Yasushi Koito

Experimental Study on Convective Thermal–Fluid Flow Transport Phenomena in Circular Tube Using Nanofluids

The aim of the present study is to disclose the thermal–fluid flow transport phenomenon of nanofluids in the heated horizontal circular tube. Consideration is given to the effects of volume fraction of the nanoparticle and Reynolds number on the turbulent heat transfer and pressure loss. It is found that (i) each viscosity, for three different nanofluids, increases with an increase in the volume fraction of nanofluid, (ii) the corresponding pressure loss increases slightly in comparison with that of pure water, and (iii) substantial heat transfer enhancement is caused by suspending nanoparticles, but its effect is attenuated by large particle aggregation.

Evaluation of Effective Thermal Conductivity of Multilayer Printed Circuit Board

To perform a rational thermal design of a printed circuit board (PCB) with highly anisotropic heat transfer nature in its initial stage, effective thermal conductivities in thickness direction and in in-plane direction must be given depending on the electric circuit of the board. However, a simple evaluation method for the effective thermal conductivities of such PCB has not been developed yet. In this study, as the first step to propose a simple evaluation method, the heat transfer coefficient by natural convection around a horizontal disk, which is indispensable for measuring the effective thermal conductivity, has been evaluated. Furthermore, the thermal conductivity of the glass epoxy resin in in-plane direction has been evaluated by applying the evaluated heat transfer coefficient, and then, the validity of the proposed thermal conductivity measurements of the anisotropic PCB has been confirmed.Copyright

On simple prediction method for thermal contact resistance between wavy surfaces with thermal interface material under low mean nominal contact pressure (fundamental study based on 1-D model)

The thermal contact resistance (TCR) is the crucial issue in the field of heat removal from systems like electronic equipment, satellite thermal control systems, and so on. To cope with the problem, a lot of studies have been done mainly for flat rough surfaces. However, as pointed out so far, there are still wide discrepancies among measured and predicted TCRs, even for similar materials. To investigate the key factors for the abovementioned discrepancies, a fundamental analysis was conducted in our previous study [1] using a simple contact surface model, which was composed of the unit cell model proposed by Tachibana [2] and Sanokawa [3]. Furthermore, by introducing a 2-D microscopic surface model, which consists of random numbers and Abbott’s bearing area curve, the effects of surface waviness and roughness on the temperature fields near the contact interface have been investigated microscopically [4]. In this study, based on a 1-D wavy surface model, a fundamental study has been conducted to predict TCR and the thermal contact conductance (TCC), which is a reciprocal of TCR, between wavy surfaces with the thermal interface material (TIM) under a relatively low mean nominal contact pressure of 0.1–1.0 MPa. From comparison between the calculated and measured results, it has been shown that, in spite of a simple 1-D analysis, the present model predicts the temperature drop at the contact interface, which is obtained as the product of TCR and the heat rate flowing through TIM, within some 10 to 60% error for a TIM with the thermal conductivity of 2.3 W/(m·K) and the initial thickness of 0.5, 1 and 2 mm.Copyright

Fabrication of a Vapor Chamber on a Plastic Board

A vapor chamber is a flat-plate heat pipe, where a cooled (condenser) section is much larger than a heated (evaporator) section, and has been used as a heat spreader to enhance the cooling of electronic devices. An objective of this study is to integrate the vapor chamber into a polycarbonate board. Plastic materials are easy to manufacturing, light weight, low cost, flexible, and then the present study aims at performing a phase-change heat transfer and a heat spreading inside the polycarbonate board. A sintered copper powder and water are used as a wick structure and a working fluid, respectively. In experiments, the heat is applied by a heater while the cooling water is circulated between a thermostatic bath and a cooling jacket. The experiments are conducted changing a liquid volume and a heat input, and the transient temperature distribution of the vapor chamber is measured by thermocouples. For comparison, the experiment is also conducted where the working fluid is not charged into the vapor chamber. Moreover, based on a thermal resistance network, an analytical model of the vapor chamber is made and the analysis is performed on the phase-change heat transfer inside the vapor chamber. From the experimental and analytical results, the heat transfer characteristics of the polymer-based vapor chamber and the effectiveness of the phase-change heat transfer are confirmed.Copyright

A capillary-wick heat pipe fabricated on a plastic board (Fundamental experiments on heat transport characteristics)

A capillary-wick heat pipe having the dimensions of 5.0 mm × 5.0 mm × 100 mm (length) is fabricated on a surface of a plastic board, and the experimental investigations are conducted on the operational characteristics of the heat pipe. Plastics are easy to manufacturing, lightweight, low cost, flexible, and besides, the present study aims at the phase-change heat transfer inside the plastic board. A sintered copper powder and water are used as the wick structure and the working fluid of the heat pipe, respectively. In experiments, an evaporator section of the heat pipe is heated by a heater while a condenser section is water-cooled by a heat sink. A heat input and a liquid volume inside the heat pipe are changed, and the temperature distribution of the heat pipe is measured by thermocouples. Moreover, a one-dimensional thermal circuit model is made to evaluate the effective thermal conductivity of the heat pipe. From the experimental results, the continuous phase-change heat transfer inside the plastic board and its effectiveness are confirmed. It is also revealed that the effective thermal conductivity of the heat pipe is 854 W/(m·K) in maximum under the present experimental conditions.Copyright

Simple evaluation method for temperature drop at contact interface between rough surfaces under low contact pressure conditions

For heat removal from systems such as electronic equipment, satellite thermal control systems, and nuclear reactors, reduction of thermal contact resistance (TCR) is the most crucial issue to be addressed. Several studies have attempted to propose evaluation equations for predicting TCR for flat rough surfaces. However, as is well known, there are still wide discrepancies among measured results, even for similar materials. In this study, based on the conventional unit cell model for flat surfaces with roughness and the newly proposed contact surface model for wavy surfaces with roughness, thermal contact resistance under a low contact pressure of 0.I-I.0 MPa is investigated theoretically and experimentally. Comparison of the measured and calculated results shows that the measured temperature drop at the interface (that is, the thermal contact resistance) between flat surfaces with roughness lies between the values evaluated by the unit cell model for the cases with and without the heat flow constriction. Furthermore, when the rough surface has waviness, the introduction of macroscopic constriction resistance is shown to be important for evaluating the temperature drop at the interface.

Fabrication of Heat Pipes on an Acrylic Resin Board

As a first step to develop an electronic wiring board in which micro or miniature heat pipes are internally fabricated, the experimental and analytical studies are performed on a wickless gravity-assisted heat pipe, namely thermosyphon, fabricated on a surface of an acrylic resin board. This proposal aims at performing a phase-change heat transfer inside an electronic wiring board having a low thermal conductivity. In experiments, the evaporator section of the heat pipe is heated by a heater while the condenser section is water-cooled by a heat sink. Water is used as a working fluid. Changing a heat input and a liquid volume ratio inside the heat pipe, the temperature distribution is measured by thermocouples and then compared to the case where the working fluid is not charged. Moreover, the simple model of the heat pipe is made based on a thermal resistance network, and the analysis is performed on a phase-change heat transfer and a conductive heat transfer inside the resin board having the heat pipe. The effective thermal conductivity of the heat pipe is evaluated. Although this study is an initial stage, the operational and the heat transfer characteristics of the resin board having the heat pipe are confirmed.Copyright

Fundamental Study on Thermal Characteristics of Heat Spreaders

A heat spreader is one of the solutions for thermal management of electronic and photonic systems. By placing the heat spreader between a small heat source and a large heat sink, the heat flux is spread from the former to the latter, resulting in a lower thermal spreading resistance between them. There are many types of heat spreaders known today having different heat transfer modes, shapes and sizes. This paper describes the theoretical study to present the fundamental data for the rational use and thermal design of heat spreaders. Two-dimensional disk-shaped mathematical model of the heat spreader is constructed, and the dimensionless numerical analysis is performed to investigate the thermal spreading characteristics of the heat spreaders. From the numerical results, the temperature distribution and the heat flow inside the heat spreaders are visualized, and then the effects of design parameters are clarified. The discussion is also made on the discharge characteristics of the heat spreaders. Moreover, a simple equation is proposed to evaluate the heat spreaders.Copyright

Numerical visualization of heat transfer in a vapor chamber

This paper describes the three-dimensional numerical simulation to visualize the thermal-fluid transport characteristics inside the vapor chamber. The computational domain is composed of three regions of a vapor, a liquid-wick and a solid wall, and it is placed between a small heat source and a large heat sink. The bottom of the vapor chamber receives the heat from the heat source, while the top is entirely cooled by the heat sink. The equations of continuity, motion and energy for each region are solved numerically with boundary conditions. From the numerical results, the vapor velocity, liquid velocity and temperature distributions inside the vapor chamber are clarified, and the operational characteristics of the vapor chamber are confirmed. The numerical visualization is expected to provide a new clue for optimizing and improving the vapor chamber.

Theoretical investigation on effective thermal conductivity of a disk-shaped composite system

This paper addresses the methodology to estimate the effective thermal conductivity of the wiring board, where the metal wiring network is very complicated and then the thermal conductivity of the metal wiring is more than 1000 times higher than that of the resign board. Based on the concept of analogy between the electric and the thermal resistance network, two types of estimation equations are derived by dividing the composite system parallel or perpendicular to the heated/cooled surface. When the ratio of higher to lower thermal conductivities is less than 10, the estimated values by these equations agree with each other. However, the difference is clearly found between them when the ratio is larger than 100. The estimated values are moreover compared with the exact solutions, which are obtained by numerical simulation of heat transfer using Microsoft Excel, and then the applicability of the present estimation methodology is discussed. It is found that the thermal resistance network obtained by dividing the composite system perpendicular to the heated/cooled surface is effective to estimate the effective thermal conductivity of the composite system.Copyright