-Yeul Jung

Forced Convective Heat Transfer of Nanofluids in Microchannels

Convective heat transfer coefficient and friction factor of a nanofluid in rectangular microchannel were measured. An integrated microsystem consisting of a single microchannel on one side and two localized heaters and five polysilicon temperature sensors along the channel on the other side were fabricated. Aluminum dioxide (Al2 O3 ) nanofluids with various particle volume fractions were used in experiment to investigate the effect of the volume fraction of the nanoparticles to the convective heat transfer and fluid flow in microchannels. The convective heat transfer coefficient of the Al2 O3 nanofluid in laminar flow regime was measured to be increased up to 15% compared to the distilled water at a volume fraction of 1.8 volume percent without major friction loss. The Nusselt number measured increases with increasing the Reynolds number in laminar flow regime. A new type of convective heat transfer correlation was proposed to correlate experimental data of heat transfer coefficient for nanofluids in microchannels.Copyright

A capillary-pumped loop (CPL) with microcone-shaped capillary structure for cooling electronic devices

A MEMS-based integrated capillary-pumped loop (CPL), which can be used for cooling electronic devices such as the CPU of a personal computer or notebook, was developed. The CPL consists of an evaporator and condenser both with the same size of 30 mm × 30 mm × 5.15 mm, which were fabricated using two layers of glass wafer and one layer of silicon wafer. A key element of the CPL is that the 480 ± 15 µm thickness silicon wafer where an array of 56 × 56 cone-shaped microholes that generates the capillary forces was fabricated and inserted above the compensation cavity for liquid transportation instead of a porous wick in the evaporator. The same cone-shaped microstructure was used in the condenser to create a stable interface between the liquid and vapor phases. The CPL fabricated was tested under various conditions such as different relative heights, fill ratios and heat fluxes. The operation conditions of the CPL were varied according to the relative height and fill ratios. With an allowable temperature of 110 °C on the evaporator surfaces, the CPL can handle a heat flux of about 6.22 W cm−2 for the air-cooled condenser. Steady-state operation conditions were achieved within 10 min.

Behavior of particles in an evaporating didisperse colloid droplet on a hydrophilic surface.

It is well-known that the liquid and the nanoparticles in an evaporating colloid droplet on a hydrophilic surface move radially outward for the contact line to maintain its position. However, the motion of micro/nanoparticles in an evaporating didisperse colloid droplet has not been reported to date. In this study, an experiment on an evaporating didisperse colloid droplet on the hydrophilic surface is carried out. It is found that nanoparticles move radially outward and remain at the contact line while microparticles move inward toward the center of the droplet. Furthermore, the mechanism of the microparticles moving toward the center of the droplet is found to be due to the surface tension force of the liquid.

Bubble nucleation on micro line heaters under steady or finite pulse of voltage input

Abstract Nucleation temperatures on micro line heaters under steady voltage input were measured precisely by obtaining the I–R (current–resistance) characteristic curves of the heaters having dimensions of 50 μm in length, 3 or 5 μm in width, and 0.583 μm in thickness. The bubble nucleation temperature on the heater with 3 μm width is higher than the superheat limit, while the temperature on the heater with broader width of 5 μm is considerably less than the superheat limit. The nucleation temperature under a finite voltage pulse input was also measured for 5-μm width heater. The nucleation temperatures were also estimated by using the molecular cluster model for bubble nucleation on the cavity free surface with effect of contact angle. The bubble nucleation process was observed by a microscope/35 mm camera unit with a flash light of μs duration.

Bubble nucleation on micro line heaters

Nucleation temperatures on the micro line heaters were measured precisely by obtaining the I-R(current-resistance) characteristic curves of the heaters. The bubble nucleation temperatures on the heater with 3㎛ width are higher than the superheat limit, while the temperature on the heater with broader width of 5㎛ are considerably less than the superheat limit. The nucleation temperatures were also estimated by using the molecular cluster model of bubble nucleation with effect of contact angle. The bubble nucleation process was observed by microscope / 35 ㎜ camera unit with a flash light of ㎲ duration.

Pollution risk assessment of oil spill accidents in Garorim Bay of Korea.

This study presents a model to assess the oil spill risk in Garorim Bay in Korea, where large-scale oil spill accidents frequently occur. The oil spill risk assessment is carried out by using two factors: 1) The impact probability of the oil spill, and 2) the first impact time of the oil that has been spilt. The risk assessment is conducted for environmentally sensitive areas, such as the coastline and aquaculture farms in the Garorim Bay area. Finally, Garorim Bay is divided into six subareas, and the risks of each subarea are compared with one another to identify the subarea that is most vulnerable to an oil spill accident. These results represent an objective and comprehensive oil spill risk level for a specific region. The prediction of the oil spill spread is based on real-time sea conditions and can be improved by integrating our results, especially when sea conditions are rapidly changing.

Cylindrical Water Triboelectric Nanogenerator via Controlling Geometrical Shape of Anodized Aluminum for Enhanced Electrostatic Induction

We demonstrate a cylindrical water triboelectric nanogenerator (CW-TENG) that generates sustainable electrical output. The inner surface of the cylinder was patterned into superhydrophobic and hydrophilic parts to control water flow inside the packaged design of CW-TENG. Here, various thicknesses and roughnesses of the superhydrophobic surface, generated using aluminum oxide nanostructures for enhanced electrostatic induction, were measured to obtain the maximum output and superhydrophobicity. Also, we demonstrate the possibility of using a hydrophilic surface for energy harvesting and as a water reservoir in the packaged design.

PARAMETRIC STUDY ON TRANSIENT HOT-WIRE METHOD TO MEASURE NANOFLUID CONDUCTIVITIES

Recently, nanofluid is a hot research topic among thermal engineers to enhance the heat transfer. However, there are deviations between research groups in the effective thermal conductivity measurements by the transient hot-wire method. Since there has been no report for the researchers to select a proper data range of temperature to estimate the thermal conductivities, the deviation could be partly attributed to the poor selection of the temperature data range. In this study, the impacts of the data range selection, the power supplier response delay, the thermal coefficient of resistors, the hot-wire type and test section size on the thermal conductivity measurement using the transient hot-wire method are analyzed. The proper selection of the temperature data range to be used to estimate the thermal conductivities of fluids using transient hot-wire method is suggested by investigating the impact of it on the measurement by both computationally and experimentally. The thermal coefficient of resistor is found to affect the measurement seriously whereas the type of hot-wire does not. The test section size of the measuring device is compared with the thermal penetration depth.

Experimental Study on N 2 Impurity Effect in the Pressure Drop During CO 2 Mixture Transportation

Carbon-dioxide capture and storage (CCS) process is consisted by capturing carbon-dioxide from large point source such as power plant and steel works, transporting and sequestrating captured in a stable geological structure. During CCS process, it is inevitable of introducing impurities from combustion, capture and purification process into stream. Impurities such as , , CO, , Ar, , , can influence on process efficiency, capital expenditure, operation expense of CCS process. In this study, experimental apparatus is built to simulate the behavior of transport under various impurity composition and process pressure condition. With this apparatus, impurity effect on mixture transportation was experimentally evaluated. The result showed that as ratio increased pressure drop per mass flow and specific volume of mixture also increased. In 120 and 100 bar condition the mixture was in single phase supercritical condition, and as ratio increased gradient of specific volume change and pressure drop per mass flow did not change largely compared to low pressure condition. In 70 bar condition the mixture phase changed from single phase liquid to single phase vapor through liquid-vapor two phase region, and it showed that the gradient of specific volume change and pressure drop per mass flow varied in each phase.

Development of waterborne oil spill sensor based on printed ITO nanocrystals.

Oil spill accidents occasionally occur in coastal and ocean environments, and cause critical environmental damage, spoiling the marine habitats and ecosystems. To mitigate the damages, the species and amount of spilled oil should be monitored. In this study, we developed a waterborne oil spill sensor using a printed ITO layer. ITO is a compatible material for salty environments such as oceans because ITO is strong against corrosion. The fabricated sensor was tested using three oils, gasoline, lubricant and diesel, and different oil thicknesses of 0, 5, 10, and 15mm. The results showed that the resistance of the sensor clearly increased with the oil thickness and its electrical resistance. For sustainable sensing applications in marine environments, XRD patterns confirmed that the crystal structure of the ITO sensor did not change and FE-SEM images showed that the surface was clearly maintained after tests.