Specific comments:
In order to improve the manuscript, a few minor remarks are suggested before the paper is to be accepted:
1) The introduction needs to be better organized in order to show the new contribution of the paper. Please clearly define a novelty statement in the introduction.
2) The introduction section should be extended with the highlight of the practical usage of the investigated subject matter.
3) The major remark is the lack of qualitative/quantitative comparison with experimental data. No experimental work to verify the numerical results. How the proposed computational method was validated?
4) Commercial software was used. Is there a novel mathematical model implemented?
5) Manuscript preparation doesn't meet the requirements of the journal, please check the requirements in the "guide for authors" and correct it following the instructions.

Author Response

Attached please find our Rebuttal.

Author Response File: Author Response.docx

Reviewer 2 Report

The manuscript describes a comparative study of the thermal efficiency among three different types of pipe flow models through numerical simulations. Although the manuscript is well-structured, some comments are given below to aid the authors to enrich the overall quality of the manuscript:

 

• Please state in the text the finite element spaces for the velocities and pressure. If they are equal, also indicate the stabilization scheme selected in the software.
• Please also state in the text the boundary conditions at the outlet face. In addition, make a comment on the differences in the results of an applied velocity on the inlet face instead the mass flow rate.
• A conjugate heat transfer model has been employed to take into account the heat conduction at the copper wall. Please make a comment on the influence of the heat conduction in the results. Can such a heat conduction model be neglected at the wall?.
• Due to the high Peclet number, please state in the text the stabilization scheme selected in the software for the energy equation.
• Please insert the temperature contour plot over a plane at the heated section.
• The friction factor in fully developed laminar flow depends only on the Reynolds number and, thus, it is a constant. However, in fig. 8(a) for example, one can observe a variation along the pipe length. Maybe such a graphic should describe the pressure drop.
• Finally, please carefully proof-read spell check to eliminate grammatical errors.

Author Response

Attached please find our Rebuttal.

Author Response File: Author Response.docx

Reviewer 3 Report

The manuscript presents a study focusing on the thermal performance of a circular pipe with two types of inserts filled with nanofluids. The authors discuss the influencing factors and thermal properties of the pipes. The manuscript starts with introduction on previous work, experimental and theoretical, on the thermal properties of nanofluids, important parameters, and thermal efficiency of various configurations. The problem is clearly described and the proposed finite element model is well presented. The results and discussion is clear, which leads to conclusive points that summarize the results well. However, the authors should address the following issues before accepted by the journal:
1. The authors need to re-write the introduction part. Instead of using each paragraph to discuss one specific previous study, the authors should group the previous studies by topic or findings. For example, the authors can separate previous studies into experimental work, numerical studies, and those covering both. More concise wording is needed when introducing these work.
2. In the results and discussion part, the authors should compare the current work with previous work, particularly those that have been cited in the introduction part.
3. The authors need to discuss more recent literature and its findings. For example, the references below should be discussed and compared with other previous work, and with current work in the results and discussion part:
Physics Letters A (2020): 126500.
Applied Thermal Engineering 168 (2020): 114843.
Renewable Energy 146 (2020): 2316-2329.

Author Response

Attached please find our Rebuttal.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I acknowledge the replies from the authors and I accept the detailed explanations to the main comments. The manuscript has been improved and all suggestions were taken into account in the revised version. In my opinion, the article may be accepted in the present form.

Author Response

thank you

Reviewer 2 Report

• Please insert in fig. 4(a) also the temperature variation at the inner wall in order to compare to that of the outer wall and, thus, analyze the influence of the heat conduction in the wall.
• Concerning the friction factor, the authors stated that the velocity is a function of the pipe length (z). The friction factor given by Eq. (12) is originated from a balance of forces, which means that the advective acceleration is zero. In addition, the velocity in such an expression should be the mean velocity, which is constant. Please clarify this issue.
• Please explain the difference in the flow development of applying a velocity profile at the inlet face as a boundary condition rather than the mass flow rate.

Author Response

Thank you for being patient

Reviewer 3 Report

The introduction part still needs improvement. There should be a clear and logic flow of briefing previous studies. Sorted by different groups of authors are not good enough. There needs to be more summarizing sentences to connect between paragraphs. 

Author Response

Thank you for your comments

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

Accept in present form.

Reviewer 3 Report

can be accepted.