A microgroove evaporator is expected to realize a high heat transfer coefficient despite its simple structure. However, the theory for designing the groove structure with regard to given thermal transport properties and thus, a high performance micro-grooved evaporator has not been established, so far. In this study, the feasible numerical model for calculating capillary-rise length (mass transfer capability) and evaporation rate (heat transport rate) of single rectangular microgroove was proposed. We found that the calculated transported heat and capillary-rise length in single microgroove are in good agreement with the experimental value under the wide range of conditions. We also designed a high performance microgroove evaporator composed of the combination of different size of grooves. The calculated effective heat flux of the proposed groove stracture suggests a large potential for improving the performance of micro-grooved evaporator.

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Development of a hybrid cooling system with the combination of heat pipes and phase change material for the lithium-ion battery in electric vehicles is being studied in our laboratory. It is also necessary to investigate the performance of the heat pipe for the development of this hybrid cooling technology. This study attempted to observe the motion of the vapor phase in the heat pipe. Furthermore, this observation attempted to measure the velocity of the vapor phase by using PIV (Particle Image Velocimetry) technique. The final aim of this study is to observe a dryout state of the heat pipe.

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The lithium-ion battery used for an electric vehicle has high capacity of electric generation and long life in repetitive use, and it is very beneficial for efficient energy usage. However, the establishment of thermal management system which ensures the safety in case of abnormal heat generation of the lithium-ion battery cell, is not yet sufficient. Therefore, we proposed a new cooling system using phase change material and heat pipes and investigated its cooling effect. As a result, we found that the temperature of the lithium-ion battery of electric vehicle in driving, could be maintained under 45 deg C by the cooling system.

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In recent years, the heat flux of electronic devices has significantly increased due to higher output and downsizing. A JEST type Loop Heat Pipe with a water-cooled heat exchanger has been developed. In the present study, the total thermal resistance was divided to each part of the equipment, and the effect of each thermal resistance on the cooling performance was investigated. As a result, the effect of vapor line inner diameter on the total thermal resistance was clarified. And it was also revealed that the evaporator thermal resistance was 65% of the total thermal resistance. Therefore, an approach to decrease the evaporator thermal resistance is effective for improving the cooling performance.

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This study experimentally investigated the heat transfer characteristics of a fin with built-in closed-loop self-oscillating heat pipe. The fin part of the test plate consists of small closed-loop channel. The effects of temperature of fin base, channel structure of heat pipe and fin thickness are investigated. The result revealed that the fin efficiency was affected by the base temperature in the case of fin with built-in heat pipe. The fin with meander channel structure case showed higher fin efficiency by increasing channel cross sectional area. On the other hand, the fin with pool channel structure case showed the lower fin efficiency. Moreover, the thinner fin case decreased the fin efficiency due to its higher flow resistance.

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Oscillation characteristics of self-oscillating boiling cooler with a moving part such as bellows are experimentally investigated. Two oscillation behaviors are observed when heat flux is increased. At low heat fluxes, the frequency is kept almost unchanged and the amplitude is increased. At high heat fluxes, the frequency is increased and the amplitude is almost unchanged since the liquid working fluid reaches the end of the boiling surface. It is also shown that average operating pressure corresponds to the applied pressure on the bellows. In addition, critical heat flux and heat transfer coefficient are increased as the applied pressure is increased.

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In recent years, the performance of electronic devices has been significantly improved, while the heat generation density from these devices has increased. In this study, a prototype of loop typed thermosyphon, which consists of an evaporator with vertical finned surface and air-cooled condenser, has been investigated experimentally. Refrigerant HFC-134a is used as working fluid. The effects of volume filling rate of the working fluid on the cooling performance was examined. The thermal resistance and heat receiving surface temperature were measured by varying the input heat flux from 15 to 100 W/cm^2. In the case of 40% volume filling rate, the cooling performance was the highest and heat receiving surface temperature of evaporator is around 85 ℃ at the effective heat flux of 80 W/cm^2 (heat generation of 2.0 kW).

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Cooling characteristic of a loop thermosyphon is investigated experimentally. The evaporator height and the vapor tube diameter is changed as well as liquid height, and then surface temperature of dummy CPU is measured. The surface temperature is decreased with decrease in liquid height, especially at low liquid height. And more, the larger the evaporator height or the vapor tube diameter is, the lower the surface temperature is. On the other hand, at high liquid height, the effect of the evaporator height and the vapor tube diameter on the surface temperature is not evident.

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Concerning the heat removal from the systems like electronic equipment, satellite thermal control device and so on, the reduction of the thermal contact resistance is a quite crucial issue to be tackled. In this study, the thermal contact resistance between dissimilar materials with flat rough surface has been investigated experimentally under comparatively low mean contact pressure of 0.1 to 1.0 MPa. From comparison between the measured and predicted results, it has been shown that, even under such low mean contact pressure, the temperature drop at the contact interface can be predicted satisfactorily by using the conventional prediction equation based on the unit cell model.

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In this paper, we describe the thermal characterization of a multifunctional hybrid photovoltaic panel for the conversion of sunlight into microwave that is used in a solar power satellite. We designed a practical configuration of the hybrid panel and investigated the thermal characteristics by experiment and numerical analysis. First, thermal-vacuum test using a prototype panel was conducted to obtain practical thermal resistances between each layers of the panel. Using the obtained thermal resistances, we then simulated daily temperature variation and thermal deformation during an orbit. The simulation results show that the deformation is maintained within allowable range in the present panel configuration; panel temperature is, however, too low when top and bottom surface do not receive sunlight.

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Subcooled pool boiling is investigated for binary mixtures of water and Ethylene-glycol of 30vol%,50vol% and 70vol% under the atmospheric condition. The heat transfer performance decreases with increasing of Ethylene-glycol concentration.According to the experimental data, the heat flux is indicated with liquid subcooling for the concentration of Ethylene-glycol. As increasing Ethylene-glycol concentration, CHF decreases and Microbubble Emission Boiling, MEB, is difficult to be generated. Stable MEB is observed at liquid subcooling of 50K for the mixture of water and Ethylene-glycol of 30vol %. The concentration of coolant is one of the difficult factors to be resolved in boiling heat transfer for the application to cooling technology.

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This paper evaluates pool boiling heat transfer with lotus-type porous copper on a heated surface. The experiments are performed under atmospheric and saturated conditions. The lotus porous medium has unidirectional pore structure and the averaged size of the pore hole is 0.4 mm. The porous plate of 1.0 mm or 2.0 mm in thickness is mechanically attached onto the heated surface. The boiling curves suggest that utilization of the lotus porous medium definitely leads to boiling heat transfer enhancement. However, the results also prove that the thickness of the lotus porous doesn't affect the boiling heat transfer rate and that there is a big contact thermal resistance between the heated surface and the lotus plate.

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Recently, Computational Fluid Dynamics is widely used in electronic equipment thermal design process. With regard to semiconductor packages in electronic equipment, two-resistor thermal models are usually available in order to reduce thermal simulation calculation time. However, it is known that two-resistor models include several kinds of calculation error. In this study, a calculation error reduction method for two-resistor thermal models is newly developed. Novel error reduction method applying to BGA packages consists of three steps to correct package thermal resistances measured by JEDEC method. Usually, more than 20% junction temperature rise error occurs in various cooling condition such as fan cooling and heat-sink cooling. In our study, it is clarified that less than 5% error of calculated temperature rise could be achieved in almost cooling conditions.

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In order to improve the accuracy of the coil temperature prediction, detailed fundamental experiments have been conducted on thermal resistances that are caused by the void air gap and contact surfaces. The thermal resistance of the coil around the air gap can be calculated by an air gap distance and air heat conductivity. Contact surface thermal resistance between the core and the housing was constant regardless of the press-fitting state in this experiment. Prediction accuracy of the coil temperature is improved by including the heat resistance characteristics that is obtained by the basic experiment to conjugate heat transfer analysis model.

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Recently smartphones have thermal problems because heat dissipation of CPU increases with high performance and high density packaging. Hot spots on the rear case are a critical issue for cause of low temperature burn. Cu or graphite sheets which put on the rear case decrease its temperature, but antennas near the rear case are unavailable by the electric conductive sheets. In this paper, cooling behavior of smartphone with high thermal conductivity rear case is studied. It is confirmed that high thermal conductivity resin which does not influence antenna characteristics are effective to cancel hot spots on the rear case.

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Viscoelastic fluids in low Reynolds number flow regime have important potential for heat transfer performance in contrast to Newtonian fluuids. To provide a high efficiency heat exchanger, the authors are aiming at controlling the heat transfer rate actively, in particular, with photo-rheological fluids (PRFs) which include photoisomer, the rheological properties of which can be tuned by light. The viscoelasticity of the fluids as a working fluid of heat transfer becomes weaker as the exposure time increases. As a result of the experiment, both relaxation time and pressure loss penalty were decreased with UV irradiation. The corresponding heat transfer rate also has a decreasing trend as the exposure time is lengthened more. These changes are supposed to be caused by weaker viscoelasticity of the aqueous solution that may significantly influence flow structure, as OMCA, one component of the solute used in the present study, undergoes a photo-isomerization.

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In this study, the effect of the geometry of the micro asymmetric converging-diverging nozzle was evaluated based on the numerical simulation for the supersonic air flow inside the micro-channel. This nozzle was designed to generate low temperature air flow for the high heat flux heat sink and configured by simple arc curvature. The channel length was 10 mm. The nozzle width was varied from 20 to 400 μm. The inlet pressure, outlet pressure, and wall temperature were 0.7 MPaA, 0.1 MPaA, and 350 K, respectively. Then, in order to evaluate the effect of back pressure, outlet pressure was varied from 0.1 to 0.4 MPa for the case of 200 μm of nozzle width. From calculation results, the pressure and temperature distributions inside the each channel were compared and discussed.

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This paper describes cooling performance of miniature cooling fans and blowers. In recent years, several types of miniature devices that can generate cooling airflow into electronic equipment have been developed. These devices are enough thin and small and these are usable in high-density packaging electronic equipment such as laptop computers and portable devices. In this study, we are trying to evaluate an optimum selection of these devices in order to obtain higher cooling performance in high-density packaging electronic equipment by using these devices. In this report, we compared effectiveness of three types of the miniature fans and the piezoelectric micro blower mounted in the narrow flow passage from the viewpoint of cooling performance through heat transfer experiment.

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This study describes a heat transfer characteristic of pulsating airflow around an obstruction mounted in a rectangular duct. Several researchers have reported the possibility of the heat transfer enhancement by pulsating flow. Our study is trying to apply pulsating flow to the heat transfer enhancement method in electronic equipment. In this report, the heat transfer performance of the pulsating airflow around the cylinder type obstruction while changing the diameter of the obstruction. The obstructions simulate components in electronic equipment. It is found that heat transfer around the obstruction can be enhanced by the flow pulsation regardless of the dimension of the obstruction.

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A shape optimization method for volume minimization of thermoelastic body is presented. In this study, the volume minimization problem is formulated, while mean compliance is constrained to less than a desired value, in the thermoelastic body. The shape gradient of the shape optimization problems was derived theoretically using the adjoint variable method, the Lagrange multiplier method and the formulae of the material derivative. Reshaping was accomplished using a traction method that was proposed as a solution to the shape optimization problems. The validity of the proposed method was confirmed based on the results of 2D numerical analysis.

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In order to contain the thermal expansion of the positioning device consisting of motor, bearing and ball screw, a special water jacket was designed and made. A cooling system was established to cool the positioning device by circulation temperature-controlled water. During experimental verification, the positioning device is was operated under several motor rotation speed. Temperature profile of the ball screw was measured and the thermal expansion was calculated. As a result, the thermal expansion of the positioning was contained below 10μm

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For the fabrication of groove on FTO thin film, substrate (glass) side laser scribing is usually used to obtain several tens μm groove width. In this study, in order to achieve smaller groove width, Bessel beam, which has extremely small beam waist, was used instead of Gaussian beam. Novel optical system has been designed to shape Bessel beam with 4〜5 μm width and 20 mm focal depth. Experimentally, we achieved perfectly isolated grooves with only 2.0〜3.0 μm width along 10 mm on focal direction.

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This paper presents a method for measuring the emissivity and temperature simultaneously by a spot infrared thermometer. Theoretically, it is possible to determine both the emissivity and temperature by providing the two ambient temperatures and measuring the corresponding outputs of the thermometer. To verify this, a simple experimental apparatus was fabricated having a hemisphere aluminum bowl to uniform the ambient temperature of the test surface. The ambient temperature was varied by the electric heating wire attached on the aluminum bowl. As a result, it was suggested that this measurement is possible if the emissivity was not less than about 0.5. Since various error factors were considered to exist in this measurement system, it is necessary to reduce the main factors to improve the measurement accuracy.

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Marangoni convection becomes dominant under microgravity and micro-scale environments. In the micro-scale environment, the theory of macro-scale transport phenomena including fluid flow, heat and mass transfer can not be maintained. In addition, Marangoni convection with concentration gradient of surfactant is very difficult problem to analyze. To analyze mass transfer of some surfactants, we have to consider adhesion of surfactant to the interface, interaction between surfactants and macro-scale mass transfer. In this study, a numerical model for calculating the micro-scale phenomena has been developed. Multiphase simulation model including heat and mass transfer was developed and coupled with Discrete Element Method to analyze micro-scale mass transfer.

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We observed the condensation process of submicron-scale droplets on silicon (100) surface by using environmental scanning electron microscope. We found that the condensation growth of a submicron-sized droplet is classified into two modes. In the first mode, the contact angle is smaller than the macroscopic contact angle, which is the contact angle observed for millimeter-sized droplets, and increases linearly with time until reaching the macroscopic contact angle. In the second mode, which follows the first mode, the base diameter increases with time while the contact angle remains constant. The first mode is a growth mode peculiar to droplets with diameters less than 10 μm.

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Nanobubbles exist at solid-liquid interfaces between pure water and hydrophobic surfaces with very high stability, lasting in certain cases up to several days. However, the generation and stability mechanisms are still unclear. In this study, surface nanobubbles at a pure water-highly ordered pyrolytic graphite (HOPG) interface were investigated by peak force quantitative nanomechanics (PF-QNM). Multiple isolated nanobubbles generated by the solvent-exchange method were present on the terraced areas, avoiding the steps of the HOPG surface. Adjacent nanobubbles coalesced and formed butterfly-shaped nanobubbles. Coalescence was enhanced by the PF-QNM measurement. We determined that nanobubbles can exist for a long time because of nanoscale contact angle hysteresis at the water-HOPG interface. Moreover, the hydrophilic steps of HOPG were avoided during coalescence. This result indicates that it is possible to control the nanobubble generation by the hydrophobic-hydrophilic combination surface.

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In the present study, wettability of focused ion beam (FIB) irradiated hydrophobic FOPA surface was investigated. It was evaluated by contact angle measurement and a chemical composition of surface was analyzed by glow discharged-optical emission spectroscopy. The result shows that the hydrophobicity of the surface decreased after FIB irradiation because fluorine atoms of FOPA were sputtered from the surface. In addition, FIB irradiation was used to fabricate a submicron-scale hydrophilic spots on hydrophobic surface. Condensation behavior was observed by environmental scanning electron microscope and condensed tiny droplets appeared on hydrophilic spots while no droplet are observed on pristine FOPA surface near these spots. Observed result shows that the submicron-scale nucleation behavior can be controlled by surface wettability modification.

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Experiments of pool saturated boiling of water were conducted using micro/nano hierarchical structured surfaces. We tried to make hierarchical structures without MEMS fabrication processes such as photolithography and dry etching, and in this study, the structured superhydrophilic surfaces were fabricated by two kinds of thermal spray method and chemical oxidation using an alkaline solution. The hierarchically structured surface fabricated by chemical oxidation of a plasma sprayed copper surface indicated the maximum critical heat flux of 1.8 MW/m^2 corresponding to 1.8 times the CHF of rough copper bare surface. Finally, the mechanism of CHF enhancement on superhydrophilic surface was discussed based on the water permeability in the micro and nano strucutures on the surfaces quantitatively evaluated by the capillary tube method.

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Carbon nanotube (CNT) is receiving a lot of attention as gas sensor material because it has the properties suitable for gas sensor such as its large surface area, chemical stability and good sensitivity at room temperature. At present two detection mechanism were proposed: Schottky barrier between electrodes and CNT and charge transfer between gas molecules and CNT. Regardless of many researches, sensing mechanism of multi-walled CNT gas sensor has not been investigated unlike that of SWNCT. In this study, we focused on simple CNT gas sensor which consisted of as-grown Multi-walled CNT and tried to elucidate the detection mechanism using transmission line method.

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Free convection generated in the fluid temperature difference by local heating, influences that can`t be ignored in heat transfer characteristics. Therefore, it is required to measure the amount of heat generated up to the free convection occurs. In previous studies, we've been estimated for the temperature distribution and the free convection around the micro heating magnetic sphere of mm size in the water. In this study, the micro particles under the alternating magnetic field over time imaging the absorbance at a wavelength of 1150 nm or 1412 nm, to estimate the temperature distribution by obtaining the absorbance of images. This time, we set up micro particles in the syrup, to estimate the temperature distribution from the absorbance image.

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The Scanning Thermal Microscope (SThM) measuring temperature in micro to nanoscale has been studied. In our previous studies, the active thermometry was developed for taking real temperature image. Unfortunately, the active thermometry does not perform well with a multifunctional cantilever probe, because of the probe heating makes thermal interference of a neat flow detector, the thermopile. We have tried to eliminate the interference by introducing proportionate heating with the body heater and the calibration heater of the probe. Active thermometry with the proportionate heating test showed effective in elimination of the interference. Moreover, a new probe for the active thermometry is under development. Through the simulation, the new probe is designed for avoiding the interference.

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Compact power generation systems which consist of methanol reformer and Polymer Electrolyte Fuel Cell (PEFC) are greatly required as emergency power source. In this system, PEFC exhaust heat can be utilized for evaporating water/methanol solution before reforming. To use PEFC exhaust heat (about 80〜90 °C), a microchannel heat exchanger is suggested as an evaporator because of supplying heat more efficiently. However, phase change phenomena of water/methanol solution in a microchannel are not clarified. In this study, the double-pipe heat exchanger including the microchannel was developed. Then, phase change phenomena in the microchannel were visualized with varying concentration, flow rate and pressure.

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This study focuses on an analysis of the compressor performance of a micro gas turbine to realize an improved micro gas turbine design. For this purpose, the performance of the compressor is determined, and the results are feedback for an experiment. Heat transfer impacts greatly the performance of whole micro gas turbine than other gas turbine because the distance between the compressor and the turbine or combustor is small in micro gas turbines. The obtained values when heat transfer is not considered did not satisfy the design requirements. Therefore, the compressor model could not perform sufficiently well. Furthermore, when heat transfer is considered in the model, a decrease of approximately 32% is observed in the compressor performance in comparison to models in which heat transfer is not considered.

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Pool boiling has been used for cooling in numerous thermal energy dissipation systems, such as high-power electronics, heat exchangers, and nuclear reactors. FC-72 is often used as a coolant for electronic cooling. While Critical Heat Flux(CHF) was increased by attaching a honeycomb porous plate to heated surface in a saturated pool boiling of water. For the case of water, CHF was enhanced up to about twice compared to that of plain surface (2 MW/m^2). In this study, we investigated experimentally the effect of honeycomb porous plate on CHF in a saturated pool boiling of FC-72.

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To improve the heat transfer performance during nucleate boiling applied to the cooling of semiconductor chips and of power electronics with high heat generation density, the present authors proposed a promising method of using immiscible mixtures. The nucleate boiling of immiscible mixtures with optimized fluid combination, composition and distribution of phases can make simultaneously possible i) drastic increase of CHF, e.g. 3×10^6W/m^2 for aqueous mixture without using enhanced surfaces, ii) reduction of surface temperature, iii) reduction of surface temperature overshoot at the incipience of boiling, and iv) elevation of system pressure preventing the mixing of air. Because detailed heat transfer characteristics remain unknown, the paper summarizes the recent results from pool boiling and flow boiling experiments performed in our laboratory.

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This study involved conducting subcooled flow boiling experiments using various different dissolved air concentrations in the Perfluorohexane (PFH) in order to clarify its effects on flow boiling heat transfer through a tube with an inner diameter of 4 mm, the following parameters were calculated and discussed excess temperature, boiling curve. Consequently, dissolved air was shown to affect boiling characteristics. In particular, dissolved air significantly affected the boiling curve in the low heat flux region, and the boiling curve was continuously decreased with increasing the effective heat flux for G = 300 [kg/m^2s], respectively. Also dissolved air significantly affect the onset of boiling.

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Flow induced vibration due to internal gas-liquid two-phase flow is a significant issue for various engineering applications. Gas-liquid two-phase flow is known for its unsteady and oscillatory behavior, and fluctuating force is generated when undergoing fluid-structure interaction. In order to properly assess the fluctuating force behavior of two-phase flow, identification of the flow regime is particular importance for the safety of the operation. Aim of the present study is to identify two-phase flow regime from fluctuating force signal. In this study, a new methodology of two-phase flow regime identification is presented by patterning the fluctuating force signal obtained from the experiment using neural network.

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This paper describes the formulation of two-phase flow equation under phase equilibrium and flow dynamics in one-dimensional nozzle. The derivatives of average specific volume and enthalpy were derived by using thermodynamic relationships. By introducing Clapeyron-Clausius equation, these derivatives were expressed with the small differentials of vapor quality and temperature. Three-dimensional two-phase thermos-fluid equations were formulated with these derivatives of average specific volume and enthalpy. One-dimensional nozzle flow was calculated with the derived formula. The flow transits from subsonic to supersonic by changing the area ratio.

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In order to predict the liquid film dryout, estimations of evaporation of liquid film, entrainment and deposition rates, are required. In this investigation, the liquid film flow rate was measured by using the liquid film extraction method, and droplet flow rate was also estimated under thermodynamic equilibrium assumption. The qualitative applicability of this method to the boiling condition was confirmed by the experimental results. Experimental results also showed the importance of the influence of the heating on the entrainment, especially under high heat-flux condition.

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CHF characteristics of downward flow are quite different from that of upward flow. In this investigation, CHF experiments were carried out on a forced convective boiling system with a stainless steel tube of 20 mm in inner diameter and 400 mm in heating length. On the basis of the experimental results, typical CHF mechanisms of downward flow were classified into three modes, i.e., the dryout of falling liquid film at high quality condition, CHF due to flooding at low quality condition, CHF caused by the instability of two-phase flow structure at near subcooled condition. These CHF characteristics were estimated by using simple CHF model.

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Numerical simulation is performed for silicon nanopowder production by means of an argon thermal plasma jet. A simple model is developed to describe a collective growth and convective-diffusive transport of nanopowder. A high-temperature plasma flow which entrains ambient low-temperature gas is simulated successfully; and the spatial distributions and time evolutions of the nanoparticle concentration and mean volume diameter are also clarified. A large number of small nanoparticles are produced especially around the plasma. The regions where the nanopowder has large sizes almost correspond to those where the nanopowder shows low concentrations because the nanopowder grows by coagulation as well as condensation.

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Numerical simulation of subcooled flow boiling in a vertical rectangular duct was carried out using a one-way bubble tracking method. This method treats each bubble as an individual particle on the Lagrangian coordinates. Whilst, correlations were used to evaluate the parameters including liquid velocity profile, wall superheat, nucleation site density, bubble lift-off frequency, mean bubble lift-off diameter, bubble condensation. Investigation was reformed for the effects of lift-off bubble size distribution on the void fraction. It was demonstrated that the void fraction in subcooled boiling flow is strongly affected by bubble size distribution.

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Focusing on the heat transfer mechanism of the MEB we are trying to express the heat transfer characteristics of subcooled boiling based on the microlayer model. Experimental study was carried out by using a platinum wire, and the cycle of individual bubble was measured. The comparison to the theoretical estimation indicates that there is a possibility to express the heat transfer of MEB by the microlayer model for subcooled boiling.

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