Jonggan Hong

Effect of laser irradiation on thermal conductivity of ZnO nanofluids

This work examines a laser particle fragmentation process to enhance the thermal conductivity of nanofluids. A Q-switched Nd:YAG laser is employed to change the size of the suspended ZnO nanoparticles in water. The influence of laser irradiation on the suspended particles is analyzed by transmission electron microscopy and absorption spectroscopy. The thermal conductivity of the nanofluid is measured by the transient hot-wire method. The results show that laser irradiation leads to partial fragmentation of some particles. However, the partial size reduction results in substantial enhancement of the thermal conductivity.

Development of a micro liquid-level sensor for harsh environments using a periodic heating technique

This paper describes the development and testing of a novel micro thermal sensor for point sensing of lubrication oil level in industrial compressors. The results reported in this work can be applied to various harsh environments that feature high temperature/pressure, limited space and flow/vibration. The sensor employs an ac (alternating current) thermal technique with a single heating/sensing element. As the sensing scheme is based on the so-called three-omega method, the sensing signal is noise-resistant and hardly affected by flow in the liquid being measured. Experiments with DI water, ethanol and ethylene glycol confirm that the sensor performance is satisfactory under atmospheric pressure. Also, to mimic harsh conditions as in an industrial compressor, tests are performed in a pressure vessel containing R410A gas and polyvinylether lubrication oil under high temperatures and pressures. The results indicate that the sensitivity and response time of the developed sensor are appropriate for practical usage in harsh environments. As the sensor can be easily mass-produced at low cost using photolithography, it has strong potential for industrial applications.

Measurement of the Thermal Conductivity of Alumina/Zinc-Oxide/Titanium-Oxide Nanofluids

The thermal conductivity of water- and ethylene glycol-based nanofluids containing alumina , zinc oxide (ZnO) and titanium dioxide nanoparticles is measured by varying the particle diameter and volume fraction. The transient hot-wire method using an anodized tantalum wire for electrical insulation is employed for the measurement. The experimental results show that nanofluids have substantially higher thermal conductivities than those of the base fluid and the ratio of thermal conductivity enhancement increases linearly with the volume fraction. It has been found that the ratio of thermal conductivity enhancement increases with decreasing particle size but no empirical or theoretical correlation can explain the particle-size dependence of the thermal conductivity. This work provides, for the first time to our knowledge, a set of consistent experimental data over a wide range of nanofluid conditions and can therefore serve as a basis for developing theoretical models to predict thermal conduction phenomena in nanofluids.

Measuring the Thermal Conductivity of Flowing Liquid Samples Using the Three Omega Method

The thermal conductivity of a liquid is generally measured under conditions that suppress bulk flow in the sample. However, in situ measurement of the thermal conductivity of a flowing liquid would be useful in various scientific and engineering applications. This work demonstrates that a thermal wave technique, such as the three omega method, can effectively measure the thermal conductivity of flowing liquid if the frequency range is adjusted such that the thermal boundary layer is sufficiently thinner than the momentum boundary layer. A new dimensionless number was defined to assess the convection effects, and a criterion for thermal conductivity measurements was obtained for water flowing in a circular tube.

Development of a Novel Thermal Sensor to Monitor the Fluid State in a Microchannel

This work presents development of a novel thermal sensor to measure the fluid phase and composition in a microchannel. As the sensor employs an AC hot-film technique to monitor the fluid state, it is more sensitive and noise-resistant than the conventionally used DC technique. The sensor is composed of a thin-film heater integrated into a PDMS microchannel fabricated on a glass substrate. Monitoring the liquid-gas phase interface in a microchannel is demonstrated by examining water injection and liquid evaporation processes. The results show good agreement with those by optical inspection. Also, real-time monitoring the composition of ethanol/water mixtures flowing in a microchannel is demonstrated. The presented sensor is expected to be used in various potential applications, including multiphase flow sensors, lab-on-a-chip devices, micro heat exchangers and micro fuel cells.Copyright

A novel thermal sensor to monitor the gas-liquid phase interface in microfluidic channels

We designed and fabricated a novel microscale sensor for monitoring the motion of the gas-liquid interface in microchannels, based on the 3omega thermal-analysis method. As the sensor employs an AC hot-film technique to probe the movement of phase interface, it is more sensitive and noise-resistant than the already developed DC technique. The sensor is composed of a thin-film heater integrated into a PDMS microchannel fabricated on a glass substrate. The performance of the sensor is characterized by examining water injection and liquid evaporation processes. The results show good agreement with those by optical inspection. It is also demonstrated that the sensor can effectively monitor the long-term evaporation process of water in a microchannel. The result demonstrates strong potential of the proposed sensor as an integrated real-time probe to monitor a variety of microfluidic phenomena.

Experimental Analysis of Bubble Dynamics Induced by Pulsed-Laser Heating of Absorbing Liquid

The bubble dynamics induced by direct laser heating is experimentally analyzed as a first step to assess the technical feasibility of laser-based ink-jet technology. To understand the interaction between laser light and ink, the absorption spectrum is measured for various ink colors and concentrations. The hydrodynamics of laser-generated bubbles is examined by the laser-flash photography. When an Ar ion laser pulse (wavelength 488 nm) with an output power up to 600 mW is incident on the ink solution through a transparent window, a hemispherical bubble with a diameter up to can be formed with a lifetime in a few tens of microsecond depending on the laser power and the focal-spot size. Parametric study has been performed to reveal the effect of laser pulse width, output power, ink concentration, and color on the bubble dynamics. The results show that the bubble generated by a laser pulse is largely similar to that produced by a thin-film heater. Consequently, the present work demonstrates the feasibility of developing a laser-actuated droplet generation mechanism for applications in ink-jet print heads. Furthermore, the results of this work indicate that the droplet generation frequency is likely to be further increased by optimizing the process parameters.