An experimental investigation was conducted to examine the effects of variations in the temperature and volume fraction on the steady-state effective thermal conductivity of two different nanoparticle suspensions. Copper and aluminum oxide, CuO and Al 2 O 3 , nanoparticles with area weighted diameters of 29 and 36 nm, respectively, were blended with distilled water at 2%, 4%, 6%, and 10% volume fractions and the resulting suspensions were evaluated at temperatures ranging from 27.5 to 34.7°C. The results indicate that the nanoparticle material, diameter, volume fraction, and bulk temperature, all have a significant impact on the effective thermal conductivity of these suspensions. The 6% volume fraction of CuO nanoparticle/distilled water suspension resulted in an increase in the effective thermal conductivity of 1.52 times that of pure distilled water and the 10% Al 2 O 3 nanoparticle/distilled water suspension increased the effective thermal conductivity by a factor of 1.3, at a temperature of 34°C. A two-factor linear regression analysis based on the temperature and volume fraction was applied and indicated that the experimental results are in stark contrast to the trends predicted by the traditional theoretical models with respect to both temperature and volume fraction. The available models are reviewed and the possible reasons for the unusually high effective thermal conductivity of nanofluids are analyzed and discussed.

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On the boil: Nanostructured Cu interfaces display enhanced boiling performance at low superheated temperatures compared to untreated Cu, caused by a large increase in the density of active bubble nucleation sites. The enhancement is caused by nanobubbles, which percolate through the interconnected network of nanopores and enable stable nucleation of bubbles at microscale cavities (defects) on the film surface (see picture).

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Presented here is the first of a two-part investigation designed to systematically identify and investigate the parameters affecting the evaporation/boiling and critical heat flux (CHF) from thin capillary wicking structures. The evaporation/boiling heat transfer coefficient, characteristics, and CHF were investigated under steady-state conditions for a variety of capillary structures with a range of wick thicknesses, volumetric porosities, and mesh sizes. In Part I of the investigation we describe the wicking fabrication process and experimental test facility and focus on the effects of the capillary wick thickness. In Part II we examine the effects of variations in the volumetric porosity and the mesh size as well as presenting detailed discussions of the evaporation/boiling phenomena from thin capillary wicking structures. An optimal sintering process was developed and employed to fabricate the test articles, which were fabricated using multiple, uniform layers of sintered isotropic copper mesh. This process minimized the interface thermal contact resistance between the heated wall and the capillary wick, as well as enhancing the contact conditions between the layers of copper mesh. Due to the effective reduction in the thermal contact resistance between the wall and capillary wick, both the evaporation/boiling heat transfer coefficient and the critical heat flux (CHF) demonstrated dramatic improvements, with heat transfer coefficients up to 245.5kW∕m2K and CHF values in excess of 367.9W∕cm2, observed. The experimental results indicate that while the evaporation/boiling heat transfer coefficient, which increases with increasing heat flux, is only related to the exposed surface area and is not affected by the capillary wick thickness, the CHF for steady-state operation is strongly dependent on the capillary wick thickness and increases proportionally with increase in the wick thickness. In addition to these observations, the experimental tests and subsequent analysis have resulted in the development of a new evaporation/boiling curve for capillary wicking structures, which provides new physical insights into the unique nature of the evaporation/boiling process in these capillary wicking structures. Sample structures and fabrication processes, as well as the test procedures are described in detail and the experimental results and observations are systematically presented and analyzed.

To better understand the mechanisms that govern the behavior of pool boiling on horizontal highly conductive microporous coated surfaces, a series of experimental investigations were designed to systematically examine the effects of the geometric dimensions (i.e., coating thickness, volumetric porosity, and pore size, as well as the surface conditions of the porous coatings) on the pool-boiling performance and characteristics. The study was conducted using saturated distilled water at atmospheric pressure (101kPa) and porous surfaces fabricated from sintered isotropic copper wire screens. For nucleate boiling on the microporous coated surfaces, two vapor ventilation modes were observed to exist: (i) upward and (ii) mainly from sideways leakage to the unsealed sides and partially from the center of porous surfaces. The ratio of the heater size to the coating thickness, the friction factor of the two-phase flow to single-phase flow inside the porous coatings, as well as the input heat flux all govern the vapor ventilation mode that occurs. In this investigation, the ratio of heater size to coating thickness varies from 3.5 to 38 in order to identify the effect of heater size on the boiling characteristics. The experimental results indicate that the boiling performance and characteristics are also strongly dependent on the volumetric porosity and mesh size, as well as the surface conditions when the heater size is given. Descriptions and discussion of the typical boiling characteristics; the progressive boiling process, from pool nucleate boiling to film boiling; and the boiling performance curves on conductive microporous coated surfaces are all systematically presented.

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