Two-Phase Annular Flow in Vertical Pipes: A Critical Review of Current Research Techniques and Progress
Round 1
Reviewer 1 Report
At first, this is a very good review paper, which has made substantial effort and contribution to this topic. I would like to have following comments/suggestions to improve the paper quality/completeness:
1. Despite this is a review, for readers to quickly catch your contribution, it would be better to highlight major difficulties and challenges that are commonly existent in this field, in a clearer way in abstract and introduction.
2. This review paper has included almost the key papers on this topic. But the manuscript could be substantially improved by relying and citing more on recent literature about real case studies of applications of CFD models or related topics, such as the following that can be found from online literature searching (note: these are not from reviewer's work to avoid conflict of interest):
- Pengxin Cheng, Jinsong Zhang, Nan Gui, Xingtuan Yang, Jiyuan Tu & Shengyao Jiang (2022) Numerical investigation of two-phase flow through tube bundles based on the lattice Boltzmann method, DOI: 10.1080/19942060.2022.2077835.
- Yiyi Ma, and D Z Zhu, (2021). Axis Switching of Free-Falling Elliptical Water Jets. Journal of Hydraulic Engineering 2020-07-01 146(7): 06020009. DOI: 10.1061/(ASCE)HY.1943-7900.0001772.
- Xiaofei Liu, Chengcheng Gong, Lite Zhang, Haozhe Jin & Chao Wang (2020) Numerical study of the hydrodynamic parameters influencing internal corrosion in pipelines for different elbow flow configurations, DOI: 10.1080/19942060.2019.1678524.
- Wenchao Lv, Guofu Ou, Xiaofei Liu & Chengcheng Gong (2020) Experimental and numerical studies on the corrosion properties of AISI 316L stainless steel in two-phase upward slug flows, DOI: 10.1080/19942060.2020.1780155.
- Junling He, Qingzhi Hou, Jijian Lian, Arris S. Tijsseling, Zafer Bozkus, Janek Laanearu & Lei Lin (2022) Three-dimensional CFD analysis of liquid slug acceleration and impact in a voided pipeline with end orifice, DOI: 10.1080/19942060.2022.2095440.
- Lujia Liu; Weiyun Shao; and David Z. Zhu (2019). Experimental Study on Stormwater Geyser in Vertical Shaft above Junction Chamber. Journal of Hydraulic Engineering, 146(2). doi: 10.1061/(ASCE)HY.1943-7900.0001660.
3. In the results section, an in-depth discussion should be made for enhancing the practical implication of the review results and findings from current study, which will be more useful than that for only performing the statistical data collection and analysis.
4. In the conclusion section, the limitations of current/previous studies, suggested improvements and future research direction of this topic should be further highlighted.
Author Response
Please see the attachment
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper presents a review on papers studying gas-liquid annular flow in vertical pipes. The review is split into sections devoted to “Investigation methodologies”, “Fundamental understanding of the wavy liquid film”, “Correlations of the film thickness”, “Disturbance wave characteristics”, “Mechanism of droplets”, “Droplet entrainment and void fraction”. About 250 papers are in the reference list.
The authors state in the Abstract that “there has not been comprehensive review”, especially on the latest decades. I would agree with that, but the problem is that the present review is not comprehensive either.
A comprehensive review should tell the readers in a clear and concise manner what is happening in annular flow. This should be the main purpose of the review, whilst the references only serve as the source of information, as a means of bringing knowledge to the audience.
The present review is made in an opposite manner. Any attempts to make a structured, systematic description ebb at the stage of naming the sections (this stage also causes questions).
Inside the sections, everything is a mess. Each section consists of straightforward retelling of several papers, usually without any logical “bridges” from one paper to another. Often, the description is a direct copy&paste from abstract or conclusions of the cited paper. Some papers are mentioned by wholesale (e.g. [53-72]), some have verbose descriptions. If would have been reasonable if the latter papers were especially relevant or informative, but this is not the case.
Literally every PhD student presents a literature review of similar scale and quality in his/her PhD thesis. I guess the present review is of the same origin, namely, a PhD thesis written around the border between 2020 and 2021. It is unclear why the authors think that this review should be published as a journal paper.
The only reason for not rejecting the paper is that its publication would not bring much harm. Someone new to the field might use its reference list as a first step to create his/her own literature analysis. Unfortunately, this is all good the paper can bring in its present state. I strongly recommend the authors to make an attempt to bring the paper to order and remove or edit the incorrect statements. Some advice for such a revision is given below.
1. The scope of the review is very wide: the authors aim at describing all the aspects of the flow. It also makes the review quite shallow in terms of knowledge. At the same time, the scope is limited to the flow in vertical pipes. But the flow in horizontal/inclined ducts and pipes is generally the same annular flow with qualitatively the same phenomena in continuous and dispersed phase. Could the authors justify this choice?
2. The “Methods” section should be about the purpose (measured parameters), physical principles, capabilities (resolutions, etc.), and limitations of the technique. It should not include the results: there are other sections for them.
3. Just the same, in sections 3-7 there is no need to repeatedly and verbosely describe the methods. You have done it already in section 2. Just name the method, and only if it is important for understanding the results.
4. The “Methods” should have additional subdivision on film thickness measurement techniques and droplet studies. At least, they should be presented in a consecutive order, not as a mess they are now. Visualization serves mainly for qualitative analysis, it may go first. Fig. 7 and related text should go to 2.9. Film thickness measurements (2.1-2.3, 2.5, 2.10-2.12) should go before droplet measurements (2.7-2.9, 2.13).
5. Often, there is unnecessary information. Usually, it appears due to thoughtless copy-paste from the cited paper. Please, reduce the amount of such copying to the minimum level. What is the point in specifying, e.g., pipe diameter, sampling frequency, range of conditions for particular experimental works? It can be done as a brief summary over all the papers using a particular technique, and only in form of limitations analysis, not as a thoughtlessly compiled list.
6. Selection of illustrations is also hectic and ambiguous. Fig. 1 and 8 are of the same type; why should they both be given, especially in different parts of the paper? The case with Fig. 13 and 22 is even worse: it is the same model and even the same modeled event.
7. There should not be thoughtless copy-paste of self-praising statements from the papers. Of course, the experimentalists often say that their resolution or quality or sampling frequency is high, good, the best, or outstanding. Despite those claims, the corresponding parameter is often much worse than in the competing papers even among those already published by that time. It can be forgiven to the authors of an experimental paper, but not the authors of a review paper.
8. There are only two references published in the beginning of 2021 and nothing from 2022. Why would not the authors refresh the reference list?
9. The words “non-intrusive” are mentioned too many times. Majority of the considered methods are non-intrusive. It would be better to note the intrusiveness for those techniques which have this limitation.
10. Section 2.6 contains only one reference. There are more, even among the papers cited in this review. Please make a thorough search.
11. Please, do your best to describe the references inside the sections in some logical order. There should not be sudden jumps from one parameter to another, then returning to the same parameter, then another jump in a completely different direction. The paper is full of such cases.
12. For the empirical correlations given in the form of tables, the ranges of conditions used to develop a correlation should be specified. Otherwise these data are nearly useless.
13. The reference list should have all the co-authors specified; it also must have the same formatting.
Below are some particular examples requesting corrections. Unfortunately, the list is far from complete.
L. 47. Why Prof. Hall Taylor’s co-authorship of this book is ignored?
L. 67-68. There are more reviews on methods. Ref. 53 and 135 are good examples. Please, make a more thorough search.
L. 86. This flow should be called “dispersed”; or “droplet” instead of “bubble”, at least. By the way, why is a paper on a liquid-liquid flow is cited in this review?
L. 175. Why 2009 is “recently”?
L. 187. It is Cuadros, not Cuadrosa.
L. 309-312. Why is it emphasized that PLIF technique does real-time measurements? PLIF gathers large images, and the buffer memory is usually limited. Besides, PLIF requires complex post-processing to merely extract the film thickness information. I would say that PLIF is among the worst techniques for real-time measurements.
L. 327. Ref. [98, 123-126] used BBLIF, not PLIF.
L. 348-349. Fan et al. did not use PLIF technique.
L. 354-360. In ref. 135, severe flaws of PLIF were also demonstrated. Moreover, the problems attributed to BBLIF are generally true for nearly all film thickness measurement techniques. If the authors know any film thickness measurement technique properly dealing with steep interfacial slopes and/or bubbles, they should specify them and justify these statements.
L. 463-465, 609-612. These statements are too obvious even for the present shallow review.
L. 611. Please, specify the difference between non-invasive and non-intrusive.
L. 616-618. What is “effective” viscosity? Please elaborate.
L. 621-622. “… flow conditions such as the liquid flow rate, gas-liquid, and flow directions”. What is meant by “gas-liquid” here? Physical properties of gas and liquid? If yes, please, write it properly.
L. 656-659. "In a vertical pipe with an internal diameter of 127 mm, the variation of average liquid film thickness against gas superficial velocity showed a minimum at a gas superficial velocity of 12.4 m/s, which was in close agreement with the literature [32]". Again a thoughtless copy-paste. With what literature? If this fact was already known before [32], maybe this "literature" should be cited instead?
Fig. 18 and related text: Could the authors comment why the time-averaged film thickness in a vertical pipe could become circumferentially non-uniform? Is there any physical mechanism for this? Or is it just an imperfect experiment?
L. 669-670. “The film thickness and velocity profiles within the wavy film have been successfully investigated using optical techniques [128-130]”. Does this statement mean that other optical techniques were unsuccessful? Please just remove it.
L. 773-777 and 801-804 repeat the same text.
L. 798. “Zhao et al. [31] conducted high-frequency film thickness measurements…” The frequency in this paper is 1 kHz. There any many papers with order of magnitude higher sampling frequencies. Please remove the words “high-frequency”.
L. 809. “Wave celerity increased when increasing superficial gas velocity, although this increase became slightly smaller for higher gas velocities”. This statement (especially the part on “slightly smaller for higher gas velocities”) should be justified.
L. 810. “Wave celerity increased with the decrease of liquid superficial velocity”. This statement contradicts to all available data: the real tendency is the opposite. Please, do not repeat the mistakes of [5].
L. 821. The wisps observed at large liquid flow rate are not waves. They are large lumps of liquid torn from the waves by the gas stream; rather droplets than waves. Please, remove all mentions of “wisp waves” from the paper.
L. 856. “As a source of entrainment–deposition, and eddy, the disturbance wave had significant impacts on the interfacial shear stress and the density in the gas core [66]”. This statement is unclear. What is meant by source of deposition, source of eddy? How is it justified in [66]?
L. 887. Instead of "Zhao et al." it should be “Zhang et al”.
L. 892. What is “droplet quantity flow rate”?
L. 946. “The rate of atomization was found to vary linearly with gas velocity and liquid film flow rate”. This is only true for excess liquid flow rate and only within the range of conditions used by Schadel et al.
L. 958. The expression (‘‘increasing-decreasing” void fraction trend) is uninformative. Please clarify.
L. 996-997. “… are well summarised in the current work”. Please, remove the self-praising. Just “summarized”.
L. 1000. Please replace “droplet mechanism” by “droplet entrainment/creation/formation mechanism” throughout the paper.
L. 1004-1005. “New correlations … have been proposed”. Were the new correlations proposed in this paper or in the reviewed literature? If in the literature, are they new? Compared to what?
L. 1105. “Neither the analytical expression…” What expression is meant?
L. 1107. “It should be also emphasized that the gas flow in the central core region is not an essential reequipment for annular two-phase flow in vertical pipes”. What is meant by reequipment? Requirement?
L. 1109-1111. This statement is unclear. It was not analyzed in the paper and suddenly appears in the conclusions. Either justify it or remove it.
The nomenclature section is made in a negligent manner. What is the difference between entrainment and entrainment rate? What does mean Greek letter gamma in the correlations? The non-dimensionless numbers should be accompanied by their definitions, since they may vary from one study to another.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper is a good review of two-phase flow in vertical pipes. I have the following suggestions：
(1) It will be perfect if the authors could introduce some work from their labs.
(2) In the introduction part, give a high-level workflow of how this review is organized.
Author Response
Please see the attachment
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
In fact, the comments were not well addressed.
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
Reorganization in accordance to part of the recommendations makes the review more understandable. Though it does not remove the major flaw of the review, there would not be much harm in publishing this paper.
Author Response
We appreciate your valuable comments.
