Visualization of Boiling Heat Transfer on Micro Porous Coated Surface in Confined Space

Numerous researches have been developed for pool boiling on microporous coated surface in the past decade. The nucleate boiling heat transfer was found to be increased by up to 4.5 times than that on uncoated surface. Recently, the two-phase micro heat exchangers have been considered for high flux electronic devices cooling. The enhancement techniques for improving the nucleate boiling heat transfer performance in the micro heat exchangers have gotten more importance. Previous studies of microporous coatings, however, have been restricted to boiling in unconfined space. No studies have been made on the feasibility of using microporous coatings for enhancing boiling in confined spaces. This study provides an experimental observation of the vapor generation and leaving processes on microporous coatings surface in a 1-mm confined space. It would be helpful for understanding the mechanism of boiling heat transfer and improving the design of two-phase micro heat exchangers. Aluminum particles of average diameter 20 μm were mixed with a binder and a carrier to develop a 150 μm thickness boiling enhancement paint on a 3.0 cm by 3.0 cm copper heating surface. The heating surface was covered by a thin glass plate with a 1 mm spacer to form a 1 mm vertical narrow space for the test section. The boiling phenomenon was recorded by a high speed camera. In addition to the three boiling regimes observed by Bonjour and Lallemand [1], i.e., isolated deformed bubbles, coalesced bubbles and partial dryout at low, moderate and high heat fluxes respectively in unconfined space, a suction and blowing process was observed at the highest heat flux condition. Owing to the space confinement, liquid was sucked and vapor was expelled periodically during the bubble generation process. This mechanism significantly enhanced the boiling heat transfer performance in confined space.

Analysis of Water Transport in the Vicinity of Micro-Porous Layer in PEFC With Freezing Method

This paper examines the role of micro porous layers (MPLs) in Polymer Electrode Fuel Cells (PEFCs) by observing the cross-sectional distribution of condensed water inside a cathode side MPL In addition, the forms of water condensation in the vicinity of a MPL are also compared between two places, under flow channels and under lands, by observing both inside the MPL and an interface between the MPL and a catalyst layer (CL). The freezing method and a cryo-scanning electronic microscope (cryo-SEM) are used for the observation. The result under the non-flooded condition shows that condensed water does not accumulate inside the MPL. This result indicates that the water produced by PEFC power generation passes through the MPL as vapor state under non-flooded conditions.

Simulation of Magnetic Actuation of Ferrofluids in Microtubes

Magnetic actuation of ferrofluids with dynamic magnetic fields is one of the most promising research areas with its wide and different potential application areas such as biomedical and micropumping applications. Ferrofluid has the potential of opening up new possibilities. To have more understanding about various fields of engineering, more research should be conducted by considering both the experimental and modeling aspects. The most important parameters determining the flow property, flow rates and overall system efficiency are the quality and the topology of magnetic fields used in these systems. Therefore, the methods of dynamic magnetic field generation constitute a central problem to obtain desired performance. This study includes modeling and simulation of ferrofluid actuation with dynamic magnetic fields by using the COMSOL software and reports that ferrofluid actuation can be successfully used and the simulation results agree well with the experimental results.

Diffusion and Mixing in Microchannel Analyzed by the Luminol Chemiluminescence

This study focused on the diffusion and mixing phenomena investigated by using luminol chemiluminescence (CL) to estimate the local chemical reaction rate in the T-junction microchannel. Generally, the degree of mixing in microchannel is calculated by the deviation of the obtained concentration profiles from the uniform concentration profile by using fluorescence technique. Thus, the degree of mixing is a macroscopic estimate for the whole microchannel, which is inappropriate for understanding the diffusion and mixing phenomena in the mixing layer. In this study, the luminol CL reaction is applied to visualize the local chemical reaction and to estimate the local diffusion and mixing phenomena at an interface between two liquids in microchannel. Luminol emits blue chemiluminescence when it reacts with the hydrogen peroxide at the mixing layer. Experiments were carried out on the T-junction microchannel with 200 microns in width and 50 microns in depth casted in the PDMS chip. The chemiluminescence intensity profiles clearly show the mixing layer at an interface between two liquids. The experimental results are compared with the results of numerical simulation that involves solving the mass transport equations including the chemical reaction term. By calibrating CL intensity to the chemical reaction rate estimated by the numerical simulation, the local chemical reaction profile can be quantitatively estimated from the CL intensity profile.

Characterization and Modeling of Micro Swimmers With Helical Tails and Cylindrical Heads Inside Circular Channels

Micro swimming robots offer many advantages in biomedical applications, such as delivering potent drugs to specific locations in targeted tissues and organs with limited side effects, conducting surgical operations with minimal damage to healthy tissues, treatment of clogged arteries, and collecting biological samples for diagnostic purposes. Reliable navigation techniques for micro swimmers need to be developed to improve the localization of robots inside the human body in future biomedical applications. In order to estimate the dynamic trajectory of magnetically propelled micro swimmers in channels, that mimic blood vessels and other conduits, fluid-micro robot interaction and the effect of the channel wall must be understood well. In this study, swimming of one-link robots with helical tails is modeled with Stokes equations and solved numerically with the finite element method. Forces acting on the robot are set to zero to enforce the force-free swimming and obtain forward, lateral and angular velocities that satisfy the constraints. Effects of the number of helical waves, wave amplitude, relative size of the cylindrical head of micro swimmer and the radial position on angular and linear velocity vectors of micro swimmer are presented.

A Photographic Study on the Effects of Hydrophobic-Spot Size and Subcooling on Local Film Boiling

The effects of hydrophobic circle spot size and subcooling on local film boiling phenomenon from the copper surface with single PTFE (Polytetrafluoroethylene) hydrophobic circle spot at low heat flux has been investigated. The experiments were performed using pure water as the working fluid and subcooling ranging from 0 and 10K. The heat transfer surfaces are used polished copper block with single PTFE hydrophobic circle spot of diameters 2, 4 and 6 mm, respectively. A high-speed camera was used to capture bubble dynamics and disclosed the sequence of the process leading to local film boiling. The result shows that local films boiling occurs on the PTFE circle spot at low heat flux and was triggered by the merging of neighboring bubbles. The study also showed that transition time required for change from nucleate boiling regime to local film boiling regime depends on the diameter of the hydrophobic circle spot and the subcooling. A stable local film boiling occurs at the smallest diameter of hydrophobic spot. Subcooling cause the local film boiling occur at negative superheat and oscillation of bubble dome.

Air Bubble Injection Into a Liquid Stream in a Minichannel

Air bubble injection experiments have been performed to obtain a better understanding and detailed data on bubble behavior and liquid velocity profiles to be used for validation of 3-D Interface Tracking Models and CFD models. Two test sections used were vertical rectangular minichannels with a width and gap of 20 mm × 5.1 mm and 20 mm × 1.9 mm, respectively. Subcooled water at near atmospheric pressure flowed upward under laminar and turbulent flow conditions accompanied by air bubbles injected from a small hole on one of the vertical walls. The experiments yielded data on bubble formation and departure, and interactions with laminar or turbulent water flow. Instantaneous and ensemble-average liquid velocity profiles have been obtained using a Particle Image Velocimetry technique and a high speed video camera.

Effects of Chemical and Physical Shear-Stress Stimulation of Human Placenta-Derived Multipotent Stem Cells in Microchannel

Multipotent cells obtain from human postpartum term placenta is an ethically conductive, easily accessible and high-yielding stem cell source. In this conference presentation, we demonstrate using microchannel platform to culture and differentiate the human placenta-derived stem cells. Both chemical and shear stress stimulation effects were investigated.

3D Velocity Measurement by Orthogonal-Plane Micro-PIV for Electrokinetic Enhancement of Surface Reaction

In microfluidic systems, surface reaction is diffusion-limited because the effect of convection on mass transport decreases due to low Peclet number. It is indicated that an externally induced flow toward the reactive interface is effective to enhance the efficiency of the surface reaction. However, it is difficult to evaluate the flow velocity normal to the substrate, which directly contributes to the enhancement of the surface reaction, due to the monolithic dimension of microfluidic device. This paper reports the development of a 3D flow velocity measurement method by orthogonal-plane micro-PIV to evaluate the contribution of flow distortion by alternating-current electrokinetic phenomena on the reaction enhancement. 3D velocity field is reconstructed from two orthogonal velocity fields measured by 2D micro-PIV with different measurement planes; one is based on normal observation with the measurement plane parallel to the bottom wall and the other is based on a lateral observation with the plane perpendicular to the bottom wall through a sidewall of a fluidic channel made of PDMS (100 × 50 μm). Complete 3D velocity field is determined by scanning the measurement plane in each observation scheme. Validity of orthogonal-plane micro-PIV for the 3D velocity measurement was confirmed by the measurement of three component velocities in a tilt rectangular microchannel. Then, we investigated AC-driven electrothermal (ACET) effect induced by the property gradient of fluid due to temperature nonuniformity under an electric field application. Effective flow structure of ACET for the enhancement of surface reaction, a circular stirring fluid motion which conveys bulk fluid to the surface region, was observed. This stirring motion of fluid could improve the binding opportunities between suspended and immobilized species and result in the promotion of reaction efficiency. It is clarified that 3D flow of ACET contributes to the localized enhancement of the surface reaction efficiency.

Heat Transfer Enhancement Effect of Nanostructured Surface Made of Carbon Nanotube on SiC Ceramics

The Japan Atomic Energy Agency (JAEA) has been conducting research and development on the thermo-chemical iodine–sulfur (IS) process, which is one of the most attractive water-splitting hydrogen production methods using the nuclear heat of a high-temperature gas-cooled reactor (HTGR). In researching this IS process, a silicon carbide (SiC) heat exchanger with good corrosion resistance was used in a corrosive situation in boiling sulfuric acid. With the aim of enhancing heat transfer in the SiC heat exchanger, a nanostructured surface made of carbon nanotubes (CNTs) was produced on a SiC substrate by surface decomposition. Two types of SiC, one produced by pressureless sintering (PLS-SiC) and one by chemical vapor deposition (CVD-SiC), were used as substrates. CNTs formed by the surface decomposition of SiC can vary depending on the crystal structure of the substrates. Additionally, in order to investigate surface wettability, nanostructured surfaces on the CVD-SiC with hydrophilicity and hydrophobicity were produced. The effects of heat transfer enhancement by the nanostructured surfaces were evaluated by a convective heat transfer test using de-ionized water. The nanostructured surface on the CVD-SiC with hydrophilicity was the only surface that showed any heat transfer enhancement. However, this enhancement was much smaller than those previously reported. The experiment showed that the small size of the nanopores influenced the heat transfer enhancement and that the wettability of the nanostructured surface was related to heat transfer enhancement.