Introduction
Since 1995, nanofluids with suspended nano size solid particles [1] have shown tremendous promise as heat transfer fluids. Since then, the mechanism of enhanced heat transfer in nanofluids has remained elusive, e.g., how heat transfer is enhanced by suppressing natural convection. In 2003, NPs were found [2] to suppress the thermal gradients for natural convection to develop in enclosures heated at one end and cooled at the other. NPs stand for nanoparticles. Similarly, heat transfer is found [3,4] enhanced in laminar flow of water through heated horizontal tubes by suppressing natural convection. Movement of NPs due to Brownian diffusion, thermophoresis, viscosity gradient and non-uniform shear rate are thought to have an insignificant effect [3] on heat transfer. However, it was recently concluded [4] that nanofluids may only be understood as two-phase media where nanoparticles move freely with Brownian diffusion neutralizing the thermal gradients necessary for convective heat transfer.

Problem
Browian diffusion based on macroscopic particles moving through horizontal tubes follows classical physics. However, NPs < 100 nm are nanoscopic - not macroscopic. Therefore, QM – not classical physics - governs whether NPs can carry heat and neutralize thermal gradients. QM stands for quantum mechanics. Contrarily, QM given by the Planck law precludes NPs from carrying heat because the high EM confinement of the atoms causes their heat capacity to vanish. What this means is Brownian motion of NPs thought to carry heat and neutralize thermal gradients cannot be the mechanism of suppressing natural convection. But if so,

How then do NPs neutralize thermal gradients?

Proposal
NPs neutralize thermal gradients in horizontal tubes by emitting non-thermal QED induced EM radiation that passes through thermal gradients to enhance heat transfer by directly heating the tube walls upon absorption, i.e., natural convection is indeed suppressed, but heat transfer is enhanced. QED stands for quantum electrodynamics. QED is a consequence of QM that denies atoms the heat capacity to conserve absorbed thermal energy from the nanofluid by an increase in temperature. Instead, the thermal energy absorbed by the NP is conserved by the emission of QED radiation.

QED produces a steady source of non-thermal EM radiation in NPs from the thermal surroundings, the EM radiation produced from the UV to the VIS depending on the NP material and diameter. UV stands for ultraviolet and VIS for visible. In the thumbnail, QED radiation is shown emitted from a NP at the center of the heater, although NPs in the tube removed from the heater are also emitting QED radiation, albeit at lower intensity. Of note, the experiment shows the wall temperature Tw near the exit to approach that at the heater which is difficult to explain. By QED, water is transparent in the UV and VIS, and therefore can pass down the tube to increase Tw upon being absorbed in the tube wall at the exit.

Theory
In the nanofluid, the thermal energy absorbed by NPs is confined almost totally to their surface because NPs have high surface to volume ratios. Therefore, NP atoms are placed under high, but temporary EM confinement. There is no physical EM confinement - the thermal energy traps itself to the NP surface. Since the temperature cannot increase by QM, QED conserves the trapped thermal energy by creating EM radiation having half-wavelength λ/2 that stands across the NP diameter d, i.e., λ/2 = d. Upon expending the trapped thermal energy in the NP surface to create the standing QED radiation, the EM confinement vanishes allowing the QED radiation to escape the NP. The QED radiation has frequency ν = (c/n)/λ = c/2nd, where c is the velocity of light and n the refractive index of the NP. See diverse QED applications at http://www.nanoqed.org/, 2010 – 2015.

Discussion
Classical physics allows NPs to fluctuate in temperature from the thermal surroundings, and therefore NPs can only emit in the IR which is immediately absorbed and does not penetrate the water. IR stands for infrared. In contrast, QM produces higher frequency UV and VIS radiation that enhances heat transfer by significantly penetrating the water thereby suppressing the thermal gradients of natural convection.

Conclusions
Brownian motion of NPs is unlikely to be the mechanism of heat transfer enhancement in nanofluids because QM precludes NPs < 100 nm from having the heat capacity to carry heat and neutralize thermal gradients.

Nanofluid enhancement of water depends on the NP material and size that has led to much confusion about the advantages, if any, of nanofluids over water alone. By NPs emitting QED radiation from the UV to VIS, nanofluids may be understood by the absorption of liquid water. NPs having, say n = 2 and d < 50 nm emit QED radiation at λ < 200 nm which is readily absorbed in water, and therefore should not show heat transfer enhancement. But for 100 > d > 50 nm, the QED induced UV and VIS penetrate water and should show enhancement. However, aggregation that increases the size of the NPs can make NPs < 50 nm show heat transfer enhancement.

References
[1] U. S. Choi, “Enhancing thermal conductivity of fluids with nanoparticles,” FED vol. 231/MD-Vol. 66, ASME, New York, 99–105, 1995.
[2] N. Putra, et al., “Natural convection of nano-fluids,” Heat and Mass Transfer, 39, 775–784, 2003.
[3] A. T. Utomo, et al., “The effect of nanoparticles on laminar heat transfer in a horizontal tube,” Int. J. Heat and Mass Transfer, 69, 77–91, 2014.
[4] L. Colla, et al., “Nanofluids Suppress Secondary Flow in Laminar Pipe Flow,” Proc. World Congress on Mechanical, Chemical, and Material Engineering (MCM 2015) Barcelona, Spain – July 20 - 21, 2015.