Heat transfer coefficient of nanofluids is higher than base fluid when the comparison is done keeping equal Reynolds number. Is this comparison criteria accurate enough or misleading?
Dear Nilesh
As you said, Heat transfer coefficient of nanofluids is higher than its' bulk fluid in the equal Reynold number. and because Reynold number is a dimensionless number, I assert that this comparison is correct and it has an agreement with my previous studies about nanofluids properties. I will noted the reason of this difference in the heat transfer coefficient in my further messages.
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Dear Nilesh
1. The activity of nano-particles in nanofluids is more than its' bulk materials and this is because of more surface contact with the surrounding fluid or atmosphere. a group of nano-particles have a more area to contact with their surroundings in compare to bulk material of them. so nano-particles could do more reaction because more surface contact and it results more heat transfer coefficient.
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Dear Nilesh
As my friend Eleonora Stefanutti mentioned before
In general you can say that a nano-particles presents an higher ratio between the number of atoms with the particle surface and the number of atoms in the particle inner volume. This property leads the nano-particles to achieve in the more surface contact and more heat transfer in comparison to the bulk material. In fact, as the particle becomes smaller, the ratio between superficial atoms and volume atoms becomes higher. This means that superficial atoms have indeed more free energy with respect to inner ones. This condition (i.e. absence of dimension) makes superficial atoms very reactive.
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It is not significant whether thermal conductivity enhancement or reduction in viscosity influence nanofluid heat transfer enhancements. These parameters might indicate which nanofluid can give higher heat transfer coefficients. It might not be important to determine enhancement at similar Reynolds numbers. What is important is the condition at which higher heat transfer coefficients are obtained and which nanofluids can give. We should also investigate whether the nanoparticle properties have any influence on nanofluid heat transfer coefficients.
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