In the measurement of thermal conductivity of nanofluid, usually the experimental value is higher than the value predicted by Maxwell's theory. Is there any experimental of theory evidence which report that Maxwells' prediction is higher than the experimental value? If yes, what may be the possible reason? Can anyone suggest this.
All investigators have reported their experimental values are higher than the values predicted by Maxwell's equation. Theoretical modelling of thermal conductivity is based on certain assumptions. Maxwell has developed the theory for spherical particles whereas the particles may not be. The size of the particle, concentration range, ignoring the influence of temperature in the development of the equation can be additional reasons. Hence the deviation between them.
Maxwell theory is based on Mean Field theory and it predicts that for spherical well dispersed nanoparticles thermal conductivity in diluted regime is 3φ where φ is the volume fraction of nanoparticles. Increase in the thermal conductivity beyond this limit occur due to various reasons like presence of clusters, polar nature of nano-particles, addition of other fluids or other larger sized particles. To account this two theories are successful namely extended Mean Field theory and equilibrium molecular dynamics. There are several interesting papers. 1 W. Evans, J. Fish, and P. Keblinski, Appl. Phys. Lett. 88,093116 (2006). 2 R. Prasher et al., Appl. Phys. Lett. 89, 143119 (2006).
3. J. Eapen, J. Li, and S. Yip, Phys. Rev. Lett. 98, 028302 (2007).
Most of the experimental value is higher than the predicted value by Maxwell's theory and other models based on mean field theory. R. Prasher et al., Appl. Phys. Lett. 89, 143119 (2006) discusses the enhancement in experimental value over theoritical value by considering the particle agglomeration effect. But we are observing the reverse of this situation, i.e., experimental value is lower than the predicted value. W. Evans, R. Prasher, J. Fish, P. Meakin, P. Phelan and P. Keblinski, Int. J. Heat Mass Trans., 51 (2008) 1431-1438 discussing such situation due to interfacial resistance which poses a barrier to heat flow. So, is there any other papers which describes similar experimental observations and also explains such unusual behavior?
The observation is interesting. Can you disclose the conditions under which the thermal conductivity is observed to be lower than the predicted value?
The Maxwell model is for micro particles and larger, where heat transfer mechanism in nano-systems is not considered.
It depends on the method of preparation of nanofluids (using surfactant...). Therefore it is possible to find a nanofluid with lower thermal conductivity than the value predicted by Maxwell model.
@Parekh, I am glad to see that there is another person besides myself who has observed a reduction of thermal conductivity of nanofluids. I tried the steady state as well as the unsteady hot wire methods. When the thermal conductivity of the particles was less than that of the base fluid, there was a clear drop in mean conductivity. Even change of surfactants did not give us the anticipated increase. Many others claim a large increase at even as low as 1% volumetric concentration of the nanoparticles. In our experiments, the difference between Maxwell's results and experiments was only within the error range.
If your particles have a thermal conductivity higher than that of the base fluid, then you might try with another surfactant. Also, use a freshly prepared sample, not one that has been allowed to stand for many days.
There are so many theories for nanofluid thermal conductivity that it is difficult to say which of those is correct or if any of them is correct even. Of course, each researcher is attached to his pet idea. Perhaps clarity will emerge after the dust settles.
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