Polyethylene Pyrolysis Products: Their Detonability in Air and Applicability to Solid-Fuel Detonation Ramjets

Round 1

Reviewer 1 Report

This manuscript describes experimental work about the use of Polyethylene pyrolysis products as an alternative fuel for Detonation ramjet engine. The manuscript is composed of two parts. The first part deals with the measurements of pyrolysis products of Polyethylene produced in the pulsed detonation tube. The second part deals with the applicability of pyrolysis products for ramjet detonation engine. The authors have already issued a patent for design of the solid-fuel Detonation Ramjet demonstrator. This paper is interesting and may be useful. In my opinion the design of ramjet detonation engine is quite challenging. I have performed preliminary thermochemical calculations by using “gaseq” code in order to validate the detonation velocity results presented in figures 7 and 10. The calculations have been performed on detonation of Methane and Air and Hydrogen and air at decomposition temperature of Polyethylene 850°C (In this temperature the mass fraction of Methane and Hydrogen are dominant). The detonation velocity of Hydrogen and air is about 1750 m/s. The detonation velocity of Hydrogen and air is about 1880 m/s (by using the C-J Detonation method). Thus the detonation velocities reported in this paper is similar to the detonation velocity calculated by “gaseq” software. Good agreement has been obtained. This ramjet motor is prone failures such as: combustion instability and high temperature issues, large pressures and blast loads, Hydrogen embrittlement, and HTHA. The authors should address these issues in the revised version. This interesting contribution needs major revisions before it is published.

Comments and Suggestions for Authors

1) The abstract needs to include the main impact of this study. The novelty of this study should be strengthen and emphasized based on gaps of current literature.

2) The following great classical book should be cited in the introduction:

Lewis, B.; Von Elbe, G. Combustion Flames and Explosion of Gases, 2nd ed.; Academic Press Inc.: London, UK, 1961, https://doi.org/10.1016/C2013-0-12402-6.

3) The abbreviations used (such as DR, SCM, SDT, GG, SW, DDT etc.) inside the manuscript should be defined in the beginning or at the end of the manuscript.

4) The combustion instability should be discussed in this manuscript. It recommended citing the following new paper:  

Zonglin Jiang, Zijian Zhang, Yunfeng Liu, Chun Wang, Changtong Luo, Criteria for hypersonic airbreathing propulsion and its experimental verification, Chinese Journal of Aeronautics, Volume 34, Issue 3, 2021, Pages 94-104, https://doi.org/10.1016/j.cja.2020.11.001.

This insightful paper describes the parameters influencing the combustion stability (such as: equivalence ratio, Inlet Mach number).

Another paper which deals with supersonic pulsed detonation Ramjet engine is:

Alexandrov, V.G. & Kraiko, A. & Reent, K.S., 2001, Mathematical model of a supersonic pulsed detonation Ramjet engine, High Speed Deflagration and Detonation: Fundamentals and Control. 20, 333-344.

5) The temperature measured during the pulsed detonation tube experiment is quite high (According Fig. 4 the maximal temperature is about 850ºC). The exposure of the pipe wall to the hot exhaust gas mixture is short. The inner wall of the ramjet may be exposed to flame for a long time. It should be noted that there are several failure modes which may contribute to structural failure of the ramjet (such as: Creep and Fatigue, oxidation corrosion and fracture). Since Hydrogen is generated by the polyethylene pyrolysis. At decomposition temperature of about 850ºC, the volume fraction of Hydrogen in the mixture is about 53% (table 3). Thus the engine metal parts may be prone to Hydrogen embrittlement or High Temperature Hydrogen Attack (HTHA). The service life of this engine may depend on these factors. These failures should be mentioned.

6) (Section 3.2 - Preliminary test fires) the thrust of the solid fuel ramjet engine should be presented.

7) The conclusion section needs to include the novelty of this research.

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

The English needs improvement.

I have made a few comments in the attached annotated version. 

My main issue is the assessment of stability and hazard.  It is good that system experiments have been performed but more consideration needs to be given to scale and to duration.  How critical are the properties of the fuel prior to pyrolysis?  I suspect that is not important but would like reassurance.

Scientifically sound as far as it goes and some further discussion of questions and further work including combustion stability and safety is needed as part of the test bed work. 

 

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The subject of the submitted paper is very topical. The concept of hybrid engine (solid hydrocarbon fuel and gaseous/liquid oxidizer) is under development by many research groups. In these engines, fuel is always organic polymer. The work is experimental and is done at a high level. The manuscript is very long. The work consists of three separate experimental studies: the study of the polyethylene pyrolysis, the study of the detonability of pyrolys products, and the hot-fire tests of a new ramjet combustor.
A. The three parts of the work are connected with each other only ideologically. The results of the first part are not used in the second part, the results of the second part are not used in the third one.
B. The theoretical background of the work is virtually not presented. At least the results of the first part should be compared with theoretical calculations.
C. The second section of the paper should be divided into two parts.
D. The DDT run-up distance is defined in the paper but is never used.
E. The DDT run-up time, exactly what does it characterize, tube, fuel mixture, ignition method? Why is it more informative than the run-up distance? Why was it necessary to measure it? What would happen if the run-up time would be 20% longer?

F. The manuscript (language and typeface) is terrrible. The manuscript needs a nomenclature because the authors extensively use their own jargon and abbreviations: STD volume, STD shot, etc. Many sentences should be spitted; they consist of 40+ words and sometimes the end does not match the beginning. There are letters of tiny size, smaller than 6 pt. The manuscript is very tough for reading. There are a lot of missed articles. Most of the nouns have no preceding articles; it is not clear are they nouns or verbs or attributive nouns? The whole text is a very long charade.

The paper needs a revision, during which issues A.-F. should be resolved.

Please find further comments in the attached file.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors,

Thank you very much. In my opinion, this contribution should be considered for publication. 

Author Response

Thank you. We appreciate your comments very much.