Numerical study of natural convection in a horizontal cylinder filled with water-based alumina nanofluid

Natural heat convection of water-based alumina (Al2O3/water) nanofluids (with volume fraction 1% and 4%) in a horizontal cylinder is numerically investigated. The whole three-dimensional computational fluid dynamics (CFD) procedure is performed in a completely open-source way. Blender, enGrid, OpenFOAM and ParaView are employed for geometry creation, mesh generation, case simulation and post process, respectively. Original solver ‘buoyantBoussinesqSimpleFoam’ is selected for the present study, and a temperature-dependent solver ‘buoyantBoussinesqSimpleTDFoam’ is developed to ensure the simulation is more realistic. The two solvers are used for same cases and compared to corresponding experimental results. The flow regime in these cases is laminar (Reynolds number is 150) and the Rayleigh number range is 0.7 × 107 ~ 5 × 107. By comparison, the average natural Nusselt numbers of water and Al2O3/water nanofluids are found to increase with the Rayleigh number. At the same Rayleigh number, the Nusselt number is found to decrease with nanofluid volume fraction. The temperature-dependent solver is found better for water and 1% Al2O3/water nanofluid cases, while the original solver is better for 4% Al2O3/water nanofluid cases. Furthermore, due to strong three-dimensional flow features in the horizontal cylinder, three-dimensional CFD simulation is recommended instead of two-dimensional simplifications. Electronic supplementary material The online version of this article (doi:10.1186/s11671-015-0847-x) contains supplementary material, which is available to authorized users.

collected from experimental investigations instead of prediction models" has nothing to do with the use of a single or two-phase models.
Our thoughts: "The mechanisms of nanofluid thermal conductivity enhancement are still not very clear" is a commonly accepted conclusion in nanofluid research [8,9]. So far, no paper can be found to disclose the reasons for sure why nanofluid has such a good heat transfer performance enhancement [9,10]. Currently, nearly all prediction models for nanofluid thermal conductivity and viscosity are proposed based on some assumptions [11,12], e.g. nanoparticles are spherical [13] or cylindrical [14]. That is the reason why we use the data (regression data) collected from experimental investigations instead of those prediction models which may cause inaccurate simulation. According to the experimental data, our numerical solver updates the non-uniform nanofluid properties in the whole computational region after each iteration to make the simulation more practical. This is the way how do we use the experimental data. Problem configuration'. They can also be found in 'Nomenclature' at the end of paper.

Referee4's comments and our revisions
2. On page 6, Cp should be "specific heat at constant pressure".
Reason for no revision: Nanofluid is considered as incompressible fluid as water, specific heat property is default as a stable value without variation induced by pressure change.
3. Units on page 7, "Kg" should be "kg" and "k" should be "K" (absolute temperature). On the same page, give an explanation why the values of "3970 kg/m3 and 765 J/kg.K" are used in the study other than other values. Viscosity v differs with dynamic viscosity, which needs to be defined. In addition, (T-273) is to change absolute temperature to Celsius scale, which should give units or explanation.
Revision: Corrected "Kg" to "kg" and "k" to "K" on page 7. A note is given for 'Celsius scale' on page 7. 3970 / 3 and 765 / • are the most commonly accepted parameters for Al2O3 nanoparticle properties. Viscosities are defined with 'dynamic' and 'kinetic' on page 7 and 8. The explanation of (T-273) is given on page 7 as 'Based on regression analysis from Das's study [15,16]'. 4. On page 8, "heat transfer coefficient" should be convection heat transfer coefficient". Units of the parameters h, Lc, etc. should be given.
Revision: "heat transfer coefficient" has been corrected to "convection heat transfer coefficient" on page8. Units of the parameters h, Lc, etc are defined in 'Nomenclature' at the end of this paper.

On page 11, what is the definition of X, section area or length?
Revision: The definition of 'X' is added on page 11 as 'cylinder cross section position'. 6. What is the conclusion of this study: Heat Transfer Deterioration or Enhancement?
In Abstract, the conclusion is not given either. Readers will interested in the study results, not numerical study itself.
Revision: 'At same Rayleigh number, Nusselt number is found to decrease with nanofluid volume fraction.' can be found in abstract, as well as the similar conclusion in the main body of this paper on page 12. In other word, heat transfer deterioration is found in our present work.