Chih-Jung Kuo

Suppression of Boiling Flow Oscillations in Parallel Microchannels by Inlet Restrictors

parallel channel and upstream compressible volume instabilities commonly exhibited during flow boiling in parallel microchannels have been investigated. The heat fluxes at the onset of unstable boiling have been obtained over effective heat fluxes ranging from 9t o614 W/cm 2 and mass fluxes from 115 to 389 kg/m 2 s. A dimensionless parameter M, which accounts for the pressure drop increase imposed by the inlet restrictors, has been used to correlate the extent of flow instability suppression. It has been shown that the onset of unstable boiling asymptotically increases with M. At sufficiently high M values, parallel channels and upstream compressible volume instabilities are completely eradicated although it gives way to another instability to develop, namely, the critical heat flux conditions. A correlation has been developed in terms of M to predict the conditions leading to unstable boiling. DOI: 10.1115/1.2150837

Bubble Dynamics During Boiling in Enhanced Surface Microchannels

An experimental investigation is presented on flow boiling of deionized water in 227-mum hydraulic diameter microchannels with reentrant type cavities. Key features of nucleate boiling are discussed. Active nucleation site density, bubble frequency and departure diameter, and flow patterns over mass velocities ranging from 41 kg/m2-s to 302 kg/m2-s and heat fluxes ranging from 28 to 445 W/cm2 are studied. Similarities and differences with results obtained on large-scale systems and unenhanced microchannels are discussed

Reduced Pressure Boiling Heat Transfer in Rectangular Microchannels With Interconnected Reentrant Cavities

Boiling flow of deionized water through 227 μm hydraulic diameter microchannels with 7.5 μm wide interconnected reentrant cavities at 47 kPa exit pressure has been investigated. Average two-phase heat transfer coefficients have been obtained over effective heat fluxes ranging from 28 to 445 W/cm 2 and mass fluxes from 41 to 302 kg/m 2 s. A map is developed that divides the data into two regions where the heat transfer mechanisms are nucleation or convective boiling dominant. The map is compared to similar atmospheric exit pressure data developed in a previous study. A boiling mechanism transition criterion based on the Reynolds number and the Kandlikar k 1 number is proposed.

Cavitation Enhanced Heat Transfer in Microchannels

Heat transfer has been investigated in the presence of hydrodynamic cavitation instigated by 20-μm wide inlet micro-orifices entrenched inside 227-/μm hydraulic diameter microchannels. Average surface temperatures, heat transfer coefficients, and pressure drops have been obtained over effective heat fluxes ranging from 39 to 558 W/cm 2 at mass flux of 1814 kg/m 2 s under noncavitating and three cavitating conditions. Significant heat transfer enhancement has been recorded during supercavitating flow conditions in comparison to noncavitating flows with minimal pressure drop penalty. Once supercavitating conditions were reached, no apparent heat transfer augmentation was detected with the reduction of the cavitation index. Visualization of the flow morphology and the heat transfer coefficient characteristics aided in the evaluation of the dominant heat transfer mechanism under various thermal-hydraulic conditions.

Hydoroil-Based Micro Pin Fin Heat Sink

An experimental study on thermal-hydraulic performance of de-ionized water over a bank of shrouded NACA 66-021 hydrofoil micro pin fins with wetted perimeter of 1030-μm and chord thickness of 100 μm has been performed. Average heat transfer coefficients have been obtained over effective heat fluxes ranging from 4.0 to 308 W/cm2 and mass velocities from 134 to 6600 kg/m2 s. The experimental data is reduced to the Nusselt numbers, Reynolds numbers, total thermal resistances, and friction factors in order to determine the thermal-hydraulic performance of the heat sink. It has been found that prodigious hydrodynamic improvement can be obtained with the hydrofoil-based micro pin fin heat sink compared to the circular pin fin device. Fluid flow over pin fin heat sinks comprised from hydrofoils yielded radically lower thermal resistances than circular pin fins for a similar pressure drop.Copyright

Interaction of a liquid flow around a micropillar with a gas jet

An experimental study was conducted to investigate two-phase flow characteristics resulting from gas jet injection into a 225 μm high by 1500 μm wide microchannel. The jet was injected from a 25 μm wide slit on the downstream side of a 150 μm diameter pillar. The liquid Reynolds number (Re = ρUD/μ) based on pillar diameter ranged from 100 to 700, and the average gas momentum coefficient (ρjetUjetAjet/ρmainUmainAref), defined as the ratio of gas momentum to liquid momentum, ranged from 1.6 × 10−5 to 3.368 × 10−1. Flow visualization, micro particle image velocimetry (μPIV), and micro particle tracing velocimetry (μPTV) were used to elucidate the two-phase flow patterns, liquid velocity field, and bubble dynamics. Two modes of gas jets were observed in which bubbles either formed and detached at the pillar or formed an attached ligament that sheared bubbles from its trailing edge. The modes were determined to be primarily Reynolds number dependent. Both modes were observed to positively affect turbulent kine...

Colloidal self-assembly on internal surfaces of partially sealed microchannels

Self-assembly of micro- and nanoparticles on internal surfaces of micromachined structures can enhance the functionality of a wide range of microfluidic systems. Here, we report experimental investigations of the self-assembly of colloidal microspheres on the sidewalls of partially sealed silicon microfluidic channels. Two different approaches were studied. The first, convective self-assembly, is known to be effective in open-domain microsystems, but was found to result in assemblies with poor uniformity in the partially sealed devices. Real-time video microscopy of the assembly process indicates that these results can be attributed to the prolonged duration of evaporation and the complicated geometry of the evaporation front. In contrast, the second approach, which is a novel combination of sedimentation and hydrodynamic particle removal using a turbulent flow, resulted in the assembly of microparticles on predefined locations on the sidewalls of the microchannels. The important parameters controlling this process are discussed in the context of the literature on hydrodynamic particle removal from flat surfaces.

Enhanced Boiling Heat Transfer in Second Generation Microchannels: Part A — Methods and Preliminary Results

This paper reviews literature on conventional scale boiling enhancement techniques by means of reentrant cavities and discusses various avenues by which the knowledge obtained from that research can be used to enhance boiling in microchannels. Fabrication techniques developed by the Micro Thermal-Fluids Laboratory at Rensselaer Polytechnic Institute together with the Advanced Microsystems Materials Laboratory at McGill University are discussed, and preliminary data are given. These results demonstrate the potential for improving boiling heat transfer characteristics in microchannels and introduce the next generation of microchannel heat transfer technology.Copyright

Boiling in Enhanced Surface Microchannels

An experimental investigation is presented on flow boiling of de-ionized water in 227-μm hydraulic diameter microchannels with reentrant type cavities. Key features of nucleate boiling are discussed. Active nucleation site density, bubble frequency and departure diameter, and flow patterns over mass velocities ranging from 41 kg/m2 -s to 302 kg/m2 -s and heat fluxes ranging from 28 to 445 W/cm2 are studied. Similarities and differences with results obtained on large-scale systems and unenhanced microchannels are discussed.Copyright

Heat and Mass Transfer Enhancement by Active Flow Control in Micro Domains

A flow control method is presented that employ liquid and gas jets to enhance heat and mass transfer in micro domains. By introducing pressure disturbances, mixing can be significantly enhanced through the promotion of early transition to a turbulent flow. Since heat transfer mechanisms are closely linked to flow characteristics, the heat transfer coefficient can be significantly enhanced with rigorous mixing. The flow field of water around a low aspect ratio micro circular pillar of diameter 150 μm entrenched inside a 225 μm high by 1500 μm wide microchannel with active flow control was studied and its effect on mixing is discussed. A steady control jet emanating from a 25 μm slit on the pillar was introduced to induce favorable disturbances to the flow in order to modify the flow field, promote turbulence, and increase large-scale mixing. Micro particle image velocimetry (μPIV) was employed to quantify the flow field, the spanwise vorticity, and the turbulent kinetic energy (TKE) in the microchannel. Flow regimes (i.e., steady, transition from quasi-steady to unsteady, and unsteady flow) were elucidated. The turbulent kinetic energy was shown to significantly increase with the controlled jet, and therefore, significantly enhance mixing at the micro scale.Copyright