Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper is about the evaluation of the impact of different cavity flow configurations in a LPT stage. The authors use the available experimental data to validate a numerical approach for the study of the test rig. The activity is well developed and the paper is well organized. The topic is of interest for the turbomachinery community. The outcomes may provide useful information to the interested audience. However, a few modifications are necessary to reach journal-quality level. My comments follow:

- Line 54, paper [10] is cited but no outcome from that work is reported to correctly define the state-of-the art.

- Figure 2, the authors define planes (A) and (B) but they don't name the kiel probe planes, which are often used in the paper. It may be useful to give them a name and use it in the text to better refer to them.

- Line 117, the authors cite the CADENCE grid generator but they never explicitly report that they used CADENCE as CFD solver, why?

- Line 135, can you please provide more information about grid dependence? What did you use to evaluate it? Is the GCI calculated?

- Line 145-153, a bulleted list is necessary.

- Line 155-156, could you include a computational time-step or better a numerical sampling frequency to be compared with the experimental sampling freq?

- Line 170, "the diagram on the right-hand side" is not sufficiently clear, please include in Figure 4 some letters like (a) (b) (c) etc... and refer to them.

- Figure 4, data are extracted from the "green plane" (see my comment above) but to what radius? The span as in Figure 5 should be included. Also, Figures EXPERIMENTAL for MC = 0.9% and 2.7% don't show a high value for the total pressure coefficient, while CFD does but this is not commented.

- Line 177-185, a bulleted list is necessary. However, most of the comments are qualitative, a more detailed evaluation is necessary (ie., quantify the variation of radial/tangential position of the cores).

- Line 183-185,  "an enlargement ... in this region" is true only for the experimental data, while CFD tends to remain in a central position.

- Line 188-193, "In the hub ... of the blade", I don't see the effect discussed by the authors. I agree that the BL is ingested in case of no purge but it is not demonstrated by that figure.

- Figure 8, comments are correct but it seems that the authors are comparing situations with different mass flows, which explains the different Vr values. The sealing effectiveness values as defined in many papers should be used to compare the different cases.

- Lines 256-272, how is defined h2? why did you chose that height? Also, the whole discussion does not seem to add anything to the paper.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This is a manuscript  with an interesting and well approached topic which is suitable for the journal profile.

It should be noted the comparison between experimental and numerical results considering the validation of the CFD model. 

Perhaps the introduction and conclusions could have been more extensive, but the main ideas are contained in these chapters.

Recommendation:

>To better emphasize the imposed boundary conditions I recommend that on page 5 , the parts starting with:

"Inlet Section:", (line 145), "Outlet Section:" (line 147), "Solid Walls" (line 148), "Rotating Components:" (line 149) and "Cooling Domains:" (line 151) should be written on separate lines. 

>For the same purpose, I recommend that on page 6, the sentences "Migration and Intensification of Loss Core:" (line 180) and "Enlargement of Lower Channel Lose Core:" (line 183), can be written at the beginning of a new lines.

>On pg. 12, for y⁺, write "Dimensionless wall distance" instead of "Non-dimensional wall coordinate" 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

1. The originality and the novelty of this paper needs to be made clear in the introduction. The interaction of purge flow and main flow have been studied by several researchers and there are a number of unsteady numerical studies. The introduction part needs to be carefully rewritten to highlight specifically what is original and new in this paper.

2. The description of numerical methods need to be improved. No information is provided on the solver numerics and algorithms. Please include appropriate grid sensitivity analysis and turbulence model selection parts in some words - these are at present just referred to the other studies.

3. Include zoomed in views of the mesh of the different components. An assembly screen shot is not sufficient.

4. The extent of numerical uncertainty has not been quantified in the results and discussions. No information on time step value as the simulation progresses is presented in the paper - please include a plot since dual time stepping is considered.

5. The extent of discussion of results is very brief. Please include additional discussions around some aspects of the purge flow in the cavity.

6. Additional plots on span wise variation of M numbers and flow angles would add to the discussion - particularly with temporal changes.

Comments on the Quality of English Language

Several grammatical and typographical errors noted throughout the paper. This must be thoroughly proof read.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Paper: machines-3416885-v1

Full Title: Effect of Purge on Secondary Flows due to Interaction between Cavity Flow and Main Channel

 

I - General Comments

The manuscript deals with Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations aiming to investigate the unsteady interactions within the stator/rotor cavity of a low-pressure turbine. The work has particularly focused on unsteady loss mechanisms occurring downstream of the vane row and in areas, where cavity flow re-enters the main channel, showing how cooling flow rates affect these losses. As an important conclusion, the paper has cited that that increasing the cooling flow rate injected into the cavity produces an increase of the hub’s passage vortex effect and, consequently, there is a more intense interaction between main flow and cavity flow. There is some comments and recommendations addressed to the authors, before my final decision of accept the manuscript for publication in machines.

 

II - Specific Comments

(i) The paper has cited 25 “References”. It is suggested citations more updated and a better contextualization that clarifies the intended publication in the specialized literature.

(ii) In “Introduction, please includes a description linking the hypothesis that the research has assumed with the chosen numerical approach. It is more important to demonstrate the new features, benefits, and advantages of the chosen numerical strategy.

(iii) In “Section 2.1”, on page 3, line 107, it is unclear the concept of “total pressure measurements upstream and downstream of the vane”. Please, introduce the concept of pressure and of instrumentation used to measure it.

(iv) A Table summarising measurement uncertainties is welcome.

(v) The manuscript is poor in terms of differential equations and boundary conditions that govern the investigated problem. The authors need to give attention for that problem. What are the dimensionless parameters associated with the investigated physical phenomenon?

(vi) Can the authors provide the Kolmogorov scale and grid size used in their simulations?

(vii) The authors should provide the hardware specification, computational cost, and mesh size for comparison with other numerical approaches. The purpose of these comparisons is for helping readers understand the performance of the chosen numerical method.

(viii) It would be desirable that the authors demonstrate the limitations of the present method. Also, it would be useful to discuss about their drawbacks as well as the ways to solve them.

(ix) Figure 5 needs of a better discussion too.

(x) In “Conclusions”, it is important a concise explanation concerning the limitations of the present numerical approach and a short discussion showing perspectives for future research. The contribution of the present manuscript must be justified too.

 

III - Recommendation for the machines’ editor

The manuscript needs attend all topics above evaluated by reviewer. Upon consideration of all points above, I think the paper could be considered for publication in machines.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I want to thank the authors for successfully addressing my comments.

Reviewer 3 Report

Comments and Suggestions for Authors

Authors have addressed comments.

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript has been sufficiently improved to warrant publication in Machines.