Takeshi Yokomori

Characteristics of rare earth (RE = Eu, Tb, Tm)-doped Y2O3 phosphors for thermometry

The temperature-dependent photoluminescences of Y(2)O(3) : Eu (6% Eu), Y(2)O(3) : Tb (4% Tb) and Y(2)O(3) : Tm (1% Tm) were investigated for high-temperature phosphor thermometry. Two different phases, the monoclinic phase and cubic phase, were considered because the fluorescence spectra vary with the phase. To employ the intensity ratio method, we investigated their photoluminescence spectra under the excitation light of an Hg-Xe lamp as the temperature was elevated from room temperature to more than 1200 K. As a result, it was confirmed that the luminescence intensity of all of the phosphors varied with elevating temperature, i.e. thermal quenching, with the variations depending on the type of rare earth impurity and their phases. The results indicate that Y(2)O(3) : Eu phosphors are applicable to the intensity ratio method because they show appropriate variations in the intensity ratio of two emission lines, and they also have strong and sharp peak intensities without excessive optical noise or black body radiation over a wide range of temperatures. The intensity ratios for Y(2)O(3) : Tb provide such small variations with temperature that the temperature resolution is low, despite the strong emission intensities. As for Y(2) O(3) :Tm, the intensity ratios also have a low temperature resolution and their emission intensities are weak. Therefore, Y(2)O(3) : Tb and Y(2)O(3) : Tm are not appropriate for the intensity ratio method for phosphor thermometry.

Co-doped Y2O3:Tb3+/Tm3+ multicolor emitting phosphors for thermometry.

In this study, the new temperature-dependent phosphor, Y2O3:Tb3+/Tm3+, was investigated for high-temperature thermometry. The photoluminescence intensity at 456 nm emitted from Tm3+ was strong at temperatures higher than 1100 K, whereas the peak intensities emitted from Tb3+ decreased due to the thermal quenching effect. Thus, the intensity ratio between those emissions showed an appropriate variation for thermometry over a wide temperature range. In addition, the phosphors showed a distinct change of visible emission colors from green to blue with increasing temperature. These findings suggest the applicability of these phosphors in visual thermo-sensors.

Extinction characteristics of premixed flame in heated microchannel at reduced pressures

The effects of pressure, flow velocity and mixture composition on extinction limits of premixed methane/air flames in a mesoscale channel (2 mm) with a temperature gradient were studied both experimentally and numerically at low pressure conditions (0.2–0.05 atm). An external heat source was employed to form a steady temperature gradient along the channel for simulating the state of the heat recirculation. At low pressure conditions, combustion in a small-scale channel can be simulated since flame becomes thicker with the decrease of pressure. Both experimental and numerical results showed that maximum upper blow-off limits were found to be at the fuel-leaner side under low pressure conditions. That is, the flammable regions under low pressure conditions showed tendencies asymmetric to the stoichiometric mixture ratio, while the conventional flammable region at atmospheric pressure is symmetric to it. Furthermore, computational results implied the existence of a lower extinction limit of stable flame although heat loss was compensated for by the external heater.

Interaction of adjacent flame surfaces on the formation of wrinkling laminar premixed flame

To gain further understanding of the flamelet regime of turbulent combustion, the interaction betweenadjacent flame surfaces of a wrinkling laminar premixed flame has been investigated experimentally. The wrinkling flame was formed with three adjacent laminar V-shaped flames stabilized on a multiple-slitburner of a propane/air mixture. In addition, a single V-shaped flame was also formed on the burner. We compared flame shapes and flow velocity distributions between those two types of flames. The results show that the unburned flow field was expanded outwardly by the convex flame surface toward unburned side. In the case of the wrinkling flame, however, the outward deviation of the flow field caused by the existence of the inner convex was prevented by the existence of the surrounding flames. Consequently, the inner convex flame surface of the wrinkling flame is more oblique, and the curvature is sharper than that of a single convex flame (single V-shaped flame) due to the interaction of adjacent flame surfaces. Moreover, the formation of the inner convex flame of the wrinkling flame is more unstable than that of a single convex flame. This is due to the curvature of the inner convex of the wrinkling flame being sharper than that of the single convex flame so that the burning velocity is decreased as a result of the curvature effect.

Micro particle image velocimetry investigation of near-wall behaviors of tumble enhanced flow in an internal combustion engine

A micro particle image velocimetry has been performed to investigate tumble enhanced flow characteristics near piston top surface of a motored internal combustion engine for three inlet valve open timing (−30, −15, 0 crank angle degrees). Particle image velocimetry was conducted at 340, 350 and 360 crank angle degrees of the end of the compression stroke at the constant motored speed of 2000 r/min. The measurement region was 3.2 mm × 1.5 mm on the piston top including central axis of the cylinder. The spatial resolution of particle image velocimetry in the wall-normal direction was 75 µm and the vector spacing was 37.5 µm. The first velocity vector is located about 60 µm from the piston top surface. The micro particle image velocimetry measurements revealed that the ensemble-averaged flow near the piston top is not close to the turbulent boundary layer and rather has tendency of the Blasius theorem, whereas fluctuation root-mean-square velocity near the wall is not low. This result shows that revision of a wall heat transfer model based on an assumption of the proper characteristics of flow field near the piston top is required for more accurate prediction of heat flux in gasoline engines.

EXPERIMENTAL STUDY OF THERMO-ACOUSTIC INSTABILITY TRIGGERING IN A STAGED LIQUID FUEL COMBUSTOR USING HIGH-SPEED OH-PLIF

A staged injector developed by JAXA and fueled with kerosene is studied in a high-pressure combustion experiment. With a stable pilot fuel flow rate, the fuel flow rate in the main stage is progressively increased. A high-speed OH-PLIF system is used to record the flame motion at 10,000 fps. In the beginning of the recording, the flame behavior is dominated by relatively low-frequency rotation due to the swirling motion of the flow. These rotational motions then coexist with a thermo-acoustic instability around 475 Hz which increases the amplitude of the pressure fluctuations inside the chamber. DMD analyses indicate that this instability is associated with a widening of the flame occurring when the pressure fluctuations are the highest, giving the instability a positive feedback. The instability frequency then abruptly switches to 500 Hz while the mode shape remains

Numerical Investigation on Hydrogen Permeation with Flat Sheet Pd/Ag Membrane for Various Geometries

The variation in hydrogen concentration distribution on Pd/Ag membrane surface for permeation with H2/N2 mixture was investigated numerically, by varying the diameter of the mixture inlet. A 77wt.%Pd/23wt.%Ag purification membrane with thickness of 25μm and effective surface area of 3.14x10-4m2 was considered. The membrane temperature, total upstream pressure, and inlet H2 mole fraction were set to 623K, 0.25MPa and 0.75, respectively. Meanwhile, the downstream pressure was set at atmospheric. Mean mole flux (feed flow rate/effective membrane surface area) was varied between 0.19-0.95 mol.s-1.m-2. The hydrogen concentration distribution was investigated for various ratios of inlet radius to membrane radius (0.22:1, 0.50:1 and 0.75:1). It is found that hydrogen permeation flux obtained from various hydrogen concentration distributions are almost same, and quantitatively fit with analytical result based on the Sieverts’ equation with the effect of H2 permeation.

Characterization of Flat Sheet Pd/Ag Membrane Using Pure Hydrogen

The permeation characteristic of a flat sheet Pd/Ag membrane was investigated experimentally by using pure hydrogen. A 77wt.%Pd/23wt.%Ag membrane with 25μm thickness and 0.02m diameter was used. The hydrogen permeation mole flux was investigated under various reference membrane temperatures (423-723K), for upstream pressures of 0.21-0.29MPa, and feed flow rate of 1.39x10-4mol/s. In all cases, the downstream pressure was set at atmospheric. Experimental results demonstrate the linear relationship between the hydrogen permeation flux and difference in square root of hydrogen partial pressure between the upstream side and downstream side, regardless of reference temperatures. This indicates the compatibility of the Pd/Ag membrane used in the present study with Sieverts’ Law. At sufficiently low temperature level, the hydrogen permeation flux is found to be very sensitive with the changes in the temperature.

Initial Study on Hydrogen Permeation with Flat Sheet Pd/Ag Membrane for Mixture of Hydrogen and Carbon Dioxide

The hydrogen permeation with a flat sheet Pd/Ag membrane for mixture of hydrogen and carbon dioxide was investigated experimentally. A flat sheet, dense 77wt.%Pd/23wt.%Ag purification membrane with thickness of 25μm and diameter of 0.02m was used. The membrane temperature, total upstream pressure and inlet H2 mole fraction were set to 623K, 0.25MPa and 0.75, respectively. Meanwhile, the downstream pressure was set at atmospheric. Mean mole flux (feed flow rate/effective membrane surface area) was varied between 0.089-0.62mol.s-1.m-2. When the mean mole flux is increased, hydrogen permeation flux increases, which is supposed due to the effect of flow velocity. In addition, the analytical result based on Sieverts’ equation with the effect of permeation is found to be very close with the experimentally obtained hydrogen permeation flux. For permeation with H2:CO2 mixture, future work is necessary to investigate the variation in permeation flux with time.

Two-dimensional non-contact temperature measurement of high temperature air jet using phosphor thermometry

The two-dimensional non-contact temperature measurement of high temperature air jet was demonstrated by using the phosphor thermometry based on the two color intensity ratio method. The temperature could be measured up to 1000 K.