Overall statement:

The review paper presents a very important topic which is the stability of nanofluids’ used in heat pipes. The justification for the review is provided initially in the abstract of the paper; inconsistencies associated with findings due to nanofluid stability issues which occur due to operation cycles.

In the review, the authors presented preparation methods, stability issues, surfactant effect, stability after operation cycles, performance of heat pipe when using nanofluids as working fluids and the associated heat transfer mechanisms when using nanofluids as working fluids.

The strengths

1) The paper is well structured and presents critical aspects associated with the topic of the review. 2) The introduction presents a good introduction, and research background, to the topic. 3) The paper provides a good introduction, showing concept, components and application, to heat pipes and two-phase closed thermosyphon (refer to page 2 column 1). 4) Comparison tables are well-produced and provided to aid the objective of summarizing the experiments reviewed from different references. 5) The references are relevant to the topic and are relatively up to date (relative to the date of the paper’s publication, 2018).

Weaknesses

1. The authors claimed “Therefore, in the two-step method, the use of chemical additives (such as surfactants) or nanoparticle functionalization to minimize nanoparticle agglomeration and improve nanofluid stability is a common practice [39].” They cited a study that support the claim as an example. However, it is preferable to cite another review paper that supports the claim entirely, as in the claim ‘a common practice’.
2. The review mentions the effect of pH value on stability of nanofluid but does not show to what extent or highlight the impact of pH, which I believe is very important.

Concluding remarks:

A paragraph is provided in the introduction section, which I believe represents both problem statement and conclusion of the study which is:

“The field of nanofluids is still under development, and further experiments and simulations need to be conducted in order to quantify and validate the effectiveness of nanofluids in different thermal devices such as heat pipes”.

Moreover, the main reason for the heat pipe performance is a porous layer which is formed on the evaporator surface during cycle operation, and the review recommended additional research to understand the concepts surrounding the porous layer.

Recommendations for future work

In my opinion, I recommend including the effect of dispersant on nanofluid stability and differentiate between dispersant and surfactant. Moreover, to study the impact of pH value specifically on the stability of nanofluids.

• This is a post-publication review. The adherence to journal guidelines is not considered as the paper has already been processed by the editorial team of the journal