Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this study, the two-phase friction pressure drop during boiling and condensation in horizontal microchannels was comprehensively experimentally studied, and its effect on the energy efficiency of steam compression system was emphasized.

This is a classical fluid flow topic and fits the scope of journal Energies.

This is a huge experimental fluid dynamics work, I appreciate the work in this paper.

However, the key issue is the novelty of this study should be addressed. The introduction section 1 and 2 can be merged, the research status on this micro channel fluid flow should be addressed.

In line 49, typically the “published in Energies” can be deleted. Although this is the identical journal that the authors submitted. As well in line 231.

Try to avoid just list the reference result in section 1 and 2. Highlight the relationship with this present study.

There are many key information are referred to the authors’ previous work in ref 18 to 20. However, as an independent paper, the authors are required to present the details in this paper. And clarify the difference between the published papers, this is to avoid duplicate publications. After reading the reference 18 to 20, it seems this current manuscript covers more data then the previous papers.

Besides, the fluid properties of the different refrigerants should be given.

The unit in table 3should be given, is it percentage?

The descriptions of Reynolds number, Weber number, and the Lockhart–Martinelli number should be presented in an earlier part before the regression of formula 3 and 5.

Besides, it is noted that the (1-x) are appears in the equations of dimensionless numbers, this should be explained.

Only the predicted pressure gradient are shown in figure 20, how about the other parameters? For example, friction factor?

Overall this manuscript is well organized and can be accepted after minor revisions.

 

Author Response

Dear Reviewer,

Please check the attached file. 

Regards,

Authors

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper provides empirical data for frictional pressure drop during boiling and condensation of six low-GWP refrigerants in horizontal microchannels. The data is relatively extensive, covering a wide range of fluids, saturation conditions, flow rates, qualities and tube types. Results are qualitatively consistent with expected behavior and are of quantitative significance to the field. A new correlation is developed using the data and presented as an improvement over existing correlations for the specific fluids and application considered.

The paper certainly contains valuable information which, in principle, justifies publication in a scientific journal. I have several comments and questions that I feel need to be addressed before I can recommend the article for acceptance.

1. The earlier sections of the article, notably between sections 1 and 2, contain a significant amount of repetition regarding the pros and cons of minichannels (increased heat transfer vs pressure drop). For example, the entire first paragraph of section 2 (and much of the final paragraph of section 2) is effectively repetition of information provided in section 1. The authors should please review these introductory chapters and try to reduce the amount of repetition to keep the paper more concise.
2. Section 2 discusses papers [9] – [17] in quite a lot of detail but does not contain a good synthesis. I encourage the authors to attempt to reduce the amount of detail per paper (through critical reflection on what information really needs to be included here), and rather to place emphasis on critical analysis and synthesis of the literature. It is important that the specific gap addressed in this work be clearly identified in the discussion of the literature in this section.
3. Please quantify the test section length(s) for the experiments. Table 1 gives several dimensions but nothing that can be used to determine length.
4. Are the pressure transducer accuracies given on lines 351 and 399 / 400 percentage of measurement value or percentage of transducer range? Please qualify.
5. Line 356 – “minimal fluctuations” for all recorded variables is mentioned as the check for steady state and therefore data acquisition. Please quantify the stability criterion. How is minimal defined?
6. Table 3 – are the provided uncertainties percentages? Please clarify or add units if needed. If these are percentages, the uncertainty on dp/dz is quite large (>13% max. for all fluids). How many of the data points are subject to such large uncertainty? Understanding this is essential in the evaluation of the various empirical equations later. I admit that showing error bars on the figures in the results would be too noisy. Perhaps a histogram of uncertainty distribution would be valuable?
7. The data analysis considers pressure gradient (Eq. 1 and 2) but gradient is not measured, change is measured with a differential transducer. I assume then that the reported gradient is (P_i – P_o)/L (where L is the test section length). This is therefore the average gradient across the test section.
8. Following from (7) – results are given as dp/dz vs quality but quality must change over the test section (otherwise values are for two-phase flow but not boiling / condensation). For the condensation tests, there is some discussion on the change in quality but nothing for the boiling case. Are these qualities inlet values or mean values? By how much does quality change over the tests? If quality is changing meaningfully then dp/dz is unlikely to be constant over the test section and the use of an average value vs quality becomes problematic. Please clarify.
9. The authors claim “significantly improved prediction” with their correlation but it doesn’t seem that it offers significantly better correlation than Muller Steingahen and Heck’s correlation (Fig. 18). To better judge the improvement, R^2 (or RMSE or some other quantitative metric) needs to be provided for the existing correlations as well (as in Fig. 20). To me it seems MS&H also achieves +-20% accuracy and is therefore as good as the new correlation (line 591).

Minor items (incl. editorial)

1. Lines 60 – 65: the two sentences in “For instance, Xu et al……and COP calculations.” Don’t make sense to me. Please consider rewriting.
2. Please pay attention to superscripts in units.
   1. Line 255 - “800 mg/m2s” – 2 should be superscript. Multiple places.
   2. Line 510 – including exponent “1” isn’t necessary. Multiple places.
3. Line 329 - the sentence starting on this line seems to be incorrect. It seems like possibly two sentences have been combined somehow with missing text?
4. Line 362 – missing word “variables” (or similar) at the end of the sentence. Please correct.
5. It seems like subheadings have been left in or not formatted as such in the discussion of the results. Start of lines 470, 479 and 488. Please correct.
6. The captions for Figs 4 – 6 should be improved to be more descriptive (later figures e.g. Fig 7 are better). Please consider specifying boiling / condensation in the captions as well for clarity.
7. Line 490 – “Friede” should be “Friedel”
8. Line 511 - please correct “propnae".
9. Line 549 – “Raynolds numer” should be “Reynolds number”.
10. 3 - please qualify if Re is Re_liq, Re_vap or Re for a homogenous mixture.
11. 5
    1. Is X and X_Martinelli the same? Please consolidate.
    2. Fr_2ph and N_conf need to be defined.
    3. Weber number should be “We” as per nomenclature, not “Weber”. Applies after Eq. 5 too.
12. 9 – please “Re” for Reynolds number.
13. The discussion on “design implications” (lines 479 – 493) includes comparison of boiling results to condensation results and to existing correlations. Condensation results etc. have not yet been presented. This section should be moved to later in the paper where the statements are supported by the preceding results.

Author Response

Dear Reviewer,

Please check the attached file. 

Regards,

Authors

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors, below I provide some comments that may help you improve your manuscript to meet the scientific standards required by the journal.

 

This work presents a large experimental database (3,553 points) for boiling and condensation in minichannels (Dh = 0.715 and 1.16 mm) with six refrigerants, and proposes a new empirical correlation for the frictional pressure gradient based on dimensionless groups (Re, We, Lockhart–Martinelli) and a specific friction factor. The experimental setup, sensors, and operation protocol are well described; there is a comparison with classical models (Cavallini, Müller-Steinhagen & Heck, Friedel) and a declared validation of ±20 % with R²≈0.99 for the new correlation.

(L25–28): It is claimed that the reduction of losse “contributes to lowering compressor workload and improving COP”. I suggest adding that the actual impact on COP depends on the hydraulic coupling with the compression cycle (effective pressure ratio). A quantitative example would strengthen this conclusion.

 

L41–47: It is explained that minichannels offer improved heat transfer “but at the cost of elevated pressure losses,” which is correct. However, L96–97 states that reduced geometry implies “reduced pressure drop,” which is contradictory. I suggest correcting this: in smaller channels, friction per unit length increases, although the overall drop can be reduced if the length is shortened.

 

L98–113: Good description of the interaction between ∆p and the transfer coefficient. I suggest reinforcing the thermodynamic concept of the “trade-off” between transfer efficiency and exergy losses.

 

L114–126: Visual and numerical studies are cited. It could be added here that flow instability (slug/annular) also impacts the irreversibility of the process and, therefore, the cycle efficiency.

 

L143–169: The reviewed correlations focus on the effects of G, q", and Tsat. It would be useful to include a physical discussion of the limits of validity of conventional correlations (developed for larger channels) when applied to minichannels, where confinement numbers (Nconf) are relevant.

 

L231–244: Kuczyński's model incorporates dimensionless analysis. I suggest emphasizing that including variables such as the Froude number or confinement would be more physically appropriate than extrapolating classical correlations from macrosystems.

 

L338–344: Good temperature instrumentation. I recommend showing propagation uncertainty toward x and (dp/dz)f.

 

L380–381: It is stated that x “always remains below 0.08.” This contradicts Table 2 (x up to 0.92). It needs to be clarified: is this for a subset of conditions?

 

L449–454: Correct description of the increase in ∆p with x. I suggest discussing how this increase is linked to the increase in entropy generated by friction (thermodynamic irreversibility).

L549: Typo: “Raynolds number” should be corrected to Reynolds number.

 

L547–552: The inclusion of Re, We, and X is physically justified. A discussion of dimensionality is missing (to ensure homogeneity of units).

 

L576–578: It is stated that boiling requires more critical design than condensation. It would be desirable to quantify with an example how much the auxiliary power required increases in a reference cycle.

 

L598–603: Good proposal for future work. I suggest explicitly adding the extension to thermodynamic criteria such as ecological function or efficient power.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English is generally good and understandable, suitable for Energies.

Needed improvements:

“Raynolds numer”, change to Reynolds number.

“the the” (double article in L318).

“dala logger” (L382) change to data logger.

Review consistency in use of “pressure drop” vs. “pressure gradient” (sometimes interchanged).

Long sentences (L33–38; L91–97) could be split for clarity.

Author Response

Dear Reviewer,

Please check the attached file. 

Regards,

Authors

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for the constructive responses to my feedback. All of my comments and concerns have been well addressed.