Pressure Analysis in the Draft Tube of a Pump-Turbine under Steady and Transient Conditions

Round 1

Reviewer 1 Report

Please fine the attached file.

Comments for author File: Comments.pdf

Author Response

To reviewer 1 Dear Sir/Madam, Thank you very much for the detailed comments. Your suggestions for this paper were quite good. According to the results and analysis, the content was re-structured to make them more clearly. Besides, I revised the title to “Pressure analysis in the draft tube of a pump-turbine under steady and transient conditions” which is more accurate overview of the content. In this revised paper, several important papers have been added in the literature review. The further explanations on the simulated cases were complemented. A check had been done for English grammar, expression problems. Once again, thank you for revising my article so carefully, which made this paper more professional. Sincerely yours, Jing Yang

Reviewer 2 Report

Dear authors of the paper "Prediction of cavitation in draft tube of pump turbine based on pressure pulsation". Please find attached a few recommendations in order to improve the paper quality:

• first of all, I recommend that the title of the paper should be: "Prediction of cavitation in the draft tube of pump-turbine based on pressure pulsation".
• Abstract: please verify the text, there are some mistakes: "... inside the draft tube , One of the advantages ...".
• I suggest reading the entire paper, there are too many mistakes in the paper. There are phrases without sense, the grammar, and the punctuation has serious problems.
• please add a picture or sketch of the turbine with the main dimensions presented on lines 135-139.
•  for figure 2 - the dimensions cannot be seen.
• a main discussion regarding the paper is that from the beginning in the introduction is discussed the formation of the vortex rope and the pressure pulsations effects and after the cavitation. The apparition of the vortex rope is connected by the operating regime and appears in the draft tube cone with pressure pulsations. If you discuss cavitation you can look at the runner blades - there is a problem with cavitation, not the draft tube cone.
• all the results present the registered pressure only. From the absolute values, you cannot analyze the cavitation as the title of the paper suggests.
• in the operation of a hydraulic turbine or the pump-turbine, the cavitation occurs in the operating regimes, and on these regimes, the paper should focus. 

Author Response

To reviewer 2 Dear Sir/Madam, Thank you very much for the detailed comments. Your suggestions for this paper were quite good. According to the results and analysis, the content was re-structured to make them more clearly. Besides, I revised the title to “Pressure analysis in the draft tube of a pump-turbine under steady and transient conditions” which is more accurate overview of the content. In this revised paper, several important papers have been added in the literature review. The further explanations on the simulated cases were complemented. A check had been done for English grammar, expression problems. Once again, thank you for revising my article so carefully, which made this paper more professional. Sincerely yours, Jing Yang

Round 2

Reviewer 1 Report

The quality of the revised manuscript has improved and thus I now recommend that it is acceptable for publication in Energies journal.

Reviewer 2 Report

Dear authors, thank you for the modification of the paper.

In my opinion it can be  accepted in present form.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Review of energies-1171744: Prediction of pressure fluctuation and cavitation in draft tube of pump turbine

 

I think that the quality of the work have to be improved:

1) Explain better the CFD model used to simulate cavitation. I would also suggest to mention advanced models employed to simulate cavitation as the Lattice Boltzmann Method described in the following work:

Kähler, G., Bonelli, F., Gonnella, G., Lamura, A, Cavitation inception of a van der Waals fluid at a sack-wall obstacle Physics of Fluids 27, 123307 (2015); doi: http://dx.doi.org/10.1063/1.4937595

2) Provide more details on the numerical method employed: space and time discretization etc.

3)  Improve  figures quality and description: for example it is not clear to me the meaning of 0.25T, 0.5T etc. in figures 3 and 4; provide a reference to planes that are given on x-axis in figure 5. Without a better description of this figures it is very difficult for me to evaluate the paper.

Author Response

Dear reviewer,

Thank you very much for your suggestion. I have revised the article according to your suggestion.

F I have supplemented my calculation method and model parameters in detail according to your suggestions

I have also corrected other problems in the article for your review. Thank you very much for your careful review and professional comments.

Sincerely

Reviewer 2 Report

The Authors investigated a parameter, which is important for operation, which is the formation of cavitation. The results described in the article show that the Authors are experts with the operating principles of water turbines. The processing of the literature background is thorough, the cited articles are up-to-date. However, I have a high number of comments to make about the structure of the articles. If they are repaired, the article can achieve a quality improvement.

First of all, the structure of the article. In my opinion, the five pages of the literature background, without any drawing, figures or tables are too much for the article. In engineering field, a figure, a graph or a table could tell more information, and could tell more precisely the information for other researchers or engineers, than more pages text.

Secondly, the Authors write about the comparison of one- and three-dimensional numerical analysis. How can I imagine the one-dimensional analysis at this case? You should take figures in the article about your investigation and about the literature too. In Page 4, the for the comparisons should be use tables also.

In Page 6, Figure 2 the Authors show the place of planes and points. Is this figure related to the fluid domain shown in Figure 1? Figure 2 is not correct in this form. You have to show the dimensions of the draft tube, and the space between the different planes. Without specifications this is not a scientific paper.

In Section 3, the Authors described three equations, which means all the equations in the full article. But I do not now, what is the meaning of the n, the D, the M and the H. Please specify them. The situation is similar in Figure 3 and Figure 4; I do not know what does the T mean.

In Section 4, there are schematic drawing about the turbine. Firstly, in engineering practice, this kind of drawing called schematic drawing, not line diagram. Secondly, there is no reference to Figure 6 and Figure 7. Thirdly, you write the dimensions in text, which is not so elegant and accurate. Please improve the drawing and make the references. This part is too small part compared to the introduction, but this should be the most important part of the article. Moreover, which calculation method did you use for these results? How can I imagine the one-dimensional modelling at this case? From Figure 8 to Figure 11, you show the comparison of simulation and the experiment. My questions are: what kind of simulation and what kind of experiment? If you take experimental investigations, please take photos about the measurement. In Figure 12, the average of what is represented?

• Hereinafter, I would like to speak about the formatting. There are lot of mistypes and mistakes in the text. Without claiming completeness, the most important are the following:
• there are multiple sentences without a closing sentence (Line 46, 89, 105, 253)
• in my opinion, in English the structure “larger than that of…” is not correct (Line 47, 51, 131)
• the citations have lot of mistakes too. For example, in Line 89, the correct method is: Zhang et al. [8] and Liang et al. [10]… There are mistakes in Line 126, 151, 154, 168, 191, 194, 207. Please use the international form.
• the next are the signs of dimensions. The correct method is: value space sign of dimension (for example 50 m/s). You write these without space, wrong sign of dimensions and any other variation (Line 186, 187, 239, 240, 270, 268, 285, 345, 348, 350, 351, 369, 373, 388392, 395, 399)
• in the name of the Section 2, there are two points, and in 2.1. the research is small r, instead of R.
• in Line 266, not SST k-W, is SST k-omega modell
• you use the reference the monitoring point with P1-1, p1, plane7, pane1-5. Please unify them.
• the names of the vertical axes are unreadable.
• there are mistakes in the reference list too. In the name of a publication, we cannot use et al in case of the authors, we have to write everybody’s name. And there are publications, where are missing details

If you correct your mistakes according to the comments, this could be a very valuable publication.

Comments for author File: Comments.pdf

Author Response

Dear reviewer,

Thank you very much for your suggestion. I have revised the article according to your suggestion.

First of all, I have simplified background of the paper.

As for the meaning of n, D, M and H, n is working rotation speed, D is impeller diameter, M is working torque and H is working head. All of the parameters are actual values which are relative to the unit parameters. These parameters are familiar to people who study on hydraulic machine , sorry not explain them clearly. T is the rotation period of impeller. I add the description of draft tube size.

About your doubt of calculation methods, in this paper, I used the method of one-dimensional and three-dimensional coupling to reduce the calculation amount and used the result of one-dimensional calculation as the boundary condition of three-dimensional calculation to analyze the internal flow.

I also tried my best to correct the questions you raised about the format. I have also corrected other problems in the article for your review. Thank you very much for your careful review and professional comments.

Sincerely

Reviewer 3 Report

Dear Authors

In my opinion I find your paper interesting and worth investigating. However, I have also found a huge number of inaccuracies and ambiguities, so I presented it in the original pdf file. Please, see attachment (if you have not received it, please contact Editor). When you answer all questions and correct shortcomings, please write explanations in text as much and as clear as possible.

Below, I present only general remarks as the most important and critical points concerning the quality of paper.

Keywords. I suggest to change ‘installation elevation’ for term, which is more commonly understood for readers. Now it is not understood even after reading the paper.

General comment. Paper contains a lot of sentences in which the dots are missing at their ends. There are also a few more mistakes and inaccuracies. Additionally, the Authors use small first letters of names. It should be corrected for respect of cited Authors. For details, please see original submitted and reviewed file.

Further on, paper contains some careless writing of text, e.g. velocity in ‘M /s’ or ‘m / S’, moment in ‘N M’, pressure in ‘pa’, water level in ‘M’, names of subchapters with small first letters. Correcting such inaccuracies was quite annoying. This is negligible mistakes for the merit content of the paper but imply for the readers a nonchalance of the writers. My advice is to try to avoid it in the future for being taken seriously and respected. Once again for details, please see your original submitted and reviewed file. As I have read, prof. Zhengwei Wang has been a supervisor and sponsor of the paper, so I would like to address another writing Authors – prof. Wang is the famous researcher in the world from renowned research centre, so please keep the quality of your papers for yourselves and for him.

Chapter ‘Research object’.

1. Authors should use instead of description of water levels the range of heads that are encountered in power plant acting on the hydro unit. It is more impressive for each reader/researcher. The mentioned descriptions can be left as is but please insert also heads.
2. The information about research object is very poor. I think Authors should present meridional cross-section of hydro unit under current view. Main dimensions should be presented in figure and/or in text (runner diameter, diameters of inlet and outlet of spiral case and draft tube).
3. Nothing is mentioned about specific speed of unit (together with definition) and the elevation of runner according to tailwater level. Generally the reader nothing can compare to other similar machines. As we all know Francis turbines or pumpturbines are widely studied for a long time and there are a lot of research studies concerning similar units. How to estimate Authors’ results when poor description is presented? How would you have written your Introduction if other Authors had not presented details about their research objects? If you want to be cited it is important and your business to do it as the best as you can do it.
4. Computational domain and Figure In my opinion (gained from my own experience) the domain of the presented pumpturbine should also include pre-channel before current inlet (inlet to spiral case, volute) and post-channel behind current outlet (draft tube outlet). That would shift the influence of boundary conditions, especially at outlet, which is outlet of important part of machine that is draft tube (just investigated in paper). The boundary conditions are ideal (no boundary layer and other phenomena), that means artificial conditions influencing flow field (forcing no vortices, no secondary flows at inlet etc.). In such a case it is extremely important to add sentences that explain that some flow effects may not have been modelled properly. You should add it. Authors must inform the readers of deficiencies of simulations. Domain should be justified. Fortunately for Authors, the investigated Francis pumpturbine, as I see, is the very high-head machine even for Francis applications, which means that influence of error resulting from applying ideal BC close to inlet and outlet is of a lower importance.
5. Please provide e.g. in table number of elements for the main parts of turbine (spiral case, stationary and non-stationary guide vanes, runner, draft tube). Use a percentage referring to total number of grid elements.

Figure 1. Firstly, it should be enlarged as much as possible (in MDPI there is no restriction on the length of the papers). Secondly, and more important, is that references 2 and 4 indicate the same part that is runner. Pointer 2 should show fixed guide vanes, which are not presented at all in this figure. Should be carefully corrected. Additionally, the caption contains twice used word ‘vane’. It should be used ‘vanes’.

L249. What does ‘grid quality is greater than 0.4’ mean? Should be explained in text more clearly.

L268. 3540 Pa. This value corresponds to temperature of 25degC (298K). This is a quite high temperature value (river in warm region in China?). I hope Authors are aware of this. In my opinion it is too high temperature, which is default taken to simulations (try avoiding default values), and which may influence the results as vapour pressure significantly lowers when temperature decreases few degrees. Now, the conclusions may be more critical than it is in reality. Please refer to that.

Numerical calculations. Solving program is not mentioned (for meshing it was done so why is not it done for simulations?). Additional information is also poor. What was discretization model used? Zwart-Gerber-Belamri cavitation model contains more boundary conditions than Schnerr-Sauer model. They should be written down in text. The strategy of simulation is not written. How many iterations were used?

L274. “… the pressure values of the volute inlet and draft tube outlet are given at a certain time”. What does ‘a certain time’ mean?

L277. I see missing verb in the sentence and dot before.

L285. “The unit parameters Q11 and M11 corresponding to the test opening are obtained by interpolating the model test data of the unit according to the calculated opening.” This sentence requires more explanation in text. What is ‘calculated opening’? This is not clear.

Table 2. Add percentage of errors to table.

Figure 3. What do ‘0T’ and three other denotations mean? They are not referenced in text. Is it time (here are steady state calculations)? Should be added. In my opinion all pictures should be one under another and enlarged to margins.

Figure 3. Additionally, the presented results indicate strong stochastic type of flow in draft tube, probably due to full load of machine it was not converged sufficiently. Please justify your simulations. In my opinion Authors should discuss the results more intensively in paper. I can also see the influence of ideal boundary condition of pressure at outlet (this is what I mentioned above about ideal BC). Please refer to that.

Figure 4. It is not referenced in text. Should be added and results discussed precisely. Additionally, pictures should be enlarged and placed two under another two for more clarification.

L343-L353 and Fig.6 and 7. This paragraph and corresponding figures should be moved up to ‘Research object’ subchapter. At last the Authors have presented some information about hydrounit. Figure 7 should be named 6b and fig. 6 to 6a.

L364. What does ‘the node water head’ mean?

L368. Why do Authors use term ‘hysteresis’ here? Not understood. Hysteresis means something else.

L371. “The outlet pressure of the ball valve ... reaches ... 446.895m.” Completely not understood. Should be re-arranged.

L397. “than – 2 to 2m”. Not understood.

Figure 8. What does a term ‘relative opening degree’ mean here? Explain in text.

Conclusions. Be precise what was done. Add that in steady state condition 3D CFD aproach was used and in transient 1D aproach was used. Expand text according to it.

References. They should be unified, especially names of Authors (whole name or only first letters?).

Comments for author File: Comments.pdf

Author Response

Dear reviewer, Thank you very much for your suggestion. I have revised the article according to your suggestion. As for your doubt of the temperature value, the station is located on the south area of China, and in the future study, I'm going to think a little bit more carefully about the values. In ‘The grid quality is greater than 0.4’, 0.4 Refers to the evaluation factor used to check grid quality in ICEM. Detailed description of the calculation method is given in 2.2. I have supplemented the details of mesh according to your suggestions I also try my best to modify the relevant formatting issues. I have also corrected other problems in the article for your review. Thank you very much for your careful review and professional comments. Sincerely

Round 2

Reviewer 1 Report

The authors have addressed all my comments.

Author Response

I have completed the reviewer's comments.

Reviewer 2 Report

Compared to the previous version, changes have been made in the article, mainly in the introduction. However, many of the errors in my previous criticism have not been corrected. For example:

From Page 5: the value unit of measurement is still not standardized (with space, without space, meter sign with capital M.

Page 7: There are no dimension in the schematic drawing, and it is called still line diagram.

Also Page 7: There are still no information about the positions of the investigated planes (diameter, distance to each other).

From Figure 7, the label of the y axis is not readable.

The citations in the text still do not follow the standards.

I still could not find any information on what equipment was used to carry out the experiments, the results of the experiments were compared with the numerical results.

On this basis, I do not feel that the article meets the basic criteria for a scientific publication.

Author Response

Dear Reviewer,

Thanks for your suggestion. I have tried my best to modify the article according to your suggestions. The modifications and explanations are as follows:

From Page 5: the value unit of measurement is still not standardized (with space, without space, meter sign with capital M.

(It has already modified.)

Page 7: There are no dimension in the schematic drawing, and it is called still line diagram.Also Page 7: There are still no information about the positions of the investigated planes (diameter, distance to each other).

(It has already modified.)

From Figure 7, the label of the y axis is not readable.

(It may be the problem of my submission of documents, and I have corrected it.)

The citations in the text still do not follow the standards.

(It has already modified.)

I still could not find any information on what equipment was used to carry out the experiments, the results of the experiments were compared with the numerical results.

(In this paper, the three-dimensional calculation of steady-state condition is compared with the unit parameter of the experimental data, and the one-dimensional calculation is compared with the experimental results.)

I hope you are satisfied with the modifications and explanations. If there are any suggestions, I will make further corrections.

Reviewer 3 Report

Dear Authors

The paper still has many shortcomings that need to be corrected. Due to the huge number of changes the revised version is completely unclear, so please provide except revised (marked up) version also clear version.

Please, see attachment (if you have not received it, please contact Editor). Below I present general remarks as the most important and critical points concerning the quality of paper.

Generally you have not answered many questions regarding my last review. Paper is corrected with a dose of nonchalance. Please provide answers and corrections. When you are unable to do it, please state it clearly. Some of questions I repeat once again.

General comment to ‘Introduction’. I had wondered why so much text is going to be removed from Introduction until I read the suggestion of the one of the Reviewer. Generally, it is a pity that so much text is going to be deleted but I respect the decision of the Reviewer. However, consider if as much text has to be deleted.

General comment. Do not use spaces in units e.g.: ‘m / s’, ‘N m’. Should be ‘m/s’, ‘Nm’.

Why a word ‘runner’ is changed to ‘impeller’? The original text was good.

Fig. 1. The arrows are shifted. Now it is not understood.

Fig. 2. The caption ‘Single line diagram’ sounds weird. My proposal is: 'Views of the volute (a) and the draft tube (b)'. I suggest to add main dimensions to drawings.

Formula (2). Why good formula is going to be deleted and instead of it, the wrong one is inserted (missing nominator)?

Why is not presented meridional cross-section of hydro unit? If you are unable to do it, then state it clearly.

What is the specific speed of machine? I see all values are in text so I think it can be given directly.

L327. “…T means a rotation period of impeller.” Authors use steady-state condition in this chapter, which means that runner domain is standing still in calculations. So what does period mean here? Explain in text as much as possible.

L336. The tailwater level I still not understood. Is runner above or under the written down (5.7m) value?

I still do not see the boundary conditions for Zwart-Gerber-Belamri cavitation model e.g.: bubble diameter, nucleation site volume fraction, evaporation/condensation coefficients. They should be written down in text. The strategy of simulation is not written. How many iterations were used? What was discretization model used in calculations? More information is still needed.

Figure 4. Please justify your simulations. In my opinion Authors should discuss the results more intensively in paper. Please refer to that.

Chapter 4. Why is nothing written about the measurement device used in pumpturbine investigations? Should be added.

L402. Why do Authors use term ‘hysteresis’ here? Not understood. Hysteresis means something else.

L405. “The outlet pressure of the ball valve ... reaches ... 446.895m.” Pressure in metres? Should be re-arranged.

Figure 8. What does a term ‘relative opening degree’ mean here? Explain in text.

Comments for author File: Comments.pdf

Author Response

My reply is in the attached Word file.

Author Response File: Author Response.docx

Round 3

Reviewer 2 Report

As I wrote in the previous review, the biggest problem is with the model and the presentation of the results. Even after my many reactions, I cannot see the size of the model, the position of the planes being tested, where the results are displayed. Without these, researchers are not provided with any information content.

Reviewer 3 Report

Dear Authors

I asked you about cross-section of turbine under Fig.1. You presented it in cover letter. That’s good but why is not it introduced to paper? Main dimensions should be attached.

In my previous review I asked about: ‘The tailwater level I still do not understood. Is runner above or under the written down (5.7m) value?’ The given answer (‘It refers to the installation elevation of draft tube, that is, the elevation of draft tube center line.) still does not explain it. It is important for cavitational danger whether runner is under or above tailwater level. It is not explained.

In my previous review I asked about: ‘Why do Authors use term ‘hysteresis’ here?’ I still think the term hysteresis is not used properly here and the answer does not explain clearly what Authors meant using term ‘hysteresis’. In my opinion it should be clearly explained and stated in text.

My advice for the future. When you answer the Reviewers’ questions. Please do not use only statements e.g. ‘It has already modified’ or similar. Explain clearly and point the place in text where the new text is introduced. This helps find answers quicker.

According to current version of paper please see comments below.

Introduction. When citing multi-author paper citation should be e.g.: ‘Li ZJ et al. [5]’. Now in many places is e.g. : ‘Li ZJ [5] et al.’. When citing 2-author paper should be e.g.: Ruprecht and Helmrich [10]. Now is Ruprecht [10]. These all should be corrected.

I asked you in my first review about specific speed value and its definition. Now is only value given but the definition should also be added to compare it with the other similar machines and analysis results. The reason is that different definitions are used in the research centres.

You have not justified your simulations. It was pointed in last review. It is important.

L154. “The detail statics…”. What is this? In my review it was pointed to be corrected to “The detailed statistics...”. Why is not it corrected?

Table1. Caption - the as above. “Detail statics”? It was pointed to be corrected as above. Why is not it corrected?

Table1. “unstrutured”? It was pointed to be corrected: “unstructured”. It is not.

The numeration of formulas in chapter 2.2 is not adequate to other chapters.

Table 4. Caption is not corrected: “laod tuebine” It was pointed to be corrected. ” It is not.

Dear Authors. As you can see I still see a lot of ambiguity in your paper sometimes important and sometimes small. I think the problem is that the only one Author corrects the text. Over and over again the same errors are indicated as not being corrected. He just needs help. It is already tiring and even annoying to read the same uncorrected text a third time and wait for the next version again. All authors should read each revision of the text and reviewers' comments and together try to answer. Explanations should be integrated into the text so that readers' doubts are also reduced. I suggest to do it with your paper, which I believe is valuable. Now I am recommending you to submit the paper once again to 'Energies' with carefully corrected all problem pointed by Reviewers from the first Reviews.

With best wishes