۱٫ Introduction

۲٫ Materials and methods

۳٫ Results

۴٫ Conclusions

Appendix A. Supplementary data

References

Abstract

The thermal system design strongly depends on material selection. Nanofluids offer design flexibility and finetuning of properties by incorporation of nanoparticles in base fluids. This flexibility is guided by particle-particle communication, which may be beneficial in creating ballistic routes in heat transfer but also detrimental due to affecting nanofluid properties. The transition of nanofluids to industrial use requires application-based examinations. For this purpose, different types of nanofluids were investigated in this work in terms of their thermal efficiency in a flat plate solar collector (FPSC) and some figure-of-merits (FOMs), under laminar and turbulent flow conditions. Investigation of both aims at clarifying the correlation between FOMs and FPSC thermal efficiency, and further reporting on the validity of FOMs in assessing thermal efficiency. Results indicate that nanofluids’ eligibility as a heat transfer fluid depends on the flow condition, since a base fluid could outperform a nanofluid under turbulent flow. Nanofluid type and nanoparticle shape affects thermal performance, as suspensions of nanoplatelets/nanotubes in low concentrations (< 0.04 vol%/0.25 vol%) are shown to outperform certain spherical metal-oxide nanoparticles (< 3 vol%), according to some FOMs. It is shown that performance evaluation criteria (PEC), overall energetic efficiency, and energy ratio (ER) do not capture FPSC thermal efficiency trends, e.g., for graphene nanoplatelet nanofluid, as Mouromtseff number-based comparisons do for laminar and turbulent conditions. It must be highlighted that the FOM type to indicate thermal efficiency should be chosen depending on the application, and simultaneous consideration of thermal and hydrodynamic characteristics is required.

Introduction

In today’s world, the main emphasis in energy industry lies on the concept of “sustainability” in energy applications. Plenty of Rooms at the Bottom are available, as stated by Richard P. Feynmann [1], at the enhanced surface-to-volume ratio of nanoparticles which bring about unique characteristics to base fluids when mixed with nanoparticles. Nanofluids [2] or nano-enhanced Heat Transfer Fluids (ne-HTFs) have been under research in a variety of heat transfer-based systems. This has been by necessity due to the poor thermal characteristics of most conventional HTFs, e.g., water, ethylene glycol, mineral oils, brines, etc. compared to solids. This fact highlighted the need for their replacement by ne-HTFs as a potential way to improve thermal performance of solar collectors, electronic cooling systems, nuclear reactor cooling schemes, refrigerators, and so forth.

Nanofluids thermal conductivity and heat transfer coefficient have been among the most investigated characteristics [3–۵] in determining ne-HTFs’ potential in various heating and cooling processes. Hydrodynamic and colloid state points of view show per contra that high viscosity of nanofluids cause increments in pressure drop, pumping power, operation cost, and nanoparticle sedimentation concerns to increase; as well as sometimes thermal performances no better than HTFs in turbulent flow and fully developed flow conditions [3,6–۸]. Although the convective heat transfer coefficient and Nusselt number are important performance indicators, various types of figure-of-merits (FOMs) have been proposed to compare the heat transfer characteristics of HTFs. One of the most earliest FOM definition is the Mouromtseff number (Mo) [9] that has been used to compare heat transfer capabilities of HTFs in laminar and turbulent flow conditions. Another FOM is defined as the ratio of viscosity enhancement and thermal conductivity enhancement coefficients [10], valid in the case of fully developed laminar flow. Sekrani et al. [11] suggested two different types of FOMs to compare the heat transfer capability of nanofluids as the performance evaluation criterion, i.e., PEC, [12] and overall energetic efficiency.