Effects of Compounds in Liquefied Methane on Rocket Engine Operation

Round 1

Reviewer 1 Report

The paper deals with a discussion of compositions and impurities of liquefied methane mixtures. In particular, the phase diagrams of carbon dioxide and methane are reviewed, highlighting the critical conditions that need to be avoided for safe operations. The effects of the impurities on the combustion processes inside the thrust chamber are investigated by 1D laminar counter flow flame calculations and steady state 2D CFD simulations, highlighting that the influence on the flame temperature is minor, while a pressure drop of about 3.5% can be observed in certain conditions. 

The paper is well organized, providing qualitative and quantitative indications of interest for rocket engines processes. 

However, some open points should be addressed before publishing. 

 

1. The abstract should briefly describe the purpose of the paper, along with the most important results. Currently, the purpose of the paper is briefly addressed in lines 20-21, and the major findings are not discussed. Hence, I would suggest to improve the abstract to help the reader understanding the most relevant points of the study. 
2. I would suggest a quick review of the text to avoid repetitions and unclear sentences. Some examples are hereby provided: lines 145-147; lines 131-134 and lines 143-147; lines 204-205; lines 450-455 and lines 459-464.
3. Figure 2: please clarify the behavior of the orange curve (p=0.1 MPa) and provide a justification for the two effects highlighted when comparing methane-air and methane-oxygen flames.
4. Section 4.2: please include the information about the stoichiometric point considered in the computations.
5. Section 4.3.3: please provide details on the geometry (I.e., inlets and throat diameters) and on the boundary conditions considered to enable data reproducibility.
6. Some figures highlighting the recirculation zones and features may support the conclusion derived from figure 7. It seems that not all pollutants lead to a decrease of the recirculation zone: why? Please clarify. 
7. The influence of the combustion processes on thrust and wall heat flux are theoretically discussed in Section 4.3. I would have expected a quantification of the resulting wall heat flux, being the pressures and temperatures derived from the CFD simulations. Please, consider including some considerations and computations on the topic to complete the discussion of section 4.3

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The article submitted for review is a well-organized scientific work
in terms of the methods presented and the results obtained.
The only thing that should be expanded is the introduction
to the topic with a broader reference to the literature.

From other comments:
(a) in Table 1, the incorrect value of the boiling point temperature of methane at normal pressure.
(b) for the mixtures from Table 3, it will be useful to present
the phase envelopes in the form of p-T plots.

Author Response

Please see the attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The comments from the first round of review were addressed and no further revision is required.