Round 1

Reviewer 1 Report

Overall Impression

This manuscript covers interesting topics and is with 21 pages very long. It is more like an internal or final report for people involved in a development project and not as a publication for all kinds of readers. Due to that fact, even for an expert in this field, it is difficult for the reader to follow the context and identify a main focus of the paper. According to the abstract I got the impression that it was a sensor development project for combustion control but after having read the report I am not convinced that it is a sensor development, since it also describes combustion acoustics and the optimization of a burner. All is maybe related to the task of what the sensor should be capable to detect. Therefore I recommend that this paper focuses more on the sensor technologies by removing topics related to the combustion. The combustion topics certainly are worth of a different publication.

I also recommend using more equations as a clarification to the reader. For example, the Rayleigh criterion, should briefly explained in order to provide the understanding why the sensor could fulfil this task.

The numerical aperture of the optic is also described. Please use equations to explain why it is important in regard to the detection of high frequencies heat release fluctuations. Again, also for the explanation and discussion of the results and the validation of the sensors functions equations should be used.

Literature

The references that are mentioned are mostly related to previous work the own laboratory. In order to provide a broader background in the introduction of the topic, additional work shall be mentioned.

Since a lot of work has been done in this field, please consider the links below:

https://www.researchgate.net/search.Search.html?type=project&query=flame%20scanner

https://www.researchgate.net/search.Search.html?type=project&query=optical%20flame

https://www.researchgate.net/search.Search.html?type=publication&query=acoustic%20optical%20flame

Patents

A list of patent applications is mentioned. Please be aware that again, a lot of patents have already  be submitted  in the same topic. I am aware of some of them which nearly describe the same approach. In order to avoid infringements, please

consider the link below:

https://worldwide.espacenet.com/patent/search?q=flame%20scanner

Summary

This paper is a good contribution to instrumentation technologies for gas turbines and should be published after the suggested changes mentioned above.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper is well written and of high interest for industrial applications. Maintaining combustion dynamics at acceptable level for operation is critical for any gas turbine application.

Minor typo corrections:

• Line 188: author name Hardalupas
• Line 312: micro-controller controller
• Line 532: a heat-stop

Comments / questions to authors:

• Figure18: is the peak ~850 Hz a probe resonance? It was not mentioned.
• Figure21: in addition to the time trace signal it would have been clearer to add the FFT spectrum of both pressure and light intensity. Analysing the coherence and/or the cross correlation between the pressure and light intensity would be a plus.
• In practical systems one can find different flame stabilization mechanism (e.g. swirl, recirculation…). The flame has 3D distribution and one particular issue is that looking for light intensity in one direction could be not sufficient to properly characterize its dynamics. Having several probes as mentioned can help. However, it is not straightforward to always correlate heat release rate fluctuations and light intensity.
• Concerning the high frequency dynamics (> 1 kHz), because of its nature (small wavelength) and localized mode shapes, it would be very difficult to generalize such system. At low frequency it may work even is the probe does not detect the integral flame volume which is not the case for high frequency dynamics.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

The reviewer would like to thank the authors for their work.

This work describes a novel combustion monitoring approach relying on 3 probes, each featuring a combination of optical and acoustic sensors, with application to gas turbines and aero engines. The authors first present the technical choices that are made to overcome the limited optical access to the combustion zone under challenging conditions in terms of temperature, pressure, lifing and safety requirements. A prototype of the probe is thus proposed. They then show how such a probe can be used to monitor the combustion regime and ignition sequence by comparing the light intensities in different spectral bands (RGB) and the pressure signals of several tests under different operating conditions.

The general impression is that a thorough analysis of the test data is missing. A clear comparison to a baseline and the proof of the added value of such a probe need to be presented and detailed. Below are more specific comments/questions that need to be addressed.

1)      In line 122, it is pointed out that an optical diagnostic is useless if the optical access is not transparent. Soot can indeed settle on the quartz glass of the probe. An assumption is made about a “self-cleaning” process of the glass. Could you please give a reference or further explanations on the matter?  What experience do the authors have after the series of tests they performed?

2)      Another challenge is highlighted in line 170: the optical sensor is likely to see the light reflected by the different components of a combustion chamber. The authors state that they verified the reflected light can be used in the analysis. But I could not find anything else on the matter in the rest of the study. Could you please clarify?

3)      Font of Fig. 6 is too small. It cannot be read. The sensor selection and calibration need to be better explained.

4)      Figure 9 shows the results of a cold-flow simulation that is supposed to give some information regarding the flame position and shape. Could you please clarify this analysis by distinguishing facts from assumptions? How is the simulation used to supplement/complete the study? What does it bring?

5)      In Fig. 13, could you please comment on the 50Hz and higher harmonics peaks (up to 400Hz) that appear in both spectra? How does this impact the pressure readings during the siren sweeps in this frequency range? Also, how are the two excitation frequencies chosen?

6)      In line 344, it is stated that a single probe can detect which side the flame is coming from. Could you please explain how?

7)      Could you please identify S1, S2 and S3 on Fig. 14?

8)      Line 368, one can read that the analysis of the signal RMS is needed to get some information regarding flame propagation. The reader is referred to Fig. 5 that shows the signal RMS. Could you please detail the corresponding analysis?

9)      In Fig. 16, please use the same scale for all graphs in order to ease the interpretation. Does the black color show the full-spectrum light intensity? If so, I am surprised that its amplitude is less than B or G ones. I must have missed something. Could you also add a sketch showing where the primary and secondary combustion zones are with respect to the full frame?

10)   In Fig. 17, a red curved is used to show G data and vis versa (or maybe there is a typo in the legend). This is very disturbing. Also, why is the RMS of the signals in the secondary zone missing? On the average intensity graph, the green curve is missing.

11)   Line 418 states that the highest frequency of combustion instability is 836Hz but in Fig. 18, one can see a peak at around 850Hz. Could you please explain?

12)   A carefully place pressure probe is usually enough to detect a combustion instability. I do not see a thorough analysis of the pressure signals when the authors speak about an instability at 836Hz for example. What does it look like? What frequencies, amplitudes and phase (with respect to the light intensity signals) does it show?

13)   Also, the probe is called “Rayleigh Criterion Probe”. Is there any intent to access any information about the Rayleigh source term? If yes, then monitoring the heat release rate (via a OH* filter) needs to be considered. Do you have an idea about the excited mode shapes in the test rig?

14)   Line 455: I do not see how Fig. 19 is in agreement with Fig. 17. Could you please make the two graphs comparable?

15)   Contrary to what is stated lines 498-504, I do not see any peak at 800Hz, 1200Hz, 1400Hz and 2000Hz on the Blue light signal spectrogram. Could you please clarify?

Also, a number of repetitions and typos need to be corrected, for example:

-          Line 57: replace “whether” by “while”

-          Line 43 and 59: “of” is missing in “a set three probes” and in “in terms flame”

-          Line 137 to 140 is an exact duplicate of an earlier passage.

-          Line 172 to 176 is an exact duplicate of an earlier passage.

-          Line 234 to 236 is an exact duplicate of an earlier passage.

-          Line 283: please add units to the dimensions.

-          Line 312: “micor-controllercontroller”

-          Line 473-474: words are missing. Please clarify.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

You do not need to answer me but take my considerations in the manuscript 

best