Lin-Wen Hu

Experimental Investigation of Turbulent Convective Heat Transfer and Pressure Loss of Alumina/Water and Zirconia/Water Nanoparticle Colloids (Nanofluids) in Horizontal Tubes

The turbulent convective heat transfer behavior of alumina (Al 2 O 3 ) and zirconia (ZrO 2 ) nanoparticle dispersions in water is investigated experimentally in a flow loop with a horizontal tube test section at various flow rates (9000<Re < 63,000), temperatures (21-76°C), heat fluxes (up to ∼190 kW/m 2 ), and particle concentrations (0.9-3.6 vol % and 0.2-0.9 vol % for Al 2 O 3 and ZrO 2 , respectively). The experimental data are compared to predictions made using the traditional single-phase convective heat transfer and viscous pressure loss correlations for fully developed turbulent flow, Dittus-Boelter, and Blasius/MacAdams, respectively. It is shown that if the measured temperature- and loading-dependent thermal conductivities and viscosities of the nanofluids are used in calculating the Reynolds, Prandtl, and Nusselt numbers, the existing correlations accurately reproduce the convective heat transfer and viscous pressure loss behavior in tubes. Therefore, no abnormal heat transfer enhancement was observed in this study.

Effects of nanoparticle deposition on surface wettability influencing boiling heat transfer in nanofluids

Buildup of a porous layer of nanoparticles on the heated surface occurs upon boiling of nanofluids containing alumina, zirconia, or silica nanoparticles. This layer significantly improves the surface wettability, as shown by a reduction of the static contact angle on the nanofluid-boiled surfaces compared with the pure-water-boiled surfaces. The contact angle reduction is attributed to changes in surface energy and surface morphology brought about by the presence of the nanoparticle layer. The high surface wettability can plausibly explain the boiling critical heat flux enhancement in nanofluids.

Separate effects of surface roughness, wettability, and porosity on the boiling critical heat flux

The separate effects of surface wettability, porosity, and roughness on the critical heat flux (CHF) of water were examined using engineered surfaces. Values explored were 0, 5, 10, and 15 μm for Rz (roughness), 110° for static contact angle (wettability), and 0 and 50% for pore volume fraction. The porous hydrophilic surface enhanced CHF by 50%–60%, while the porous hydrophobic surface resulted in a reduction of CHF by 97%. Wettability had little effect on the smooth non-porous surface CHF. Surface roughness (Ra, Rq, Rz) had no effect on CHF within the limit of this database.

Experimental Study of Flow Critical Heat Flux in Alumina-Water, Zinc-Oxide-Water, and Diamond-Water Nanofluids

It is shown that addition of alumina, zinc-oxide, and diamond particles can enhance the critical heat flux (CHF) limit of water inflow boiling. The particles used here were in the nanometer range (<100 nm) and at low concentration (≤0.1 vol %). The CHF tests were conducted at 0.1 MPa and at three different mass fluxes (1500 kg/m 2 s, 2000 kg/m 2 s, and 2500 kg/m 2 s). The thermal conditions at CHF were subcooled. The maximum CHF enhancement was 53%, 53%, and 38% for alumina, zinc oxide, and diamond, respectively, always obtained at the highest mass flux. A postmortem analysis of the boiling surface reveals that its morphology is altered by deposition of the particles during boiling. Additionally, the wettability of the surface is substantially increased, which seems to correlate well with the observed CHF enhancement.

Photovoltaic-thermoelectric hybrid systems : A general optimization methodology

The present work outlines a general optimization methodology for hybrid systems consisting of photovoltaic (PV) and thermoelectric (TE) modules. Exemplarily, hybrid systems with hydrogenated microcrystalline silicon, hydrogenated amorphous silicon, and bulk heterojunction polymer thin-film solar cell for different solar TE generator efficiencies are evaluated. The proposed methodology optimizes the partitioning of the solar spectrum in order to yield the maximum conversion efficiency of a PV-TE hybrid system with a solar cell operating at ambient temperature.

Nanofluids for Enhanced Economics and Safety of Nuclear Reactors: An Evaluation of the Potential Features, Issues, and Research Gaps

Nanofluids are engineered colloidal suspensions of nanoparticles in water and exhibit a very significant enhancement (up to 200%) of the boiling critical heat flux (CHF) at modest nanoparticle concentrations (

On the effect of surface roughness height, wettability, and nanoporosity on Leidenfrost phenomena

0.1% by volume). Since CHF is the upper limit of nucleate boiling, such enhancement offers the potential for major performance improvement in many practical applications that use nucleate boiling as their prevalent heat transfer mode. The Massachusetts Institute of Technology is exploring the nuclear applications of nanofluids, specifically the following three: 1. main reactor coolant for pressurized water reactors (PWRs)2. coolant for the emergency core cooling system (ECCS) of both PWRs and boiling water reactors3. coolant for in-vessel retention of the molten core during severe accidents in high-power-density light water reactors. The main features and potential issues of these applications are discussed. The first application could enable significant power uprates in current and future PWRs, thus enhancing their economic performance. Specifically, the use of nanofluids with at least 32% higher CHF could enable a 20% power density uprate in current plants without changing the fuel assembly design and without reducing the margin to CHF. The nanoparticles would not alter the neutronic performance of the system significantly. A RELAP5 analysis of the large-break loss-of-coolant accident in PWRs has shown that the use of a nanofluid in the ECCS accumulators and safety injection can increase the peak-cladding-temperature margins (in the nominal-power core) or maintain them in uprated cores if the nanofluid has a higher post-CHF heat transfer rate. The third application can increase the margin to vessel breach by 40% during severe accidents in high-power density systems such as Westinghouse AP1000 and the Korean APR1400. In summary, the use of nanofluids in nuclear systems seems promising; however, several significant gaps are evident, including, most notably, demonstration of the nanofluid thermal-hydraulic performance at prototypical reactor conditions and the compatibility of the nanofluid chemistry with the reactor materials. These gaps must be closed before any of the aforementioned applications can be implemented in a nuclear power plant.

Infrared thermometry study of nanofluid pool boiling phenomena

In recent quenching heat transfer studies of nanofluids, it was found that deposition of nanoparticles on a surface raises its Leidenfrost point (LFP) considerably [Kim et al., Int. J. Multiphase Flow 35, 427 (2009) and Kim et al., Int. J. Heat Mass Transfer 53, 1542 (2010)]. To probe the physical mechanism underlying this observation, the effects of surface properties on LFP of water droplets were studied, using custom-fabricated surfaces for which roughness height, wettability, and porosity were controlled at the nanoscale. This approach reveals that nanoporosity is the crucial feature in efficiently increasing the LFP by initiating heterogeneous nucleation of bubbles during short-lived solid-liquid contacts, which results in disruption of the vapor film.

Alumina Nanoparticles Enhance the Flow Boiling Critical Heat Flux of Water at Low Pressure

Infrared thermometry was used to obtain first-of-a-kind, time- and space-resolved data for pool boiling phenomena in water-based nanofluids with diamond and silica nanoparticles at low concentration (<0.1 vol.%). In addition to macroscopic parameters like the average heat transfer coefficient and critical heat flux [CHF] value, more fundamental parameters such as the bubble departure diameter and frequency, growth and wait times, and nucleation site density [NSD] were directly measured for a thin, resistively heated, indium-tin-oxide surface deposited onto a sapphire substrate. Consistent with other nanofluid studies, the nanoparticles caused deterioration in the nucleate boiling heat transfer (by as much as 50%) and an increase in the CHF (by as much as 100%). The bubble departure frequency and NSD were found to be lower in nanofluids compared with water for the same wall superheat. Furthermore, it was found that a porous layer of nanoparticles built up on the heater surface during nucleate boiling, which improved surface wettability compared with the water-boiled surfaces. Using the prevalent nucleate boiling models, it was possible to correlate this improved surface wettability to the experimentally observed reductions in the bubble departure frequency, NSD, and ultimately to the deterioration in the nucleate boiling heat transfer and the CHF enhancement.

Transient and Steady-State Experimental Comparison Study of Effective Thermal Conductivity of Al2O3∕Water Nanofluids

Many studies have shown that addition of nanosized particles to water enhances the critical heat flux (CHF) in pool boiling. The resulting colloidal dispersions are known in the literature as nanofluids. However, for most potential applications of nanofluids the situation of interest is flow boiling. This technical note presents first-of-a-kind data for flow boiling CHF in nanofluids. It is shown that a significant CHF enhancement (up to 30%) can be achieved with as little as 0.01% by volume concentration of alumina nanoparticles in flow experiments at atmospheric pressure, low subcooling 20° C, and relatively high mass flux 1000 kg/ m 2 s. DOI: 10.1115/1.2818787