Nanoparticles emit EM radiation to enhance thermal conductivity and boiling heat transferThe emission of electromagnetic (EM) radiation by nanoparticles (NPs) resolves the paradox of enhanced critical heat flux (CHF) without increasing the boiling heat transfer (BHT) coefficient
By: Thomas Prevenslik Nuclear reactors and power plants rely on the continuous formation of bubbles in boiling to limit surface temperatures, the efficiency of which is quantified in terms of the coefficient of BHT. High BHT means more heat can be supplied to a surface, but there is a limit. As more heat is supplied, instead of forming bubbles, a vapor forms on the surface that acts as a thermal insulator causing the temperature to increase beyond safe limits. The heat flux defined as the quantity of heat per unit area of the surface area is then said to have reached a critical value called the critical heat flux (CHF). For water and other coolants, classical heat transfer holds that the CHF should increase or decrease in proportional to the BHT coefficient. However, classical heat transfer is not valid for nanofluids. Nanofluids are comprised of submicron NPs in a coolant, the heat capacity of NPs limited by quantum mechanics (QM). Experiments over the past decade show the QM effect as NPs in diverse coolants provide significant enhancements in thermal conductivity, although the volume fraction of NPs is paradoxically insignificant. Nanofluids in BHT are only more recently studied, e.g., the proposed nanofluid is alumina NPs in nuclear reactors. But the fraction of NPs is again very small, say < 1% by volume, and therefore like the thermal conductivity of nanofluids, the CHF and the BHT coefficient of the nanofluid are not expected to be enhanced over the pure water. Contrarily, experiments show enhanced CHF, but the BHT coefficients paradoxically remain about the same. Heat Transfer by Quantum Mechanics QM argues that classical heat transfer theory cannot explain the paradox of an enhanced CHF without increased BHT coefficient for nanofluids anymore than classical theory could explain the significant increase in thermal conductivity found for insignificant volume fractions of NPs in nanofluids without boiling. See http://www.nanoqed.org at “Nanofluids and Thin Films”, 2009. Without boiling, NPs in nanofluids gain kT energy from collisions with coolant molecules. Here, k is Boltzmann’s constant and T is absolute temperature. At 300 K, the kT energy of the atoms in NPs resides in the far infrared (FIR). In contrast, atoms in NPs under BHT collide with heated surfaces having temperatures as high as 1300K where the thermal kT energy of the atom resides is in the near infrared (NIR).Regardless, QM argues NPs are submicron and by their size exclude NIR and FIR radiation. Indeed, the Planck energy of the harmonic oscillator vanishes for submicron NPs, and therefore the NPs lack the heat capacity necessary to conserve the absorption of heat by an increase in temperature. The NPs lacking heat capacity nevertheless are required to conserve the absorbed kT energy. In BHT, the kT energy in the NIR extracted by the NP in collisions with heated surfaces may only proceed by QED induced frequency upconversion to the optical EM confinement frequency of the NP. QED stands for quantum electrodynamics. QED requires the low frequency NIR photons to be optically confined within the submicron NP geometry. Squeezing NIR photons in NPs treated as a QM box increases their frequency to UV levels and beyond. But the EM optical confinement is quasibound and the UV leaks to and is absorbed by the surrounding water. Resolution of Paradox The CHFBHT paradox is thereby explained by the NPs converting kT energy gained upon colliding with the heated surface to UV emission that is absorbed the surrounding water. In this way, QM trumps classical heat transfer theory by the UV bypassing the bubble boiling process and allowing CHF enhancement without increasing the BHT coefficient. UV sensitive fluorescent water soluble markers are recommended as a means of verifying the correctness of QED induced heat transfer in BHT. Conclusions 1. Enhanced CHF in nanofluids without an increase in BHT coefficient is caused by the EM radiation emitted by NPs bypassing the bubble boiling process. 2. Classical heat transfer theory does not apply to NPs. The QM size effect precludes the conservation of absorbed thermal kT energy in NPs by an increase in temperature. 3. Conservation of absorbed EM energy by NPs may only proceed by the QED induced frequency upconversion of kT energy in the NIR and FIR to the EM confinement optic al frequency of the NP, the latter in the UV and beyond. 4. Classically, the EM radiation in the FIR at 300 K at any point in the coolant is considered the same in all directions. QM alters this classical picture by converting the FIR to UV and higher EM radiation that penetrates further than the FIR would without NPs. Therefore, the effective thermal conductivity of a nanofluid is enhanced above that of the coolant alone. # # # About QED induced EM radiation: Classically, EM energy is conserved by an increase in temperature. But at the nanoscale, temperature increases are forbidden by the quantum mechanics restriction of vanishing specific heat. QED radiation explains how the EM energy is conserved by the emission of nonthermal EM radiation. End
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