Direct numerical simulation (DNS) of turbulent heat transfer in a channel flow has been carried out with a higher Reynolds number up to Re_τ=640,which is based on the friction velocity u_τ and the channel half width δ. The configuration is a fully developed turbulent channel flow with uniform heating from both walls. Calculated conditions are Reynolds numbers of 180,395 and 640 and the molecular Prandtl numbers of 0.025 and 0.71. Turbulence quantities such as temperature variance, turbulent heat fluxes, their correlation coefficients, turbulent Prandtl number and time scale ratio are presented to investigate the Reynolds- and Prandtl-number effects and the scaling issue up to Re_τ=640.
Momentum and thermal turbulence structures in a channel flow have been investigated through DNS database up to Re_γ=640,where Re_γ is the Reynolds number based on the friction velocity u_γ and the channel half width δ. It was found that large-scale motions exist in the outer layer at high Reynolds numbers for both momentum and thermal fields. It was shown that the near-wall turbulence structures become clustered with increasing Reynolds number for both momentum and thermal fields. It was also shown that large-scale structures exist even in the near-wall region at high Reynolds number, which is closely associated with the large-scale motions in the outer layer.
Effect of a difference in the thermal boundary condition is examined. DNS was carried out for constant wall temperature difference condition for Re_γ=180,395 with Pγ=0.71 to obtain statistical cross-correlation coefficients, turbulent Prandtl number, skewness factor and flatness factor. The obtained results are compared with those of DNS for the uniform heat flux heating. A large difference was obtained in the skewness and flatness factors of the temperature variance for the two thermal boundary conditions.
Turbulent concentric annular pipe flows with four radius ratios were investigated by means of the direct numerical simulation (DNS). In the inner region, the mean velocity shifts downward from the logarithmic law and the Reynolds stresses decrease as the radius ratio α decreases. We found that the high order statistics and instantaneous fields for very small radious ration (α=0.025) are much different from those for α=0.1 and α=0.2.
A time-developing boundary layer flow on a flat plate from laminar to turbulence has been calculated with a direct numerical simulation. The profiles of mean velocity and fluctuating velocity components obtained in the turbulent region are excellently coincided with the existing simulation and measurements, when the comparisons are made for the Reynolds number based on the displacement thickness of the boundary layer. Consequently, it is expected that the direct numerical simulation of time-developing flows can provide various information with regard to usual turbulent boundary layers.
A nonisothermal flow affected by effects of wall curvature and rotation is characteristic of a micro gas-turbine. In general, the effects of curvature and rotation are regarded as similar in nature. However, the unified parameter which represents both effects is not sufficiently clear. In this study, we performed the direct numerical simulation (DNS) of thermal turbulent flow in a rotating curved channel, and explored the fundamental characteristics of this kind of turbulent flow on the basis of the obtained statistical turbulence quantities and instantaneous velocity and thermal fields. Also, we propose a new unified parameter for curvature and rotation effects, and validate it with the results of DNS.
In order to develop an overall efficient and accurate model of simulating an unsteady three-dimensional airflow over complex terrain with characteristic length scales of the order of kilometers, we have been examining the large-eddy simulation (LES) technique using a finite-difference method (FDM). These LES codes are referred to as the RIAM-COMPACT (Research Institute for Applied Mechanics, Kyushu University, Computational Prediction of Airflow over Complex Terrain), and are based on two grid systems and corresponding variable arrangement : one is an orthogonal staggered grid; the other is a generalized curvilinear collocated grid. In this paper, using the RIAM-COMPACT with a generalized curvilinear collocated grid, we have performed the calculations of stably stratified airflows over an isolated steep hill. A strong wind area caused by the lee wave was clearly seen near the first trough behind the hill. Next, we made the calculations of stably stratified airflows over Mt. Miyake. Particularly, the behavior of the passive particles from the top of Mt. Miyake was investigated.
Fully developed turbulent premixed flames were directly simulated in different cases P_u/P_b=7.53,2.50 while u'/u_L was close to unity. Analysis of the Favre-averaged transport equation for turbulent kinetic energy showed pressure concerning terms produced kinetic energy in flame brush, and mean pressure gradient ternm, pressure dilatation term and additional dissipation components were modeled. Analysis of the Favre-averaged transport equation for turbulent scalar flux showed pressure concerning terms and velocity-reaction rate correlation term were sources for counter-gradient diffusion, and mean pressure gradient term, fluctuating pressure term, dissipation terms and velocity-reaction rate correlation term were modeled.
Direct numerical simulations of hydrogen-air turbulent premixed flames propagating in three-dimensional homogeneous isotropic turbulence are conducted to investigate effects of turbulence characteristics on the local flame structures. Calculations are conducted for the three cases : in the wrinkled flamelets, on the boarder of distributed reaction zones and well-stirred reactor regimes and in the well-stirred reactor regimes. The probability density functions of the curvature of flame fronts show the same distribution for all cases when it is normalized by the turbulence characteristics in the unburned side. The heat release rate increases linearly with the increase of the curvature and the increasing rates are nearly identical for all cases.
A Large-Eddy Simulation (LES) based on a finite volume method was appliedto turbulent liquid flows with moderately fast and rapid chemical reactions. The large-eddy probability density function (LEPDF) model and joint LEPDF model were used as a SGS model for a rapid reaction and for a moderately fast reaction, respectively. To investigate the applicability of the LES based on the proposed SGS models to neutrally, stably and unstably stratified reacting liquid flows, the results of the LES were compared with the measurements. The predictions of the LES were in good agreement with the measurements. The results also show that the present LES can accurately estimate the concentration statistics in neutrally, stably and unstably stratified liquid flows with reactions.
Gas flows in unsteady jets were calculated from a large eddy simulation (LES). Based on obtained results, the effects of velocity conditions at a nozzle exit on penetration length and jet radius were clarified. Furthermore, it was shown that vortex generation in a shear region and a jet-tip was enhanced by adding perturbation to a nozzle-exit, which promoted an entrainment into a jet and turbulent diffusion of momentum.
A priori test using the LES data base is carried out to evaluate the Algebraic stress model (ASM), the WET model and the linear eddy viscosity model. The accuracy and relative performance of their turbulent heat flux models are carefully examined in both plane wall jet and impinging jet regions. The ASM gives a good result by counterbalancing them though the modeling pressure-temperature gradient term and the algebraic approximation is not so efficient. The WET model require the use of appropriate wall damping function to preciously predict wall shear flow.
The present study considers a possibility of modeling the correlation of velocity and pressure-gradient vectors, in contrast to the conventional approach where pressure-strain correlation and pressure-diffusion terms are treated individually. A hybrid approach combining the velocity measurement by PIV and the numerical procedure to evaluate instantaneous pressure field in the wake of a rectangular cylinder has indicated that the correlation between the velocity vector and pressure-gradient vector become significant particularly when they are perpendicular to each other. The series of evaluations in necessary to understand the wider variation of complex flow problems. Well designed DNS should provide useful data.
Numerical analysis has been performed for three-dimensional developing turbulent flow in a U-bend of strong curvature with rib-roughened walls by using an algebraic Reynolds stress model. Special attention is paid for the developing turbulent flow in U-bend with positive and negative rotations in this study. It has been pointed out as a characteristic features from the experimental result that positive rotation leads to a more uniform velocity distribution within bend, whereas, the high momentum fluid remains closer to the inner side with negative rotation. The present method could relatively predict such velocity profiles and reproduce the separated flow generated near the outer wall which is just located at downstream of curved duct.
Turbulent channel flows rotating about the spanwise, streamwise and wall-normal axes are calculated with a low-Reynolds-number second-moment closure. In the case of weak streamwise rotation, a shear stress component induced by the rotaion changes its sign at a position between the wall and the centerline. This feature is captured by the model. At high rotation rates, however, the sign reversal disappears in DNS and the predicted sign disagrees with that of DNS, leading to poor predictions of the velocity profiles. A need for a redistribution process which works against the exact rotational production is suggested. The wall-normal rotation leads to relaminarization of the flow when the rotation rate becomes high. Good predictions are obtained for weak rotation but the model gives relaminarization at a lower rotation number than DNS.
This paper presents an experimental study to explore the noise reduction of an under-expanded supersonic jet. As a novel method to obtain the noise reduction, a rigid wire is inserted into the under expanded supersonicjet stream. The experimental results show that the wire device significantly changes the shock and acoustic wave structures, leading to substantial noise reduction, compared with the under expanded jets without the wire device. According to the jet noise measurement, the + type wire device leads to the overall sound intensity reduction of about 15dB.
This paper presents quasi one-dimensional numerical analysis of the interaction of particles with the carrying gas flow for high velocity oxy-fuel (HVOF) thermal spraying nozzle. The method includes gas dynamical calculations of the gas flow in a HVOF nozzle taking into account of flow friction at wall and calculations of injected particle motion and temperature. The calculations were performed for particles of tungsten carbide with diameter of 30mm and 50mm. The design Mach number of the nozzle is set as 2.0. The working gas is assumed to be perfect gas for simplicity. The gas temperature at the chamber is set as 3000K. Thermal choking takes place due to wall friction at 16.4 times the nozzle exit diameter downstream from the entrance of the nozzle throat. The velocity and temperature of working gas and particles are calculated through the HVOF nozzel.
In present paper, we investigated the effects of clearance length between a body and a duct wall, and duct height on the heat transfer characteristics and flow behavior at downstream region of body when the blunt body was set in parallel plate duct with some distance separating it from the duct wall as a turbulence promoter. At the ratio of clearance to body height, C/D=0.05∿0.1,the heat transfer was characterized by the reattachment of shear flow separated from the body. Furthermore, the heat transfer depended on both effects of the reattachment flow and the separation vortex at C/D=0.15,and effect of the separation vortex generated by separating flow through the clearance or the side vortex induced by Karman vortex at C/D=0.2 and 0.25.
In this paper, the flow of underexpanded jet issued from rectangular nozzles of aspect ratio 1,3 and 5 were investigated. Two dimensional temperature distributions on vertical flat plate were measured by means of infrared image camera. The density fields were visualized by means of optical method of schlieren. As a result, the temperature variation with the pressure ratio at the plate center show that there are two clear peaks of the temperature, only for the aspect ratio of 3. Mean the two-dimensional temperature distributions for the aspect ratio of 3 reveal that the flow pattern is not plane symmetry.
A numerical simulation using axi-symmetrical Navier-Stokes equation for gas flow inside an elliptical model was performed, in which supersonic jets emitted from inlet of the cell and the induced shock waves interfere with each other. The effects of the pulse width, initial Mach number, initial pressure ratio and initial temperature ratio of the jets emitted in the way of single pulse on the behavior of the jets and shock waves as well as their interference were investigated. As a result, it was found that an optimal condition for colliding of the jet and shock waves in the vicinity of focal point of the elliptical cell and so the jet flows out through the exit of the cell
The temperature and pressure distributions on a plate in the dual underexpanded impinging jets are experimentally investigated. The static pressure fluctuations on the plate were also measured and compared with the case of the single jet. The main parameters for the experiments are the non-dimensional distance between the two nozzle centers H/D=1.5,2.0,the nozzle to plate separation L/D=2.0,and 5.0,and pressure ratio defined by p_o/p_b=1.0∿6.0. The results show that the temperature at the middle point between the jet axes is considerably affected by the pressure ratio p_o/p__b, where p_o is the stagnation pressure, p_b the back pressure. Moreover, FFT analyses of the static pressure fluctuations on the plate suggest that relatively low frequency range between 0 and 1.5kHz for the dual jets is dominant under the condition of H/D=1.5 and L/D=2.0,and at the pressure ratio of about 3.0 compare to the single jet.
In order to reduce the time to charge a MH-tank, the heat exchange performance of a tank has to be improved. In this study, several MH-tanks with an effective storage capacity of 1.25Nm^3 were manufactured in different structures. Through the charging tests and the heat transfer simulation, influences of structure on the temperature distribution inside the tank and on the charging time were investigated. Based on the data, the structure of the MH-tank with an actual capacity as a storage device for a FCV of 31.25Nm^3 was optimized. The 80% of an effective storage capacity could be charged within 10min.
In order to reduce charging time of hydrogen storage tank with metal hydride (MH), an optimum fin arrangement enhancing poor heat conductivity of MH bed was numerically analyzed. Outline of the analysis is as follows. Radial aluminum fins are inserted around a central hydrogen filter in a simple circular MH tank where the outer surface serves as a heat transfer surface. Then the effect of fin number on the charging time is evaluated. When the fin number is below 16,the charging time considerably decreases with increase of the fin number. The effect of fins more than 16 becomes smaller and the increase of fin area (larger than 20% of total cross sectional area) may not be compensated. The optimum fin number is applied to a design of MH tank for FCEV and effectiveness of the qualitative analysis in the design is also shown.
MH (Metal Hydride) refrigeration system, which uses natural refrigerants, is a very safe and ecological system. Endothermic reaction between MH and hydride gas could be applied to a non-fluorocarbon refrigerating system. In this study, using demonstrational system and MH calculation model, the effects of MH alloy mass fraction, initial H_2 pressure, heat source temperature, middle cooling temperature and regeneration process time on refrigeration performance, are measured and calculated in order to find the optimum operating conditions. In addition, the carbon fiber is used to improve the effective thermal conductivity of MH alloy bed. As a result, mixing carbon fiber is more effective for cooling load than that of non-mixing carbon fiber.
A procedure to simultaneously estimate thermal diffusivity and thermal conductivity is improved by employing a revised inverse solution, in which the time at which this method starts being applied is independently chosen for the measured temperature to satisfy a required temperature change. As the result, the procedure becomes easier and more convenient than the previous one. The estimations of thermal diffusivity and conductivity are made for a particle of metal-hydrogen. The estimation shows that the thermal diffusivity, a, is almost constant value of 4.0×(10)^<-7> m/s^2 in a range of 10 to 30℃ and of 0.148 to 0.502 MPa, while the thermal conductivity linearly increases from 1.0 to 3.0W/(mK) for the different temperature with the pressure and the composition rate.
High power density is necessary for applying PEMFC to automotives. This paper describes with the effects of separator geometries on electric generation performance of PEMFC by the use of serpentine and column separators. The result indicates that PEMFC with serpentine separator achieves higher performance than with column separator. This can be explained by the fact that drainage capability of a serpentine type separator is superior to that of column with less contact resistance between a separator and a gas diffusion layer due to the increased contact area.
This paper describes with the influence of the operating conditions including the gas flow rate, the humidity, and the operating pressure on the performance of fuel cell with serpentine flow field. The results shows that the voltage drop at high load is depending on the cathode gas velocity and the condensed water quantity in the flow passage of the separator. Thus, it is important to secure more than 10∿15 m/sec of cathode gas velocity and to minimize the condensed water in the fuel cell without reducing the water state of the membrane by the design of separator and operating condition to prevent the voltage drop at high load.
In order to protect environment, it is necessary to use environmentally acceptable refrigerants. In the absorption heat pump system and adsorption heat pump system, natural refrigerants are used from the first. Those systems, however, need high temperature heat source, and adsorption heat pump is batch process. Therefore, those systems have no come into wide use for household. For solving these kinds of problem, study on possibility to realize a continuous type adsorption heat pump using honeycomb Rotor for low-temperature waste heat recovery is carried out. This project is carried out by three groups all together. One studies on adsorbent, one of the others study on heat exchanger and system, another one is a manufacture of honeycomb rotor. In this paper, my personal opinion about technical problems and procedure of collaboration are described
We introduce about development of a solar still in practical use, which has been done in cooperation between a venture company "topecology" and university of the Rryukyus. The solar still was newly designed, and it was found that the still has great productivity in the outdoor experiments in the university. But there are many problems in the development of the products to the market, and we have solved the problems by trial and error. Until now, the company has sold about 20 stills to the Indonesian government. The business has just begun, and the venture company continues its challenge.
Natural salt production has rapidly been increasing in Okinawa after 1997. In this paper, we report some production methods of natural salt, which have been employed in Okinawa, and propose a high performance concentrator of sea water from 3.5% to 25% and two low-cost salt-production systems, i. e. a spray drying system and a double dram drying system. The concentrator can reduce energy consumption by 85%. The two systems can drastically reduce labor costs.
As is well known, wettability of solid surface and surface tension of liquid have an important role in the field of heat transfer with phase change. Those are remarkably related upon the behavior of liquid thin films that are dominant factor in the heat transfer phenomena. Several phenomena concerning thin liquid film, which is in contact with a heat transfer surface, are discussed. That is, the measurement of condensate thickness for water-ethanol solutal Maragoni dropwise condensation, the condensation of water vapor on outer surface of vertical thin tubes on which the surface tension instability occurs and the vaporization in micro-channel type high density vapor generator.
Several researchers reported that the apparent contact angle increased with decreasing the contact line velocity when the apparent contact angle was measured from the spreading front of a liquid droplet colliding with flat surfaces at room temperature. The purpose of the present study is to discuss the reason why such an experimental result, which is against the well-know fact, was obtained. A mathematical model describing the fluid dynamics during the droplet impingement on a flat surface is numerically solved to investigate the configuration of the droplet frame near the contact line. As a result, the complicated configuration in the initial stage causes measurement errors on the apparent contact angle.
The effect of surface characteristics on the evaporation time of a single water droplet was studied. The evaporation lifetime curve for a water droplet was measured at a surface temperature up to 500℃. The results showed that the Leidenfrost point (LEP) was not sensitive to surface roughness. It slightly increases with increasing surface roughness of surfaces with arithmetic average surface roughness values of 50∿2000nm. The LFP of a Teflon-coated brass surface was up to 60-70℃ compared with a brass surface. The Leidenfrost film boiling of a water droplet on heated surface which was coated with a diecast releasing agent cannot exist in this experiment. When a water droplet dropped onto a wire netting-set surface, a water droplet evaporated rapidly at even higher temperatures over 400℃.
Experimental study has been performed on evaporation of water droplet on a copper surface. This surface is exposed by the Plasma irradiation to increase the wettability. We measured the relation between the Plasma irradiation and contact angle first, and then the evaporation time, the wetting limit temperature and Leidenfrost point of droplet, increasing the surface temperature. The effect of Plasma irradiation on evaporation curve has been examined. It is found that the evaporation time decreases, and wetting limit temperature and Leidenfrost point increases as the contact angel decreasces.
Plasma irradiation temporarily improves wettability of the metal surface. Experimental study of surface wettability has been performed on three kinds of aluminum surface; normal surface, plasma irradiated surface, plasma irradiated infinitesimal irregularity surface. The relationship between contact angle and irradiation time were examined for these surfaces. The terminal contact angle of these surfaces is about 70°, 3°(superhydrophilic), and about 0°(spreading wetting), respectively. For these three surfaces, we measured change in contact angle with lapsed time after plasma irradiation, and the evaporation time and the wetting limit temperature of water droplet.
The microchannel (height 20μm) with thin film heater was fabricated to observe phase change phenomenon. To investigate of effect of wettability on channel surface, two microchannels that have different wettability wall, were compared using high-speed camera. The test resulted in the phase change phenomena from nuclear boiling to dry-out with different wettability were almost same.
Improving the boiling heat transfer characteristics in a micro-channel vaporizer has become an important issue because of space limitations in a fuel cell vehicle. The process of bubble growth in the micro channel was observed using a high-speed camera to clarify how the surface properties of the heating plate, channel gap and water supply pressure affect heat transfer characteristics and evaporation. The effects of the surface properties of the heating surface and water supply pressure were mainly evaluated.
Pressure drop characteristic and mechanism of gas-liquid two-phase flow in micro channel is experimentally and numerically investigated. In the case of flow rate is relatively small. liquid and gas separate axially and liquid slugs tend to suspend in the channel due to the effect of surface tension force. To make the evaluation of the pressure drop due to the surface tension or viscosity force in relation to the secondly flow near the gas-liquid-solid surface, direct observation of flowing liquid slug shape by using high-speed video camera and direct numerical simulation of the flow behavior in the liquid slug is performed.
Distribution of supercritical CO_2 injected into a packed bed of glass beads containing water is visualized directly by utilizing an MRI technique. After the CO_2 displaced much of the water, some water remained near the central axis, although there were several mm-sized, CO_2-rich channels running through this water-rich region. To better understand the results, we also used the lattice Boltzmann method (LBM), to numerically simulate this system. In a parallel channel with hydrophilic surfaces, a high velocity of water between the bubbles along the flow direction results in a high relative permeability at low water saturations. The effect of capillary contraction along the flow direction was also simulated. The flow through a narrow channel consisted of water and periodic passages of a CO_2 bubble. Channels with smaller cross-sectional areas have faster flow speeds, especially for a CO_2 bubble.
Recently, it is reported that clean effect of electrolyzed alkaline water is utilized in the manufacturing processes of semiconductor and washing machine. The mechanism of this phenomenon has been explained as a hypothetical effect due to an emulsification of oils and fats by OH^- ion and a decrease in surface tension. However, the variations of thermophysical properties of water during electrolysis have never been experimentally observed. The purpose of the present study is to detect the variations of surface tension through the process of electrolysis of water. In order to measure the surface tension during electrolysis, we applied the surface laser-light scattering method. By this method, it is possible to detect the dynamic variations of the surface properties in non-contact manner.
Experimental results are presented that show the effect of the addition of small amount of high-boiling organic liquid (dioctyl phthalate) on the boiling heat transfer from silicon chips mounted on the bottom surface of horizontal duct in which FC-72 is flowing slowly. The chip size was 10×10×0.5 mm. Significant increase in the critical heat flux, about twice as large as that for pure FC-72,was obtained in the range of liquid subcooling of 10 to 20 K.
Improvement of CHF requires that cooling liquid can contact the heating surface, or a high- wettability heating surface, even if a vapor bubble layer is generated on the surface. From this point of view, an experimental study to investigate boiling heat flux and CHF was performed by use of an oxide semiconductor-coated material under a γ ray irradiation environment. The results showed that boiling heat flux and CHF of oxidized titanium were improved after <60>^COγ ray irradiated.
This paper shows a method to inversely solve the forced convection heat transfer problem. The method is based on the adjoint formulation, in which the heat flow of thermal field is reversed in both time and space. Then we can obtain the optimal thermal boundary conditions for heat transfer characteristics. As an application of the numerical solution of the adjoint problem, we show the optimal control of forced convection heat transfer under unsteady thermal boundary conditions in a two-dimensional enclosure.
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