Experimental Investigation of Heat Transfer with Various Aqueous Mono/Hybrid Nanofluids in a Multi-Channel Heat Exchanger
Round 1
Reviewer 1 Report
Dear Authors
General amendments:
Paper is interesting and valuable. However, there are a few things that limit quality of this article. For instance:
- It is mandatory to present thermal properties of studied nanofluids. Table with thermal properties is recommended to include in the article.
- Dynamic properties like Reynolds number should be presented as well.
- Authors are not certain which density of heat flux they used. For instance, in line 244, there is: “… from 3.86 W/m2 to 7.51 W/m2, while in the Abstract there is: 3.85 W cm-2 and 7.51 Wcm-2. Which data are right? Assuming it is 7.5 W/cm2 for instance, this gives 75 kW/m2, which is quite a big value.
- Authors do not use scientific language in some sentences. For instance:
- In abstract there is: “The use of nanofluids in a working fluid …”
Comment: Authors rather talk about using nanoparticles in working fluid.
- in line 35 there is: “…as compared to the original working fluid, …”.
Comment: “original”? more suitable seems to be: compared to the “liquid phase flow” or to “pure liquid” or to “carrier liquid” or to “working liquid”.
- In line 71 there is: “… TiO2 nanofluid suspended in water …”
Comment: it is rather TiO2 nanoparticles suspended in water.
Mixing “nanofluid” with “nanoparticles” appears in some other places as well (lines 128, 129, etc.). This should be improved.
- Figures 4 – 9. For instance, in line 411, there is: “… a decrease in the temperature gradient of nearly 10°C …”
Comment: This is not gradient. The gradient this is dT/dx=tg(a), while Authors talk about temperature difference of nearly 10 °C.
- Figures 1 and Figures 2 are not in the write place. Authors called Figure 2 in line 205 but they missed Figure 1. Reference to the Figure 1 appears later, i.e. in the next subsection (line 233). This should be improved.
- Figure 3. It is recommended to show coordinates in this figure.
- Figures 4 – 9 should have indicated Reynolds number.
- Authors used dimensions: m, cm, mm. Unification of dimensions is recommended where it is useful.
- Analyzing Fig. 9 for instance, it looks that for x-coordinate higher than ab. 18 mm the thermal loss appears. It would be nice to have experiments for thermally fully developed state. However, this is not objection; this is rather suggestion for future experiments.
- Authors should present dimension of all quantities in the text. For instance, it is not clear why the specific heat (J/(kg K) is not included in equation, as we talk about longitudinal temperature difference. Equation (2) should be developed.
- Authors declare flow range from 01 to 0.3 USGPM, while they present results for 0.2 and 0.3 USGPM.
- In conclusions Authors could try to explain why result in Fig. 4 and Fig. 7 differ much in reference to 0.5% wt TiO2 and 1% wt TiO2 comparing to carrier liquid.
Detailed amendments:
Lines 64 and 25, there is: “They also determined the pressure drop across the tests section with a pressure loop”.
Comment: “pressure drop across the tests section” means rather pressure difference across the stream, however, it is longitudinal pressure drop, which is a measure of pressure losses (friction losses) on tested distance. This should be clarified.
Line 67. There is ”… optimal concentration of 3% …”
Comment: is it volumetric or mass concentration?
Lines 112 and 113 Authors wrote: “… effectiveness ratio of 1105 with a concentration of 10%, …”
Comment: This is not clear how effectiveness was measured; what is dimension for 1105 ? is it still volumetric concentration?
Lines from 199 to 202. This paragraph is not important and should be cancelled.
Line 224 and further, there is: “… copper salt (11.65 mM) …”
Comment: dimension “mM” should be explained.
Figure 2. All parts of experimental stand should be described in the title of Figure 2.
Line 276. There is: “… and ∂ which when attached to a term …”.
Comment: it is strange to explain that ∂ means partial derivative.
Line 297, there is: “… The applied heat fluxes were 3.85 W cm-2, …”
Comment: If dimension of power to area is used this is not heat flux. This is density of the heat flux.
Line 339, there is: “… increase int eh experienced …”
Line 341, there is: “The boundary later thickness …”.
Comment: “layer” instead of “later”.
Line 348, there is: “… an average increase of 0.166 for the 0.5%wt TiO2 and a decrease of -2.16% for …”
Comment: dimension is missed and this is not clear how it was calculated?
Line 352, there is: “… an average improvement over the base fluid of 14.7% and 19.9% …”
Comment: this is not clear how improvement has been calculated. Does it regard dT? This should be clearly stated.
Line 409, there is: “… In the last figure …”
Comment: Instead of “last figure”, the “number” of figure should be used.
Equation (2). “e’ should be the subscript.
Author Response
Please see attachment. Thank you for your comments.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
No information on morphology of used nanoparticles has been provided. Morphology of the particles is important and need to be identified using suitable techniques (SEM/DLS etc.) before and after preparation of the dispersions.
Pg. 7 line 271. Uncertainty was ESTIMATED not CREATED
Pg. 7 line 272. This error can be determined OR WAS DETERMINED
Thermocouple uncertainty is mentioned to be 0.75% based on information quoted by the manufacturer, as thermocouples have been fixed on the base of the channel with some epoxy one needs to do the calibration with temperature calibrator to confirm exact level of error in the measured quantity.
Authors have mentioned information of flow rates, it is suggested to include Reynolds Number and flowing regimes as well.
No information is given on the validation of the test section, it is suggested that authors should include results (heat transfer and pressure drop) obtained with pure fluid and their contrast with well accepted theoretical models.
Avoid using personal nouns in scientific writing. Personal noun (we) has been used at several places in the text.
Authors have mentioned some possible origin for thermal losses in the system. However no quantification for the same has been provided, it is suggested to calculate the losses (electricity fed to the test setup and temperature and flow rates of flowing fluid are known) and comment on their neglection in heat transfer calculation.
Authors did mention surface contamination with utilization of nanofluids (pg. 8, row 320) but once again information on change of surface morphology is missing. Better approach would be to scan the roughness profile of heating surface before and after testing.
The authors did mention boundary layers effect for possible reason behind the observed parabolic trend in the temperature difference readings. It is suggested to account for the extent of entry region as per operating flow regime and use that to further strengthen the stated argument.
Rephrasing needed pg. 8 row 339.
heat convection coefficient should be replaced with “convective heat transfer coefficient”
Pg. 14 row 449 which SHOWED
In all cases local Nusselt No shows an increasing trend close to the outlet of the test channel (@ x=30-35 mm), there must be some comment on this in the text.
As nanofluids have higher viscosity than pure fluids (water in this case), for same flow rate more pumping power would have been required to sustain the flow of nanofluids. Such information on fluid flow characteristics needs to be clearly stated in the text.
Authors have not provided any information on pressure drop while working with nanofluids, this is an important characteristic and should properly be estimated and stated in the text so that the readers have an idea that how much would be the operating cost behind any improvement in the heat transfer.
Authors have not mentioned degree of uncertainty in their heat transfer results. This is very important to have an idea about quality and reliability of the data. It is suggested that authors should quantify degree of uncertainty and mention it in the text.
Authors have not used any empirical correlation for its prediction capability against their data. It is suggested that authors should test well cited correlations and include this in the submission.
Author Response
Thank you for your questions and comments. Please see the attachment for the responses.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Authors agreed with reviewer objections and made corrections. However, Authors included a new text which disqualifies the paper. In reaction on following reviewer amendment: “Figures 4 – 9 should have indicated Reynolds number”. Authors in line 372 of revised paper, wrote: “Where ? is the density of the fluid, u is the velocity of the fluid, De is the characteristic velocity of the fluid, and μ is the dynamic viscosity of the fluid”.
Comment: Bolded sentence indicates fundamental mistake which disqualifies the publication of the article.
In Line 375 there is: “Table 2. Reynolds flow for each fluid”
Comment: This is strange sentence. There is no “Reynolds flow”. We talk about Reynolds number. Therefore, title should be changed. There must be clear that Re=2647 and Re=3970 etc.
All symbols used in the article should be collected in Nomenclature, showing their names and dimensions.
Authors should examine scientific language used in the paper.
Author Response
Please see attached
Author Response File: Author Response.pdf
Reviewer 2 Report
The draft contains information on utilization of nanofluids for thermal applications. The revised draft still needs major revision before its possible publication in the journal.
Morphology of used nanoparticles is very important and should be discussed in the draft. The author have replied that it has been discussed an earlier publication (some new materials have been tested for the current study, furthermore even for same material new samples have been tested for this study and they may have different characteristics).
Uncertainty in the reported results (for temperature readings) is not correct, the information obtained from manufacturer is reported whereas there needs to be calibrated after their fixation on the test setup.
No information has been furnished on validation of test section, the authors mentioned that this has been reported in an earlier publication.
Thermal losses have been neglected and no details has been furnished on their estimation and proportion as was suggested in the earlier feedback.
No details has been furnished on estimation of extent of flow development.
Thermophysical properties (thermal conductivity, viscosity etc.) of tested nanofluids have not been reported in the draft.
Uncertainty propagation (type-B uncertainty) needs to be rechecked and corrected.
Data collected in this study is a new data set and it should be checked with prediction capability of well known correlations from the literature.
Explaination on trend observed with local Nusselt number has not been provided.
Author Response
Please see attached
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
Reynolds number was properly corrected.
Reviewer 2 Report
The authors have revised the draft as per forwarded feedback.
