Falling Snow Melting Characteristics of Warm Water Flowing along Sheet Channels Spread on a Roof

The experiment for investigating the falling snow melting characteristics of warm water flowing along sheet channels spread on a roof was performed in Tookamachi city, Nigata prefecture from February 6 to February 7, 1995. The sheet surface temperatures at 11 positions in 3 channels were measured. A physical model for a gas-water-snow system was constructed to compare the predicted results with the measured ones. A fully spread uniform water film in the sheet channel was observed in the experiments. The experimental results elucidated that it was feasible to use warm water flowing along sheet channels for melting falling snow on roofs. The temperature drop in the sheet channel mainly depended on the snowfall intensity, atmospheric temperature and wind speed. Under the influence of the roof edge, the temperature drop in the channel next to the side edge was much larger than that in middle channels. A water-snow two phase flow or a snow covered frozen water was experienced temporarily in the lower reaches of the water flowing channel. These suggest that a larger water flow rate is needed for the channel next to the roof edge, and a higher inlet temperature or a greater water flow rate is required for a severe weather condition. There was reasonably good agreement between the measured and predicted water temperatures

The Convective Instability in a Microemulsion Phase-Change-Material Slurry Layer

In the present experimental study, the stability of Rayleigh-Benard convection has been investigated in rectangular enclosures filled with microemulsion Phase-Change-Material (PCM) slurry. The PCM slurry exhibited a pseudoplastic non-Newtonian fluid behavior. Hysteresis in convection was clearly observed while the PCM were in a solid phase and in phase changing. The critical Rayleigh number decreases with the PCM mass concentration while the PCM is in phase changing. The fluid temperature at the center of the enclosure showed a time-dependent oscillation during the transition from a heat conduction state to a convection state. The maximum Nusselt number has been observed for all of the slurries while the heating plate was controlled at a temperature that most of PCM was in phase changing.</p

Heat Storage Characteristics of Phase Change Material Microcapsule Slurry in a Rectangular Cavity with a Heating Vertical Wall

This paper has dealt with the Heat storage characteristics of the microcapsule slurry composed of phase change material (PCM) as a latent heat storage material and water. A rectangular cavity with a heating vertical wall, which was filled with the microcapsule slurry, was selected as the present research model. The heating wall temperature, the width of the cavity and the PCM concentration were set as parameters. It was clarified that the transportation of latent heat evolved by melting of the PCM in the slurry exerted on a remarkable influence on the natural convection heat transfer. In addition, it was found that the heat transfer coefficient reached a local maximum value with an increase in initial temperature difference between heating wall and PCM. The heat storage completion time also have a local maximum value with an increase in heating wall temperature due to the latent heat by melting the PCM in the slurry.内壁面のうちの一側面を加熱熱源とする短形蓄熱槽は、工業的な応用が多い基礎的な形態であり、従来から多くの研究がなされている。Eftekharらはパラフィンワックスを充填した垂直フィンを有する短形蓄熱槽の下面を加熱した場合における固駅境界線の移動を詳細に検討し、熱移動に関する実験整理式の提案を行っている。平田らは氷およびパラフィンを充填した短形蓄熱槽の下面を加熱した場合において、蓄熱材固相が固定されて移動しない場合と固相が浮力により移動し加熱面と接触する場合を比較することで接触融解の効果について検討を行っている。斉藤らも分割フィンを有する短形蓄熱槽を用いた蓄熱において接触融解の効果を検討している。これらの短形蓄熱槽の潜熱蓄熱に関する既往の研究は全て固液相変化を伴う水、パラフィン、低融点の金属などの物質を蓄熱材とするものであり、その蓄熱過程における熱源から蓄熱材への熱伝達特性や蓄熱完了時間、蓄熱材の移動といった蓄熱挙動は、蓄熱材の相状態、すなわち流動可能な液相であるか、流動しない古相であるかに強く依存している。一方、本件で対象とする潜熱マイクロカプセルスラリーはマイクロカプセル芯物質がその融点温度以下で凝固した状態においても、分散媒により流動性は維持され、蓄熱材スラリーを巨視的に観察した場合には固相蓄熱材の壁面への固着も発生せず、固液境界も存在しない性質を有している。融解・凝固の際の固液境界は個々のマイクロカプセル内に存在するのみである。このため蓄熱挙動を支配する要因は前述のような潜熱蓄熱に関する従来の研究とは大きく異なり、マイクロカプセル芯物質の相状態や潜熱マイクロカプセルスラリーの温度および潜熱マイクロカプセルの質量濃度による物性と僭熱量の変化が現象を支配する要因となる。このような潜熱マイクロカプセルスラリーの蓄熱材として用いた場合の非安定な蓄熱挙動を解明するため、潜熱マイクロカプセル質量濃度、蓄熱槽寸法、蓄熱熱源となる加熱壁面の温度などを操作パラメータとし、それらが蓄熱過程における加熱面から蓄熱材への熱伝達特性、蓄熱量および蓄熱材融解割合の経時変化、蓄熱完了時間などに与える影響を本研究において数値解析により検討した

A Numerical Study on the Performance of an Open-type Flat-plate Solar Collector

A set mathematical models was developed for predicting the performance of an open-type flat-plate solar collector, and solved numerically through an implicit difference method. The effects of various parameters on the absorption of solar energy for the collector were investigated. The results showed that the solar energy absorptance of the open-type flat-plate collector was relatively high especially for the region where the weather was humid and hot, and there were an optimum length and an optimum tilt angle for the absorbing plate on which the collector could obtain the highest solar energy absorptance. It was found that the latent heat flux of water evaporation could be 5 to 15 times larger than the sensible heat flux. The effects of the magnitude of the solar incident flux, the atmospheric humidity, the atmospheric temperature, the absorbing plate tilt angle, and water film thickness on the temperature rising of the water film were clarified in numerical quantities. The increase of the solar incident flux, the atmospheric humidity or the atmospheric temperature also resulted in a rise in the energy absorptance of the collector

Droplet motion on a wrinkled PDMS surface with a gradient structural length scale shorter than the droplet diameter

Droplet transportation using a wettability gradient surface has attracted much attention owing to applications such as in microfluidic devices. A surface with a spatial structural gradient was prepared through a simple and cost-effective process even though understanding of droplet behavior on the structure was still limited. Here, we report impinging droplet motion on a gradient wrinkled surface. Surfaces were prepared through hard film deposition on soft pre-strained polydimethylsiloxane (PDMS) with a mask installed with a slit to control the amount of deposition, which is related to the wavelength of the wrinkles. Droplets were impinged with varying position with respect to the structure, and the droplet motion was observed in the direction away from the region under the slit. We found an asymmetric contact angle and alternate motion on both sides of the three-phase contact line during the motion according to the gradient of the wrinkle wavelength. These results may help not only to understand the behavior of droplet impingement on a gradient structural surface but also to further develop applications using directional droplet transfer

Heat Storage Characteristics of an Inclined Rectangular Heat Storage Vessel Packed with Plate Type of Shape-Stabilized Phase Change Material

The present study deals with the heat storage characteristics of the inclined rectangular vessel packed with spape-stabilized paraffin plates as a latent heat storage material by numerical analysis. It was found that the heat storage characteristics were remarkedly affected by the inclination angle of the rectangular vessel and the heat transfer medium flow direction, due to the presence of natural convection in the fluid flow channel between the paraffin plates. Numerical results revealed flow patterns, temperature profiles and heat storage characteristics for various inclination angles, inlet velocities and temperatures of the heat transfer medium fluid, and widths of the fluid flow channel.一般に、潜熱蓄熱槽の蓄熱過程では、熱媒体流速を極力小さくすることにより、圧力損失の軽減や熱媒体の持つ熱エネルギーの有効利用を図る試みがなされており、さらに潜熱の放熱過程では、小さな熱媒体流速により、一定の高温水準での熱抽出が可能となる。例えば、齊藤らの研究においては、球カプセルを充填した円筒形蓄熱槽内熱媒体の流速は10mm/s以下の低流速領域である。また、阿部らの報告した円筒状蓄熱材充填蓄熱槽内の熱媒体流速は約0.5mm/sと極めて小さいものである。このように、熱媒体の強制対流による流速が小さい場合には、状況によって蓄熱槽内の熱媒体流路に発生した自然対流がその熱伝達特性を大きく左右することになり、強制－自然共存対流の検討が必要となる。流路内の強制－自然共存対流に起因する伝熱現象は流路形状、重力方向に対する強制対流の流れ方向、流路の加熱または冷却条件など諸因子によって複雑な様相を呈する。強制－自然共存対流伝熱現象は、電子機器の除熱や大洋州熱器などにおいて数多く見られる。この場合の伝熱現象の解析的扱いは矩形流路壁の熱境界条件として等温そして等熱流束の固定条件の場合が大半で、その壁面温度がその内部に有る相変化物質の潜熱吸収や放出により変化するような場合はほとんど取り上げられていない現状にある。著者らは垂直配置した矩形潜熱蓄熱槽内の蓄熱及び放熱特性に大きく影響を及ぼすことを明らかにしている。例えば、熱媒体入口流速がV=0.2mm/sの放熱過程においては、下向き流れ（強制対流方向は重力方向と同じ場合）においては、上向き流れ（強制対流方向は重力方向と逆の場合）に比較して、流路内に発生した自然対流が熱伝達を阻害し、その放熱完了時間を大幅に増大する結果となる。本研究は、平板状形状安定化処理パラフィン潜熱蓄熱材が充填された矩形蓄熱槽を研究対象とし、二次元非定常数値解析により、矩形蓄熱槽を傾斜することにより自然対流を抑制した場合における強制－自然共存対流熱伝達の蓄熱特性に及ぼす効果を検討するものである。さらに、熱媒体蓄熱槽入口流速、入口温度及び熱媒体流路幅の蓄熱特性に及ぼす影響も併せて検討を行ってある

Liquid-Liquid Direct Contact Heat Exchange Using a Perfluorocarbon Liquid for Waste Heat Recovery : Heat Transfer Characteristics obtained with Perfluorocarbon Droplets Descending in a Hot Water Medium

This paper deals with the heat transfer characteristics of a liquid-liquid direct contact operation in which a Perfluorocarbon (PFC) liquid is released in a hot water stream, a low-grade heat source such as urban sewage, for the purpose of heat recovery from it. The paper reports on a set of experiments in which a PFC liquid (1800 kg/m^3 at 20℃) was continuously injected from a single, downward-facing nozzle into a slow, upward flow of hot water to be disintegrated into droplets descending in, and thereby heated from the water flow. The results of the experiments show how the size distribution and the translational motions of the droplets affect the overall coefficient for the water-flow-to-droplets heat transfer and also the temperature effectiveness for the droplets.近年、未利用エネルギー活用の観点から、工場や家庭温排水などを熱源として利用する廃熱回収用熱交換器の開発が急務となっている。従来のシェルアンドチューブなどの隔壁型交換器では、伝熱面に排水中のごみやスケールが堆積し、その伝熱効率が著しく低下する等の問題点があった。この問題解決として、このような汚濁排水中に、非水溶性熱媒体を噴射・注入し、直接接触熱交換により、熱抽出が可能となる。このような直接接触熱交換法は、個体壁伝熱面の汚れによる伝熱効率低下の問題がなくなり、廃熱回収用熱交換方式として極めて有効である。さらに、この熱交換法は、個体壁を介さないために高い熱通過率が得られ、小温度差での熱交換に有効である。また、液－液の直接接触する界面が、そのまま伝熱面に相当するため、単位体積当たりの伝熱面積が増加する利点を有する。本研究は、下水等の汚れた熱水源より効率的な熱回収をする手段として液液直接熱交換法に注目したものであり、熱回収媒体としてフッ素系不活性液体を熱源水へ噴射し、形成したフッ素系不活性液滴群と熱源水の直接接触による、流動及び熱伝達特性を検討するものである。すなわち、円形単孔ノズルから高密度のフッ素系不活性液体を熱源である温水槽へ上部より噴射し、その液滴群形成過程の観察及び液滴群の流れの特性の解明を通じて、熱水源からの直接熱交換法による熱抽出に関する基礎特性を明らかにすることを目的とする。最終的に、この種の熱源水よりの熱回収媒体としてフッ素系不活性液体を用いた場合における実用に寄与する無次元熱伝達率等に関する実験整理式の検討をも行うものである

Heat and Mass Transfer Analysis of Fluidized Bed Grain Drying

The effects of heat and mass transfer parameters on the efficiency of fluidized bed drying have been studied to optimize the input and output conditions. The analysis was carried out using two different materials, wheat and corn. Energy and exergy models based on the first and second law of thermodynamic are developed. Furthermore, some unified non-dimensional experimental correlations for predicting the efficiency of fluidized bed drying process have been proposed. The effects of hydrodynamics and thermodynamics conditions such as the inlet air temperature, the initial moisture content and well known Fourier and Reynolds numbers on energy efficiency and exergy efficiency were analyzed using the developed model. A good agreement was achieved between the model predictions, non-dimensional correlations and the available experimental results

Sorption Characteristics of Honeycomb-Type Sorption Element Composed of Organic Sorbent

This study deals with the sorption characteristics of a honeycomb-type sorption element composed of a new organic sorbent that was composed of the cross-linked polymer of sodium acrylate. Transient experiments in which moist air was passed into the honeycomb-type sorption element were conducted under various conditions of air velocity, temperature, relative humidity and honeycomb length. As a result, the effective mass transfer coefficient of the organic sorbent adsorbing the water vapor was non-dimensionalized as a function of Reynolds number, modified Stefan number and non-dimensional honeycomb length.</p

Heat Storage Characteristics of Latent-Heat Microcapsule Slurry Using Hot Air Bubbles by Direct-Contact Heat Exchange

This study deals with the heat storage characteristics of latent-heat microcapsule slurry consisting of a mixture of fine microcapsules packed with latent-heat storage material and water. The heat storage operation for the latent-heat microcapsules was carried out by the direct-contact heat exchange method using hot air bubbles. The latent-heat microcapsule consisted of n-paraffin as a core latent-heat storage material and melamine resin as a coating substance. The relationship between the completion time of latent-heat storage and some parameters was examined experimentally. The nondimensional correlation equations for temperature efficiency, the completion time period of the latent-heat storage process and variation in the enthalpy of air through the microcapsule slurry layer were derived in terms of the ratio of microcapsule slurry layer height to microcapsule diameter, Reynolds number for airflow, Stefan number and modified Stefan number for absolute humidity of flowing air.</p