A series of study has been performed on the metal hydride particle beds of Ti0.15Zr0.85Cr0.9Fe0.6Ni0.2Mn0.3Cu0.05 (MH-1, using for heat source), Ti0.73Zr0.27Cr1.2Fe0.3Ni0.1Mn0.4Cu0.05 (MH-2, using for cooling load) to measure the effective thermal conductivities. The effective thermal conductivities of activated and oxidized MH particle bed in helium have been examined. Experiment results show that pressure has great influence on effective thermal conductivity in low pressure range (<0.5 MPa). And that influence decreases rapidly with increase of gas pressure. The reason of pressure dependence at low pressure range is that the mean free path of gas becomes greater than effective thickness of gas film which is important to the heat transfer mechanism of particle bed. In order to enhance the poor thermal conductivity of metal hydride particle bed, carbon fiber mixing method has been used in this study. Three types, two insert methods and five mass percentages of carbon fiber have been examined and compared. The highest effective thermal conductivity of MH particle bed has been reached with Type B carbon fiber which has second higher thermal conductivity, and 2 weight percentage. This method has acquired 5-6 times higher thermal conductivity than pure metal hydride particle beds with quite low quantity of additives, only 2 mass% of carbon fiber. This is a good result comparing to other method which can reach higher effective thermal conductivity but needs much higher percentage of additives too.
To elucidate the possibility and availability of thermal recycling of waste plastic resin from a basic and microscopic viewpoint, a series of abrupt heating processes of a spherical micro plastic particle having a diameter of about 200 μm is observed, when it is abruptly exposed to hot oxidizing combustion gas. Three ingenious devices are introduced and two typical plastic resins of polyethylene terephthalate and polyethylene are used. In this paper the dependency of internal and external appearances of residual plastic embers on the heating time and the ingredients of plastic resins is optically analyzed, along with appearances of internal micro bubbling, multiple micro explosions and jets, and micro diffusion flames during abrupt heating. Based on temporal variations of the surface area of a micro plastic particle, the apparent burning rate constant is also evaluated and compared with those of well-known volatile liquid fuels.
A micropump driven by the thermocapillary convection is proposed. The purpose of this study is to examine the flow structure in liquid region and the effect of the geometry on the performance of the present micropump. There are two significant advantages in the thermocapillary-driven system. First, the surface forces become more dominant than the volume forces with decreasing scale. The present micropump driven by the surface forces shows an advantage in the micro scale over a diaphragm pump driven by the volume forces. Secondary, the thermocapillary driven system contains no movable parts; thus, it allows a very simple structure compared to the diaphragm one. In the present micropump system, a number of ribs are distributed along the flow circuit between a heater and a cooler. Since heat transfer from these ribs to the working liquid imposes temperature gradients along the gas-liquid interfaces, the flow from the hot to the cold side is induced by the Marangoni effect. Fundamental characteristics of the present micropump are studied on the basis of three-dimensional simulation conducted taking the gas, liquid and ribs into account. In this study, the flow structure corresponding to the temperature field was observed. The present calculation has revealed that the flow field exhibits a transition from steady flow to oscillatory flow when the Marangoni number exceeds a critical value of about 2,000-2,500. An experiment was also performed. The liquid flow driven by the present micropump system was confirmed through the experiment.
Spectral control of thermal radiation emitted from rectangular micro-cavities made on a metal surface was investigated through numerical simulation and experiment. In the numerical simulation, thermal radiation from solid surface was dealt as hemispherical emission from point sources, and the Maxwell's equations were solved using the cubic interpolated propagation method. It was demonstrated that spherical waves emitted from inside of a cavity were spectrally selected, and that the emittance could be increased around the wavelength corresponding to the standard mode of cavity resonance. Furthermore, in experiment using rectangular micro-cavities (0.5x0.5x0.5 μm3) made periodically on Ni specularly-polished surface, spectral emittance was measured in the near-infrared region. The experimental results disclosed that the emissive power only in the range of shorter wavelength than 1.2 μm was increased by the micro-cavities that played a role of a wave guide to produce cutoff effect clearly.
The results of electro-thermal analysis, which is widely known as hydrodynamic model, are strongly dependent on the mesh size of model. However, the theory and method of accurate mesh size have not been investigated. In this research, we focus on submicron Si MOSFET and show the mesh zoning method for electro-thermal analysis. In the previous study, the authors proposed the mesh zoning method for vertical direction of Si MOSFET, i.e. the direction from the gate oxide to the bottom surface of MOSFET. The mesh zoning method was derived from the theory of the semiconductor physics. In this paper, the mesh zoning method for the lateral direction, i.e. the direction from the source electrode to the drain electrode, is considered. The calculation results show the most important point of mesh zoning for lateral direction is pinch-off point in the electron channel of MOSFET. Further, in the case that the fine meshes are used around the pinch-off point and wider meshes are used for other region, the results show good agreement with the results of the fine mesh model.
The aim of the present study is to understand the basics of spontaneous ignition of irradiated solid combustibles in sub-atmospheric pressure via analytical and numerical approaches. Final goal of this work is to provide the universal description to the recently-observed ignition characters in sub-atmospheric pressure. An irradiated ignition of horizontally-placed, thin cellulose paper in sub-atmospheric pressure is considered here; the sample is heated at the bottom to lead ignition below the surface in gravimetric environment. Two analytical solutions based on classical ignition theories are referred and 3-D numerical simulation is employed for further discussions. Analytical solutions suggest that the observed ignition trend by authors recently is similar to what would be attained in pure diffusion field, not in the stagnation-point flow field which includes the flow-associated heat loss, except near the lowest pressure range to be ignited. According to the 3-D numerical simulation, it is revealed that either pure diffusion or stagnation-point flow field appears depending on the imposed conditions. When longer ignition delay time is expected, an transport process comes to play a role and ignition is close to what would be attained in stagnation-point flow field, rather than the pure diffusion field. Classical ignition theories are useful to understand the insight of ignition processes in low pressure.
Refrigerant mal-distribution in a distributor located at the inlet of a heat exchanger used for an air conditioning system plays an important role in the heat exchanger performance. Distribution performance in the distributor is greatly affected by the flow conditions as well as the geometrical parameters of the distributor. To clarify the distribution characteristics, it is essential to know flow rates of both liquid and vapor states at every branch tube after distribution. This paper proposes a relatively simple test method, which enables to measure the inlet quality and the flow rate of refrigerant at every branch of the distributor. By using the proposed evaluation method, optimization on the geometrical parameters of the distributor was conducted to reduce the refrigerant mal-distribution. It was confirmed that the optimized distributor was able to reduce the mal-distribution of the refrigerant over branches despite the flow condition changes. By the flow visualization of the refrigerant in the distributor, it was observed that certain amount of liquid refrigerant remained and swayed unstably at the bottom of the distributor. It is supposed that the liquid refrigerant behavior in the distributor great affects the distribution performance.
In the present study the quantum molecular dynamics method was applied to an energy transfer problem to an electron during ionic surface collision process in order to elucidate how energy of ionic collision transfers to the emitted electrons. Effects of various physical parameters, such as the collision velocity and interaction strength between the observed electron and the classical particles on the energy transfer to the electron were investigated by the quantum molecular dynamics method when the potassium ion was collided with the surface so as to elucidate the energy path to the electron and the predominant factor of energy transfer to the electron. Effects of potential energy between the ion and the electron and that between the surface molecule and the electron on the electronic energy transfer were shown in the present paper. The energy transfer to the observed secondary electron through the potential energy term between the ion and the electron was much dependent on the ion collision energy although the energy increase to the observed secondary electron was not monotonous through the potential energy between the ion and surface molecules with the change of the ion collision energy.
Partial dehydration by microwave vacuum drying has been applied to tuna, oyster and mackerel prior to freezing in order to reduce quality damages due to freezing and thawing. Samples were dehydrated at pressure of 4kPa and temperature lower than 25°C. Two cooling conditions were tested in the experiment by using the freezing chamber of temperatures -20°C and -80°C. The experimental results showed that decreasing the water content in tuna could lower the freezing point temperature and made the freezing time shorter. It was also found that removing some water was effective to reduce the size of ice crystal and the drip loss in mackerel. After thawing, the pre-dehydrated mackerel showed better microstructure than that frozen without pre-treatment. Furthermore, the sensory tests have been done by a group of panelist for the evaluation on aroma, flavor, and general acceptability of mackerels.
In this article, we applied tunable diode laser absorption spectroscopy (TDLAS) technique to measure variation of water vapor concentration in gas flow channels in an operating polymer electrolyte membrane fuel cell (PEMFC). TDLAS measurement offered an optical remote sensing to detect slight change of water vapor concentration, showing that electrochemical reaction in an operating PEMFC is not uniformly observed. We performed TDLAS measurement in gas flow channels in the anode and the cathode with variation of cell current density. We observed increase of water vapor concentration due to electrochemical reaction along the cathode channel as well as along the anode channel from the inlet, suggesting that generated water was exhausted through the both channels. It was also shown that less electrochemical reaction was observed in the upstream of the channel presumably due to less hydration of the membrane because of dry gas supply.
A numerical analysis was performed using DNS (Direct Numerical Simulation) databases of statistically steady and fully developed turbulent premixed flames with different density ratios and with different Lewis numbers. Firstly, local flame surfaces at a prescribed progress variable were identified as local three-dimensional polygons. And then the polygon was divided into some triangles and local flame areas were evaluated. The turbulent burning velocity was evaluated using the ratio of the area of a turbulent flame to that of a planar flame and compared with the turbulent burning velocity obtained by the reaction rate. As a result, for unity Lewis number, the turbulent burning velocity evaluated by the flame area agrees with that by the reaction rate independent of the density ratio, while for non-unity Lewis number, the turbulent burning velocity obtained by the reaction rate increases or decreases by the extent which the Lewis number contributes. Secondary, local burning velocities over the flame surface were evaluated, and then the probability density functions (pdfs) of local burning velocities were obtained. For Le=0.8, 1.0, the peak of the pdf is located at a higher value than the unstretched local burning velocity, while for Le=1.2, it is located at a lower value than the unstretched local burning velocity.
Direct numerical simulation (DNS) of supercritical CO2 turbulent channel flow has been performed to investigate the heat transfer mechanism of supercritical fluid. In the present DNS, full compressible Navier-Stokes equations and Peng-Robison state equation are solved. Due to effects of the mean density variation in the wall normal direction, mean velocity in the cooling region becomes high compared with that in the heating region. The mean width between high- and low-speed streaks near the wall decreases in the cooling region, which means that turbulence in the cooling region is enhanced and lots of fine scale eddies are created due to the local high Reynolds number effects. From the turbulent kinetic energy budget, it is found that compressibility effects related with pressure fluctuation and dilatation of velocity fluctuation can be ignored even for supercritical condition. However, the effect of density fluctuation on turbulent kinetic energy cannot be ignored. In the cooling region, low kinematic viscosity and high thermal conductivity in the low speed streaks modify fine scale structure and turbulent transport of temperature, which results in high Nusselt number in the cooling condition of the supercritical CO2.
This paper describes a time-resolved measurement of thermal property in microscale during reaction processes of polymer by using an infrared (IR) laser. Polymer or gel-like material, so-called macromolecules, have diversity in its structure and intermolecular association, and recent development of measurement and control technique in micro- and nano- scale has opened up new possibilities for the property design of materials. The intermolecular dynamics of polymer can be reflected in time-resolved information of the thermal conductivity or thermal diffusivity. A measurement system of the thermal diffusivity in real-time and non-contact manner based on the forced Rayleigh scattering (FRS) method has been developed. This system can be applied for a changing process of a wide variety of polymer material because of employing a CO2 laser with the IR wavelength. Also, it is possible to measure the micro-scale property. By using the IR-FRS system, an investigation of the relationship between intermolecular dynamics of macromolecules and energy transfer can be conducted through the time-resolved data of the thermal diffusivity. As samples, crosslinking processes of a polysaccharide aqueous solution and an ultraviolet curable polymer were measured. In these processes, the samples change their microstructure by hydrogen and covalent bonding, respectively. Time evolution of the measured thermal property from the IR-FRS system clearly indicated the difference in bonding modes of macromolecules. According to the time-resolved measurement results, the validity of this technique for a versatile instrument of intermolecular dynamics of macromolecules is demonstrated.
The effects of the attack-angle of the fin notch array against the main flow and size of the clearance at the fin-tip on the heat transfer and pressure loss performances of a channel with cut-fins (parallel fins with square notches) mounted on the bottom wall were evaluated in the present article. Three-dimensional numerical simulations and heat transfer experiments employing a modified single-blow method were conducted to discuss these characteristics. As the size of the clearance was decreased, the pressure loss reduction effect in the cut-fins case compared with the plain-fins case (parallel fins without notches) was increased. On the contrary, smaller thermal resistance ratio between the cut-fin and plain-fin cases was achieved with larger clearance size. A maximum peak, therefore, appeared in the overall performance in relation to the clearance size. Larger heat transfer coefficients were obtained with smaller attack-angles of the notch array in both experimental and numerical results, particularly under larger Reynolds number conditions. This was due to the spanwise flow generated in the area adjacent to the notch, by which renewal of the thermal boundary layer was effectively produced at the trailing edge of the notch.
An experimental study was conducted on turbulent flow and mixing in a counter-flow type T-junction. We measured the velocity and concentration fields simultaneously by combining PIV and PLIF. Special attention was directed to the concentration fluctuation near the channel wall, which might bring about the high-cycle thermal fatigue in case of mixing of hot and cold flows. The velocity ratio of the counter-channel flow to the main-channel flow was changed from 1.0 to 5.0. The fluorescent dye was mixed in the main-channel flow. The dominant structures of the fluctuating velocity and concentration fields that cause the concentration fluctuation near the channel wall were analyzed by POD. It was found that the concentration fluctuation near the channel wall was caused by the superposition of the spanwise wobbling motion of the mixing interface of two flows and the rotational oscillation of the flows.
We propose a novel technique of molecular dynamics simulation to evaluate the relaxation time of phonons in solids for investigation of solid heat conductivity. The basic idea is to observe relaxation behavior of the power spectrum of atomic velocities after energetically stimulating modes in a specific frequency region. The transient entropy S(t) is defined with the power spectrum based on non-equilibrium statistical mechanics to quantitatively evaluate the relaxation speed. In this paper, two example systems are shown; Lennard-Jones model crystal and silicon crystal. For both systems, we found that the observed S(t) is well fitted to a single exponential function, from which we can obtain a frequency-dependent relaxation time.
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Edited and published by : The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Produced and listed by : Showa Joho Process Co., Ltd.(Vol.8 No.3-) Sanbi Printing Co., Ltd.(Vol.1 No.1-Vol.8 No.2)