Fundamental Study of Premixed Methane Air Combustion in Extreme Turbulent Conditions Using PIV and C-X CH PLIF
Round 1
Reviewer 1 Report
The nature of the experimental work presented by the authors is very good, and it is very much helpful in validating turbulent combustion codes. After reading the entire manuscript, I find the manuscript to be well written but certain figures to be presented properly like figures 5 -9. For example, figure 5, all the sub-figures should have the proper common scale (or scales like 0-15 and 0-25 m/s) and the background should not be black.
The language used in the manuscript is good.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
Flow (PIV) and flame (PLIF) characteristics of highly turbulent methane/air premixed combustion over a backward-facing step are investigated. This works aims at demonstrating the experimental methodology and the post-processing techniques to image methylidyne (CH) in high-intensity turbulent premixed combustion systems. Grids and perforated plates were introduced upstream of the flame to modify the turbulence length scales. Two different Reynolds numbers and three equivalence ratios have been considered.
The following comments seem pertinent:
1. The description under sub-section 3.1 at the bottom of p. 9 is difficult to follow. One can read, for example, “The grid at the combustor inlet induces isotropic turbulence in the flow.” The turbulence behind a grid or a perforated plate is generated by interactions among multiple wakes and jets, and requires a sufficient downstream distance to homogenize. If this requirement is satisfied, it should be clearly stated and a quantitative indicator of isotropy should be provided. On the other hand, the ”flow acceleration” is local, as written in Fig. 5, due probably to thermal dilatation effects, although it is not mentioned in the text. The mixing or shear layer between the recirculation zone and the main flow is composed of vortices, shed downstream of the sharp corner of the step instead on “low-velocity recirculation zones”, as the authors narrate. The “presence of small eddies in the upper part of the flow field” seems totally irrelevant to this reviewer and the reason stated is really cryptic. The BR has, in principle, little to do with the “induced eddies”; moreover, higher BR would implies less interacting vortices in the wakes and jets of the grids or perforated plates. This referee is confused about the use of the term “vortex breakdown” in the last sentence of the las paragraph of p. 9. This whole paragraph should be completely rewritten.
2. The legend “small-scale eddies” in Fig. 5 (Grid 3, Re = 15,000) is, again, confusing, unless a detailed explanation is provided.
3. “Vortex shedding” downstream of the sharp corner of the step occurs both at Re equal to 15,000 an 30,000, and with every grid or perforated plate.
4. Fig. 5 should be redone, with legends which are really descriptive of specific and distinctive flow features.
5. Fig. 6. Average u and v velocity profiles and their rms along the interrogation line, in the absence of grids and perforated plates, would be of great interest as a reference case to assess the effects introduced by the presence of “turbulators” upon the means and fluctuations.
6. The turbulent intensity contour maps presented in Fig.7 are confusing and add very little information to understand the combusting flow structure. The text description of Fig. 7 is also somewhat insufficient.
7. Fig. 8. The vorticity profiles are rather erratic. What is the physical reason for that? The vorticity maps of Fig. 9 display interesting features, but need further physically based explanations.
8. TKE and scalar fluctuation dissipation rate maps would be very useful to try to rationalize the premixed combustion characteristics.
9. Fig. 10. The authors must describe what the reader is viewing in Fig. 10, (iii) and (iv). A color palette is also missing. Fig 10 (v) is of interest but requires a more detailed, physically sound description, and, if possible, a more precise jargon. Wrinkle formation and development are due to underlying physical mechanisms, which would be relevant to elucidate.
10. Would the authors, please, explain the meaning of “The turbulent and velocity scales generate high compressive forces in the flame, leading to accelerated flame- flame interactions.” in p. 26 one before the last paragraph?
11. A map of scalar fluctuation dissipation rate superposed on Fig. 10 (v) would probably help to better narrate the last paragraph of p. 26, continued on top of p. 27.
12. It would be rather illustrative to have the maps of Figs. 11-13 with no grids or perforated plates.
In summary:
This manuscript is focused on demonstrating the experimental methodology and the post-processing techniques to image methylidyne (CH) in high-intensity turbulent premixed combustion, and adds very little to the physical understanding of the flow/reaction interactions. This reviewer would suggest to reduce the number of cases investigated, for example, comparing results with no grids and results with one or two grids and two Reynolds number. The content of this manuscript could be broken down into several publications. Physically sound explanatory paragraphs are essential before this work can be published. Major modifications are needed.
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
The work focuses on flow and flame characteristics in environments with extreme turbulence using PIV at 10 kHz and PLIF at 10 kHz. As far as the introductory part is concerned, a greater bibliographic search is recommended, integrating with further contributions with respect to the characterization of methane flames, for example:
-Self-wrinkling induced by Darrieus-Landau instability in turbulent premixed Bunsen flames from low to moderately high Reynolds numbers. DOI:https://doi.org/10.1103/PhysRevFluids.7.053202
- Mitigation of Darrieus–Landau instability effects on turbulent premixed flames. doi: https://doi.org/10.1016/j.proci.2020.07.018.
As for the experimental setup section, it is well written and reasoned. For this section, in addition to that of the results, it is advisable to use a larger font to facilitate understanding and also a higher quality of the images.
In conclusion, the manuscript addresses a current topic using both experimental and numerical approaches. The argument is in line with the journal's arguments, but strong revision is needed regarding the introductory section before acceptance. A review of the images is also needed to make them higher quality.
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
I have quickly read the authors' responses to my queries and I am satisfied with them. I recommend publication of the new version.
Reviewer 3 Report
The manuscript has been implemented and modified in the various parts in accordance with the revision. In this form it can be accepted for publication.
