Visual computing is an advanced simulation in which the intermediate computed results are visualized during the computations. In the present paper, two visual computing systems aiming for the practical research use were developed for the fluid dynamics simulations. One is a system for rather small scale simulations, i. e. two-dimensional computations or scheme developments, and runs on a single graphics workstation. The other is a system for small to large scale simulations, i.e. three-dimensional computations, and runs on a system of one graphics workstation and one CPU server. Both systems are machine-independent. Several examples including the elapsed time measurements of the simulation, the graphics and the data transfer show various use of the visual computing, the achievement of the designed features of the present systems and the advantage of the visual computing to the "postprocess" visualization.
In the present work two-dimensional viscous flows through compressor and gas turbine blade cascades at transonic speed are analyzed by solving compressible N-S equations in the generalized co-ordinate system, so that sufficient number of grid points could be distributed in the boundary layer and wake regions. An efficient Implicit Approximate Factorization (IAF) finite difference scheme, originally developed by Beam-Warming, is used together with a fourth order Total Variation Diminishing (TVD) scheme based on the MUSCL-type approach with the Roe's approximate Riemann solver for shock capturing. In order to predict the boundary layer turbulence characteristics, shock boundary layer interaction, transition from laminar to turbulent flow, etc. with sufficient accuracy, an improved low Reynolds number κ-ε turbulence model developed by the authors is used. In this κ-ε model, the low Reynolds number damping factors are defined as a function of turbulence Reynolds number which is only a rather general indicator of the degree of turbulence activity at any location in the flow rather than a specific function of the location itself. The emphasis in this paper is on the modeling of turbulence phenomena and the effect of grid topology on results of computations. Computations are carried out for different flow conditions of compressor and gas turbine blade cascades for which detailed and reliable information about shock location, shock losses, viscous losses, blade surface pressure distribution and overall performance are available. Comparison of computed results with the experimental data showed a very good agreement. The results demonstrated that the Navier-Stokes approach using the present κ-ε turbulence model and fourth order TVD scheme would lead to improved prediction of viscous flow phenomena in turbomachinery cascades.
Complex geometry and very small working space within a modern high speed impeller passage impose severe constraints on any experimental technique to be used for detailed measurement. In this study, using a dual focus laser velocimeter, low energy regions were located in the impeller passage of a small high speed commercial centrifugal compressor passage at three rotational speeds, one close to 100000 rev/min under 'design' and 'off design' operating conditions. It was deduced that impeller rotation and mass flow rate significantly altered the flow pattern. Higher speed and lower mass flow rate enhanced the process of flow separation resulting in noticeable movement of wake regions to different locations within the impeller passage. Interestingly, however, the overall wake behaviour pattern of high speed impeller flow was quite similar to those observed previously in low speed impeller passage. This finding may have positive implications for future design.
To realize micro maintenance robots for small diameter pipes of nuclear reactors and so on, high power in-pipe mobile micromachines have been required. The authors have proposed the bellows microactuator using fluid power and have tried to apply the actuators to in-pipe mobile micromachines. In the previous papers, some inchworm mobile machine prototypes with 25 mm in diameter are fabricated and the traveling performances are experimentally investigated. In this paper, to miniaturize the in-pipe mobile machine and to make it adaptable to pipe diameters, firstly, a simple rubber-tube actuator constrained with a coil-spring is proposed and the static characteristics are investigated. Secondly, a supporting mechanism which utilizes a toggle mechanism and is adaptable to pipe diameters is proposed and the supporting forces are investigated. Finally, an in-pipe mobile micromachine for pipes with 4 - 5 mm in diameter is fabricated and the maximum traveling velocity of 7 mm/s in both ahead and astern movements is experimentally verified.
The flow and optical fields of a Q-switched supersonic flow chemical oxygen-iodine laser are simulated by solving simultaneously the gas flow model coupled with the precise chemical kinetic model and the geometric optical model. The effects of hyperfine relaxation and velocity cross relaxation for iodine atoms and nonuniformity of the three-dimensional flow field on power extraction are investigated. The results show that the peak power under the influence of hyperfine relaxation and velocity cross relaxation, which is normalized by the corresponding continuous wave value, is 15. This value agrees fairly well with previously reported experimental results of Q-switched chemical oxygen-iodine laser operations. The peak power of pulse in the three-dimensional flow field is considerably small compared to that in the equivalent one-dimensional flow field and the pulse is widened roughly twice due to insufficient mixing, shock waves, expansion waves and wakes.
An experimental study was carried out to verify the feasibility of a quantitative analysis of fuel droplet diameter at high pressures and temperatures in combustion systems. A fueling system was developed to atomize fuel into droplets of the same diameter. Using this system, fueling was made into a visualized device where high pressure and temperature nitrogen flowed under steady-state conditions. The laser induced fluorescence method was utilized to determine n-dodecane fuel and iso-octanefuel droplet behavior and the phase Doppler particle analyzer was used to measure the diameter and velocity of the droplets. As a result, it was confirmed that a quantitative analysis of fuel droplets is possible. The results indicate that due to its lower boiling point, iso-octane droplets tend to evaporate much faster the n-dodecane droplets.
This paper has dealt with the heat transfer characteristics of a liquid-liquid direct contact operation in which a Perfluorocarbon (PFC) liquid was released in a hot water stream, which was a low-grade heat source such as urban sewage, for the purpose of heat recovery from it. The paper reported on a set of experiments in which a PFC liquid (1800kg/m3 at 20°C) was continuously injected from a downward-facing single nozzle into a slow and upward flow of hot water and it was disintegrated into a lot of droplets and then they were heated from the ascending hot water. The experimental results revealed that the size distribution and flow behavior of the droplet ensembles exerted an influence on the coefficient of overall heat transfer between the droplet ensembles and hot water and also the temperature effectiveness for the droplet ensembles.
The two-dimensional characteristics of the boiling two-phase flow of liquid helium in a duct are numerically investigated to realize the further development and high performance of new cryogenic engineering applications. First, the governing equations of the boiling two-phase flow of liquid helium based on the unsteady drift-flux model are presented and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two-dimensional structure of the boiling two-phase flow of liquid helium is shown in detail, and it is found that the vapor gas phase rapidly spreads throughout the inner flow duct because of the change of the pressure gradient due to the effect of superfluidity which appears in momentum equations. Next, it is clarified that the distributions of the void fraction, the velocity field and the instantaneous streamline of two-phase superfluid helium flow show a tendency different from those of normal fluid helium because the counterflow of two-phase superfluid occurs against normal fluid flow. According to these theoretical results, the fundamental characteristics of the cryogenic boiling two-phase flow are predicted. The numerical results obtained would contribute to advanced cryogenic industrial applications.
In this paper we describe the analysis of the effects of the specific heat of a regenerator material on the refrigeration capacity of a 4K-GM (Gifford-McMahon) refrigerator. The findings revealed by the analysis are given below. When the specific heat of the regenerator material is changed from 1.5 J/(K·cm3) to 0.01 J/(K·cm3), the no-load temperature is changed from 2.11 K to 26.9 K. The lowest temperature that can be theoretically attained in this refrigerator is the temperature around the λ point of helium. It is sufficient for the regenerator material to provide a specific heat of 0.8 J/(K·cm3). The refrigeration capacity is found to be superior over a wider temperature range when the specific heat distribution is uniform than when it has a peak. When different kinds of regenerator materials are stacked, the material stacked at the cold part of the regenerator may have a low specific heat at high temperatures, but the material stacked at the hot part of the regenerator should have a high specific heat at low temperatures.
Engine performance and exhaust gas characteristics of a direct injection CI engine were investigated with neat dimethyl-ether (DME), and alternative fuel being considered for attaining low emissions. The study was conducted by comparing the new fuel with the conventional diesel fuel. Results from the present work include : (1) It was necessary to add a small amount of lubricating additives to DME in order to achieve satisfactory engine operation, to minimize excessive wear in the injection nozzle. (2) Engine performance for both fuels was mutually comparable. (3) Although negligible amounts of soot and very low THC emissions were measured, NOx emission was about the same as in diesel fuel operation. (4) The reduction of NOx emission by retarding the injection timing was highly significant with DME. (5) The NOx emission form the retrofitted engine could be further reduced by dissolving CO2 in DME.
The paper discusses the methodology of predicting combustion and NO formation in large diesel engines from scale model experiments, together with the experimental results compared with the theoretical predictions. Combustion similarity means that flow patterns and flame distribution develop similarly in differently sized engines, so allowing heat release rates and thermal efficiency to be predicted from scale model experiments. One of the objectives of this research is to validate the establishment of the combustion similarity by experiments. Additionally, an algorithm to predict NO emissions is also established in this paper, based on the combustion similarity and unique characteristics of NO formation. An experimental data was obtained with two sets of similar engines ranging from 85 to 125 and from 600 to 800 mm bore. The results showed good agreement with the theoretical predictions, indicating possibility of the scale model experiment for diesel engine combustion.
A direct numerical simulation of a two-dimensional jet diffusion flame developed in a co-flowing air stream was carried out using the GRI [Gas Research Institute] chemical reaction mechanism (NOx included) in order to elucidate the mechanism of NOx formation in turbulent flow fields. The governing equations were discretized and numerically integrated using the finite volume method. The temperature dependence of thermodynamical properties was taken into account ; transport properties were calculated according to the simplified transport model proposed by Smooke [Reduced Kinetic Mechanisms and Asymptotic Approximations for Methane-Air Flames, (1991), p. 1-28, Springer-Verlag]. Fuel jet velocity was found to have the same effects on the flame structure and the unsteady behavior as those concluded from the flame sheet approach ; by increasing the fuel jet velocity, the flow becomes turbulent with large fluctuations downstream, and the flame is disturbed by the enlarged fluctuations in the turbulent region downstream where local extinction takes place. From a comparison with the laminar counterflow diffusion flame, it was concluded that the hypothesis of the laminar flamelet model that "the instantaneous local structure of the turbulent flow field can be accurately simulated by the laminar diffusion flame", can be applied to the unsteady turbulent jet flame that includes the possibility of extinction and takes into account the NOx formation process. As a result, it was verified that the formation of NOx in the turbulent flow field has the same mechanism as that in the unsteady laminar diffusion flame even for the case of extinction.
A reconstruction method of temperature distribution in a 2-D absorbing-emitting system has been established based on radiant image processing. In this method, the radiant energy received by the image-formation elements of a CCD camera is dominated by the radiant energy emitted from the gas medium and the wall surface inside the system. The values of the ratio of the radiant energy received by the elements of the CCD camera to that emitted from every gas and wall element have been calculated using Monte Carlo method. Four CCD cameras are used to capture radiant energy images and the reconstruction problem of the temperature distribution is converted to a linear programming problem and solved through using an interior point method. After introducing random errors into the CCD's energy distribution, the temperature distribution of the system has been reconstructed with acceptable precision, which showed a great potential for application.