Characterization of Nonlinear Responses of Non-Premixed Flames to Low-Frequency Acoustic Excitations

Round 1

Reviewer 1 Report

Dear Authors,
Thank you for submitting your elaborative manuscript. I would like to accept it in current form with minor comment.
Have you considered non-excited case, if yes, have you observed any self-excitation modes?

Regards,

Additional comments

The research has experimentally characterized the non-linear response of non-premixed methane-air flame excited by forced fluctuations at air inlet. The results are discusses to understand the effects on Flame Describing function, flame length and height of center of mass (for heat release).

Many research has studied the effects on heat release with external excitations. The current study systematically provide the experimental data for studied conditions, which can be helpful to research community for validating their models.

The research provided the experimental data to validate various numerical models and develop understanding the non-linear excitations.

The presented work is sufficient, but in order to provide further concrete understanding, the authors can improve the results further by employing other non-linear analysis to extract modes of non-linear oscillations and develop time-series analysis. And compare the non-excited case with the externally excited cases.

The conclusions are consistent with the evidence & arguments presented. And the references are appropriate.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Thank you for this submission. This is a very thorough investigation of an interesting topic - non-premixed flame response to acoustic forcing. The description of your setup and methodology was very detailed, and your results were communicated in a series of clear, informative figures. I do not see a reason this study cannot be published in its current form.

However, I have one major suggestion. You make several conclusions about how flame shape changes with excitation frequency and amplitude. These conclusions seem supported by the data you present. However, they would be much stronger with an analysis of the uncertainty present in your measurements. An exhaustive uncertainty analysis would provide error bars to data points in Figures 3, 5, 6, and 7, and would provide the uncertainties for the values in Table 2. Especially for Table 2, these uncertainties would give confidence to the reader that the trends observed with changing acoustic excitation are statistically significant.

One related note: for the time-averaged chemiluminescence results (Figure 2 et al.), please specify the number of image acquisitions that are averaged together.

Only minor errors are present. Two stood out to me:

I would revise the title for clarity and brevity: "Characterization of non-linear responses of non-premixed flames to low-frequency acoustic excitations"

Line 24-25 should read "When two adjacent oscillation regions have approximately the same amplitude, ..." or "When two adjacent oscillation regions have similar amplitudes, ..."

Author Response

Please see the attachment.

Author Response File: Author Response.docx