Yew Mun Hung

Thermal Analysis of a Water-Filled Micro Heat Pipe With Phase-Change Interfacial Resistance

The progressive evaporation and condensation processes in a micro heat pipe, with which high heat fluxes at the liquid–vapor interface are associated, render it a device of high thermal conductance. By coupling the phase-change interfacial resistance model with a mathematical model based on first principles for fluid flow and heat transfer, the axial temperature variations of the liquid and vapor phases as well as those of other field variables are characterized and analyzed. The findings provide a well-defined exposition of the validity of uniform-temperature assumption for the liquid and vapor phases in the analysis of micro heat pipes. In conjunction with the acquisition of liquid and vapor temperature profiles, the heat transfer characteristics of the evaporation process can be analyzed. The local evaporative heat transfer coefficient and heat flux are evaluated. The results indicate that both heat transfer coefficient and heat flux are of considerably high values, confirming that the heat transport capability of a micro heat pipe is dominated by the phase-change heat transfer at the liquid–vapor interface. [DOI: 10.1115/1.4006898]

Analysis of Microheat Pipes With Axial Conduction in the Solid Wall

A one-dimensional, steady-state model of a triangular microheat pipe (MHP) is developed, with the main purpose of investigating the thermal effects of the solid wall on the heat transport capacity of an MHP. The energy equation of the solid wall is solved analytically to obtain the axial temperature distribution, the average of which over the entire length of the MHP is simply its operating temperature. Next, the liquid phase is coupled with the solid wall by a heat transfer coefficient. Then, the continuity, momentum, and energy equations of the liquid and vapor phases are, together with the Young― Laplace equation, solved numerically to yield the heat and fluid ftow characteristics of the MHP. The heat transport capacity and the associated optimal charge level of the working fluid are predicted for different operating conditions. Comparison between the models with and without a solid wall reveals that the presence of the solid wall induces a change in the phase change heat transport by the working fluid, besides facilitating axial heat conduction in the solid wall. The analysis also highlights the effects of the thickness and thermal conductivity of the solid wall on its axial temperature distribution. Finally, while the contribution of the thermal effects of the solid wall on the heat transport capacity of the MHP is usually not dominant, it is, nevertheless, not negligible either.

Enhanced Evaporation Strength through Fast Water Permeation in Graphene-Oxide Deposition

The unique characteristic of fast water permeation in laminated graphene oxide (GO) sheets has facilitated the development of ultrathin and ultrafast nanofiltration membranes. Here we report the application of fast water permeation property of immersed GO deposition for enhancing the performance of a GO/water nanofluid charged two-phase closed thermosyphon (TPCT). By benchmarking its performance against a silver oxide/water nanofluid charged TPCT, the enhancement of evaporation strength is found to be essentially attributed to the fast water permeation property of GO deposition instead of the enhanced surface wettability of the deposited layer. The expansion of interlayer distance between the graphitic planes of GO deposited layer enables intercalation of bilayer water for fast water permeation. The capillary force attributed to the frictionless interaction between the atomically smooth, hydrophobic carbon structures and the well-ordered hydrogen bonds of water molecules is sufficiently strong to overcome the gravitational force. As a result, a thin water film is formed on the GO deposited layers, inducing filmwise evaporation which is more effective than its interfacial counterpart, appreciably enhanced the overall performance of TPCT. This study paves the way for a promising start of employing the fast water permeation property of GO in thermal applications.

Coupled effects of hydrophobic layer and vibration on thermal efficiency of two-phase closed thermosyphons

Condensed droplets in a two-phase closed thermosyphon (TPCT) are subject to two competing forces: contact line pinning force between the droplet and the wall of the TPCT and the body force due to gravity. Either reducing the contact line pinning force or increasing the body force can lead to significant enhancement in the heat transport capability. This study aims to scrutinize the coupled effects of hydrophobic surface coating at the condenser wall and high-acceleration induced vibration on the thermal efficiency of a TPCT. We explore an approach to reduce the contact line pinning force by applying a thin layer of hydrophobic coating, which also facilitates dropwise condensation to further increase the heat transport capability. The body force of the condensed droplets can be increased by introducing a low-frequency (f ∼ 102 Hz) high-acceleration ( ∼ 103 m s−2) vibration. The formation of elongated liquid jets and entrainment of droplets induced by the high-acceleration vibration counteracts the enhancing effect from the increased body force of condensed droplets. Nanofluid with distinguished thermo-physical properties is charged to the TPCT to further enhance the thermal efficiency. By incorporating the coupled effects of hydrophobic layer and vibration, we can obtain a maximum augmentation in the heat transfer coefficient exceeding 47.7%. The factors contributing to the enhancement of thermal efficiency of a TPCT are identified and the underlying physical significance of the coupled effects is delineated.

Amplitude modulation schemes for enhancing acoustically-driven microcentrifugation and micromixing

The ability to drive microcentrifugation for efficient micromixing and particle concentration and separation on a microfluidic platform is critical for a wide range of lab-on-a-chip applications. In this work, we investigate the use of amplitude modulation to enhance the efficiency of the microcentrifugal recirculation flows in surface acoustic wave microfluidic systems, thus concomitantly reducing the power consumption in these devices for a given performance requirement-a crucial step in the development of miniaturized, integrated circuits for true portable functionality. In particular, we show that it is possible to obtain an increase of up to 60% in the acoustic streaming velocity in a microdroplet with kHz order modulation frequencies due to the intensification in Eckart streaming; the streaming velocity is increasing as the modulation index is increased. Additionally, we show that it is possible to exploit this streaming enhancement to effect improvements in the speed of particle concentration by up to 70% and the efficiency of micromixing by 50%, together with a modest decrease in the droplet temperature.

Acoustically-controlled Leidenfrost droplets.

Suppressing the Leidenfrost effect can significantly improve heat transfer from a heated substrate to a droplet above it. In this work, we demonstrate that by generating high frequency acoustic wave in the droplet, at sufficient vibration displacement amplitudes, the Leidenfrost effect can be suppressed due to the acoustic radiation pressure exerted on the liquid-vapor interface; strong capillary waves are observed at the liquid-vapor interface and subsequently leads to contact between the liquid and the heated substrate. Using this technique, with 10(5)Hz vibration frequency and 10(-6)m displacement amplitude of the acoustic transducer, a maximum of 45% reduction of the initial temperature (T0∼200-300°C) of the heated substrate can be achieved with a single droplet of volume 10(-5)l.

MOMENTUM INTEGRAL METHOD FOR FORCED CONVECTION IN THERMAL NONEQUILIBRIUM POWER-LAW FLUID-SATURATED POROUS MEDIA

Forced convection heat transfer for power-law fluid flow in porous media was studied analytically. The analytical solutions were obtained based on the Brinkman-extended Darcy model for fluid flow and the two-equation model for forced convection heat transfer. As a closed-form exact velocity profile is unobtainable for the general power-law index, an approximate velocity profile based on the parabolic model is proposed by subscribing to the momentum boundary layer integral method. Heat transfer analysis is based on the two-equation model by considering local thermal nonequilibrium between fluid and solid phases and constant heat flux boundary conditions. The velocity and temperature distributions obtained based on the parabolic model were verified to be reasonably accurate and improvement is justified compared to the linear model. The expression for the overall Nusselt number was derived based on the proposed parabolic model. The effects of the governing parameters of engineering importance such as Darcy number, power-law index, nondimensional interfacial heat transfer coefficient, and effective thermal conductivity ratio on the convective heat transfer characteristics of non-Newtonian fluids in porous media are analyzed and discussed.

Thermal analysis of Al2O3/water nanofluid-filled micro heat pipes

Micro heat pipe is an effective micro-scale cooling device and its operation is sustained by two-phase heat transfer and capillary suction. Based on the first-principles calculation to yield the heat and fluid flow characteristics, this work delineates the thermal performance of micro heat pipes utilizing a nanofluid as a working fluid. Heat transport capacity and thermal resistance are compared as the performance indicator of nanofluids at different nanoparticle concentration fractions in an optimally charged micro heat pipe. The performance of a nanofluid-filled micro heat pipe is deemed to decline if we use heat transport capacity as a performance indicator, which is contrary to the indication of thermal resistance. We elucidate the factors contributing to the contradictory results in the thermal performance of a nanofluid-filled micro heat pipe using different performance indicators to explain the underlying physical significance of the use of a nanofluid on the performance of micro heat pipes.

Viscous Dissipation Effect on Entropy Generation of Nanofluid Flow in Microchannels

An analytical study of the viscous dissipation effect on entropy generation of forced convection of water-alumina nanofluid in a circular microchannel subjected to exponential wall heat flux is reported. Closed form solutions of the temperature distributions in the streamwise direction for the models with and without viscous dissipation term in the energy equation are obtained. The two models are compared by analyzing their relative deviations in entropy generation for different Reynolds number and nanoparticle volume fraction. The incorporation of viscous dissipation prominently affects the temperature distribution and consequently the entropy generation. The increase in the entropy generation is mainly attributable to the increase in the fluid friction irreversibility. The addition of nanoparticle increases the effective thermal conductivity and viscosity of nanofluid which induces escalation in the heat transfer and fluid friction irreversibilities, respectively. By taking the viscous dissipation effect into account, the exergetic effectiveness for forced convection of nanofluid in microchannels attenuate with increasing nanoparticle volume fraction. From the aspect of the second law of thermodynamics, the widespread conjecture that nanofluids possess advantage over pure fluid associated with higher overall effectiveness is invalidated.© 2013 ASME

Effective micro-spray cooling for light-emitting diode with graphene nanoporous layers

A graphene nanoplatelet (GNP) coating is utilized as a functionalized surface in enhancing the evaporation rate of micro-spray cooling for light-emitting diodes (LEDs). In micro-spray cooling, water is atomized into micro-sized droplets to reduce the surface energy and to increase the surface area for evaporation. The GNP coating facilitates the effective filmwise evaporation through the attribute of fast water permeation. The oxygenated functional groups of GNPs provide the driving force that initiates the intercalation of water molecules through the carbon nanostructure. The water molecules slip through the frictionless passages between the hydrophobic carbon walls, resulting an effective filmwise evaporation. The enhancement of evaporation leads to an enormous temperature reduction of 61.3 °C. The performance of the LED is greatly enhanced: a maximum increase in illuminance of 25% and an extension of power rating from 9 W to 12 W can be achieved. With the application of GNP coating, the high-temperature region is eliminated while maintaining the LED surface temperature for optimal operation. This study paves the way for employing the effective hybrid spray-evaporation-nanostructure technique in the development of a compact, low-power-consumption cooling system.