Yuki Kameya

Three-dimensional flow characterization of a square array of multiple circular impinging jets using stereoscopic PIV and heat transfer relation

AbstractImpinging jets are one of the most industrially essential methods of cooling, for example, the cooling of gas turbine blades and electronic devices, drying or annealing of glasses. Usually, jets are configured for a specific purpose, but the flow tends to be very complicated as each jet interacts with the others, especially near the impingement surface. In the present study, complicated three-dimensional flow from a square array of circular impinging jets was revealed experimentally to investigate the effect on heat transfer characteristics. The flow fiel d was measured by scanning stereoscopic particle image velocimetry to confirm the detailed spatial features of the multiple circular impinging jets as nozzle-to-surface distance and jet-to-jet spacing were changed. Adjacent jets generated vortex rings and roll-up toward the nozzle plate, vortex rings and roll-up sizes changing depending on the experimental parameters. Differences in vorticity and dispersions of velocity from the jets were also observed. The temperature field of the impingement surface was measured using a thermosensitive liquid crystal technique. The spatial distribution of heat transfer coefficient was related to the flow field near the impingement surface.Graphical abstract

Kinetic Monte Carlo simulation of nanoparticle film formation via nanocolloid drying

A kinetic Monte Carlo simulation of nanoparticle film formation via nanocolloid drying is presented. The proposed two-dimensional model addresses the dynamics of nanoparticles in the vertical plane of a drying nanocolloid film. The gas–liquid interface movement due to solvent evaporation was controlled by a time-dependent chemical potential, and the resultant particle dynamics including Brownian diffusion and aggregate growth were calculated. Simulations were performed at various Peclet numbers defined based on the rate ratio of solvent evaporation and nanoparticle diffusion. At high Peclet numbers, nanoparticles accumulated at the top layer of the liquid film and eventually formed a skin layer, causing the formation of a particulate film with a densely packed structure. At low Peclet numbers, enhanced particle diffusion led to significant particle aggregation in the bulk colloid, and the resulting film structure became highly porous. The simulated results showed some typical characteristics of a drying nanocolloid that had been reported experimentally. Finally, the potential of the model as well as the remaining challenges are discussed.

Three-Dimensional Measurement of Near-Wall Velocity in Millimeter Channel by a Single View Imaging

It is well known that there is a strong correlation between heat transfer and near-wall flow. It is important to obtain the detailed near-wall flow field, but it has a lot of difficulties to measure near-wall region by traditional approaches for example hot wire anemometry and particle image velocimetry (PIV). The purpose of this study is to determine the three-dimensional velocity field at near-wall area in micron resolution by the astigmatism particle tracking velocimetry (APTV). In this study, an estimation of depth location of tracer particles by applying a specialized imaging optics controlling the astigmatism [1] was employed. We have developed a measurement system to get the particle location within 15 μm from wall using a long-working-distance microscope with astigmatic optics. As a proof-of-concept, near-wall velocity field in a millimeter-ordered parallel plate channel was measured with low Reynolds numbers (Re = 1 ∼ 5) Poiseuille flow to confirm the validity of it. As a result, we can obtain the near-wall velocity within 15 μm from the wall precisely. From the velocity distribution, the standard deviation of the velocity at each location was calculated and the dispersion of velocity was evaluated. As a result, it was confirmed that the measurement was carried out more accurately in high-speed area. Comparison of the measured velocity distribution with a theoretical calculation and micro-PIV results were also done. From these velocity distributions, the wall shear stress on the wall was determined.Copyright

Spectral Control of Thermal Radiation Using Micro-Structured Emitter-Surface for Thermophotovoltaic Generation of Electricity

Spectral control of thermal radiation emitted from micro-cavities made on a mirror-like-polished Ni metal surface was investigated through numerical simulation and experiment. In simulation, thermal radiation from solid surface was dealt as hemispherical emission from point sources, and Maxwell’s equations were solved using CIP (Cubic Interpolated Propagation) method. It was demonstrated that the emittance could be increased around the wavelength corresponding to the standard mode of cavity resonance, while the emittance at wavelengths corresponding to the higher modes was much the same as that of smooth flat surface. Furthermore, in experiment using rectangular micro-cavities (0.5×0.5×0.5μm3 ) made periodically on Ni smooth surface, spectral emittance was measured in the near-infrared region. The experimental results disclosed that the emissive power only in the range of shorter wavelength than 1.2μm was increased by the micro-cavities that played a role of a wave guide to produce cutoff effect clearly.Copyright

Hydrogen-rich gasification of biomass using porous catalyst

Publisher Summary This chapter examines the gasification of cellulose and lignin that are the main components in woody biomass through a constant heating-rate experiment and a constant temperature experiment, using a sponge-like porous Ni catalyst instead of the catalytic fluidized bed. The gasification reactor consists of a cylindrical electric heater and a stainless tube. In the tubular reactor, a sponge-like porous Ni catalyst is installed around the downstream end of the cylindrical heater. For the constant heating-rate experiment, the powder (cellulose and lignin) is put on the porous catalyst beforehand and the reactor is heated with a heating-rate of 60K/min; then the temperature rises from 20°C to over 1000°C. On the other hand, for the constant temperature experiment, the catalyst temperature is kept at constant and then the powder is promptly put on the catalyst surface. By controlling the mole number of oxygen adsorbed on the Ni catalyst surface, almost all of carbon in the cellulose is gasified, which indicates a high possibility of the completely char/tar free gasification. For the lignin with a high carbon/oxygen ratio, much amount of oxygen through gasifying agents or oxygen adsorbed on the catalyst surface is required for the char/tar free gasification.

A Combined Type of a Flow Control Actuator Composed of the Synthetic Jet and Vortex Generator

The purpose of the present paper is to study the flow structure induced by a combined type of actuator system which is composed of a synthetic jet and fixed vortex generator. The synthetic jet promotes the flow mixing and prevents flow separation, because it introduces a periodic disturbance which is more effective for the flow separation control at low range of the Reynolds number. The negative point is that the jet is not located near the wall, but away from the wall. The fixed vortex generator provides with a longtitudinal vortex structure with a common flow down motion. The vortex structure downstream of the combined actuator was measured by a scanning stereo-scopic particle image velocimetry(SSPIV). An experiment was conducted in the two-dimensional channel flow with the aspect ratio of 12 and channel height of 10 mm where Reynolds number is 1300. Orifice diameter of synthetic jet is 1.0 mm and Stokes number based on the operation condition is 6.3-12.7. Nondimesional stroke length is 2.7-16. Then, it is confirmed from the experiement that strong interaction between the two vortices introduces an effective vortex structure for flow control of the near-wall flow.