Impingement heat transfer from a circular orifice jet by using latent heat of water mists was studied experimentally. The amounts of mists of about Zauter's mean diameter 14 µm were from 60 to 200 g/h within a range where liquid films were not formed on the target plate and mists were added near the orifice edge. Experiments covered Reynolds numbers from 12,500 to 50,000 and a heat flux is 1,400 W/m2. The experimental results indicate that adding mists had little influence on free jet mean velocity profiles and target plate pressure coefficients. On the other hand, mists had a strong influence on temperature and humidity profiles of a free jet and they also influenced Nusselt number distributions on the target plate. Increases of mists and Reynolds number caused increases in Nusselt number on the developed region. In addition, we investigated influence of the way mists were added and these results showed that Nusselt number was influenced not only by the amounts of mists but also by the adding method. Local Nusselt number profiles with mists were closely related to temperature distributions of the free jet at the location corresponding to the target plate.
In this paper, the effect of ignition area on the propagation of a laminar premixed flame is investigated numerically in a two-dimensional channel. A single-step irreversible overall exothermic chemical reaction is applied to model combustion chemistry. The time-dependent system of governing equations for reacting flows is discretized using the finite volume method (FVM) on the hexahedral structure grid cells. The discretized system of equations is solved by adopting Front Flow Red, a multi-scale and -physics computational fluid dynamics (CFD) solver. The computed results show that the flame oscillates during the propagation owing to the strong roll-up of the vortices generated by the strong shear layer originating from the sudden high gas expansion flow at the large ignition area. The instantaneous acceleration of the vortices increases the flame surface area which gives rise to higher propagation speed; consequently, combustion time is shortened. These results suggest that the rapid increase in flame surface, caused by the large ignition area induced strong vortices, could be one of the potential methods in improving combustion efficiency by reducing the burning time in the internal combustion devices.
In this study, an in-line pre-heating effect on the propagation of a laminar premixed flame in a two-dimensional channel is investigated numerically. An in-line pre-heating thickness of 1mm is generated by imposing uniform external high-temperature along the center line in the prescribed channel. It is found that the shapes of flame and propagation speeds are strongly affected by the pre-heating effect. The distribution of velocity pointed to ahead of the propagating flame (called negative velocity hereafter) is clearly observed which is susceptible to the generation of the pair of vortex in ahead of the flame tip. The evolution of negative flow field in front of premixed flame is successfully captured first ever in a time-dependent manner. The obtained flame propagation accompanied with the aforementioned negative velocity field is not only useful to elucidate the laser induced flame instability reported recently by Tsuchimoto et al. (2007), but also to consider the pure thermal effect on flame evolution, which is hard to obtain experimentally.
This is a study of the leading-edge characteristics of a methane-air triple flame. Few experiment results are available for physical examination of such characteristics, so further experimental investigations are strongly needed to understand the stability mechanism in a mixture with a steep concentration gradient. To this end, we measured concentration profiles at the leading edge of a flame using acetone laser-induced fluorescence (acetone LIF). The results demonstrated that the lifted height of the flame changed when acetone was added to the mixture and correlated well with increased C2 radical behind the flame edge. However, the OH radical luminous intensity, measured with a spectroscope, did not change with addition of acetone. Moreover, the burning velocity obtained by the Bunsen-burner method remained constant when acetone was added to the mixture. Therefore, acetone had little influence on burning intensity. Acetone LIF can thus be employed to measure the local concentration gradient at the leading edge of a flame. The acetone LIF signals could be corrected to consider the thermal effect by using silicone oil vanishing-plane data. From the corrected acetone LIF data, the width between the lean and rich flammability limits (flammability limit width) in the flow upstream of the flame with a steep concentration gradient was clearly observed and could be quantitatively compared with the recent numerical results.
Nanoporous TiO2 photoelectrodes of dye-sensitized solar cells (DSSCs) were fabricated from various sized TiO2 particles and the electrical performance was investigated. We found that the cell that made from double-layered photoelectrode, the first layer composed of 21 nm TiO2 particles, and the second layer composed of 7 nm TiO2 particles, can give high performance nearly equal to well-establish cells that have the photoelectrode combined with a scattering layer. To clarify the effects of radiative transfer on cell performance, numerical simulation of the radiative transfer through TiO2 photoelectrodes was carried out. The results revealed that while the light absorption within the photoelectrode is important for improvement of the cell performance, local light absorption within the photoelectrode is also essential. Higher light absorption in the deep photoelectrode region close to the electrolyte layer effectively improves the cell performance, because in this region the electron carriers can easily transfer the electrons from the counter electrode to the photoelectrode.
Friction stir spot welding (FSSW) is a new metal-joining process, and a numerical simulation code to calculate optimal welding conditions is desired. In this paper, we analyzed temperature distribution and plastic deformation flow during the FSSW process with the fluid flow model and the elastic-plastic deformation model using the particle method. Spot welds are made with a cylindrical pin tool having flat shoulder with a fixed tool rotational speed. Simulation results predict an axisymmetric temperature distribution with the temperature below the tool in the region of 300 °C. The material flow predicted by the elastic-plastic deformation model is similar to experimental results. The model predicts the material flow at the pin periphery is in the upward direction. Near the shoulder, there are two flow patterns observed - beneath the shoulder, the material is pushed downward due to the force acting from the shoulder face, whereas on the shoulder periphery the material flows upward and outward due to extrusion of the material that is caused by the shoulder plunge. This extruded material shows up on the specimen surface as burr.
Investigation of a plant shoot configuration is used to obtain valuable information concerning the received light system. Additionally, analysis results concerning a plant shoot configuration interaction with direct solar radiation were taken from a past study. However, in order to consider a plant shoot as a received sunlight system, it is necessary to understand the received light characteristics of both direct solar radiation and diffused solar radiation. Under a clear sky, the ratio of direct solar radiation to diffused solar radiation is large. However, under a clouded sky, the amount of diffused solar radiation becomes larger. Therefore, in this paper, we investigate the received light characteristics of a plant shoot configuration under the influence of diffused solar radiation. As a result, we clarify the relationship between the amount of diffused solar radiation and the amount of received light as a function of the characteristics of the plant shoot configuration. In order to obtain diffused solar radiation, it is necessary to correspond to the radiation of the multi-directions. In the analysis, the characteristic of the difference in arrangement of the top leaf and the other leaf was obtained. Therefore, in analysis, leaves other than the top were distributed in the wide range.
The surface tension of some high-carbon alcohol aqueous solutions increases as the temperature rises above a certain temperature. There have been attempts to use such special solutions in thermal devices to promote heat transfer. In this study, the authors analyzed the temperature dependence of surface tension of these solutions to investigate this peculiar characteristic in detail. The test fluids were butanol and pentanol aqueous solutions as peculiar solutions, while pure water and ethanol aqueous solution were normal fluids. First, the authors adopted Wilhelmy's method to measure the surface tension during heating, but found that the influence of evaporation of the solution could not be completely eliminated. In this study, the maximum bubble pressure method was employed, which made it possible to isolate the measured solution from ambient air and eliminate the influence of evaporation of the solution. The authors succeeded in measuring the temperature dependence of surface tension, and obtained more reasonable data.
The simulation of the internal flow of a kind of high-pressure swirl injector was carried out with multiphase and cavitation models basing on the movement mesh. The spray was also simulated with the result of the internal flow simulation. The result of spray simulation was compared with experimental results. In addition, the effect on the internal flow which comes from the swirl flutes, the parts for generating the swirl flow, was investigated. It is shown that the amount and configuration of swirl flutes have crucial influence on the swirl velocity and cavitation of the internal flow.
Spark ignition is considered one of the most difficult and complex problems because it involves complicated physical and chemical processes, and it has not yet been explained sufficiently. The minimum ignition energy (MIE) is an important parameter for judging the ignition ability of combustion systems. In the present study, the spark ignition characteristics of a methane-air mixture were investigated by numerical analysis using detailed chemical kinetics consisting of 53 species and 325 elementary reactions. Two different analytical models, with and without electrodes, were applied to research the effect of electrode temperature and energy channel length on flame propagation and the relationship between the MIE and equivalence ratio. The electrode temperature was set as 300 K, 1000 K, and 2000 K for analytical models with electrodes, and the energy channel length was set as 1 mm, 2 mm, and 3 mm for analytical models without electrodes. The obtained computational results showed good agreement with experimental results. We determined that with increasing electrode temperature, the minima of the curve indicating the relationship between the MIE and equivalence ratio move toward the leaner side, that a leaner mixture is more sensitive to heat loss to the cold surrounding gas, and that heat loss to the electrodes is an unignorable factor for the initial formation of the flame kernel.
The objective of this study is to develop a new simulator for laser therapy. In order to minimize undesired effects on the laser therapy, it is important to precisely predict for heat and photon transport inside a biological tissue. In the present study, the photon transport is solved with the Radiation Element Method by Ray Emission Model (REM2), and then coupled with bioheat transfer equation. The dimensionless results become valuable guidance for developing a laser therapy system.