Modeling of Chemical Vapor Infiltration for Fiber-Reinforced Silicon Carbide Composites Using Meshless Method of Fundamental Solutions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents a modeling approach for studying densification by CVI of fibrous preforms. The numerical model is outlined along with a verification case following which, it is applied to simulate densification in randomly placed fibers. The work is interesting and can aid further development of models for various CVI techniques as has been pointed out in the introduction and future work.

 

There are some elements of the manuscript that need further clarification/explanation. Following are my specific comments/questions.

 

1.     Please perform a thorough language/grammar check.

 

2.     Dimensions or units are missing for most quantities in the model. For e.g. ‘c’ is referred to as the concentration. Is it molar or mass concentration? Or is it a mass or mole fraction? Further, it might be helpful for the reader if the constants/properties are specified in the manuscript along with references. E.g. activation energy, diffusivity, etc. This is useful to know because SiC kinetics involves several species and intermediate reactions while the model used in this work appears to be a reduced single step reaction. So, the activation energy, diffusivity, gas density, etc. should be some representative or assumed quantities.

 

3.     Abstract, Eq. (7) and conclusion: the reactive boundary condition is referred to as the equality of reaction rate and diffusion rate. The equation technically is the equality of reactive and diffusive flux. Not critical, but good to specify.

 

4.     Several places in the manuscript: SiC should be silicon carbide instead of silicon-carbon.

 

5.     Lines 117 – 129: This is a very good and concise description of the assumptions and objective.

 

6.     Eq. (2) and line 155: Infiltration is actually very fast compared to densification which is why Eq. (1) can be reduced to a steady state equation. Further, the only difference between Eq. (1) and (2) seems to be the transient term. That does not explain how “D” turns to “Pe” and that Eq. (2) is dimensionally incorrect (lhs has dimensions of C per second while rhs has dimensions of c per m2). The conversion needs some characteristic length and velocity scale. Further, Eq. (3) should be in terms of the characteristic quantities and not u and L. Anyways, Eq. (4) can be understood directly by saying the advective effects are negligible in CVI process due to very small Peclet numbers involved. So, Eq. (2) can be omitted.

 

7.     Eq. (8): The exponential should have (-Ea/RT). Further, line 202 better reads “Reaction rate is dependent on the temperature only”.

 

8.     Line 240: Please elaborate what the “backslash” operator does. Is it basically a matrix inversion?

 

9.     Line 265: Figure 4 does not seem to illustrate growth rate of the deposited material.

 

10.  Line 271 and Eq. (15): Should the density in the denominator be “rho_film”?

 

11.  Line 315: Symbol for time should be “t” instead of “T”.

 

12.  Table 1: The table shows some very interesting data but lacks discussion. For instance, consider the following:

 

a.     Comparing case 2 with 3, although they had similar initial porosity and were processed for the same amount of time, the final porosity in the low temperature case is smaller. This might be due to the faster kinetics closing the outer surfaces faster at high temperature.

b.     Comparing case 1 with 5 and 2 with 4, Shows that the samples have reached terminal porosity by almost 25-30 hours. Processing at similar conditions for twice the time did not show much change in final porosity.

c.     Is there a comparison of case 1 with 6? They seem to have similar processing conditions but very large difference in final porosity. May be adding a visualization of the cases would help. One possible factor could be formation of large inaccessible pores in case 6 that limits the densification in some regions. Again, a picture showing the geometry would help.

Comments for author File: Comments.docx

Comments on the Quality of English Language

Please perform a thorough grammar check.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

My detailed comments are as follows:

1.      If only to apply a mature method to a new field, the innovation of this paper is not outstanding. The innovation of this paper should lie in the construction of algorithms for CVI process with surface chemical reaction based on MFS. It is suggested that this article should be edited to highlight the innovation and focus on how to build CVI process simulation models based on MFS.

2.      In the introduction part, the author should provide more detailed background information about the current research status of CVI process simulation methods, and discuss the advantages and disadvantages of existing methods, so as to introduce the innovation of the work in this paper. There is only one relevant research mentioned in this paper (Ref [13]), which is an insufficient discussion.

3.      The introduction section should be simplified, and some contents not closely related to the topic of the article are suggested to be deleted. For example, line 131~138.

4.      Both line 236~240 in Page 8, and line 307~310 in Page 12 discussed the relevant contents on representative nests of fibers used to build the computational domain. It is suggested to provide more details about how to build the computational domain by place fibers randomly, especially the procedure and the rule to ensure the desired porosity is achieved.

5.      Please provide the explanation of the physical quantities represented by the symbol of δ and c_wall in the Formula (14).

6.      As shown in Figure 5 b, there is an interface layer on the surface of the fiber. Is the CVI process simulation in this paper based on the SiC fiber or the SiC fiber with an interface layer? No information about the interface layer is mentioned in the paper, and neither Figure 4 nor Figure 5 show the existence of the interface layer. If considering the existence of the interface layer, please explain whether the meshless method of fundamental solutions developed in this paper is applicable.

7.      According to the description in the paper, the computational domain is generated by placing fibers randomly at a time until the desired porosity is achieved, however, the position and size of the fibers in Figure 5 b which shows the computed infiltrated geometry by MFS and that in Figure 5 b which is obtained experimentally are exactly consistent. Please provide a method for generating representative nests of fibers with the same fiber distribution as observed from the experiment.

8.      Page 317，line 307~317. The discussion on the computational results in this section is not clear enough, please re-discuss the comparison between the computational results by using MFS and the experimental results.

9.      Page 318, in the paragraph above Table 1, please remove the contents that does not make sense, for example, the last sentence which discusses the reranking of the samples from the least porous to most porous after infiltration.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

 

1.      The aim of this work should be introduced clearly in the introduction section. Not the paper structure.

2.      Before theoretical model and numerical methodology, the physical model in three directions should be introduced. For the convenience of simulating calculations, some basic assumptions need to be made.

3.      The software used in this work needs to be introduced.

4.      In table 1, the accuracy of the decimal places must be consistent.

5.      It’s better to draw the conclusions in sections briefly.

6.      The format of the references is severely chaotic.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Comments for author File: Comments.doc

Comments on the Quality of English Language

Author Response

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Author Response File: Author Response.doc

Reviewer 3 Report

Comments and Suggestions for Authors

1.      The physical model in three directions should be introduced with 3D graphics or diagrams.

2.      In Table 1, the Initial computed porosity of specimen 6 was 0.702 with three decimal points. The other computed porosities had four decimal points. Process these computed porosities in the same accuracy.

3.      It’s better to draw the conclusions in sections briefly. Not to summarizing at the end of each section. Suggest to write the conclusion in several clauses.

Author Response

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Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Accept

Author Response

We appreciate your decision to accept the manuscript