No Anomalous Enhancement of Nano-fluid Thermal Conductivity

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Nano was a buzz word a decade back in the USA. The buck was on anything prefixed with nano. Some useful technology did burgeon, identified now as nano-technology. Solar power, electronics and biotechnology were impacted with nano-engineered materials, capacitors and nano-probes and drug delivery methods. Chemistry and material science research in nano prospered with carbon nano tubes (CNT) and nano-composites. Even the e-ink that is employed in Amazon Kindle and similar e-readers seems to use nano-particle suspensions. These innovations and material modifications are seen by some as sufficient results for the decade long investment into nano, while other researchers remained pragmatic. One such case of nano prefixed engineering research that evoked strong internal criticism almost from day-one is the research with nano-fluids.

Nanofluid is a misnomer. It is not the fluid that is of nano-size. In a nanofluid, nano-sized particles of one substance – usually a solid – is dispersed or suspended in a base fluid – usually a liquid. Few years back there was excitement in the thermal science research community about the anomalous behavior of the thermal conductivity of such a nano-fluid. The thermal conductivity of some nano-fluids were reported to have shown phenomenally high value — more than 300 percent from its base fluid [see discussed Reference below and the meta-references. Also see my earlier note on nano-fluids].

This claim of exorbitant — unexpected hence anomalous — raise is possible, as long as the thermal conductivity of the base fluid is very low. But in subsequent talks in conferences we hear the thermal conductivity of nano-fluids were even higher than that of the metal nano-particles used as suspensions. This is where one smells a rat because such a claim is against the basic understanding of the subject. If material A has a thermal conductivity of k_A which is much lower than k_B for material B, then any combination of materials A and B is expected to yield a material with thermal conductivity k_AB which is between k_A and k_B. This is intuitive and deductive. For instance, as early as 1881, following this line of reasoning, Maxwell has derived even an expression for the effective thermal conductivity (and also for electrical conductivity, where he started this approach), when the material is spherical and well dispersed (nano) particles in a base material (fluid).

So, when someone claims k_AB for a nano-fluid to be larger than k_A it is possible. If aluminium nano-powder is mixed with a water base to form a nano-fluid colloid, it can exhibit a k_AB which is much larger than that of water (which is around 0.6 W/mK). But the k_AB would be expected to be lesser than that of alumnium (around 250 W/mK). When some one claims k_AB is grater than both k_A and k_B it is not normal. In the words of Carl Sagan, such extraordinary claims require extraordinary evidence.

Over the last decade of bounteous “nano” grants, many research groups around the world have reported thermal conductivity data for nano-fluids. But such extraordinary evidence where k_AB greater than both k_A and k_B wasn’t convincingly provided. Along the way, many theories were proposed to explain the purported anomalous thermal conductivity of a nano-fluid. For instance, particle Brownian motion agitates the fluid, thus creating a micro-convection effect that increases energy transport (see my earlier note on nano-fluids for other theories and few related references).

During the 2006 International Heat Transfer Conference held in Sydney, there was a talk which consolidated the then available experimental data for nano-fluid thermal conductivity, showing conclusively that the thermal conductivity increase was not phenomenal. The speaker also argued that wherever anomalous increase was reported, it is plausible that there could be problem with the data or in the means of obtaining them. Raising many a high brows (and associated hands and voices), the speaker concluded the talk with an assertion that (s)he will not pursue any further research with nano-fluids.

Subsequently, there was a concerted effort to conclusively establish whether the anomalous increase of thermal conductivity is real and if so, does it require a new theory to explain that behavior. In a nice gesture more than fifteen groups of researchers round the globe participated in collecting indigenous data on the measurement of thermal conductivity for identically prepared nano-fluid. The data was collected in a website and analyzed and reported in the Journal of Applied Physics late in 2009. The paper was authored by more than twenty researchers, some of whom (and their groups) were authors of earlier research that reported anomalous behavior of nano-fluid thermal conductivity.

Here is a partial abstract from that paper — which involves two of my colleagues and some Ph.D. students (not mine) from our lab.

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches […] nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. […] The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. J. Appl. Phys. 81, 6692, 1997, was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

[The bold emphasis is mine]

Related note: https://arunn.me/nanofluids

Reference

Buongiorno, J., Venerus, D., Prabhat, N., McKrell, T., Townsend, J., Christianson, R., Tolmachev, Y., Keblinski, P., Hu, L., Alvarado, J., Bang, I., Bishnoi, S., Bonetti, M., Botz, F., Cecere, A., Chang, Y., Chen, G., Chen, H., Chung, S., Chyu, M., Das, S., Di Paola, R., Ding, Y., Dubois, F., Dzido, G., Eapen, J., Escher, W., Funfschilling, D., Galand, Q., Gao, J., Gharagozloo, P., Goodson, K., Gutierrez, J., Hong, H., Horton, M., Hwang, K., Iorio, C., Jang, S., Jarzebski, A., Jiang, Y., Jin, L., Kabelac, S., Kamath, A., Kedzierski, M., Kieng, L., Kim, C., Kim, J., Kim, S., Lee, S., Leong, K., Manna, I., Michel, B., Ni, R., Patel, H., Philip, J., Poulikakos, D., Reynaud, C., Savino, R., Singh, P., Song, P., Sundararajan, T., Timofeeva, E., Tritcak, T., Turanov, A., Van Vaerenbergh, S., Wen, D., Witharana, S., Yang, C., Yeh, W., Zhao, X., & Zhou, S. (2009). A benchmark study on the thermal conductivity of nanofluids Journal of Applied Physics, 106 (9) DOI: 10.1063/1.3245330