Nanofluids have been demonstrated as promising for heat transfer enhancement in forced convection and boiling applications. The addition of carbon, copper, and other high-thermal-conductivity nanoparticles to water, oil, ethylene glycol, and other fluids has been determined to increase the thermal conductivities of these fluids. The increased effective thermal conductivities of these fluids enhance their abilities to dissipate heat in such applications. The use of nanofluids for spray cooling is an extension of the application of nanofluids for enhancement of heat dissipation. In this investigation, experiments were performed to determine the level of heat transfer enhancement with the addition of alumina nanoparticles to the fluid. Using mass percentages of up to 0.5% alumina nanoparticles suspended in water, heat fluxes and surface temperatures were measured and compared. Compressed nitrogen was used to provide constant spray nozzle pressures to produce full-cone sprays in an open loop spray cooling system. Heat fluxes were measured for single-phase and evaporative spray cooling regimes.
Using molecular dynamics simulations, an analysis of the thermal conductivity enhancement of a copper/argon nanofluid is performed. First, verification of an increase of as much as ∼30% in the thermal conductivity of the theoretical nanofluid over the corresponding base fluid, due to increasing nanoparticle concentration, is presented. Thermal energy transport is then decomposed into potential, kinetic, and virial components, based on the Green-Kubo autocorrelation function used to calculate thermal conductivity from the microscopic properties of the system. Analysis of these components showed that as the concentration of the nanoparticle increases, the energy transported through the system, due to collisions within the fluid, decreases by as much as 80%. Additionally, the nanofluid system increasingly displays characteristics of an amorphous-like material with increasing concentration. The decrease in energy exchange, due to collisions, suggests another physical mechanism is present for thermal energy transport. Therefore, it is proposed that thermal diffusion is the physical mechanism that more significantly affects thermal energy transport within a nanofluid than had been previously suggested.
Carbon nanotubes (CNTs) have been thoroughly documented to demonstrate superior heat transfer properties. It has also been determined that these properties decrease substantially as overall dimensions increase from the nanoscale to the microscale. Using non-equilibrium molecular dynamics simulations and finite element analysis, the influence of both internal and external thermal boundary resistance effects on the thermal conductivity and specific heat capacity of single walled carbon nanotube bundles were investigated. Comparisons were made between accepted property values for single CNTs and for CNT bundles. Also, energy transfer between varying sized bundles of single-walled carbon nanotubes (SWCNTs) and a surrounding pressure-driven Lennard-Jones (LJ) fluid were calculated.
Preliminary results of an experimental study to test the effectiveness of metallic fibrous heat sinks for electronic cooling applications are presented. In these initial measurements, low heat inputs were employed. Within a higher range of porosity of the aluminum fibrous sinks, lower porosities lead to lower thermal resistance and higher heat transfer rates. Over a range of Reynolds number, the surface to ambient temperature difference remains fairly constant indicating the possibility that, with the use of fibrous heat sinks, low velocities are enough to keep the surface temperature to a desired value.
Experiments were performed to determine heat transfer characteristics of water sprays impacting a flat, inverted surface. Using a compressed gas tank to provide motive force in an open loop spray cooling system, droplet sprays were produced without the assistance of an atomizing gas stream. A range of droplet volumetric fluxes was produced for cooling the inverted heated surface using a full-cone spray nozzle. Heat transfer curves were plotted in the form of heat flux as a function of wall temperature difference, for volumetric flow rates up to 627 mL/min, dissipating up to 451 W/cm2. Heat transfer coefficients were also determined as functions of heat flux. The results were compared to prior data for standard, downward spraying onto heated surfaces.
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