Investigation into the Effect of H₂-Enriched Conditions on the Structure and Stability of Flames in a Low-Swirl Combustor Derived from Aero-Engine Design

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Very interesting study into the flame structure of CH4/H2 flames. However, I do not really see the connection to aero-engines, as they are usually operated with liquid fuels (as you point out). This work could be more interesting for stationary gas turbines, though they tend to operate with premixed combustion at higher air excess ratios, which makes H2 admixture more of a challenge.
I would suggest describing your experimental conditions in a bit more detail, that this is a non-premixed burner and that the air excess ratio is almost constant due to the minimum air requirements of both CH4 and H2 being very close in terms of m3/MJ.
It would be interesting to compare your findings with LES results.

Author Response

Reviewer #1: Very interesting study into the flame structure of CH4/H2 flames. However, I do not really see the connection to aero-engines, as they are usually operated with liquid fuels (as you point out). This work could be more interesting for stationary gas turbines, though they tend to operate with premixed combustion at higher air excess ratios, which makes H2 admixture more of a challenge.

Reply: Thank you for your thoughtful comments and valuable feedback. In the revised manuscript, we have included an explanation of the relevance of our study to aeroengines. Specifically, we highlight the significance of investigating flame behavior in a low-swirl combustor derived from an air-blast atomizer designed for aero-engine applications. We believe that this clarification will enhance the overall understanding of the study's context and applicability to aeronautical combustion systems.

Reviewer #1: I would suggest describing your experimental conditions in a bit more detail, that this is a non-premixed burner and that the air excess ratio is almost constant due to the minimum air requirements of both CH4 and H2 being very close in terms of m3/MJ.

Reply:  Thank you for your constructive suggestion. In the revised manuscript, we have included a more detailed description of the experimental conditions, specifically highlighted the non-premixed nature of the burner and explaining how the minimum air requirements of CH4 and H2 contribute to an almost constant air excess ratio.

Reviewer #1: It would be interesting to compare your findings with LES results.

Reply: Thank you for your suggestion. We have included a reference to a relevant LES study and highlighted its findings in the revised manuscript. This LES study investigated the same combustor but was fueled with methane and jet A. By referencing this work, we aim to provide a basis for comparison between the different fuel scenarios and their impact on flame structure and stability.

Reviewer 2 Report

Comments and Suggestions for Authors

Introduction:

The introduction serves as the gateway to the study, providing context and motivation. Below are suggested enhancements:

1.1. Broadening Context: The introduction outlines not efficiently the increasing demand for sustainable combustion processes. To enhance its context, consider expanding on the global transition towards cleaner energy and the role of hydrogen within this broader landscape.  In addition to the areas for further exploration highlighted in the review, it's recommended to consider recent works analyzing the flashback phenomenon (https://doi.org/10.1016/j.ijhydene.2023.04.252). This additional literature review could provide valuable context and insights into the practical implications of hydrogen utilization in diverse applications.

2. Highlighting Research Gap: The introduction currently sets the stage for the study but could be more explicit in highlighting the existing research gap. Consider adding a sentence that explicitly addresses what is not well-understood or explored in the current literature and how this study aims to fill that void.

3. Framing Objectives: While the introduction touches upon the potential influence of hydrogen on flame dynamics, explicitly stating the study's objectives could provide a clearer roadmap for readers. Outline what specific aspects of flame structures and dynamics the study aims to investigate.

Conclusion: The conclusion should succinctly summarize findings and propose avenues for future research. Here are suggested improvements:

1. Summarizing Key Findings: The conclusion aptly summarizes the key findings but could provide a bit more detail on the implications of these findings. Consider elaborating on why the observed alterations in flame structures are significant and how they contribute to the broader understanding of combustion dynamics.

2. Future Research Recommendations: While the conclusion touches on the need for future research, consider providing more concrete recommendations. For instance, suggest specific aspects of hydrogen combustion that could be explored in more detail, such as different hydrogen-methane ratios or variations in combustion conditions.

3. Link to Practical Applications: Connect the findings more explicitly to potential practical applications. Discuss how the observed alterations in flame characteristics might impact combustion efficiency in real-world scenarios. This can provide a more tangible takeaway for readers.

 

Overall Comments:

 

The manuscript effectively presents a comparative analysis of flame structures under different fueling conditions, and the suggested revisions aim to refine the introduction and conclusion for a more comprehensive and reader-friendly presentation. Please consider these suggestions to further enhance the clarity and impact of your study.

Author Response

Reply: Thank you for the constructive and insightful feedback. We appreciate your thorough evaluation of our manuscript and the valuable suggestions provided.

We enhanced the introduction by incorporating relevant information on the demand for sustainable combustion processes and recent works, as suggested.

We included sentences in the introduction that clearly identifies what is not well-understood or explored in the current literature and how our study aims to fill that void.

We also enhanced the introduction by outlining the specific aspects of flame structures and dynamics that our study aims to investigate.

Regarding the conclusions we provided more details on why the observed alterations in flame structures are significant and how they contribute to a broader understanding of combustion dynamics. We also suggested specific aspects of hydrogen combustion that could be explored in more detail, such as different hydrogen-methane ratios or variations in combustion conditions.

Thank you once again for your valuable feedback.

 

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have modified the manuscript in accordance with the reviewer's comments. In this form the work can be accepted for publication