Spontaneous vapor explosion can occur when a layer of high temperature molten material is deposited on a pool of water or on a moist floor. This is called a base-triggered vapor explosion. In order to clarify the micro-mechanism of base-triggered vapor explosions, an experimental apparatus was designed and constructed to observe the base-triggered vapor explosion from the bottom upwards. This experimental apparatus also allowed detailed observation of the microscopic behavior at the interface between the molten material and the water. The experiments used U-Alloy95 as a simulant material. As a result, it was found that water was trapped between molten material and the floor when the molten material droplet was released into the water and covered the floor. Particle imaging velocimetry (PIV) analysis and the digital auto-correlation method were applied to the observation images to evaluate the behavior of the molten material. The upward blowout velocity of the molten material was measured from the images observed from the side of the test section and the ratio of the kinetic energy to the thermal energy of the molten material was estimated from the blowout velocity.
Heat and mass transfer characteristics in a reforming catalyst bed have been experimentally investigated. Experiments were carried out with a single bench-scaled reforming tube which was filled with reforming catalyst. The tube wall was uniformly heated, and mixtures of steam and methane or propane were reformed through the catalyst bed. Most part of the reaction was completed in the upper part of the test tube. The effects of space velocity, which is a ratio of volumetric flow rate of process gas to the catalyst volume, steam carbon molar ratio, wall temperature, bed temperature, and catalyst particle diameter on the transport phenomena with chemical reaction, were determined. A correlation to heat transfer coefficient was determined by Nu, Rep, Pr, dp/d, and Da. The prediction of the overall methane conversion rate was also presented.
Experiments and numerical simulation are performed to investigate the mechanism of heat and mass transport inside a ribbed tube induced by reciprocating flow. The reciprocating flow is generated by a crank-piston device without producing a net throughflow. It is disclosed from the study that the reciprocating flow causes the generation and extinction of separation vortices behind each rib to manipulate "trap and release" mechanism of heat and mass transport in the axial direction. The transport performance thus produced inside the ribbed tube is far superior to those inside the smooth tubes or so-called dream pipes.
A micro cogeneration system composed of a solid oxide fuel cell (SOFC) and a microturbine (MT) and an absorption refrigerator is analyzed thermodynamically. The performance analysis is conducted on the basis of the balance of macroscopic mass and energy with additional empirical correlations and operating data. First, the basic characteristics of the power generation (SOFC+MT) section and the absorption refrigerator section are clarified. Second, under the conditions of the cell temperature of 900 °C and the turbine inlet temperature of 900 °C, the optimum design points are determined. Furthermore, the annual energy saving obtained by the present system is also evaluated in the light of energy-use data for Japan. As a result, the annual fuel consumption is reduced by 32%, 36% and 42%, for apartments, offices and hotels, respectively.
Heat transfer to the wall of a small pressure vessel during filling with some different kinds of gas was investigated experimentally. The vessel was orientated vertically with the inlet at the top. The space-averaged Nusselt number for the curved wall was found to be a function of both the Reynolds and Rayleigh numbers indicating a mixed convection heat transfer situation. A correlation is proposed for the heat transfer coefficient during charging of the vessel. For the six positions where measurements were taken, the local heat transfer coefficient typically did not differ from the space-averaged value by more than about 30 percent. Measurements were also taken during discharging to atmospheric pressure. For discharging, some of the data was found to agree with a correlation for natural convection in cylindrical geometry. Local Nusselt numbers for discharging tended to be higher towards the bottom of the vessel.
Nanocrystalline silicon particles with grains smaller than 5 nm are widely recognized as a key material in optoelectronic devices, lithium battery electrodes, and bio-medical labels. Another important characteristic is that silicon is an environmentally safe material that is used in numerous silicon technologies. To date, several synthesis methods such as sputtering, laser ablation, and plasma-enhanced chemical vapor deposition (PECVD) based on low-pressure silane chemistry (SiH4) have been developed for precise control of size and density distributions of silicon nanocrystals. In this study, we explore the possibility of microplasma technologies for efficient production of mono-dispersed nanocrystalline silicon particles on a micrometer-scale, continuous-flow plasma reactor operated at atmospheric pressure. Mixtures of argon, hydrogen, and silicon tetrachloride were activated using a very-high-frequency (144 MHz) power source in a capillary glass tube with volume of less than 1 μl. Fundamental plasma parameters of the microplasma were characterized using optical emission spectroscopy, which respectively indicated electron density of 1015 cm-3, argon excitation temperature of 5000 K, and rotational temperature of 1500 K. Such high-density non-thermal reactive plasma can decompose silicon tetrachloride into atomic silicon to produce supersaturated silicon vapor, followed by gas-phase nucleation via three-body collision: particle synthesis in high-density plasma media is beneficial for promoting nucleation processes. In addition, further growth of silicon nuclei can be terminated in a short-residence-time reactor. Micro-Raman scattering spectra showed that as-deposited particles are mostly amorphous silicon with a small fraction of silicon nanocrystals. Transmission electron micrography confirmed individual 3-15 nm silicon nanocrystals. Although particles were not mono-dispersed, they were well separated and not coagulated.
In the HCCI (Homogeneous Charge Compression Ignition) engines, inhomogeneity in fuel distribution and temperature in the pre-mixture exists microscopically and has possibility to affect the ignition and combustion process. In this study, the effect of charge inhomogeneity in fuel distribution on the HCCI combustion process was investigated. Pressure profiles were measured and two dimensional chemiluminescence images were captured by using a framing camera with a 4-stroke optically accessible engine in order to understand the spatial distribution of the combustion. DME (di-methyl ether) was used as the test fuel. By changing the way of mixing air and fuel in the intake manifold, inhomogeneity in fuel distribution in the pre-mixture was varied. The result shows that luminescence is observed in a very short time in a large part of the combustion chamber under the homogeneous condition, while luminescence appears locally with considerable time differences under the inhomogeneous condition. It is also shown that the local luminescence durations are almost the same under both conditions.
In order to observe detailed behavior of micro plastic particles under rapid heating, a fundamental investigation is made by introducing two ingenious devices; one is the construction of a plane laminar premixed burner exhibiting extremely excellent two-dimensionality, the other is the construction of a magnifying particle-tracking system composed of a pair of rotating plane mirrors and a fixed high-speed video camera. Taking account of the flow patterns obtained using a PIV/PTV system, a series of heating processes from melting to burning of micro PET particles passing through a laminar flame sheet is optically observed. It is found that the proposed high-speed and magnifying tracking system can realize a wide straight range of particle tracking up to 50 mm and clarify many interesting facts concerning the ignition and burning processes of micro PET particles.
The transient method of measuring heat transfer coefficients that uses liquid crystals, since its beginnings in the early 1980s, has become one of the best ways of determining surface distributions of heat transfer coefficient. In this paper, experimental results are presented of local surface heat transfer coefficients in a turbulent jet, impinging onto a curved surface which has the convex and concave surfaces. The preheated wall transient method is applied with liquid crystals. Different Reynolds numbers, distances between impingement jet and collision surface, and inclined angles are considered. This paper reports the correlation at the stagnation point heat transfer coefficients of Reynolds number, distances, and impingement inclined angles with Nusselt numbers.
In order to reduce the CO2 emission, in May 2004, the European Union (EU) started an experimental approach known as the “naturalhy Project” in order to transport hydrogen by mixing it with the existing high-pressure natural gas in the pipelines. Naturalhy represents a mixture of hydrogen and natural gas. In other words, this gas is also known as hythane, which is an abbreviation of hydrogen and methane. The name “hythane” is the registered trademark of Hydrogen Consulting Inc., USA. Why will this gas gain importance? It is generally considered that the sudden realization of a hydrogen energy society cannot take place. It is normally assumed that the present status of methane as an energy carrier gradually changes to a state of hydrogen-methane mixed gas and finally to 100% hydrogen. This is why the authors investigate the properties of this mixture. This study is considered to be the first to measure the temperature dependence of the viscosity of hydrogen-methane mixed gas. In order to measure the viscosity, the authors used a capillary method that measures the pressure drop in the laminar flow through a pipe. It was conducted in an electrically polished, ultra clean and smooth tube and the pressure drop between the upstream and downstream was carefully measured using a capacitance manometer. In order to remove the effect of temperature dependence, the tube was placed in a constant temperature bath, and the temperature fluctuation was maintained within ±0.3°C throughout this experimental study. The authors obtained the viscosity of the hydrogen-methane mixed gas within a temperature range of 20-70°C.
Although diesel engines have an advantage of low fuel consumption in comparison with gasoline engines, exhaust gas has more particulate matters (PM) including soot. As one of the key technologies, a diesel particulate filter (DPF) has been developed to reduce PM. When the exhaust gas passes its porous filter wall, the soot particles are trapped. However, the filter would readily be plugged with particles, and the accumulated particles must be removed to prevent filter clogging and a rise in backpressure, which is called filter regeneration process. In this study, we have simulated the flow in the wall-flow DPF using the lattice Boltzmann method. Filters of different length, porosity, and pore size are used. The soot oxidation for filter regeneration process is considered. Especially, the effect of NO2 on the soot oxidation is examined. The reaction rate has been determined by previous experimental data. Results show that, the flow along the filter monolith is roughly uniform, and the large pressure drop across the filter wall is observed. The soot oxidation rate becomes ten times larger when NO2 is added. These are useful information to construct the future regeneration system.
The bioethanol reforming system (FBSR) using sunlight as a heat source is a fuel production system for fuel cells with little environmental impact. However, because solar radiation and outside air temperature are unstable, it is difficult to predict operation of the system with accuracy. Therefore, an operation prediction program of the FBSR using a layered neural network (NN) with the error-correction learning method has been developed. We developed a method of analyzing the operation of a natural energy system with sufficient accuracy. The weather pattern (the amount of global solar radiation and the outside air temperature) and energy-demand pattern for the past one year are inputted into the NN. Moreover, training signals are calculated by a genetic algorithm (GA). The training signals are given to the NN, and the operation pattern of the FBSR is made to learn. Operation of the FBSR on arbitrary days can be predicted by inputting the weather pattern and the energy-demand pattern into this learning NN. In this paper, the operation prediction program of the FBSR is developed, and details of the analytic accuracy are clarified. As a result of analyzing using the developed algorithm, when ±20% or less of power load fluctuation occurred, the operation plan was analyzable in 14% or less of error span. On the other hand, in operation prediction when ±50% or less of fluctuation is added to the outside temperature and global solar radiation, there was 16% or less analysis error.
The effect of positive stretch on the local flame properties of turbulent propagating flames in the flamelet regime was investigated experimentally for methane-, hydrogen- and propane-air mixtures with lean and rich having nearly the same laminar burning velocity (SL0=25cm/s). The ratio of the turbulence intensity u' to SL0 was varied as 1.4 and 2. A 2D laser tomography technique was used to obtain the temporal local flame configuration and movement in a constant-volume vessel. Some of the key parameters of turbulent combustion quantitatively measured are the local flame displacement velocity SF, curvature and stretch of turbulent flames. Additionally, the Markstein number Ma was obtained from outwardly propagating spherical laminar flames, in order to examine the effect of positive stretch on burning velocity. It was found that the obtained SF was distributed over a wide range with flame stretch as well as curvature, even for low turbulence (u'/SL0=1.4). There also existed a good relationship between SF and the turbulent burning velocity. A quantitative relationship between Ma based on laminar flames and the SF for positive stretch and curvature of turbulent flames could be observed only for mixtures with Lewis number Le greater about one.
To examine the effect of low co-axial flow on soot formation in a laminar jet diffusion flame, microgravity experiments have been conducted. The tested co-axial flow velocity range is 0-7.3 cm/s, which is very difficult to provide on the ground because of suffering from the additional external flow induced by buoyancy force. The result showed that the soot formation characteristics were greatly affected by co-axial flow velocity at the low flow velocity range, that is, soot concentration increased with increase in the external flow velocity. According to the radial distributions, the effect of external flow velocity on the soot formation was prominent near the outer edge of visible flame. A comparison with numerical calculation suggested that the increase of soot concentration was caused by increased flame temperature in the area of low oxygen and fuel excess region.
Two-phase flow nozzles are used in the total flow system of geothermal power plants and in the ejector of the refrigeration cycle, etc. One of the most important functions of the two-phase flow nozzle is converting two-phase flow thermal energy into kinetic energy. The kinetic energy of the two-phase flow exhausted from a nozzle is available for all applications of this type. In the case of non-best fitting expansion conditions, when the operation conditions of the supersonic nozzle are widely chosen, there exist shock waves or expansion waves at the outlet of the nozzle. Those waves affect largely the energy conversion efficiency of the two-phase flow nozzle. The purpose of the present study is to elucidate character of the expansion waves at the outlet of the supersonic two-phase flow nozzle. High-pressure hot water blowdown experiments have been carried out. The decompression curves of the expansion waves are measured by changing the flowrate in the nozzle and inlet temperature of the hot water. The back pressures of the nozzle are also changed in those experiments. The expansion angles of the two-phase flow flushed out from the nozzle are measured by means of the photograph. The experimental results show that the decompression curves are different from those predicted by the isentropic homogeneous two-phase flow theory. The regions where the expansion waves occur become wide due to the increased outlet speed of the two-phase flow. The qualitative dependency of this expansion character is the same as the isentropic homogeneous flow, but the values obtained from the experiments are quite different. When the back pressure of the nozzle is higher, these regions do not become small in spite of the supersonic two-phase flow. This means that the disturbance in the downstream propagates to the upstream. It is shown by the present experiments that the expansion waves in the supersonic two-phase flow of water have a subsonic feature. The measured expansion angles become larger with increasing flowrate of the two-phase flow and decreasing back pressures.
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)