Physically Consistent Implementation of the Mixture Model for Modelling Nanofluid Conjugate Heat Transfer in Minichannel Heat Sinks

Round 1

Reviewer 1 Report

In the present study, single phase flow and two-phase flow models of nanofluids were utilized to investigate the forced convection flow and heat transfer in a mini channel heat sink. The results of both models were compared. Generally, it is an interesting study; however, it seems the authors have presented the models vaguely, or I cannot understand the feasible physics behind their model. From what I can read, in the modeling section of the mixture model (2.1.2), the authors are going to assume the nanoparticles as bubbles. So, as I understand, there are two phases 1- Al2O3 nanoparticles (bubbles) and the second, the host liquid (water). Then, the authors introduce another phase, and in the assumptions, they stated, “The diameter of the nanoparticles is smaller than the diameter of the bubbles so that the volume fraction ? of the nanoparticles is analytically smooth and continuous and that the spherical shape of the bubbles is unaffected by the presence of the nanoparticles in it.” So here, there is a distinction between bubbles and nanoparticles.

Moreover, in the single-phase model, it has been stated “3. Both the radiative heat transfer and gravity have been disregarded” while in the mixture model, it has been stated: “Gravitational effects are modeled, and radiative heat transfer is disregarded.”. So, why in one model the gravity effect was considered, and in another gravity, the effect was neglected? If the authors mean the buoyancy effects were ignored, the gravity impact on the slip velocity of nanoparticles was considered. Then they should state it clearly.

In any case, I invite authors to read the paper of “Buongiorno, J., 2006. Convective transport in nanofluids, J. Heat Transfer. Mar 2006, 128(3): 240-250; https://doi.org/10.1115/1.2150834” Using a scale analysis demonstrates the gravity impact and the drag forces for nanoparticles are insignificant. They could be important for micro-sized particles. Thus, the mixture model built here based on the gravity and drag forces seems inaccurate. Authors should adequately justify the correctness of their model first.

After addressing the above comments, I can review the results and other parts of the manuscript.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

I recommend that the authors should make more and more discussion to improve the journal quality. This paper investigated the inconsistent heat transfer predictions of two-phase mixture models and/or single-phase homogeneous models using the commercial software ANSYS FLUENT 19.5. However, the current version of the paper doesn’t include enough variety of calculations to lead to the conclusions. The authors calculated only the simple and small schematic as seen at Figure 1 (a) and (b), so that couldn't show the validity. Furthermore, this model couldn’t calculate the physical model for nano Al2O3 particles, i.e. agglomeration, sedimentation and so on.

(1) Figure 3 (a) and (b): Number of elements at the horizontal axis might be lack of “million“. The current elements show less than 1.6.

(2) Figure 6 (a): The calculation results show the only good trend for the experimental data, don’t show the reasonable predictions. There is a large difference between them.

(3) P.14, L.7: “92 > Re > 455” in the paper, but this should be “92 < Re < 455”.

(4) P.14, L. 18: “a_nf=1%, f=100%” seemed to be an anomaly point in the simulation results. Is it a realistic parameter? What physical phenomenon does this parameter show? Can the used calculation model treat this parameter?

(5) P.15, L. 6-12: The authors calculated only the very small and simple schematic. I am worried about the same heat transport as the nanoparticles approach might be caused because of the small and simple calculation schematic. Could the same heat transport at the various calculation schematics be reproduced by both single and two-phase models?

(6) P. 15, L. 15-16: “The absence of significant physical processes of nanoparticle clustering”. The calculation models are lack of very important physical process, i.e. agglomeration, sedimentation and so on. Therefore, I am worried that the different physical phenomena could happen from the calculation.

(7) P. 18, L.8-9: “Whilst not meeting the residuals convergence criterion, the results support

the substantial independence of the model predictions from the second phase bubble diameter ?nf.” The authors should write more and more descriptions for the readers’ understanding.

(8) P. 22-23: “These results, albeit limited in applicability by the microchannel simple geometry used in this study, have important practical implications for the progress of heat sink technology. It is shown that, within reason, a lack of uniformity in the nanofluid coolant does not impact directly its cooling performance. This provides an important design margin for heat sinks required to work within tight temperature tolerances.”

Was there anything in the text that led to this conclusion?

Comments for author File: Comments.docx

Author Response

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Reviewer 3 Report

This work needs the following revisions:

- There is a need to enhance the discussion of this work. Please compare your results with others from the literature and state if your results lead to higher or to lower performance enhancements.

- Please improve the conclusion by adding more numerical results.

- Section 2.1.2: Please add references for the given assumptions.

- The data in the tables needs references, for example tables 1 – 2 and 3.

- In figure 6, plot in the right axis the deviations.

 

- Please put the symbols of the nomenclature in the proper alphabetical order.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors have satisfactorily addressed my concerns and the revised manuscript is of better quality than the original submission. Therefore,  the manuscript can be recommended for publication. 

Reviewer 3 Report

The paper has been revised in a good way.