The Nu number for a nanofluid increases based on the increase of the heat transfer coefficient. At the same time, the thermal conductivity of the nanofluids were highly increased.
In cases in which Nu increases due to adding nanoparticles, there are many reasons for this enhancement. Beside the enhanced thermal conductivity of nanofluids, the interactions and Brownian motion of nanoparticles especially at high Reynolds number and the resulting disturbance of the boundary layer may be responsible for the observed enhancement of heat transfer coefficients of nanofluids. Note that I mentioned "cases", where there are cases having contrary effect as in free convection and some cases of boiling heat transfer.
In cases in which Nu increases due to adding nanoparticles, there are many reasons for this enhancement. Beside the enhanced thermal conductivity of nanofluids, the interactions and Brownian motion of nanoparticles especially at high Reynolds number and the resulting disturbance of the boundary layer may be responsible for the observed enhancement of heat transfer coefficients of nanofluids. Note that I mentioned "cases", where there are cases having contrary effect as in free convection and some cases of boiling heat transfer.
Yes, In Nanofluids, Nusselt number increase or decrease for different situations.
Heat transfer in smooth microchannels can be obtained by using the conventional correlations or experimental results for channel with large scale. This gives us a similarity between the micro- and the large-scale heat transfer. You could read a review article related to the discussion --- Microscale Thermophysical Engineering, 6:155–173, 2002
In addition to the above discussion, also you have to specify which nusselt number you are speaking on ? it's depending on your case !!!
By definition Nu=hL/k
here L is specific length
Maybe you are speaking on the geometry of the conducted in which your nanofluid is moving in this case the Nusselt number will be different to the case in which you are speaking about the geometry of the nanoparticles.
You can see this article for more details
New Nusselt number correlations to predict the thermal conductivity of nanofluids Advanced Powder Technology 25 (2014) 1124–1131
Hello everyone,
Generally, in fluids, the Nu will increase by increasing Re, and in nanofluids it is same. But in a specific Re number for a specific nano particle, the Nu will increase by increasing the concentration of nanofluid, and from a specific concentration, this trend is inversed, and increasing the nano concentration leads to decrease the Nu. This specific concentration is different for various materials. In our recent experimental research on Carbon nano fluid, we found this as 0.21% weight concentration of Carbon nano fluid. Our article is under review now, but the figure of Nusselt variation has been attached too. (entitled as: Carbon Nusselt result)
Moreover, you should read the article, entitled as:
'' Experimental study of turbulent convective heat transfer and pressure drop of dilute CuO/water nanofluid inside a circular tube ''
In Table 4, some relations for calculating Nu were presented. Moreover, In Figure 7, they were compared.
Finally, it is recommended to download the attachments.
Best Regards.
M.Firoozzadeh,
Department of Mechanical Engineering,
Jundi-Shapur university of technology, Dezful, Iran
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