A Numerical and Experimental Investigation of Dimple Effects on Heat Transfer Enhancement with Impinging Jets
Round 1
Reviewer 1 Report
The article presents a study of the effects of adding dimples to a heat exchange surface that is operated by jet impingement followed by radial flow away from the jet through a narrow gap. Both experiments and numerical simulations were used. The experimental procedures were adequate to characterize the heat transfer and flow effects of adding dimples to the heat transfer surface, and the simulation results corroborated the experimental observations. Unsurprisingly, a treatment which increases both the heat transfer surface area and the turbulence of the flowing liquid increased heat transfer.
However, the English language in the paper is moderately to significantly bad. The paper itself is good work, but the language definitely inhibits understanding.
Author Response
The English language in this paper was already re-checked by Language Centre, SWU, Thailand.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The authors present in my opinion valuable work, in order to enhance the article please improve the article by answering/ including following comments:
- please compare direct area averaged htc values from exp and simulations
- please include temperature profile of the target plate during experiments
- how Re is calculated? based on which characteristic dimension
- I do not fully understand the scope of the study, authors write :
"However, the half sphere shape is difficult in the production process, the cylindrical dimple surface is then modified to use in this work." but authors display cfd simulations of half sphere dimples. Will authors provide cfd comparison of half sphere and cylindrical dimples?
- improve literature review with more recent jet/microjet articles for example:
https://doi.org/10.1016/j.expthermflusci.2018.08.010
https://doi.org/10.1016/j.applthermaleng.2017.05.082
- improve language /grammar , eg ;
line 98 Yunfei X. and Bernhard W. [17] were studied... etc
- please add fig for every configuration with dimensions similar to 1b with radial dimensions of the dimples placement
Author Response
The authors present in my opinion valuable work, in order to enhance the article please improve the article by answering/ including following comments:
- please compare direct area averaged htc values from exp and simulations
Averaged htc (heat transfer coefficient) shown in averaged Nu term () (Figure. 7). Nu is dimensionless from htc.
- please include temperature profile of the target plate during experiments
The temperature profile is added in this paper in Figure. 4.
- how Re is calculated? based on which characteristic dimension
Re was calculated from jet diameter and the eqation.10 is added already.
- I do not fully understand the scope of the study, authors write :
"However, the half sphere shape is difficult in the production process, the cylindrical dimple surface is then modified to use in this work." but authors display cfd simulations of half sphere dimples. Will authors provide cfd comparison of half sphere and cylindrical dimples?
Figure. 3 is changed to be the cylindrical dimples.
- improve literature review with more recent jet/microjet articles for example:
https://doi.org/10.1016/j.expthermflusci.2018.08.010
https://doi.org/10.1016/j.applthermaleng.2017.05.082
In this research, we are interested in the impinging plate only not the jet nozzle. So the recommended articles will not be added in this paper.
- improve language /grammar , eg ;
line 98 Yunfei X. and Bernhard W. [17] were studied... etc
This sentence is already used “were” and the English language in this paper was re-checked by Language Centre, SWU, Thailand.
- please add fig for every configuration with dimensions similar to 1b with radial dimensions of the dimples placement
The Figure 1b and 1c are added to show how it is different in Er and Eq.
Author Response File: Author Response.docx
Reviewer 3 Report
This paper is a combination of experimental and numerical studies .Hovewer the authors does not present enough novelty for the study. What exactly the problem was solved? Authors should more clarify their original contribution. There is no net information about the conclusion in the Abstract. In the Chapter 1.Introduction the authors presented the state of art, unfortunately they did not discuss it with their research.
The authors should remove some similarities from the publication https://www.aidic.it/cet/18/70/213.pdf
The fifth column in table 2 is incomprehensible (d /D-Er-Eθ-B)
Please give some information how are meshes created? Whether the software itself meshed the
geometry or authors were interfered for a better result.
The Simulations paragraph must be vastly improved. No validation was carried out. This is the standard way to prove the goodness of a prediction method.
Chapter 6. Conlusions is poor and insufficient.
Unfortunately, the study seems like the report of an unfinished study. In my opinion the manuscript is far off the level required by an international journal. Based on the previous discussion, I am afraid to say I am not in favour of the publication.
Author Response
This paper is a combination of experimental and numerical studies. However the authors does not present enough novelty for the study. What exactly the problem was solved? Authors should more clarify their original contribution. There is no net information about the conclusion in the Abstract. In the Chapter 1.Introduction the authors presented the state of art, unfortunately they did not discuss it with their research.
This paragraph is more added in the last section of Introduction chapter.
“This research work aims to develop the heat exchanger plate to improve the heat transfer from impinging jet. The surface was designed and tested in many different cases. However, the experiment does not clearly explain the flow behavior on the test-section plate. This is because any sensors cannot be set on the plate, it will obstruct the flow. So, the numerical studies was used to analyzed and describe the experiment. In the simulation part, the friction factor (f) value on the test-section plate is the variable to present the flow obstacle of that device. In this work, the test-section plate with higher f value needs to use the higher pump power to drive the air flowing through the device than the test-section plate with lower f value. However, the TEF is the important variable used to compare the heat exchanged efficiency of the test-section plate. The TEF represents both heat exchanged efficiency and a pump power which are analyzed from Nu and f, respectively.”
The authors should remove some similarities from the publication https://www.aidic.it/cet/18/70/213.pdf
We already removed/re-written some sentences.
The fifth column in table 2 is incomprehensible (d /D-Er-Eθ-B)
In the fifth column we change the title from “(d /D-Er-Eθ-B)” to “Case Study Name (d /D-Er-Eθ-B)”.
We delete the “close” from each case study name in the 4th and 5th column.
Please give some information how are meshes created? Whether the software itself meshed the
geometry or authors were interfered for a better result.
This sentence is added in chapter 4. Simulation of Flow Configuration.
“Fluent in Ansys Program was used to simulate in this work. Grids were manually created by the researcher for defining grid in the line. To resolve the near test plate region, much finer grids were set near test plate. The mesh configuration was tested to be sufficient to provide grid independent results.”
The Simulations paragraph must be vastly improved. No validation was carried out. This is the standard way to prove the goodness of a prediction method.
The simulation and experiment are the same condition shown in Fig.3 and 4. This study also aims to present experimental data which can be used as a validation tool in numerical modeling studies flow on the test plate see in Fig. 7. The validation results are explained in chapter 5.4.
Chapter 6. Conlusions is poor and insufficient.
This paragraph was added in the first sentence of chapter 6.
“The experiment was carried out in this work and the results were used to analyze in the simulation part. In the simulation, the heat transfer behavior and friction factor value on the test plate were investigated for thermal enhancement factor (TEF). TEF can be explained the heat transfer efficiency for the impinging jet on the test plate. Therefore, the optimum condition of the fluid pump power and heat transfer value can be obtained.”
Unfortunately, the study seems like the report of an unfinished study. In my opinion the manuscript is far off the level required by an international journal. Based on the previous discussion, I am afraid to say I am not in favour of the publication.
The authors have re-written and added more important data in this article. We hope that this article will be accepted for the publication.
Author Response File: Author Response.docx
Reviewer 4 Report
A comprehensive numerical and experimental study has been conducted to investigate the efficacy of dimples in increasing the rate of heat transfer of an impinging jet. The work presented is novel and is of publishable value. However, before the paper is published, there are questions which the authors should address (as listed below).
Major concerns
1) There are grammatical errors and issues with sentence structure throughout the paper. Some of them are highlighted below. Professional checking of the paper has to be conducted prior to the publishing of the paper.
2) What CFD software is used? Is it a commercial or an open source code?
3) The number of grid cells is not mentioned in the paper. Has a grid independence study been conducted?
4) Is the size of the outlet of the domain sufficient?
5) From the temperature contours in figure 11, it appears that the simulations might not have fully converged as the color contours are not homogeneous.
6) The paper is also lacking insight on what are the fundamental physics of the flow which contributes to the increase in TEF. This is an important finding and perhaps it can be elaborated further.
Minor concerns
1) Abstract – last sentence – repeat in sentence, `5.5 times’.
2) Table 2 - Mistake in the identifying code for the last two cases.
3) Line 232 - ‘The channel models were analysed by Continuity equation ….’. This paragraph has to be rewritten as it is not clear what the ‘channel models’ are.
4) Repeat sentences - line 229 with line 253.
5) Line 255 - ‘The solutions can be converged…’ to ‘The solutions are converged …’
6) Line 270 - ‘The experiment tested on the spherical plate which can ….’. This sentence is not clear and has to be reworded.
7) Line 273:274 – ‘The inlet air flow velocity ….’ to ‘The inlet air flow velocity is set corresponding to Reynolds number of 1500, 4400, 7300, 10200 and 14600.’
8) Figure 9 and 10 – location of the dimples not shown.
9) Figure 11 – labels (a) to (h) not shown in the figure.
10) Line 437 – ‘The highest TEF…’ to ‘The highest TEF (=5.5) was found for the configuration with a dimple diameter, d = 25px, dimple distance in the radius, Er = 2d, dimple distance in the circumference, Eth = 1.5d, the distance between test plates and jet, B = 2 and Reynolds number, Re = 1500. ’
Author Response
A comprehensive numerical and experimental study has been conducted to investigate the efficacy of dimples in increasing the rate of heat transfer of an impinging jet. The work presented is novel and is of publishable value. However, before the paper is published, there are questions which the authors should address (as listed below).
Major concerns
1) There are grammatical errors and issues with sentence structure throughout the paper. Some of them are highlighted below. Professional checking of the paper has to be conducted prior to the publishing of the paper.
The paper was rechecked already from Language Centre of SWU, Thailand.
2) What CFD software is used? Is it a commercial or an open source code?
Fluent in Ansys was used to simulate in this work.
3) The number of grid cells is not mentioned in the paper. Has a grid independence study been conducted?
Grids were manually created by the researcher for defining grid in the line. To resolve the near test plate region, much finer grids were set near test plate. The mesh configuration was tested to be sufficient to provide grid independent results.
4) Is the size of the outlet of the domain sufficient?
It is sufficient as you can see in the result in Figure 7.
5) From the temperature contours in figure 11, it appears that the simulations might not have fully converged as the color contours are not homogeneous.
The researcher would like to show the color contour at the highest temperature on the test plate and how different. So the color contour range was 2000-3000K. Moreover an almost area on the plate presents the high temperature so the low temperature was deleted around the center of the test plate which is quite different from the high temperature value. The lowest temperature is about 300-400K
6) The paper is also lacking insight on what are the fundamental physics of the flow which contributes to the increase in TEF. This is an important finding and perhaps it can be elaborated further.
This paragraph is added in the last section of Introduction.
This research work aims to develop the heat exchanger plate to improve the heat transfer from impinging jet. The surface was designed and tested in many different cases. However, the experiment does not clearly explain the flow behavior on the test-section plate. This is because any sensors cannot be set on the plate, it will obstruct the flow. So, the numerical studies was used to analyzed and describe the experiment. In the simulation part, the friction factor (f) value on the test-section plate is the variable to present the flow obstacle of that device. In this work, the test-section plate with higher f value needs to use the higher pump power to drive the air flowing through the device than the test-section plate with lower f value. However, the TEF is the important variable used to compare the heat exchanged efficiency of the test-section plate. The TEF represents both heat exchanged efficiency and a pump power which are analyzed from Nu and f, respectively.
Minor concerns
Answer: All of these was rechecked already.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Responses are laconic.
Authors do not provide text with included changes. And do not include requested modifications.
Literature was not updated with newer positions ( find other newer works if suggested are not within the scope).
"line 98 Yunfei X. and Bernhard W. [17] were studied... etc
This sentence is already used “were” and the English language in this paper was re-checked by Language Centre, SWU, Thailand."
is not existent in the text anymore.
My other remark:
"However, the half sphere shape is difficult in the production process, the cylindrical dimple surface is then modified to use in this work." but authors display cfd simulations of half sphere dimples. Will authors provide cfd comparison of half sphere and cylindrical dimples?
Figure. 3 is changed to be the cylindrical dimples."
What does it change?
How the shape of the dimple affect the cfd result ? Can the offset from experiment be explained by different geometry?
Author Response
Comments and Suggestions for Authors
Responses are laconic.
Authors do not provide text with included changes. And do not include requested modifications.
Literature was not updated with newer positions ( find other newer works if suggested are not within the scope).
"line 98 Yunfei X. and Bernhard W. [17] were studied... etc
This sentence is already used “were” and the English language in this paper was re-checked by Language Centre, SWU, Thailand."
is not existent in the text anymore.
Answer. This sentence is already added “were”.
Yunfei X. and Bernhard W. [13] were studied a jet array impinging on flat and dimpled plates by using the transient liquid crystal method. The best heat transfer performance was obtained with the minimum cross flow and narrow jet-to-plate spacing, whether on a flat or dimpled plate. The dimples on the target plate resulted in higher heat transfer coefficients than with the flat plate.
My other remark:
"However, the half sphere shape is difficult in the production process, the cylindrical dimple surface is then modified to use in this work." but authors display cfd simulations of half sphere dimples. Will authors provide cfd comparison of half sphere and cylindrical dimples?
Figure. 3 is changed to be the cylindrical dimples."
What does it change?
How the shape of the dimple affect the cfd result ? Can the offset from experiment be explained by different geometry?
Answer
In the 1st drat, the shape of dimple in figure 3 is not correct for drawing (It shows the haft spherical dimple) but the simulation is correct (the cylindrical shape of dimple was used to simulation). So, the last draft article, the figure 3 was changed to be the cylindrical dimple already (new drawing).
In this research work, we studied the cylindrical dimple because the half spherical dimple results showed the moderate/low heat transfer rate in our previous simulation work (not publish). And in the figure 10, the simulation results indicate that the air can flow into the cylindrical channel, that why the cylindrical dimple is higher in heat transfer rate. Moreover, the cylindrical dimple is easier for machine.
Author Response File: Author Response.docx
Reviewer 3 Report
The authors have improved the manuscript.
Author Response
Dear reviewer
This article is revised for 4 reviewers comments already.
Thank you very much for your comments and suggestions.
Best regards.
Round 3
Reviewer 2 Report
I hope authors now see how unclear their responses are without attaching modified file,
I do not have time to read the whole article then and again and comparing it with the original submission.
Unfortunately I had to, and after changes another issues arise:
- what is the depth of the dimples?
- what is the surface increase after inserting dimples?
- how did authors ensure constant heat flux density condition on the impinged surface?
( the dimple would result in lower thermal resistance and surely higher local heat flux, line 275)
- is jet diameter Nusselt number correct when analyzing wall jet region?
- why outher once cite their work ( figs. 5, 6 ) and then use it as original data ( fig 4 table 2)
- why reference to literature correlation in eq 9 when authors provide own reference case ( flat) ?
Overall provided data analysis is poorly done, and gives an engineering data on various surfaces without any in-depth analysis.
Authors did not respond to all my previous comments, the literature was not updated with newest research beside authors own article.
Overall publication of the article is not purposeful and not recommended.
P.S.
"Answer. This sentence is already added “were”.
Yunfei X. and Bernhard W. [13] were studied a jet array impinging on flat and dimpled plates by using the transient liquid crystal method. "
this statement should be written without "were".
Author Response
Answer the mentions from reviewer no.2
I hope authors now see how unclear their responses are without attaching modified file,
I do not have time to read the whole article then and again and comparing it with the original submission.
Unfortunately, I had to, and after changes another issues arise:
- what is the depth of the dimples?
The depth of the dimples is 0.5 cm. The dimple depth figure is added in Figure 1d and Table 1.
- what is the surface increase after inserting dimples?
The surface is increased a little which is not effect to the result.
- how did authors ensure constant heat flux density condition on the impinged surface? ( the dimple would result in lower thermal resistance and surely higher local heat flux, line 275)
The heat plate was used in this work and the test plate is the copper.
- is jet diameter Nusselt number correct when analyzing wall jet region?
Yes, it is correct, because this work would like to know the heat transfer on the test plate.
- why other once cite their work (figs. 5, 6 ) and then use it as original data ( fig 4 table 2)
Sorry, for this mistake.The Figure 4 and Table 2 are added more for the reference.
- why reference to literature correlation in eq 9 when authors provide own reference case (flat) ?
For the equation no. 9, it can use for all type of modified plate such as flat rib, dimple plate, cylindrical rib etc.
P.S.
"Answer. This sentence is already added “were”.
Yunfei X. and Bernhard W. [13] were studied a jet array impinging on flat and dimpled plates by using the transient liquid crystal method. "
this statement should be written without "were".
“Were” was deleted from the sentence.
All the revised data show with yellow highlight.
Author Response File: 
Author Response.pdf
