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Experimental Analyses of Flow Pattern and Heat Transfer in a Horizontally Oriented Polymer Pulsating Heat Pipe withMerged Liquid Slugs

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Frontiers in Heat and Mass Transfer 2024年第5期22卷 1381-1397页

作者： Zhengyuan Pei yasushi koito Department of Mechanical and Mathematical Engineering Graduate School of Science and TechnologyKumamoto UniversityKumamoto860-8555Japan Division of Industrial Fundamentals Faculty of Advanced Science and TechnologyKumamoto UniversityKumamoto860-8555Japan

Extended experiments were conducted on the oscillation characteristics of merged liquid slugs in a horizontally oriented polymer pulsating heat pipe(PHP).The PHP’s serpentine channel comprised 14 parallel channels with a width of 1.3 mm and a height of 1.1 *** evaporator and condenser sections were 25 and 50 mm long,respectively,and the adiabatic section in between was *** a plastic 3D printer and semi-transparent filament made from acrylonitrile butadiene styrene,the serpentine channel was printed directly onto a thin polycarbonate sheet to form the *** PHP was charged with *** the experiments,the evaporator section was heated,and the condenser section was cooled using high-temperature and low-temperature thermostatic baths,*** patterns of the working fluid were obtained with temperature distributions of the PHP.A mathematical model was developed to analyze the flow *** liquid slugs were observed in every two channels,and their oscillation characteristics were found to be approximately the same in time and *** was also found that the oscillations of the merged liquid slugs became slower,but the heat transfer rate of the PHP increased with a decrease in the filling ratio of the working *** is because vapor condensation was enhanced in vapor plugs as the filling ratio ***,the filling ratio had a lower limit,and the heat transfer rate was maximum when the filling ratio was 40.6%in the present experimental range.

关键词： Pulsating heat pipe polymer heat pipe visualization experiment flow pattern analysis heat transfer enhancement

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Numerical Investigations on Fluid Flow and Heat Transfer Characteristics of an Ultra-Thin Heat Pipe with Separated Wick Structures

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Frontiers in Heat and Mass Transfer 2024年第3期22卷 869-887页

作者： yasushi koito Akira Fukushima Division of Industrial Fundamentals Faculty of Advanced Science and TechnologyKumamoto UniversityKumamoto860-8555Japan Department of Mechanical and Mathematical Engineering Graduate School of Science and TechnologyKumamoto UniversityKumamoto860-8555Japan

Thermal and fluid-flow characteristics were numerically analyzed for ultra-thin heat *** studies have been conducted for ultra-thin heat pipes with a centered wick structure,but this study focused on separated wick structures to increase the evaporation/condensation surface areas within the heat pipe and to reduce the concentration of heat flux within the wick structure.A mathematical heat-pipe model was made in the threedimensional coordinate system,and the model consisted of three regions:a vapor channel,liquid-wick,and container wall *** conservation equations for mass,momentum,and energy were solved numerically with boundary conditions by using a code developed by one of the *** numerical results with the separated wick structures were compared with those with the centered,which confirmed the effectiveness of the separation of the wick ***,the effectiveness of the separation was affected by the position of the separated wick structure.A simple equation was presented to determine the optimum position of the separated wick *** analyses were also conducted when the width of the heat pipe was increased with the cooled section,which clarified that the increase in the cooled-section width with the addition of wick structures wasmore effective than the increase in the cooled-section length.A 44%reduction in the total temperature difference of the heat pipe was obtained under the present numerical ***,a comparison wasmade between experimental results and numerical results.

关键词： Ultra-thin heat pipe vapor chamber heat transfer surface CFD thermal design

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Flow Patterns and Heat Transfer Characteristics of a Polymer Pulsating Heat Pipe Filled with Hydrofluoroether

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Frontiers in Heat and Mass Transfer 2024年第1期22卷 49-63页

作者： Nobuhito Nagasato Zhengyuan Pei yasushi koito Department of Mechanical and Mathematical Engineering Graduate School of Science and TechnologyKumamoto UniversityKumamoto860-8555Japan Division of Industrial Fundamentals Faculty of Advanced Science and TechnologyKumamoto UniversityKumamoto860-8555Japan

Visualization experiments were conducted to clarify the operational characteristics of a polymer pulsating heat pipe(PHP).Hydrofluoroether(HFE)-7100 was used as a working fluid,and its filling ratio was 50%of the entire PHP channel.A semi-transparent PHP was fabricated using a transparent polycarbonate sheet and a plastic 3D printer,and the movements of liquid slugs and vapor plugs of the working fluid were captured with a high-speed *** video images were then analyzed to obtain the flow patterns in the *** heat transfer characteristics of the PHPwere discussed based on the flowpatterns and temperature distributions obtainedwith *** starting heating,because of high wettability,large liquid slugs positioned at the evaporator section of the *** starting heating,since the occurrence of boiling divided the large liquid slugs,oscillatory flowof smaller liquid slugs and vapor plugs was found in the *** circulation flow of liquid slugs and vapor plugs was observed when the power input to the PHP was larger than 12.0 *** flow patterns and temperature distributions confirmed that the circulation flow enhanced the heat transfer from the evaporator section to the condenser section of the *** the circulation flow mode,large growth and contraction of vapor plugs were found one after another in all even-numbered PHP ***,the analysis of flow patterns clarified that the phase-change heat transfer rate by large growth and contraction of vapor plugs was 19%of the total heat transfer rate of the *** the generation of large vapor plugs was found in the PHP,most of the heat was transferred by the sensible heat of the working fluid.

关键词： Polymer heat pipe visualization oscillatory flow circulation flow thermal management 3D printer

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Experiments and Analyses on Heat Transfer Characteristics from a Solid Wall to a Strip-ShapedWick Structure

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Frontiers in Heat and Mass Transfer 2024年第3期22卷 687-702页

作者： Kenta Hashimoto Guohui Sun yasushi koito Department of Mechanical and Mathematical Engineering Graduate School of Science and TechnologyKumamoto UniversityKumamoto860-8555Japan Division of Industrial Fundamentals Faculty of Advanced Science and TechnologyKumamoto UniversityKumamoto860-8555Japan

Centered or striped wick structures have been used to develop ultrathin heat *** from traditional heat pipes,the centered or striped wick structures leave noncontact container surfaces with the wick *** this study,experiments andnumerical analyseswere conductedtoinvestigate the influenceof thesenoncontact *** the experiments,a strip-shaped wick structure was placed vertically,the top was sandwiched between wider rods and the bottom was immersed in a working *** rod width was greater than the wick width;thus,noncontact surfaces were left between the rod and the wick *** heat was applied from the rod to the wick structure,and the evaporation heat transfer characteristics of the working fluid from the wick structure were *** was used as the working *** experiments were conducted by varying the rod and wick *** experimental results were obtained when the wick structures were placed *** the numerical analyses,the temperature and heat flux distributions in the rod were *** the experimental and numerical results,it was confirmed that the noncontact surfaces caused the heat flux in the rod near both surfaces of the wick structure to concentrate,which increased the evaporation thermal resistance of the wick structure.A reduction in the noncontact surface area by increasing the wick width was found to be effective in decreasing the evaporation thermal resistance and increasing themaximumheat transfer rate of the wick *** separation of the wick structure increased the evaporation surface ***,its effectiveness was limited when the heat transfer rate was small.

关键词： Ultra-thin heat pipe centered wick structure striped wick structure conduction heat transfer evaporation heat transfer constriction thermal resistance

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Evaporation Heat Transfer Characteristics from a Sintered Powder Wick Structure Sandwiched between Two Solid Walls

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Frontiers in Heat and Mass Transfer 2023年第1期21卷 33-46页

作者： yasushi koito Shoma Hitotsuya Takamitsu Takayama Kenta Hashimoto Division of Industrial Fundamentals Faculty of Advanced Science and TechnologyKumamoto UniversityKumamoto860-8555Japan Department of Mechanical and Mathematical Engineering Graduate School of Science and TechnologyKumamoto UniversityKumamoto860-8555Japan

An ultra-thin flattened heat pipe has been developed with a centered wick *** structure is essential to make the heat pipe ***,the centered wick structure reduces the evaporation and condensation surface areas of the wick structure because it is sandwiched between heat pipe *** this study,because detailed discussion has not been made,heat transfer experiments were conducted for the wick structure sandwiched between two solid *** study focused on the evaporation heat transfer characteristics from the sandwiched wick *** experiments were conducted with three wick structures,that is,strip-shaped sintered copper powders with thicknesses of 0.5,1.0,and 1.5 *** was used as working *** capillary pumping performance,that is,the liquid lifting velocities of the three wick structures were the *** experimental results of the three wick structures were compared regarding the relation between the evaporation heat transfer rate and the superheat of the working *** heat transfer experiments were also conducted when one of the solid walls was removed from the wick *** was confirmed that even if the wick structure was sandwiched between the solid walls,sufficient evaporation of the working fluid occurred from the thin sides of the wick structure.

关键词： Heat transfer ultra-thin heat pipe centered wick structure restricted evaporation surface thermal design electronics cooling

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Numerical Study on Heat Transfer Characteristics of Heated/Cooled Rods Having a Composite Board in between: Effect of Thermal Vias

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Journal of Electronics Cooling and Thermal Control 2017年第4期7卷 91-102页

作者： yasushi koito Toshio Tomimura Division of Energy Science Faculty of Advanced Science and Technology Kumamoto University Kumamoto Japan

By placing a sample between a heated and a cooled rod, a thermal conductivity of the sample can be evaluated easily with the assumption of a one-dimensional heat flow. However, a three-dimensional constriction/spreading heat flow may occur inside the rods when the sample is a composite having different thermal conductivities. In order to investigate the thermal resistance due to the constriction/spreading heat flow, the three-dimensional numerical analyses were conducted on the heat transfer characteristics of the rods. In the present analyses, a polymer-based composite board having thermal vias was sandwiched between the rods. From the numerical results, it was confirmed that the constriction/spreading resistance of the rods was strongly affected by the thermal conductivity of the rods as well as the number and size of the thermal vias. A simple equation was also proposed to evaluate the constriction/spreading resistance of the rods. Fairly good agreements were obtained between the numerical results and the calculated ones by the simple equation. Moreover, the discussion was also made on an effective thermal conductivity of the composite board evaluated with the heated and the cooled rod.

关键词： Heat Transfer Enhancement Thermal Via Constriction/Spreading Resistance Steady-State Method Effective Thermal Conductivity

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Two-Dimensional Numerical Investigation on Applicability of 45<sup>。</sup>Heat Spreading Angle

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Journal of Electronics Cooling and Thermal Control 2014年第1期4卷 1-11页

作者： yasushi koito Shoryu Okamoto Toshio Tomimura Department of Advanced Mechanical Systems Kumamoto University Kumamoto Japan

The 45。heat spreading angle is familiar among thermal designers. This angle has been used for thermal design of electronic devices, and provides a heat spreading area inside a board, e.g. printed circuit board, which is placed between a heat dissipating element and a relatively large heat sink. By using this angle, the heat transfer behavior can be estimated quickly without using high-performance computers. In addition, the rough design can be made easily by changing design parameters. This angle is effective in a practical situation;however, the discussion has not been made sufficiently on the applicability of the 45。heat spreading angle. In the present study, therefore, the extensive numerical investigation is conducted for the rational thermal design using the 45。heat spreading angle. The two-dimensional mathematical model of the board is considered;the center of the top is heated by a heat source while the bottom is entirely cooled by a heat sink. The temperature distribution is obtained by solving the heat conduction equation numerically with the boundary conditions. From the numerical results, the heat transfer behavior inside the board is shown and its relation with the design parameters is clarified. The heat transfer behavior inside the 45。heat spreading area is also evaluated. The applicability is moreover discussed on the thermal resistance of the board obtained by the 45。heat spreading angle. It is confirmed that the 45。heat spreading angle is applicable when the Biot number is large, and then the equations are proposed to calculate the Biot number index to use the 45。angle. Furthermore, the validity of the 45。heat spreading angle is also confirmed when the isothermal boundary condition is used at the cooled section of the board.

关键词： 45.Heat Spreading Angle Electronic Board Electronics Cooling Thermal Design Heat Conduction

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Heat Transfer Characteristics of Square Micro Pin Fins under Natural Convection

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Journal of Electronics Cooling and Thermal Control 2014年第3期4卷 59-69页

作者： Naoko Matsumoto Toshio Tomimura yasushi koito Department of Advanced Mechanical Systems Kumamoto University Kumamoto Japan

In order to comply with the recent demand for downsizing of the electric equipment, the minia- turization and the improvement in heat transfer performance of a heat sink under natural air-cooling are increasingly required. This paper describes the experimental and numerical investigations of heat sinks with miniature/micro pins and the effect of the pin size, pin height and the number of pins on heat transfer characteristics of heat sinks. Five types of basic heat sink models are investigated in this study. The whole heat transfer area of heat sinks having the different pin size, pin height and the number of pins respectively is kept constant. From a series of experiments and numerical analyses, it has been clarified that the heat sink temperature rises with increase in the number of pins. That is, the heat sink with miniaturized fine pins showed almost no effect on the heat transfer enhancement. This is because of the choking phenomenon occurred in the air space among the pin fins. Reflecting these results, it is confirmed that the heat transfer coefficient reduces with miniaturization of pins. Concerning the effects of the heat transfer area on the heat sink performance, almost the same tendency has been observed in other three series of large surface area, that is, higher pin height. Furthermore as a result of studying non-dimensional convection heat transfer performance, it was found that the relation between the Nusselt number (Nu) and the Rayleight number (Ra) is given by Nu = 0.16 Ra0.52.

关键词： Natural Air-Cooling Heat Sink Micro Pin Fin Heat Transfer Performance Experiment Numerical Analysis

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Numerical Analysis of Printed Circuit Board with Thermal Vias: Heat Transfer Characteristics under Nonisothermal Boundary Conditions

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Journal of Electronics Cooling and Thermal Control 2013年第4期3卷 136-143页

作者： yasushi koito Yoshihiro Kubo Toshio Tomimura Department of Advanced Mechanical Systems Kumamoto University Kumamoto Japan

A thermal via has been used to enhance the heat transfer through the printed circuit board (PCB). Because the thermal conductivity of a dielectric material is very low, the array of metal vias is placed to make thermal paths in the PCB. This paper describes the numerical analysis of the PCB having metal vias and focuses on the heat transfer characteristics under the nonisothermal boundary conditions. The mathematical model of the PCB has the metal vias between two metal sheets. Under 2nd and 3rd kinds of boundary conditions, the temperature distribution is obtained numerically by changing the design parameters. The discussion is also made on the effective thermal conductivity of the PCB. In industry, the use of effective thermal conductivity is convenient for thermal engineers because it simplifies the calculation process, that is, the composite board can be modeled as a homogeneous medium. From the numerical results, it is confirmed that the placement of metal sheets and the population of metal vias are important factors to dominate the heat transfer characteristics of the PCB. It is also shown that although the nonisothermal boundary conditions are applied at the boundary surface, the temperature difference between the heated and the cooled section is almost uniform when the metal vias are populated densely with the metal sheets. In this case, the effective thermal conductivity of the PCB is found to be the same irrespective of the boundary conditions, that is, whether the isothermal or the nonisothermal boundary conditions are applied.

关键词： Thermal Management Printed Circuit Board Thermal Via Effective Thermal Conductivity Nonisothermal Boundary Condition

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