Experimental Characterization of Hydronic Air Coil Performance with Aluminum Oxide Nanofluids of Three Concentrations

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper presents an experimental investigation aimed to compare the performance of the heating coil filled with Al2O3/60% EG nanofluid in various particle concentrations to the coil's performance with 60% EG at higher temperatures, which is a continuation of experimental investigations conducted by the authors to measure and compare the thermal and fluid dynamic performance of a residential hydronic air coil using nanofluids. An improved version of the testing apparatus is used.

The paper is interesting and deserves publication after some improvements:

1. For better clarity, the caption of Tables 1 and 2 must be completed by mentioning a reference [ ].

2. As a rule to make a paper readable, the meaning of all variables and constants involved in equations should be explained the first time they appear in the text, or within the nomenclature. Fortunately, some of them are explained but unfortunately some of them are not explained. All of them should be explained.

3. In the conclusion part, the limitations of this study, suggested improvements to this work and future directions should be highlighted.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

1. A reference is added to the caption of Table 1 in the manuscript. It is [9], which is the thesis of the first author, Strandberg.
2. All variables appearing in Equations (1) through (7) are explained below them. These additions are highlighted in the manuscript for the ease of reading by the reviewer.
3. The following three paragraphs were added to the Conclusions section at its end.

The principal limitation of this study was the use of nanofluids, which was old. It was purchased over a year ago. Although it was sonicated before use, it had possibly lost the strength of the surfactant and the dispersant. As a result, the repulsive forces between nanoparticles were weak, which caused particle agglomeration, and the characteristic of stable dispersion was lost. This increased viscosity and decreased thermal conductivity made the nanofluids perform poorly in heat transfer and pum** power as the concentration increased. 

 Suggested improvements to this work would be to repeat these experiments with new fresh nanofluids, which possess more stable surfactants and dispersants. Perform experiments on 1 % concentration because that dilute concentration showed us the promise in our experiments. Minimize the bends, valves, traps, and crevices in the nanofluid loop because those are the spots where nanoparticles tend to separate and collect in crevices, diminishing the homogeneity of well-dispersed nanofluids. Measure nanofluids' viscosity and thermal conductivity before and after the experiments to ensure the properties were preserved.

Future directions advisable to researchers from this study is to focus on develo** the best quality surfactants and dispersants that could keep the nanofluids stably dispersed for a reasonable long period. The nanofluids available nowadays generally have an average particle size (APS) range of about 40 to 50 nm. Future work should concentrate on particle size of 10 nm or less. They will be much better for heat transfer, and even unavoidable low-scale agglomeration may keep the aggregated size below 100 nm, which is the threshold of nanoparticles.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Author, 

The article is mainly correct. There are minor issues:
1. Instability and aglomeration of the nanofluids can have a significant impact for the heat transfer coefficient. How the article considering this important issue?

2. Fluid Instability causes should be describe in more detail.

Author Response

1. We agree that the instability and agglomeration of nanoparticles adversely affect the heat transfer coefficient.

It reduced the convective heat transfer coefficient and the heat transfer rate in our experiments. As the particle concentration increased, the nanofluids performed in an inferior manner when compared to the pure single-phase base fluid. The experimental data in Figures 9, 10 and 11 exhibit such behaviors.

The cause of this important issue and how to overcome this shortcoming has been discussed under Section 4.4, “Challenge of Unstable Dispersion and Agglomeration.” 

Suggestions to overcome this drawback are through the development of very effective, long-lasting, robust surfactants and dispersants. This explanation has been added to the Conclusions section of the paper.  

2. The following explanation section has been added to Section 4.4 describing causes of nanofluid instability.

 Nanofluids become unstable when the particle mass becomes large enough so that the gravitational pull exceeds the buoyancy force and drag force, making the particle’s settling velocity higher than the upward component of the Brownian motion. The nanofluid is unstably dispersed at this condition, becoming microfluid, and loses all heat transfer benefits. The cause of particles growing larger is through mutual attraction, which can be minimized via surface coating and adding liquids to the base fluid that prevents particles from attaching. When maintained within a desired range, The Zeta potential, the electrical charge characteristic of these nanoparticles, ensures stable colloidal or nanoparticle dispersion. Another important characteristic of the nanofluid found from the research is that the pH value of the nanofluid is important in maintaining the stable dispersion of nanofluids. For different nanofluids (e.g. Al₂O₃, CuO, SiO₂, TiO₂, etc.), optimum pH and zeta potential ranges can be determined in which the nanofluids behave the most efficiently in heat transfer. The nanofluids in heat exchangers should always be maintained to operate in those ranges of pH and Zeta potential to obtain the gain promised by nanofluids. 

Reviewer 3 Report

Comments and Suggestions for Authors

The authors discuss performance degradation due to nanoparticle addition to their hydronic air coil. Representation of negative results is not common in the literature, preventing new researchers from advancing their research by knowing possible obstacles. Therefore, I recommend publishing this paper after the following improvements are examined.

1. The introduction part lacks recent papers, thus a decent storyline. The existing literature reporting on experimental work concerning the performance of nanofluids in a hydronic heating coil was not available, but there are some recent papers (not limited to) also representing possible challenges and their reasons due to nanofluid use.

   https://doi.org/10.3390/en16237885

   https://doi.org/10.1007/s11051-009-9657-3

2. Nanofluids are not considered as ‘solutions’ but ‘dispersions’. Please replace that word throughout the paper.

3. What is the nanoparticle concentration of the so-called mother fluid? How and at what temperature did you measure the volumetric concentration? Please disclose more information about the mother fluid and preparation protocol.

4. Have you used any measured thermophysical properties or just calculated based on empirical correlations from the literature?

5. What is RH in line 250?

6. Please be consistent in the nomenclature. “H” and “h” are not interchangeable (also C and c). See Eqn.2. “$E_x$” in Eqn.3 is also not given in the nomenclature. I recommend authors write the meaning of each variable just after they are used to prevent any confusion.

7. The authors stated in line 234 that their nanofluid will be stable according to the literature. Conversely, they expect instability and agglomeration as stated in line 331. Please clarify this conflict.

8. Considering the superior thermal conductivity expectance in line 555. Did you measure the thermal conductivity of the nanofluids used in the study?

9. The papers I have recommended also discuss the degradation in the material as you suspect in line 558. You can look for evidence and similarities, accordingly, e.g., color change in the nanofluid.

10. Can you give more information about the viscometer and its measurement ranges to be accepted as evidence (line 604)?

Comments on the Quality of English Language

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Author Response

1. The following paragraph has been added to the Introduction Section (toward the end) in response to the comments made by the Reviewer.

In our experimental investigation, we encountered challenges due to the poor performance of nanofluids due to the degradation of surfactant and dispersant, which made the fluid vulnerable to particle agglomeration and subsequent loss of thermophysical properties, as well as a reduction in the convective heat transfer coefficient and the heat transfer rate. Other researchers who conducted experimental studies have also experienced such challenges. Sommers and Yerkes [14] conducted experiments with 10 nm Al2O3 particles dispersed in propanol in a 0.75 inch ID copper pipe. They found that only 1% of the concentration gave a better heat transfer coefficient than the base fluid. With the increase in concentration, the thermal performance degraded. The 3% concentration with high pressure drop gave inferior thermal performance compared to the base fluid. They also observed nanofluid discoloration at high flow rates and temperature after circulating them.

Eneren et al. [15] encountered similar challenges in their experiments on water-based Al₂O₃, TiO₂, and polystyrene nanosuspensions circulated inside silicon microchannels. The thermal performance of Al₂O₃ nanofluid was slightly better than the base fluid for a 0.2% nanoparticle concentration. Beyond that, the performance steadily declined from 0. 3 to 0.5 to 1 % concentration. They also observed the discoloration of nanofluids, which they attributed to the erosion of materials caused by larger nanoparticles on the surface of silicon microchannels. Therefore, significant challenges exist to achieve the success of nanofluids in practical systems.

Two papers were added to the Reference Section:

[14] Sommers, A. D., and Yerkes, K. L., 2010, “Experimental Investigation into the Convective Heat Transfer and System-Level Effects of Al2O3-Propanol Nanofluid,” J Nanopart Res, 12(3), pp. 1003–1014.

[15] Eneren, P., Aksoy, Y. T., and Vetrano, M. R., 2023, “Practical Challenges in Nanofluid Convective Heat Transfer Inside Silicon Microchannels,” Energies, 16(23), p. 7885

2. A global change was made throughout the article to express nanofluids as ‘dispersions’ and not

‘solutions’.

3. The following paragraph has been added to the manuscript under the Section 2.1 “Nanofluids Suspension Preparation.”

The nanoparticle concentration of the mother fluid was 50 percent by mass. It was procured from Alfa Aesar [23] as concentrated aqueous suspensions. The average particle size of this test fluid was 53 nanometers. Knowing the density of all components from the manufacturer’s specifications, we calculated how much of this mother fluid should be added to the base fluid, 60:40 EG/Water, to create concentrations of 1, 2, and 3 % of Al₂O₃ nanofluid. The EG and water properties and the sonication process to break up agglomeration have been described in this section. Using a precise electronic mass balance, small volumes of mother fluid drawn in a pipette were added to the base fluid to prepare 1, 2 and 3% nanofluids.   

 

4. We used the thermophysical properties empirical correlations to derive experimental convective heat transfer, heat transfer rate and pum** power for this nanofluid. The empirical properties equations had been derived from previous experiments. We did some viscosity measurements after observing the poor performance of the nanofluid, but it was at best approximate. However, we did not measure the thermal conductivity of the nanofluid immediately following the experiment, which was a major flaw. We recommend this as a lesson for future researchers to follow.
5. RH is relative humidity. Correction made to make it clearer.
6. In the Nomenclature section, significant revisions (highlighted in the manuscript) have been made to be consistent. Revisions have been made to define each variable under all equations.
7. We have added the following paragraph below Line 331 of the old manuscript.

Vajjha and Das's earlier work [22] involved property measurements. During those experiments, we used the nanofluids immediately after receiving them from Alfa Ae-sar [23]. They were fresh, uniformly dispersed. The sample sizes for viscosity, thermal conductivity, specific heat, and density were small. There was no circulation of nanofluids as these were static tests on small sample volumes confined to a contained space. The experiments were of short duration. So, we measured the optimum property value under ideal conditions.  

However, in the present experiments, the nanofluids circulated in a coil for very long durations to obtain data over a range of Reynolds, Prandtl, and Nusselt numbers varying flow rate, temperature, pressure to measure, convective heat transfer, heat transfer rate, pressure loss, and the pum** power. Maintaining nanofluid purity was a more significant challenge because we started with an older nanofluid that had lost its uniform dispersion ability. This was unknown to us initially, but the unexpected result of poor performance taught us a lesson.

8. We failed to measure the nanofluid's thermal conductivity just before and after the experiments. This was a major flaw in our experimental design. We used similar nanofluids, we had used for properties measurements and assumed that the same property will hold true when the nanofluids are circulated in a heating coil. But that assumption was wrong because old nanofluids were used in a circulatory system. We have added this explanation at the end of Section 4.4, “Challenge of Unstable Dispersion and Agglomeration.” This is a lesson we learned from this experimental investigation. We pass on these findings to future researchers planning to conduct experiments of nanofluids in practical heat exchangers.
9. The Reviewer recommended two papers, which are valuable in understanding the drawbacks of nanofluids. These papers are now included in the Reference section of this revised manuscript. These papers alert the nanofluids community to aim their research in overcoming these obstacles.
10. The viscometer used to measure viscosity of the used nanofluid was Brookfield viscometer model DV-I. It is the simplest model of the Brookfield viscometer, which does not have the capability of a controlled-temperature bath. Therefore, we were measuring viscosity basically at room temperature. At best, the fluid was heated and measured at a higher temperature very quickly. But the sample cools, so this was a crude process, since viscosity is a strong function of temperature. Therefore, we ended up with a mixed result.

However, in our earlier property measurement experiments, when we measured the viscosity of many nanofluids as a function of temperature and concentration to develop empirical correlations, we used an advanced viscometer setup consisting of two Brookfield programmable viscometers. (i) Low viscosity digital viscometer (LVDV II+) small sample adapter and thermosel (SSA and T) (ii) LVDV II+ cone/plate.

There was a Julabo temperature bath connected to a computer to control the temperature of the fluid sample whose rheological properties were being measured. 

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The author sufficiently addressed the earlier comments. I recommend the article for publication.