Using Modeling to Select the Type of Microwave Field Emitter for Dense-Layer Grain Dryers

Round 1

Reviewer 1 Report (Previous Reviewer 3)

Different radiating structures are described in the paper "Microwave field distribution in grain drying installations for different types of emissions" to provide even heating while using microwave heating. The second review process is this one.

Since the majority of the concerns highlighted in the prior review have not been resolved, I will once again list them here before I can recommend publication:

- Although the authors' study is titled "...in grain drying installations...," it is surprising that they neither test the radiating structures with grain at the appropriate moisture level nor inside the microwave applicator. Authors should change the title or change the results they show in their contributions. Due to the title they propose this is not a "a separate topic of research." 

- The authors state in response #3 that "..this allows us to estimate the depth of penetration of the field into the processed material...." Since the permittivity information is ignored, this statement is completely false. To calculate the temperature distribution in microwave heating, a crucial parameter is the material's penetration depth. The electric field distribution is also significantly influenced by the applicator geometry. Since these parameters are not considered the obtained results are not very useful for microwave grain drying.

- Response 4 is not how microwave applicators are typically designed. Typically, when designing a microwave cavity, the modes of the applicator, the material's dielectric properties, its geometrical properties, and the radiating structures are all taken into account simultaneously. However, because the authors do not offer any experimental validation, it is impossible to evaluate their suggestion. Once a microwave applicator is simulated, the consistency of the microwave heating process is then examined using uniformity indicators. In this work, no uniformity indicators are used at all.

- All of the radiating estructures' dimensions, including the size of the slots and their separation from one another, should be specified. Authors should take into account that reproducibility of research is a very important point for the research progress.

- In answer to comment #9, authors employ radiation diagrams to demonstrate the homogeneity of microwave heating. This is completely useless. As mentioned in the prior review, authors should employ standard deviation-based uniformity indicators from the literature or other statistical techniques to investigate how the electric field is distributed in the vicinity of radiating structures.

Additionally, the radiating behaviour of the proposed structures are not providing a uniform radiation pattern: rectangular waveguide shows differences of 15 dB of radiating amplitudes whereas semicircular waveguide show differences around 10 dB. This is not acceptable as uniform radiation.

- Regarding point#12, I insist that usual notation should be used such as propagating modes, cut-off frequency, etcetera... so that readers do not get confused. Please refer to "Industrial Microwave Heating" book or David Pozar's book in order to see the usual concepts used in microwave and microwave heating technologies.

- The propagation features inside figures 3 and 4 should be provided and related to the fundamental modes of propagation. In fact, again I emphasize this, in figure 3 the magnetron is placed in the narrow wall of the rectangular waveguide and this should be explained and analyzed in detail since the fundamental mode of the rectangular waveguide , TE10 is not used.

- No experimental validation is performed for the simulations of the radiating structures. Usually, I recommend rejecting contributions that do not provide proper validation.

- Point 17 of the prior review indicates the radiating structure should be redesigned to obtain a proper matching at 2.45 GHz. This has not been done.

To finalize with this review I summarize the main points that should be fixed so that I can recommend accepting the paper:

- Changing the title of the paper or its content.

- Adapting the terminology to common microwave and microwave heating concepts.

- Providing all the dimensions, antennas used, etcetera so that the paper results can be reproduced if needed by other researchers.

- Validate with proper tests the simulation results: radiation patterns, heating uniformity, etcetera

- Including uniformity heating indicators that make use of the electric field distribution data and material permittivity.

- Improving the radiating features of the proposed slot antennas since now they are not radiating uniformily at all.

I hope that these comments help authors to improve their contribution and their research methods in this complicated area of microwave heating and applicator design.

 

 

 

 

English needs to be polished. Use of standard microwave and microwave heating vocabulary is very crucial. I suggest using skilled academic writers with writing expertise or specialized technologies for grammar checking and paraphrasing.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

The manuscript called "Microwave field distribution in grain drying installations for different types of emissions" consists of valuable, up-to-date research, the reviewer has minor comments that could further enrich the manuscript.

In the "Introduction" section, it would be valuable to simulate the costs of the proposed installation compared to traditional grain processing methods (optional request, please include if possible).

In the case of Figures 11 and 13, I am asking for better quality, because the descriptions on the legend are hard to read.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report (New Reviewer)

1. The title should revise, better to revise according to the study. 

2. The abstract is not clear, better to be specific in using certain issues like "dense grain layer"

3. In the introduction give some information related to the role of dryers in the storage of grains

4. Inte text used (see figure),  just use as the (figure )

5. Materials and methods need revision, give clarity and justification for each methodology selection. 

6. There are many images/ figures in the text, you can reduce them or collage them. 

7.  Need English language corrections

8. Chekc for the English clarity

9. Discussion is limited and poor

10. Revise the conclusions according to the objective of your study.

 

   

 

 

Need to revise the English language by a professional. 

Sentence construction is poor

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report (New Reviewer)

The manuscript has investigated the effects of microwave field distribution on grain drying for different types of emissions. The topic is interesting; However, the manuscript has several problems:

1. The title should be more specific. What kind of grain? It should be mentioned in the title.

2. Please check the style of citations; for example, L 29 should be "[3,4]".

3. Add "Materials" and "Statistical analysis" subsections into the Materials and Methods part.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report (Previous Reviewer 3)

While some of the issues brought up throughout the previous review process have been resolved, others of great importance have not. This paper's goal is to choose the ideal antenna arrangement for uniform grain drying with microwave radiation.

Authors, however, fail again to include accurate uniformity indicators. They use the radiation patterns as indicators although these radiation patterns are not useful when the material and/or the applicator are not considered. What justifies the claim that one radiation structure provides more uniform radiation than another? The authors do not at all explain this with mathematical tools. For example, figures 13 and 16 demonstrate how the irradiation amplitude of slotted waveguides varies significantly with respect to the irradiation angle but they choose the semi-cylindrical slotted waveguide as the better option. The authors do not take into account the propagating or resonating modes of the microwave applicator, sample movement, or sample permittivity.

Again, it is not possible to reproduce the results because all the dimensions in figures 2,3, and 4 are not provided.

 In general, the primary issues raised in the prior review process have not been resolved. Authors are recommended to read appropriate contributions for heating uniformity and apply appropriate uniformity indicators to assess this sort of irradiating structures. None of the most pertinent references regarding microwave irradiation uniformity are mentioned.

I now comment on the points of the previous review and the answers provided by the authors:

POINT 1:

My main concern here was that the authors should apply and test their proposed irradiating structures with the materials to be dried. Although the title has been changed to "Selection of microwave field emitter types for dense layer grain dryers," they still insist on grain as test material, but they do not test the antennas versus any type of grain.

I agree with the authors that this type of dryer is very particular, but the authors should provide a technique or indicator that can help designers select the best antenna in terms of heating uniformity. They again fail on this point. So, I do not see any improvement at this point.

 POINT 2:

The authors provide in their answer very interesting figures (2, 3, and 4) showing the distribution of absorbed power density (microwave heating term) in wet grain. Unfortunately, they do not introduce them in the contribution and do not use them to extract heating indicators to select the best antenna structure.

In fact, from those figures, it seems that a rectangular slotted waveguide might produce more uniform heating patterns than a semi-cylindrical slotted waveguide. Depending on the uniformity indicator used, the decision might change.

 Therefore, I insist that the proper design of these radiators must take into account the material surrounding the antenna and the computation of proper heating uniformity indicators.

In the case of figure 4, where the electric field travels a greater distance, the boundary conditions of the applicator walls and other antennas’ radiation patterns may influence the uniformity.

The authors try to explain an algorithm for the design of these applicators, but this is not explained in the paper and is not based on proper indicators. They ask this reviewer to judge the paper based on this algorithm, but they should understand that the paper should be self-explanatory and based on proper mathematical tools for choosing the best radiating structure.

 POINT 3:

Although I agree with the authors in the fact that "high-quality experimental studies require significant capital investments," an experimental validation is very important to know if what they propose is true or not. I would suggest measuring the radiation patterns of the proposed antennas at least. But I usually reject any contribution that does not offer validation of the simulation results.

 POINT 4.

Many details about the dimensions of the radiating structures are still missing, so the simulations cannot be reproduced. The dimensions and structure of the magnetron antennas should also be provided. Reading the paper, I do not know how they simulate the magnetron. I again miss the electric field distribution inside the radiating structure, especially to see how the magnetron is working in the rectangular slotted waveguide. The dimensions provided are still insufficient to be able to reproduce simulations. This is also a critical point.

 POINT 5:

I insist that authors provide a heating indicator and not use the radiation diagram as a uniformity indicator. I am not capable of choosing the best radiating structure from figures 11, 13, 15, or 16. For instance, in figure 16, the radiation pattern of the semicircular waveguide and the horn have very similar differences for the maximum and minimum values.

 POINT  6: OK

 POINT 7:

 My intention in this point is that authors also show the electric field inside the waveguides in order to see the magnetron antenna’s coupling to the waveguides. It is interesting, especially for the rectangular waveguide, because this configuration is not usual. This has not been shown in the paper.

 POINT 8:

Authors should provide, at least, a validation of the radiation patterns. If this is not done, nobody can be sure that the provided results are correct. This is a quality standard for the published research.

 

  POINT  9: OK

 I hope that these comments clarify my review and help authors to improve their contribution in this important and difficult area of microwave heating uniformity. I am sad to say that the main points I raised in my previous review have not been fixed and therefore I recommend again rejection.

No comments

Author Response

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Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

The manuscript presented for review covers interesting topic.
I have some questions: 1. What was the economic cost of making the best waveguide?
2. What kind of oven or dryer could you use for a given type of waveguide?
3. It is possible to use another mathematical model for your research, such as CST Microwave
Studio Software?

 

 

Reviewer 2 Report

 

Abstract section: please improve the English gramar in this sentence: Application of microwave electromagnetic field in processing plants for drying, pre-seeding treatment and disinfection of grain makes it possible to increase their performance rate, as well as to reduce their energy consumption.

Introduction section:

-Replace “grain of the wheat arrives” by the grains …arrive..

-Several authors developed drying systems with combined microwaves and hot air. Please add bibliographic references

Materials and methods section

-How do you design the waveguides? How do you set the distance between the slots?

-Please replace “chequer-wise stacked grid”, because the mesh is not always square

-Please add the versión of the finite element software and the solution implemented in CST Studio.

Results section

-Line 174: You said: Opposite field directionality of horns, in two located in series zones, is used in the upper half of the processing plant, where moisture content of grain under processing gets lower and, therefore, the intensity of microwave field applied to the grain layer can be increased.

I think the grain with lower moisture is not able to interact with microwaves, therefore the intensity decreases not increases…

-What do you mean with “Phase of the field in the aperture changes in accordance with a nearly square law”? Please clarify

-Line 222: which dependences do you refer?

-Line 224: replace elected by selected

English grammar has to be improved in the whole manuscript

Reviewer 3 Report

To the authors:

The three different radiating techniques discussed in this study make use of horns and slots in both rectangular multimode waveguides and semicircular waveguides. I have the impression that the study was carried out in a far field assumption without considering the microwave oven geometry or the material permittivity.

I thank authors for their effort in analysing these radiating structures. However, there are many points that should be explained in a better and concise way, and some major points should be included in the paper so that it is really useful for designing microwave dryers for grain using these radiators.

MAJOR ISSUES

- How is grain taken into account in simulations? This should be explained in the description of the simulation scenarios.

- If the applicator constraints and grain permittivity are not taken into account in these simulations, many of the conclusions may not be applicable to near field circumstances, where the radiating structures are typically deployed. So a proper validation of these simulations in far field should be made by comparing to the situation of these radiators in near field conditions.

Additionally, the modes excited inside the applicator  by the radiating estructures would also influence the electric field distribution within the grain,  which could be very different from the free-space situation.

- There are numerous unknowns in simulation settings. Dimensions, boundary conditions, excitation ports and excitation antennas should all be included in figures, in my opinion. How are the waveguides and horns excited? Coaxial probes are employed? If this is the case, the findings could be incorrect because magnetron antennas behave significantly differently from coaxial to waveguide transitions. Therefore, it is important to accurately identify the dimensions in figures 3-5 and excitation structures.

- In figure 3, why is the magnetron placed in the narrower wall of the waveguide instead of installing it on its wider wall? Which mode or modes are exited (this is not the usual TE10)? Please explain this in detail. The details of the exciting antenna are also important to know which waveguide modes are initially excited in the waveguide. 

- In section 3, results and theoretical explanations coexist. Please create a separate new section for theoretical topics. Results section should only include figures and tables with results.

- Figures 6a and 6b might be removed from the authors' explanation of horn antennas since they are both too simple and too detailed.

- Although these antennas are usually operating in close environments, the authors use several far field equations, such as directional gain or wavelength. Comment on this matter, please.

- The authors do not offer any uniformity indicators while comparing several radiation approaches in terms of microwave heating uniformity (in lines 246-247, for example). The literature on microwave heating has some helpful indicators, most of which make use of the normalized standard deviation for the electric field module raised to 2. Please use some of these metrics to give unbiased system comparisons.

This comparison must be carried out when irradiating the material in near field and taking into account the permittivity of the material.

- Please substitute wave types for the standard notation: waveguide propagation modes (lines 269–274). Additionally, the authors use the term "multiwave" when they really mean "multiple modes."

- Waveguides allow for the propagation of modes rather than wave oscillation (line 292). Please make this modification.

- Some of the multimode propagation concepts can be summed up and are too fundamental for a lengthy exposition in section 3.2.

- A proper reference for multimode waveguide to coaxial excitation should be provided in case that multimode propagation is used. 

- Finally, the authors' suggestions for improving microwave heating uniformity are solely supported by simulations; a set of thermographies is not displayed. 

MINOR ISSUES:

- It would be preferable to show the reflection coefficient or power efficiency for the antenna in figure 9a rather than SWR data. Figure 9a, however, demonstrates that while this horn is not optimum for 2.45 GHz, it would be good for radiating at empty space at 2.2 GHz (SWR=1). Perhaps a smaller antenna might radiate more effectively. Please comment this issue.

- Additionally, depicting the power flow in vector representation (Figure 9) is not particularly helpful. SAR, or volumetric heat generation within the grain body, would be far more interesting. CST provides these field monitors.

- Are the calculations of Figure 12 referred to a mode, to multiple modes or to the coaxial feeding of the radiating structure?

- It would be better to substitue Figure 13b by volumetric heat generation taking into account grain permittivity. 

- Instead of critical wavelenth it would be also better to use the cutoff wavelenth term (line 357)

- In my opinion better results for the slotted waveguides could be obtained with non uniform lenghts for the slots. The first slots, which receive more power density could be smaller to radiate similar power levels. Please comment this.

 

 

 

No comments about English Language. Some technical terms, however, are not the usual ones such as: "wave types" (waveguide modes), "critical wavelenght" (cutoff wavelength), etcetera.