Recent progress of numerical methods for simulating flows coupled with thermo-physical and thermo-chemical problems is surveyed and the development in our group is introduced. Our numerical methods are based on the preconditioning method coupled with mathematical models of thermo-physical properties and the models of chemical events such as chemical reaction, nucleation, condensation, coagulation, phase change and so on. We defined the new CFD field as “Computational Fluid Chemistry(CFC)”. As current numerical examples, applications of our numerical method to supercritical carbon dioxide and water problems with phase change which are expected to use for fabricating nano-scale polymer and metal particles are introduced. Finally, a paradigm shift of CFD toward CFC is addressed.
Lateral growth of turbulent wedge is examined experimentally in a flat-plate boundary layer at low Reynolds numbers. Turbulent wedge is generated by exciting hairpin vortices through a small hole at a location where the Reynolds number is subcritical. The lateral growth of turbulent wedge occurs at the momentum thickness Reynolds numbers above 170, with successive growth of low-speed streaks in the ambient laminar flow region. Beyond the critical Reynolds number for the lateral turbulent contamination, the lateral spacing of low-speed streaks newly generated is 45 in wall units using the friction velocity of Blasius flow at the stage of their first appearance and increases to that of the developed wall turbulence as the instability and breakdown of low-speed streaks develop.
The laminar-turbulent transition of a boundary layer induced by an ejection of jet in the inlet region of a circular pipe was experimentally investigated. The jet was periodically inserted radially from a small hole in the inlet region into the pipe flow. Axial velocity was monitored by a hot-wire anemometer. The difference of properties in laminar-turbulent transition from developed Hagen-Poiseuille flow was examined. Isolated turbulence patches were generated by the jets, and then they propagated downstream. The leading edge of the turbulent patch was definite, whereas its trailing edge was not. This characteristic was similar to that of a turbulent spot in a flat-plate boundary layer. The threshold value of the jet flow rate to generate the turbulent patch was then obtained. The threshold value decreased and saturated finally with the increase in jet duration. The normalized jet duration when the threshold value was saturated increased with the increase in Reynolds number, contrary to the developed region. The normalized threshold flow rate tended to vary with the Reynolds number among three regions. All tendencies were different from those of the developed region. With the increase in jet flow rate beyond the threshold value, the duration of the turbulent patch increased, though the fluctuating velocity within the patch did not increase. The propagation velocities of the leading and trailing edges, the duration and fluctuating velocity within the turbulent patch were almost constant irrespective of the jet ejection frequency.
In the present paper, an attempt was made on calibrating mass fraction of oxygen atom which can exist in a free stream of a high enthalpy shock tunnel based on a measurement of shock stand-off distance. To correlate the free stream oxygen atom and the shock stand-off distance, a density relaxation function was derived by considering an effect of the free stream oxygen atom on density change along stagnation stream line behind the shock wave. The density relaxation function was numerically estimated with the mass fraction of oxygen atom predicted by using NENZF code and used to examine the correlation with the shock stand-off distance around the sphere model measured in the high enthalpy shock tunnel HIEST at JAXA Kakuda Space Center. The correlation between the density relaxation function considering the effect of free stream oxygen atom and the shock stand-off distance was successfully obtained, as the result the mass fraction of oxygen atom in the free stream of the HIEST could be calibrated.
For multiphase flow simulations, accurate evaluation of interfacial tension is an important issue, so a front-tracking method which tracks the interface explicitly by marker elements is expected to be very useful. In the front-tracking method, however, treating the elements in 3D space is very tedious. To eliminate this problem, a front-tracking method without connectivity called level contour reconstruction method (LCRM) has been developed. However, the volume conservation property of the method is not so good. In this study, three techniques in reconstruction process of the LCRM are proposed and evaluated for several curvature spheres. As the result, the proposed method shows very high volume conservation property even for 2-grid diameter sphere. And also shape of a single deformable bubble and merging of two bubbles are reasonably expressed.
We have proposed a new technique for predicting room pressure and duct pressure in pressurized clean rooms. In our technique, the pressure propagation mechanism is replaced with an equivalent electrical circuit network by considering the airflow rate as electrical current, the pressure as electrical potential, and the friction loss as an electrical resistance. In this paper our technique was applied to two pressure change situations caused by external factors. One was pressure change with starting and stopping of the local exhaust ventilation. The other was pressure change with starting of the ventilation equipment. By comparing the predicted results with experimental ones, we verified the validity of our method. Using this technique, the air-conditioning equipment can be evaluated under dynamic usage and the specification of the control devices can be optimized. The effectiveness of any new idea or new control algorithm for maintaining a stable system response can be also examined.
Numerical simulations were conducted by using LES (Large Eddy Simulation) with DSM (Dynamic Smagorinsky Model) about two kinds of axial flow small fan for cooling electronic devices. In this study, the tested fans have non-axisymmetric casing (Normal fan) and axisymmetric casing (Silent fan). The numerical simulations of the flow around those fan blades were compared. Therefore, the both of fans have low pressure (Cp≤-0.6) area in center of suction side surface in blade tip. Especially in normal fan, the pressure level of low pressure areas fluctuate by fluctuation of tip vortex. This fluctuation of tip vortex is caused by fan blades passing small radius parts with separating flow and large radius parts without separating flow alternately.
The influence of the insonation conditions of short-pulse and low-duty diagnostic ultrasound on the characteristics of microbubble volumetric oscillations in the initial bubble radius range from 0.2 to 2.0 micro meters has been numerically investigated by using a recent single bubble oscillation model. The results show that increasing insonation center frequencies with the constant values of the mechanical index reduces the maximum values of both of the in-bubble temperatures and the bubble-emitted sound pressures; increasing the values of that index with constant center frequencies reduces the resonant bubble radii; the maximum in-bubble temperatures and bubble-emitted pressures are correlated with the normalized minimum bubble radii more significantly than the normalized maximum radii.
A laser 2-focus velocimeter (L2F) has been applied for measurements of velocity and size of droplets in the dense region of diesel spray. Investigated was the fuel spray injected intermittently into the atmosphere from an injector nozzle with the orifice diameter of 0.113 mm. The injection pressure was set at 40 MPa by using a common rail system. The L2F had a micro-scale measuring volume which consisted of two foci. The focal diameter was about 3 μm, and the distance between two foci was 17 μm. The maximum data sampling rate of the L2F system was markedly high as 15 MHz. Measurement was conducted at 25 mm downstream from the nozzle. The local mass flow rate and the number density were calculated at several specified times from the measured velocity, size, and arrival time of droplets. It was clearly shown that the mass flow rate was higher at the spray front than the inside, and was higher at the spray center than the periphery. The accumulated mass flow rate in the spray section measured by the L2F was almost equal to the amount of injected fuel mass within 5%.
Rotational energy distribution in a nitrogen molecular beam was experimentally studied by (2+2) N2-REMPI(Resonantly Enhanced Multiphoton Ionization). REMPI is known to have high detection sensitivity, which allows obtaining the signal under the very low number density condition like a molecular beam, successfully. Obtained REMPI spectrum was fitted by a theoretical spectrum to determine rotational temperature. The spectrum was well fitted, showing the rotational energy distribution obeyed the equilibrium Boltzmann distribution. The rotational temperature in a molecular beam must be similar to the frozen rotational temperature in a free jet, since a free jet is used as a beam source in the Kantrowitz-Grey type. The parameter p0d, which is a product of the source pressure p0 and the orifice diameter d, is known to characterize a free jet. Therefore, the rotational temperature in a molecular beam was analyzed in terms of the parameter p0d. The rotational temperature was able to be described by the power of the parameter p0d. The rotational temperature and the obtained function were compared with the value in literatures, which showed good agreement.
To increase the power density of the inverters for power industry, the new air cooling system of high power inverters were developed. Parallel air flow inverters carrying 18 differential units were introduced without unit fan because of easier maintenance and lower cost. Realizing the systems for inverter design, we proposed a useful and epoch-making analysis method named “Air flow equivalent circuits analysis combined with finite element method”. The inverters were divided into a small block and analyzed individually, and their results of the resistances were built into equivalent circuits. The proposed analysis method enables to calculate the wind velocities and pressure drops of the whole inverter panels, which unable to be solved in case of FEM analysis with the numerous mesh for the whole ones. Air flow resistances of merging ducts including the smaller parts could be evaluated in the methods. Therefore, we contribute to decrease the cooling design time within 1 second at the process of mass-produced design.
Because electron, proton, and oxygen are necessary for the cathode reaction in polymer electrolyte membrane fuel cells, achieving the optimum structure of electrode catalyst layer and the efficient transport of the reactants is significantly effective to reduce the usage of Pt catalyst. In this study, we developed a three-phase boundary and a cathode catalyst layer models to clarify structures appropriate for the efficient transport, and the dominant effects of the catalyst layer structures and the properties were investigated using the models. Additionally, the evaluation equations were presented to understand the effect of structure simply, and the effectiveness was confirmed by the comparison of the results calculated by the equation and the model simulation. From the results, suitable structures of the porosity, catalyst layer and polymer electrolyte thicknesses, for the gas diffusion and proton conduction were clarified, and it was presented that the solubility of oxygen in the polymer is one of the dominant factors and increasing in the solubility is extremely effective for reduction of the Pt usage.
Burning velocity just after the spontaneous ignition has been examined not only experimentally but also theoretically, relevant to the Self-propagating High-temperature Synthesis (SHS) process, for Ti-Al system. By varying mixture ratio, degree of dilution, and compact and particle sizes, not only the spontaneous ignition temperature, determined from the inflection-point of the temporal variations of the surface temperature, but also the burning velocity, defined as the normal component to the flame surface, has been measured. It is found that the burning velocity just after the spontaneous ignition first increases and then decreases with increasing mixture ratio, due to an increase and a decrease, respectively, in the heat of combustion, that it decreases with increasing degree of dilution, due to a decrease in the heat of combustion, and that it increases with increasing size ratio, defined as a ratio of compact and particle sizes, due to an increase in the reaction surface per unit spatial volume in the compacted mixture. Experimental comparisons with theoretical results have also been conducted and a fair degree of agreement is demonstrated, indicating that the formulation used has captured the essential features of the SHS flame propagation that passes through the compacted mixture. Since this kind of particle size effect, especially, relevant to the flame propagation after the spontaneous ignition, has not been captured in the previous studies, its elucidation can be considered not only notable but also useful, especially, in manipulating combustion process in materials synthesis.
Non-equilibrium molecular dynamics (NEMD) simulations have been carried out to obtain new evidence for the inverted temperature profile. We find that the inverted temperature profile occurs due to the excess energy transported by the reflecting molecules. A new definition of the accommodation coefficient for the reflecting molecule is proposed based on the comparison of the energy transferred by the reflecting molecule between the equilibrium and non-equilibrium conditions. The accommodation coefficient decreases with increasing the mass flux in the vicinity of the liquid surface and this is the reason for the inverted temperature profile. Also, a direct simulation of Monte Carlo (DSMC) method has been performed by applying the molecular boundary condition developed in the non-equilibrium molecular dynamics simulation. An inverted temperature profile is obtained without any contradiction to the second law because the reflecting molecules cannot accommodate to the equilibrium state.
In recent years, fossil fuels such as petroleum, coal, and natural gas have become limited resources. In addition, bad effects caused by excessive carbon dioxide (CO2) emissions have now begun destroying our global environment seriously. Since current living and economical standards depend strongly on the fossil fuels, it is necessary to realize a new society that utilizes biomass as one of major sources of energy. In this background, we manufactured a practical Stirling engine using woody biomass fuels for the first time in Japan in 2005. Further we proposed a unique co-generation system with the Stirling engine that uses woody biomass fuels such as sawdust, firewood, and wood pellets. In this co-generation system, 43% of the input energy is wasted as heat loss from the exhaust smoke into the atmosphere. Therefore we tried to recover the waste heat by using a thermoelectric conversion module in this study. In this report, the results of basic performance test and demonstration experiment as a co-generation system combined the waste heat recovery with a power generating system are reported.
This study shows an efficient configuration method of a multiple units of cogeneration system (CGS) fueled by biogas in a sewage treatment facility. The efficient configuration was clarified by classifying a relation between exhaust heat recovery efficiency (ηehr) of the CGS, and the ratio of yearly average heat demand to yearly average biogas production of the facility (Qh.d/Qb.p). The CGS was assumed to be used under Qh.d/Qb.p < ηehr, Qh.d/Qb.p ≈ ηehr, Qh.d/Qb.p > ηehr conditions. Results shown that, although the CGS was sufficiently capable to cover the total heat demand of the facility by only using biogas produced, on the point of view of energy utilization efficiency, reduction of unutilized biogas efficiency and reduction of electrical demand efficiencies, the most efficient CGS was obtained under Qh.d/Qb.p ≈ ηehr condition. It was found that under Qh.d/Qb.p ≈ ηehr condition, energy utilization, reduction of unutilized biogas and reduction of electrical demand efficiencies were 0.64, 0.99, and 0.32, respectively. Whereas, under Qh.d/Qb.p < ηehr and Qh.d/Qb.p > ηehr conditions, energy utilization, reduction of unutilized biogas and reduction of electrical demand efficiencies were in a ranges of 0.56-0.64, 0.43-0.99, and 0.16-0.20, respectively. If a CGS with a lower ηehr such as fuel cell is used under Qh.d/Qb.p < ηehr condition, or a CGS with a higher ηehr such as steam turbine is used under Qh.d/Qb.p > ηehr condition, more efficient system can be obtained.
Fatty acid methyl ester (FAME) has the properties of high viscosity and poor volatility. Therefore, the atomization of its spray seems to be inhibited, which results in problems such as severe engine deposits and injector coking. In this research, an improvement of the fuel properties is aimed by mixing the low boiling fuel in FAME. This method has the effect that physical characteristic such as viscosity or volatility is improved. In this research, light cycle oil (LCO) is used as the fuel to mix with FAME. LCO tends to the surplus in the process of the refinery because it includes much aromatic components and sulfur content. From the viewpoint of effective utilization of LCO and improvement of spray characteristics of FAME, FAME is mixed with LCO. This paper describes spray characteristics of the mixed fuel of FAME and LCO. In the experiment, the spray characteristics were measured by shadowgraph photography and Mie-scattered light photography. From the experimental results, it is found that the liquid phase penetration is shortened and the spray angle is broadened as mixing ratio of LCO increase.
The purpose of this study is to investigate the cause of PM reduction by diesel fuel (Solvent:C12, C13, C14) mixed with bio-diesel fuel (Methyl Decanoate:C9H19COOCH3) with focusing on the thermal decomposition in diesel combustion atmosphere. To make a heavy duty diesel combustion atmosphere, a flow reactor and a co-flow diffusion burner were used. The thermally decomposed fuel produced in the flow reactor was introduced into the co-flow diffusion burner directly. To study the relation between thermal decomposition and PM reduction, the chemical substances produced in the reactor and PM produced by the combustion of the burner were quantitatively and qualitatively analyzed by some analytical devices. The emission measurement with diffusion burner shows that the bio-diesel blended fuel exhausts PM in concentration 20-30% less than diesel fuel. The analysis indicates that, due to oxygen-containing fuel, the bio-diesel fuel can oxidize thermally decomposed components of C2H2 and PAHs that may become pre-cursers of PM.
This paper describes that selective liquefaction characteristics of polystyrene, PS, in waste plastics and fuel characteristics of biodiesel fuel, BDF, liquefied PS as diesel fuel. Firstly, solubility of PS in BDF was predicted by analysis based on Hansen Solubility Parameters, HSP. From the result, it was evaluated that BDF will be a good solvent for PS. In experiment, expanded polystyrene, EPS, and polystyrene paper, PSP, were liquefied in soybean oil methyl ester. From the experimental result, it was found that the kinematic viscosity of liquid increases with an increase of PS concentration in BDF. It was clarified that the increase of kinematic viscosity of BDF liquefied PS was due to molecular weight of PS. The self-ignition quality is decrease with an increase of PS concentration in BDF. The BDF with EPS concentration of 2 mass% is little change as compared with neat BDF.
Understanding of the burning velocity for micro-scale flames is inevitable in the improved design of micro-combustors for miniaturized power supplies, and also useful for modeling local burning characteristics of turbulent flames. The present study is performed to examine experimentally the burning velocity characteristics of micro-scale spherical laminar flames in the range of flame radius rf approximately from 1 to 5 mm for methane and propane mixtures as hydrocarbon fuel, and also macro-scale laminar flames with rf >7 mm for comparison. The mixtures have nearly the same laminar burning velocity at so-called unstretched flames and different equivalence ratio φ (φ=0.8-1.0 for CH4, φ=1.0-1.4 for C3H8). The radius and the burning velocity of micro-scale flames are obtained by using sequential schlieren images recorded under appropriate ignition conditions. It is found that the burning velocity of all micro-scale flames has a tendency to increase with increasing rf , and approach that of macro-scale flames. The propane mixture with φ1.4 , however, shows that the burning velocity of micro-scale flames can not be explained based on that of macro-scale flames. This suggests that the optimum size and Karlovitz number to improve the burning velocity are existed, depending on φ and fuel types.
The flow accelerated corrosion in a pipe is an important topic of interest associated with pipe-wall thinning phenomenon in a highly aged nuclear power plant. In the present study, the velocity field behind an orifice in a pipe is studied by PIV measurement in some combinations of swirl flow magnitudes and orifice bias. The flow observations along the flow axis and across the pipe indicate that the effect of orifice bias is not so influential on the flow behavior behind the orifice at small swirl flow magnitude. However, the asymmetrical flow pattern is observed in the flow behind the orifice at large swirl flow magnitude. The accelerated flow behind the orifice reattaches on the pipe wall of shorter orifice height and the corresponding velocity fluctuation decreases at large swirl flow magnitude, which occurs even at the small orifice bias of 0.7% of pipe diameter. This phenomenon is expected to promote the asymmetrical distribution of pipe-wall thickness due to flow accelerated corrosion in a prototype pipe flow.
Many studies on a small-sized heat sink used for cooling of small electronic parts have been reported. However, there were few studies for a large-sized heat sink used for cooling of the backlight unit of liquid crystal display device. In this study, the tested heat sink was made of extruding aluminum, had fins length of 400mm, and was natural convection type heat sink. The experimental results and numerical analysis results on the distribution of base temperature were compared, as the results, following conclusions were obtained. (1)In the conventional large-sized heat sink with long fins, temperature difference on the base surface more than 4 K along fins occurred. (2)It was clarified that the base center temperature could be decreased from 62.8°C to 60.4°C by changing fin pitch along fin and the location of division of fins under keeping the temperature distribution on the base surface within 1K.