Cu-Al2O3 Water Hybrid Nanofluid Transport in a Periodic Structure
Round 1
Reviewer 1 Report
The authors presented a model to simulate a periodically fully developed flow through a wavy channel by finite volume method. The authors concluded that It is found that increasing the slope of wavy channel and nanoparticle volume fraction will both result in heat transfer enhancement with the cost of increasing pumping power. I ask the authors to clearly address the following comments.
Major comments:
Instead of simply stating “Periodically fully developed nano-hybrid fluid flow in a wavy channel has not been investigated in the literature” in the introduction section, the authors should also list its standing position and importance in industrial/real world applications to show the impact of this work. “Not been investigated” is not a sufficient motive. The authors have even more discussions (about 1600 words) in the introduction section (which is only about what other researchers did) instead of in the results and discussion section (less than 700 words). Unless this is a review paper, otherwise the fruitfulness of technical discussion must be significantly improved. Furthermore, the current contents of result and discussion sections are simply repeated information of what I can easily obtain from those figures and tables with no technical/physical explanations.
Minor comments:
There are way too many typos and grammar errors such as
Line 15. An extra “.” after “fluid”.
Line 20. Amplitude of what?
Line 28. An extra “or” before “channels”.
Line 74. It should be “Beside these numerical studies,”.
Line 121. Should be “2” and “3” should be superscripts.
Line 123. “The SIMPLE FVM.”.
Line 133. Should be “scale form A2”, right?
Line 166. An extra space after “[43]”.
Line 167. “the either”.
It seems like the manuscript was not carefully prepared. The authors didn’t even delete the part explaining how references should be listed and Fig. 9 does not have sub-figures (a) and (b). I’ll just stop here as there are too many mistakes; please double check the whole manuscript again.
I think the authors missed the figure legend for “0%, A_0.75” curve in Fig. 5(a).
Author Response
The authors would like to thank the reviewer for all the comments.
Comment:
Instead of simply stating “Periodically fully developed nano-hybrid fluid flow in a wavy channel has not been investigated in the literature” in the introduction section, the authors should also list its standing position and importance in industrial/real world applications to show the impact of this work. “Not been investigated” is not a sufficient motive.
Reply:
The last section of the introduction has been modified to reflect the suggested change by the reviewer as follows
“ Periodically fully developed hybrid nanofluid flow in a wavy channel can be an asset for potentially a host of industrial applications were compact design combined with an efficient rate of heat removal is required, which could result in savings of material, weight, and cost. Therefore, the focus of this paper is to study hydrodynamically and thermally periodically fully developed flow and heat transfer of Cu-Al2O3-water based hybrid nanofluid in a wavy channel with a uniform boundary temperature. ….”
Comment:
The authors have even more discussions (about 1600 words) in the introduction section (which is only about what other researchers did) instead of in the results and discussion section (less than 700 words). Unless this is a review paper, otherwise the fruitfulness of technical discussion must be significantly improved. Furthermore, the current contents of result and discussion sections are simply repeated information of what I can easily obtain from those figures and tables with no technical/physical explanations.
Reply:
The introduction is shortened as recommended (about 980) and the results and discussion has been enriched with more physical explanations (1080). The section with changes has been highlighted in the revised manuscript.
Minor comments:
There are way too many typos and grammar errors such as
Line 15. An extra “.” after “fluid”.
Replay:
The extra “.” Is removed
Line 20. Amplitude of what?
Reply:
It has been modified “… five wavy amplitude of the channel shape for ….”
Line 28. An extra “or” before “channels”.
Reply:
The extra “or ” is removed.
Line 74. It should be “Beside these numerical studies,”.
Reply:
Corrected.
Line 121. Should be “2” and “3” should be superscripts.
Reply:
Corrected.
Line 123. “The SIMPLE FVM.”.
Reply:
Corrected.
Line 133. Should be “scale form A2”, right?
Reply:
Corrected.
Line 166. An extra space after “[43]”.
Reply:
Corrected.
Line 167. “the either”.
Reply:
Corrected.
It seems like the manuscript was not carefully prepared. The authors didn’t even delete the part explaining how references should be listed
Reply:
Corrected.
and Fig. 9 does not have sub-figures (a) and (b).
Reply:
Corrected.
I’ll just stop here as there are too many mistakes; please double check the whole manuscript again.
Reply:
Done. The entire manuscript has been proof read.
I think the authors missed the figure legend for “0%, A_0.75” curve in Fig. 5(a).
Reply:
Corrected.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript presents a 2D analysis of a periodically fully developed flow through a wavy channel module. The paper is generally well-written. The results are clearly presented and conclusions are supported by the results. The study is potentially significant to the nanofluid community, in terms of potential techniques to enhance heat transfers in nanofluid. I suggest the manuscript to be accepted for publication after revision. Some specific comments are:
The first paragraph of the introduction is way too long. While there is a really short paragraph. I suggest the authors to adjust the introduction, by shortening unnecessary parts. Some recent works on nanofluid with Al2O3 may be worth mentioning in the literature review:
CFD study of heat transfer and fluid flow in a parabolic trough solar receiver with internal annular porous structure and synthetic oil–Al2O3 nanofluid
by Bozorg et al.
3. What's the physical meaning of beta in Eq (7)?
4. Please specify the quantity name and unit for the color legends in Figure 10.
5. Please fix the language and formatting issues
Author Response
The authors would like to thank the reviewer for all comments.
This manuscript presents a 2D analysis of a periodically fully developed flow through a wavy channel module. The paper is generally well-written. The results are clearly presented and conclusions are supported by the results. The study is potentially significant to the nanofluid community, in terms of potential techniques to enhance heat transfers in nanofluid. I suggest the manuscript to be accepted for publication after revision. Some specific comments are:
The first paragraph of the introduction is way too long. While there is a really short paragraph. I suggest the authors to adjust the introduction, by shortening unnecessary parts.
Reply:
The introduction is shortened as recommended (about 980) and the results and discussion has been enriched with more physical explanations (about 1080). The sections with changes have been highlighted in the revised manuscript.
Comment:
Some recent works on nanofluid with Al2O3 may be worth mentioning in the literature review:
CFD study of heat transfer and fluid flow in a parabolic trough solar receiver with internal annular porous structure and synthetic oil– Al2O3 nanofluid
by Bozorg et al.
Reply:
The suggested reference has been added in the introduction
" ... systems. Bozorg et al. [39] employed a porous fin and synthetic oil-Al2O3 in parabolic trough solar receiver system, they found that the thermal and over all efficiency of the system increase linearly with Reynolds number, whilst the inclusion of the porous fin increases the required pumping power.
Comment:
3. What's the physical meaning of beta in Eq (7)?
Reply:
Please note in this case Beta is a constant that stands for coefficient of the linear component of the pressure field. The pressure field is decomposed into two parts, one that vary linearly with the coordinate direction at a constant and a portion that behaves in a periodic manner from periodic unit to another [54,55]
Comment:
4. Please specify the quantity name and unit for the color legends in Figure 10.
Reply:
Done.
Comment:
5. Please fix the language and formatting issues
Reply:
Done.
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors reported a numerical simulation study of the Cu-Al2O3 water hybrid nanofluid transport in a wavy periodic structure for Reynolds Number ranging from 100 – 1000. It has found that, by increasing the amplitude of the wavy structure and the volume fraction of the nanofluids, the heat transfer could be enhanced but the fraction factor also increased. A correlation between the Nusselt number, fraction factor and the amplitude, Reynolds number and nanofluid volume has also been proposed. I think the result is interesting and well presented. However, I still have some technique concerns that should be resolved:
From the equations and Figure 1, it seems that the computational domain is simplified to a 2D domain, with the Z-direction steady and infinite long, hence ignored. This assumption should be explained and justified. In the simulation results validation, only a “circular tube of 12.4 mm diameter” case was simulated and compared with previous simulation and experimental results. I feel this is different from your computational setup. More specifically, your setup is a 2D, with z-direction identical and ignored domain. But this case is a circular tube. The circular tube computational results are reliable, it does not strongly suggest that the 2D setup results are also reliable. If you are using some general purpose, reputed CFD software, that’s another story, but should be mentioned more clearly. For figures 3,4,7,8 etc, the x-axis shows the length along the wavy domain, ended with “1”. It seems to be a dimensionless length. Is it non-dimensionalized over the wave length? Additionally, this sort of equations are more suitable to be tackled with dimensionless equations. Nondimensionalization seems to be a good choice for these equations.
Overall, I support the publication of this manuscript, but requests that the above issues addressed. Thanks.
Author Response
The authors would like to thank the reviewer for all comments.
The authors reported a numerical simulation study of the Cu-Al2O3 water hybrid nanofluid transport in a wavy periodic structure for Reynolds Number ranging from 100 – 1000. It has found that, by increasing the amplitude of the wavy structure and the volume fraction of the nanofluids, the heat transfer could be enhanced but the fraction factor also increased. A correlation between the Nusselt number, fraction factor and the amplitude, Reynolds number and nanofluid volume has also been proposed. I think the result is interesting and well presented. However, I still have some technique concerns that should be resolved:
Comment:
From the equations and Figure 1, it seems that the computational domain is simplified to a 2D domain, with the Z-direction steady and infinite long, hence ignored.
Reply:
Indeed the computational domain has been simplified to 2D by assuming the physical domain in the z-direction is long enough to ignore the wall effects in that respective direction. In this particular case since we are also assuming the flow is laminar, we believe this assumption is reasonable. If it was turbulent flow it may not be so, since vortices and turbulent structures could be traveling cross stream which would enhance the transport exchange and the three dimensionality of the flow is indeed more critical.
Comment:
This assumption should be explained and justified. In the simulation results validation, only a “circular tube of 12.4 mm diameter” case was simulated and compared with previous simulation and experimental results. I feel this is different from your computational setup. More specifically, your setup is a 2D, with z-direction identical and ignored domain. But this case is a circular tube. The circular tube computational results are reliable, it does not strongly suggest that the 2D setup results are also reliable. If you are using some general purpose, reputed CFD software, that’s another story, but should be mentioned more clearly.
Reply:
The computational domain for the validation is an axisymmetric section for nanofluid flow in a tube, and this done out of necessity since no studies that involves asymmetric wavy periodic shape that employs nanofluid is at hand. Certainly, it would have been better to have such comparison as you suggested if it were available. It is not uncommon to validate using the tube geometry i.e. Rashidi et al. [33] investigated developing flow in a wavy symmetric channel with nanofluid whereas the validation is done on flow in a horizonal tube with constant temperature boundary condition [42].
Comment:
For figures 3,4,7,8 etc, the x- axis shows the length along the wavy domain, ended with “1”. It seems to be a dimensionless length. Is it non- dimensionalized over the wave length?
Reply:
It is true as you mentioned the results presented in Figure3,4,7 …are cast in dimensionless form to show the relative impact of i.e. amplitude, particle volume fraction, on the local wall shear stress (WSS), heat transfer, etc. Please note that all domains considered have the same foot-print, therefore, we able to normalize the results as such, which makes the comparisons visually easier.
Comment:
Additionally, this sort of equations are more suitable to be tackled with dimensionless equations. Nondimensionalization seems to be a good choice for these equations.
Overall, I support the publication of this manuscript, but
requests that the above issues addressed. Thanks.
Reply:
As you pointed out tackling the non-dimensional form of the equations is always a very good approach. In this work we selected to solve the equations using the primitive variables. Therefore, we conducted nearly 100 numerical experiments to cover the range of Reynolds number, different amplitudes, and nanofluid vs. hybrid nanofluid. Subsequently post process the results to cast it in non-dimensional parameters.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Major comments:
By comparing Figs. 7(a) and 8(a), it is quite clear that the peak Nu numbers are pretty much the same at both top and bottom surfaces. This means that the peak Nu number is nearly independent of top or bottom surfaces. However, when it comes to the comparison of Figs. 7(b) and 8(b) where the only difference other than surface locations is the Re number (500 vs. 300), the set with higher Re number results in lower peak Nu numbers (55 vs. 75 for 2% hyf, A_0.5). Generally in fluids, increasing Re number results in increasing Nu number (as also shown in Fig. 9). Why is it opposite here? Normally, I consider this as a minor issue but there are still way too many typos and grammar errors throughout the whole manuscript. For example, I found all these right after I start reading the revised manuscript:
Line 84. An “a” before “parabolic” is missing and the “,” after “system” should be “.”.
Line 89. It’s “where” not “were”, right?
Line 93. “In this work, Reynolds number range of 102 - 103, volume fraction of 1%-2% and five amplitudes of wavy channel shape.” is not even a complete sentence.
Line 95. “equations are discretization using the FVM”.
Again, I’m not going to list all of them here. It seems like not even the revised sections were carefully proof read before resubmission and the authors only corrected the part that I pointed out last time. Please do double check the whole manuscript again.
Minor comments:
Please cite some references for “a host of industrial applications were compact design combined with an efficient rate of heat removal is required, which could result in savings of material, weight, and cost.” if there are any. Since figs. 3 & 4 share the same explanation/phenomena, I suggest to have them combined as (a) ~ (d) in one single figure. Same for Figs. 5 & 6 and Figs. 7 & 8. There is no point to have a new set of figures with only one sentence stating that this shows the same situation. The authors have two ways of presenting the parameter of wavy channels, A1 ~ A5 corresponding to A_1.25 ~ A_0.25. However, A_1.5 which is not defined (150% I suppose) showed up several times while A5/A_0.25 never did. Also, the parameters shown in most figures are presented in both ways at the same time. This is very confusing. Please only use one set of symbols to present only the parameters used in the manuscript. The discussion of figure 10 is simply the figure caption. Please at least link them to the phenomena observed in the previous figures or insert them into previous figures as sub-figures to support the discussions. Otherwise, it seems pointless to include these figures here. The equation number (25) is not aligned with the corresponding equation.
Author Response
The authors would like to thank the reviewer for all the comments.
Comment:
By comparing Figs. 7(a) and 8(a), it is quite clear that the peak Nu numbers are pretty much the same at both top and bottom surfaces. This means that the peak Nu number is nearly independent of top or bottom surfaces.
Reply:
That is correct. There is only a shift in where the peak occurs, and this is mentioned in the manuscript “ … The improvement in the heat transfer is similar to the channel top surface and the shift in the location of the peak is attributed to the asymmetry of the channel.”
However, when it comes to the comparison of Figs. 7(b) and 8(b) where the only difference other than surface locations is the Re number (500 vs. 300), the set with higher Re number results in lower peak Nu numbers (55 vs. 75 for 2% hyf, A_0.5). Generally, in fluids, increasing Re number results in increasing Nu number (as also shown in Fig. 9). Why is it opposite here?
Reply:
That is correct. However, 7(b) and 8(b) **cannot be directly compared since 7(b) is Re = 500 and the wave form is A_0.5, whereas, 8(b) is Re = 300 and the wave form is A_1.0. So in Figs. 7(a) and (b) we wanted to make a point about the geometric effect. Whereas Figs. 8(a) and (b) we wanted to make a point about the effect of Reynolds number on the local Nu which increases with increasing Re as expected. Although the graph caption is stated correctly, there is a typo in the legend of Fig. 8(b) and is now fixed thanks to your comment. We also added the following to clarify Figure 8(a) and 8(b) on line 258,
“…. of the channel .Moreover, Figure 8(a) and Figure 8(b) show the effect of changing Reynolds number from 500 to 300 while having the same wave amplitude. This results in decreasing the Nusselt number by nearly 20%.”
We traced back the error of the typo … we have been using template on excel in order to have consistent figures in terms of color for the various working fluids and in this particular case the labels of the columns which makes the legend where not correctly updated.
**Please note the Figure numbers in the revised manuscript has been changed as follows
7(a) --> 5(a)
7(b) --> 5(b)
8(a) --> 5(c)
8(b) --> 5(d)
Comments:
Line 84. An “a” before “parabolic” is missing and the “,” after “system” should be “.”.
Reply:
Corrected.
Line 89. It’s “where” not “were”, right?
Reply:
Corrected
Line 93. “In this work, Reynolds number range of 102 - 103, volume fraction of 1%-2% and five amplitudes of wavy channel shape.” is not even a complete sentence.
Reply:
“ …In this work, we examine Reynolds number in the range of 102 - 103 and particle volume fraction in the range of 1%-2%. Five different amplitudes of wavy channel were tested.”
Line 95. “equations are discretization using the FVM”.
Reply:
Corrected.
Minor comments:
Please cite some references for “a host of industrial applications were compact design combined with an efficient rate of heat removal is required, which could result in savings of material, weight, and cost.” if there are any. Since figs. 3 & 4 share the same explanation/phenomena, I suggest to have them combined as (a) ~ (d) in one single figure. Same for Figs. 5 & 6 and Figs. 7 & 8. There is no point to have a new set of figures with only one sentence stating that this shows the same situation.
Reply:
Thank you for the suggestion, we merged the figures as recommended.
The authors have two ways of presenting the parameter of wavy channels, A1 ~ A5 corresponding to A_1.25 ~ A_0.25. However, A_1.5 which is not defined (150% I suppose) showed up several times while A5/A_0.25 never did. Also, the parameters shown in most figures are presented in both ways at the same time. This is very confusing. Please only use one set of symbols to present only the parameters used in the manuscript.
Reply:
Thank you for the suggestion, now we present the wave parameter (A1-A5) in a unified manner throughout the manuscript including the legends of the all the figures.
The discussion of figure 10 is simply the figure caption. Please at least link them to the phenomena observed in the previous figures or insert them into previous figures as sub-figures to support the discussions. Otherwise, it seems pointless to include these figures here.
Reply:
Thank you for the suggestion. Figure 10 is eliminated from the manuscript. It is more suited for the graphical abstract.
The equation number (25) is not aligned with the corresponding equation.
Reply:
Done.
Author Response File: Author Response.pdf
