Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper is devoted to the anaysis of the contemporary chemical kinetic schemes suitable for the analysis of hydrogen-air combustion at elevated pressures. The amount of schemes under consideration is sufficient and the analysis is fairly thorough. The results are of great importance for the CFD modeling of high-pressure hydrogen ignition and combustion. Before the paper can be published in Aerospace journal, there are several comments required to be addressed:

1. The amount of data on LFS under normal and high-pressure conditions is too small. There are certain difficulties in obtaining LFS values under high-pressures experimentally [Bradley et.al. // Comb. Flame, 2007, 162-172] and the discrepancies in experimental values can be quite high. Thus I recommend authors to extend the literature review and provide more experimental data points for the comparison [see e.g. Konnov et. al. // Prog. En. Comb. Sci., 2018, 197-267].

2. How the result on ignition delay time would change, if the reactor conditions would be isobaric instead of isochoric? Isobaric condition seems to be more suitable for the shock tube ignition experiments.

Minor comments

1. Quality of the figure should be increased.

2. Please check author contributions section.

 

Author Response

The first version of the manuscript has been revised (all the modifications have been highlighted in track-change mode in the reviewed version of the manuscript) in order to meet all the precious recommendations of the three reviewers as indicated in more detail below. Moreover, further paragraphs have been slightly modified to lower the repetition rate of the paper.

Reviewer 1

The paper is devoted to the anaysis of the contemporary chemical kinetic schemes suitable for the analysis of hydrogen-air combustion at elevated pressures. The amount of schemes under consideration is sufficient and the analysis is fairly thorough. The results are of great importance for the CFD modeling of high-pressure hydrogen ignition and combustion. Before the paper can be published in Aerospace journal, there are several comments required to be addressed:

1. The amount of data on LFS under normal and high-pressure conditions is too small. There are certain difficulties in obtaining LFS values under high-pressures experimentally [Bradley et.al. // Comb. Flame, 2007, 162-172] and the discrepancies in experimental values can be quite high. Thus I recommend authors to extend the literature review and provide more experimental data points for the comparison [see e.g. Konnov et. al. // Prog. En. Comb. Sci., 2018, 197-267].

The authors thank the reviewer for his/her valuable recommendation and the excellent paper of Konnov et al. Prog. En. Comb. Sci., 2018, 197-267 has been analysed. Therefore, the literature review on LFS has been slightly extended considering (see figures 17-19 of the revised version of the manuscript) the three experimental datasets of Aung et al. Combust. Flame 1998, 112(1), 1–15. at 1, 2, and 3 atm also examined in the paper of Konnov et al. Prog. En. Comb. Sci., 2018, 197-267.

2. How the result on ignition delay time would change, if the reactor conditions would be isobaric instead of isochoric? Isobaric condition seems to be more suitable for the shock tube ignition experiments.

The authors thank the reviewer for his/her valuable comment. However, it is worth noting that at least in the framework of the chemical-kinetic analysis and in agreement with the considerations explained in the paper of Chaos et al. (M. Chaos, F.L. Dryer, Chemical-Kinetic Modeling of Ignition Delay: Considerations in Interpreting Shock Tube Data, International Journal Chemical Kinetics, Vol. 42, pp 143-150, 2010), the shock-tube ignition delay time measurements are usually simulated by homogeneous, zero-dimensional, isochoric modelling assumptions. Indeed, in the author’s opinion, these hypotheses seem to be more suitable since they allow us to calculate also the pressure profile during the combustion process and to compare it with the experimental data when available.

Minor comments

3. Quality of the figure should be increased.

The authors thank the reviewer for his/her precious comment and all the figures have been changed splitting every graph for the ignition delay time calculation into three separate plots to enhance the visibility and readability of all the lines and points.

4. Please check author contributions section.

The authors thank the reviewer for his/her precious minor comment and the author’s contributions section has been revised and checked.

Reviewer 2 Report

Comments and Suggestions for Authors

In this paper the authors test 0D and 1D models for the simulation of hydrogen combustion. The paper provides some useful information to the scientific community. However, paper title makes a possible reader thinking that a gas turbine combustor from an aero-engine is simulated using 3D CFD. Moreover, the paper is very simple, with limited impact on the turbomachinery community since no final analysis of a realistic geometry is performed using the most promising kinetic method is performed on . Finally, the "Results" section has limited in-depth analysis. However, the paper is of interest for the combustion community. A few mandatory changes follow:

- Please change the title to correctly reflect paper content

- There are several typos and the MORE&LESS project is cited differently (MORE and LESS) at line 81.

- Introduction is long but only 7 papers are referenced. Aren't there more papers on this topic to be cited? However, Section 2 refers to all the necessary kinetic mechanisms, it may be considered a sub-section of the Introduction. What is missing at the end of the introductory part is a sentence about what is presented in the paper and how it advances the state of the art.

- Equations: I'm not completely sure about that but Equations should be numbered with progressive numbers, label are not allowed.

- Line 149: Table 3 is referenced but only one table is present in the paper.

- Figures are hardly readable. Too many lines and low resolution make them unacceptable on a journal paper. It is also hard to see line colors (e.g.: in Figure 1 Mueller and Aramco-II are both black to my eyes). Please provide high-resolution images.

Author Response

The first version of the manuscript has been revised (all the modifications have been highlighted in track-change mode in the reviewed version of the manuscript) in order to meet all the precious recommendations of the three reviewers as indicated in more detail below. Moreover, further paragraphs have been slightly modified to lower the repetition rate of the paper.

Reviewer 2

In this paper the authors test 0D and 1D models for the simulation of hydrogen combustion. The paper provides some useful information to the scientific community. However, paper title makes a possible reader thinking that a gas turbine combustor from an aero-engine is simulated using 3D CFD. Moreover, the paper is very simple, with limited impact on the turbomachinery community since no final analysis of a realistic geometry is performed using the most promising kinetic method is performed on . Finally, the "Results" section has limited in-depth analysis. However, the paper is of interest for the combustion community. A few mandatory changes follow:

1. Please change the title to correctly reflect paper content

The authors thank the reviewer for his/her precious comment and the title has been changed by the cancellation of the words “Gas Turbine Engines” which could generate possible misunderstandings.

2. There are several typos and the MORE&LESS project is cited differently (MORE and LESS) at line 81.

The authors thank the reviewer for his/her valuable comments and the name of the project MORE&LESS is cited coherently on line 81. Also, other typos have been corrected.

3. Introduction is long but only 7 papers are referenced. Aren't there more papers on this topic to be cited? However, Section 2 refers to all the necessary kinetic mechanisms, it may be considered a sub-section of the Introduction. What is missing at the end of the introductory part is a sentence about what is presented in the paper and how it advances the state of the art.

The authors thank the reviewer for his/her valuable comment. Surely the summary of the state-of-the-art is not exhaustive but it seems suitable for a concise overview of the topic. Moreover, according to the reviewer’s suggestion, in the revised version of the manuscript, the description of the investigated kinetic mechanisms has been reported as a sub-section of the Introduction. Furthermore, at the end of the Introduction section, two sentences have been added. The first sentence declares, according to the author’s opinion, the aim of this work and the contribution, it provides to the advancement of the state-of-the-art, while the second briefly anticipates the content of the following sections.

4. Equations: I'm not completely sure about that but Equations should be numbered with progressive numbers, label are not allowed.9.- Line 149: Table 3 is referenced but only one table is present in the paper.

The authors thank the reviewer for his/her valuable comments and the typo on line 149 has been corrected accordingly. The labels used for the equations refer to the different kinetic steps in the several reaction mechanisms. In any case, they will be adjusted in agreement with the Editor’s indications.

5. Figures are hardly readable. Too many lines and low resolution make them unacceptable on a journal paper. It is also hard to see line colors (e.g.: in Figure 1 Mueller and Aramco-II are both black to my eyes). Please provide high-resolution images.

The authors thank the reviewer for his/her precious comment and all the figures have been changed splitting every graph for the ignition delay time calculation into three separate plots to enhance the visibility and readability of all the lines and points.

Reviewer 3 Report

Comments and Suggestions for Authors

1. Rewrite the abstract, removing the abbreviations and describing the main results of the article

2. It is not clear from the intro section the newest and aim of the work

3. Add to Table 1 the info about the gas mixture from the reference works, as well as the conditions for temperature and pressure

4. Perhaps, it will be more visually better to place graphs with similar pressures for different exp. data side by side

5. Make the legend on all figs to the same format

6. The conclusion is made subjectively according to the figs. So, we can see that for the different conditions, different models have better correlation, which does not allow us to clearly understand the author conclusion. Perhaps a general table should be compiled with an estimate of the deviation of the calculation results from the experimental data 

 

Author Response

The first version of the manuscript has been revised (all the modifications have been highlighted in track-change mode in the reviewed version of the manuscript) in order to meet all the precious recommendations of the three reviewers as indicated in more detail below. Moreover, further paragraphs have been slightly modified to lower the repetition rate of the paper.

Reviewer 3

1. Rewrite the abstract, removing the abbreviations and describing the main results of the article.

The authors thank the reviewer for his/her precious comment and all abbreviations have been removed from the abstract. Moreover, a statement describing the main results of this work has been added to the revised version of the manuscript.

2. It is not clear from the intro section the newest and aim of the work.

The authors thank the reviewer for his/her valuable comment and in the Introduction section of the revised version of the manuscript a sentence describing the aim of this work and the contribution, it provides to the advancement of the state-of-the-art has been added.

3. Add to Table 1 the info about the gas mixture from the reference works, as well as the conditions for temperature and pressure.

The authors thank the reviewer for his/her precious comment. However, Table 1 is only the list of the number of reaction steps and chemical species for each of the kinetic mechanisms for high-pressure hydrogen combustion analysed in this work. Furthermore, the operative conditions for temperature, pressure, and composition of the reacting gas mixture have been highlighted in detail in the caption of every figure illustrating the ignition delay time calculation for each experimental dataset in the Results and Discussion section of the revised version of the manuscript.

4. Perhaps, it will be more visually better to place graphs with similar pressures for different exp. data side by side

The authors thank the reviewer for his/her precious comment and all the figures have been changed splitting every graph for the ignition delay time calculation into three separate plots to enhance the visibility and readability of all the lines and points. Moreover, in agreement with the work of Hu et al. (Hu, E.; Pan, L.; Gao, Z.; Lu, X..; Meng, X.; Huang, Z. Shock tube study on ignition delay of hydrogen and evaluation of various kinetic models, International Journal of Hydrogen Energy 2016, 41 (30), 13261-13280, 10.1016/j.ijhydene.2016.05.118.), the graphs have been subdivided mainly according to the reacting mixture composition i.e., the equivalence ratio, and then to the increasing experimental pressures.

5. Make the legend on all figs to the same format

The authors thank the reviewer for his/her precious comment and all the figures have been changed splitting every graph for the ignition delay time calculation into three separate plots to enhance the visibility and readability of all the lines and points. More attention has been paid in order to verify the uniformity of the overall stylistic format.

6. The conclusion is made subjectively according to the figs. So, we can see that for the different conditions, different models have better correlation, which does not allow us to clearly understand the author conclusion. Perhaps a general table should be compiled with an estimate of the deviation of the calculation results from the experimental data.

The authors thank the reviewer for his/her precious comment and all the figures have been changed splitting every graph for the ignition delay time calculation into three separate plots to enhance the visibility and readability of all the lines and points. Moreover, a quantitative parameter has been introduced for estimating the deviation of the numerical predictions from the experimental measurements i.e., the average absolute degree of mismatching and it has been included in all the plots for every analysed kinetic mechanism as a general summary for the IDT in Table 2 and for LFS in Table 3.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Authours thoroughly addressed all my comments and questions. The paper can be published in its current state.

Author Response

The authors thank the reviewer for his/her comment and no actions have been required.

Reviewer 2 Report

Comments and Suggestions for Authors

• I would split Table 2 into three parts (for different φ values, almost mandatory)
• φ is sometimes lower case (in Table 2) and sometimes upper case (in figure captions) and is never defined (it is not present in the List of Variables), please fix it (mandatory).
• I would split Table 3 into two parts or make it more compact (however it is just a suggestion)

Author Response

1) I would split Table 2 into three parts (for different φ values, almost mandatory)

The authors thank the reviewer for his/her comment and Table 2 has been split into three different tables (Table 2, Table 3, and Table 4 of the second revised version of the manuscript each inserted in the relevant sub-paragraph of the Results and Discussion section) according to the equivalence ratio.

2) φ is sometimes lower case (in Table 2) and sometimes upper case (in figure captions) and is never defined (it is not present in the List of Variables), please fix it (mandatory).

The authors thank the reviewer for his/her comment and the variable has been defined in the text and introduced in the List of Variables. Moreover, it has been inserted in lowercase in both figure captions and in the tables.

3) I would split Table 3 into two parts or make it more compact (however it is just a suggestion)

The authors thank the reviewer for his/her suggestion and Table 3 of the first revised version, which now has become Table 5 in the second revised version has been made more compact.

Reviewer 3 Report

Comments and Suggestions for Authors

The article is devoted to a review of research on the chemical-kinetic analysis of the ignition and combustion of hydrogen under high pressure under operating conditions commonly found in supersonic aircraft ramjet engines. The comments have been satisfactorily revised, and the text can be approved for publication in the journal.

Author Response

The authors deeply thank the reviewer for his/her comment and no actions have been required.