Hydrogen Bubble Method Research Articles

Quantitative flow visualization in the distributor of a plate-fin heat exchanger

The flow distribution and head loss in the distributor of a plate-fin heat exchanger were experimentally investigated using quantitative and qualitative flow visualization methods. The flow distribution and head loss are important parameters related to the thermal performance and economic design of the heat exchangers. The effect of the different distributor angles (30°, 45°, and 60°) and the alignment configuration at the junction in the distributor were studied. The Reynolds number based on the hydraulic diameter was varied from 100 to 1200. Flow visualization using the dye injection and hydrogen bubble methods was applied to capture the qualitative flow structure inside and around the distributor. Digital particle image velocimetry was used to measure the instantaneous velocity fields, and the flow maldistribution was quantified by using the averaged velocity information. The qualitative flow visualization showed the variation of streamwise velocity in the end of the distributor and the existence of the secondary flow at the junction. From the velocity and pressure measurement, we found that the 30° model has the worst flow distribution and the smallest head loss among other models and the 60° model with misaligned configuration has the best flow distribution and the largest head loss.

Separation of carbon fibers in water using microbubbles generated by hydrogen bubble method

Abstract Carbon fibers (CF) are widely used for the reinforcement of plastics, and recycling them using water processes must meet environmental impact requirements. To recover CFs from water with reduced environmental risk, we developed a separation system that consisted of a 125-L cubic container with electrodes to generate microbubbles (MB) by hydrolysis. Microbubbles were adsorbed onto the CFs in the container, and after 30 min of electrolysis, the CFs floated to the surface because of the buoyancy of the adsorbed MBs. The recovery rate was dependent on the CF lengths and a surface treatment using sizing chemicals. Based on our mechanical model and calculating the difference between the MB buoyancy and CF gravity, the flotation efficiency was regulated by the CF length, bundle diameter, and MB adsorption ratio, which supported the experimental results. Using this technology, CFs and glass fibers can be separated in water from their mixtures, and the toxicological and environmental risks can be reduced.

A micro-initiator realized by in-situ synthesis of three-dimensional porous copper azide and its ignition performance

Assembly of micro-initiator has received great attention on its application in micro-electromechanical systems, especially the integration between hazardous energetic materials and micro-ignitor. A micro-initiator was developed by integrating copper azide (Cu(N3)2·CuN3) nanoenergetic materials with a Ni-Cr microbridge realized onto a silicon substrate. In this work, three-dimensional (3-D) porous Cu (PCu) was prepared by electrochemical deposition with hydrogen bubble method. Copper azide has been synthesized through an in-situ reaction of hydrazoic acid (HN3) with three-dimensional (3-D) porous Cu (PCu), which facilitated integration between copper azide and microignitor. The novel materials were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry-differential scanning calorimetry (TG/DTA) and an electric spark tester JGY-50III, respectively. The electrical explosion performances and electrostatic sensitivity of the micro-initiator were investigated by high-speed photography and electrostatic sensitivity tester. SEM images show the additives in the electrolyte alter the morphology of PCu, however, copper azide possesses similar morphology. The effect of copper valence and reaction time on azide reaction were evaluated. The discharge energy at 50% of novel copper azide was 0.098mJ. XRD analyses show the oxidization of PCu and the growth of the reaction time benefit the generation of cupric azide (Cu(N3)2) in the azide reaction. Furthermore, the discharge energy at 50% of novel micro-initiator was 64mJ. The novel material is environmentally friendly and can be successfully ignited by the Ni-Cr microbridge, exhibiting a significant improvement in micro-initiator application and realizing integrated fabrication of micro electro mechanical system (MEMS).

水素気泡法による発生時マイクロバブル挙動の高速度顕微観測

We generated hydrogen MB and oxygen MB from tungsten and stainless wires by electrolysis of water, respectively, and observed MB motion and diameter change nearby electrode using high speed camera with 1000 fps. Generation of MB plume by hydrogen bubble method was related with ion strength of water. MB fusion was strongly induced by MBs with shorter diameter, and occurred nearby these electrodes. Diameters of hydrogen MBs were less than those of oxygen MBs, and these MB diameter would be related with solubility of gases to water and diffusion coefficients in water. To control the MB diameter by hydrogen bubble method, we should check electrode location and ion strength of the water, and MB fusion around electrode should be inhibited.

Cleaning polymer ink from a glass substrate using microbubbles generated by a hydrogen bubble method

To find strategies for improved cleaning with microbubbles (MBs), we used an MB cleaning and microscopic observation system to investigate (a) radius distributions of MBs generated by a hydrogen bubble method, (b) the behavior of MBs adsorbed on a polymer black ink (contact angle to water=60°), and (c) the performance of the MBs in removing the ink from a substrate. The radius distribution of MBs was not affected by changing the cathode diameter or the applied voltage between the anode and cathode. The number of MBs increased when we used thicker cathodes and applied higher voltages. MBs adsorbed onto the ink surface increased in radius during the cleaning process. The rates of increase in adsorbed MB diameters on the ink surface were lesser than those for MBs that were not adsorbed, because of ink movement on the adsorbed MB surfaces. We determined the response of ink weight loss from the substrate due to changes in height from the cathode, applied voltage, and cathode diameter. The ink weight loss increased with decreased height between the ink and cathode and increased applied voltage and cathode diameter. These results suggest that increases in weight loss were strongly influenced by the number of MBs (10–100μm in diameter). MBs adsorbed onto the substrate removed trace amounts of ink from the substrate; after removal of the ink, adsorption of other MBs was induced. The diameter distribution of MBs had a lesser effect on weight loss than the number of MBs generated. Important processes for cleaning the ink are as follows: (1) adsorption of MB onto the surface of the ink, (2) increasing diameter of the adsorbed MB, and (3) removal of the adsorbed MB from the ink surface. We also would like to suggest that the cleaning efficiency for hydrophobic ink could be improved by increasing the number of adsorbed MBs, and MBs must be generated closest to the surface of the ink.

Three-dimensional Visualization of Hydrogen Bubble Method by means of Digital Holography Using Two-wavelength Reconstruction

Three-dimensional Visualization of Hydrogen Bubble Method by means of Digital Holography Using Two-wavelength Reconstruction

G0301 水素気泡法で発生したマイクロバブルによる疎水性インクの洗浄効果(GS3 気泡・界面・マイクロ流)

Microbubble(MB) can be applied for cleaning of hydrophobic compounds on substrates. To characterize interaction between MB and the comounds, we observed surface of casted hydrophobic ink on glass, and surveyed loss of weight after cleaning using MB generating device by hydrogen bubble method and a microscope. The observed images on the ink after cleaning show the decrease in number of the holes generated by adsorption of MBs on the substrate decreased and the increase in diameter of the holes becaseue of increasing diameter of MB as the surface of the ink was distant from the electrode. We found a significant difference between the removals of the ink weights at the distance of 10 mm and 40 mm. These results suggested that the efficiency of cleaning of the hydropobic ink increased by generating MBs with diameter of 10 μm.

Effects of T-S wave amplitude on the control of boundary layer transition bystreaks

The effects of streaks on boundary layer transition depend on the initial amplitude of T-S waves introducedto excite the transition. This problem was studied in a flat-plate boundary layer in water tunnel byusing hydrogen bubble method. Three T-S wave initial amplitudes were tested. The results show thatboth narrow and wide-spacing streaks depress the transition excited by T-S waves with lower initialamplitude. However, when transition is excited by T-S waves of higher initial amplitude, thenarrow-spacing streaks depress the transition, while the wide-spacing streaks promote thetransition. Further the underlying mechanisms were also analyzed.

판형-핀 열교환기 분배면의 유동 가시화 연구

Plate-fin heat exchanger is a kind of compact heat exchangers with a good performance in heat transfer. It is widely used in various engineering fields such as aerospace, chemical and biomedical industries. Quantitative and qualitative flow visualization study were performed using the water model of commercial plate-fin heat exchanger with header angles of <TEX>$30^{\circ}$</TEX>. The Reynolds number was 100. Conventional digital particle image velocimetry was used to measure the instantaneous velocity fields of the header region and the flow visualization using dye injection and hydrogen bubble method were applied to capture the qualitative flow characteristics. The results showed the existence of separation flow region at the junction area and the bottom wall of the exit region.

E112 管内脈動流の不安定性 : 脈動周波数および振幅が流れに及ぼす影響(OS-18: 熱・物質移動の計測および現象解析(1))

This study deals with flow behavior and associated heat transfer in a pulsating duct flow. Among the several parameters that govern the transport phenomena of pulsating flow, the effects of pulsating frequency and amplitude on flow instabilities in the rectangular duct is mainly discussed in this paper. Mechanisms of the interaction between these two kinds of governing parameter's effects on flow behavior are not cleared yet. Hydrogen bubble method was employed for the qualitative understanding of the flow structure with periodical velocity fluctuation. Results showed that heat transfer enhancement in the central and downstream region was attributed to the restraint of the boundary-layer development induced by the periodical temporal velocity fluctuation not to the large-scale fluid mixing such as turbulence developed from small-scale flow instabilities occurred in the upstream region.

Flow visualization of vortex structure in a pulsed rectangular jet

Pulsating jet is visualized using hydrogen bubble method to clarify the vortex nature in the near field of the jet. This study focused on the development in space and time of vortex structures evolution in low aspect-ratio rectangular jet with pulsation. Pulsation means large-amplitude, low-frequency excitation which is expected to increase the mixing and spreading of the jet and to accelerate its transition from a rectangular form to an axisymmetric form. It was deemed appropriate to investigate whether jet characteristics of a pulsating, submerged jet flow can be altered by including pulsations. The difference of the vortex deformation process is discussed in relation to pulsating conditions. Consequently, the pulsation leads to the formation of vortices at regular intervals, which are larger than those occurring in a steady jet. The results show that the streamwise interaction, between leading vortex and trailing vortex rolled up at nozzle lips, strengthens with increasing pulsating frequency. The spanwise drift of the vortex becomes strongly apparent at large amplitude and high frequency conditions. The drifting start position does not change regardless of pulsating condition. The convection velocity of vortex increases at lower frequency and larger amplitude.

Flow Measurements in a Suction Sump by UVP

In the present study, to reveal the air entrainment mechanism into a suction pipe in a suction sump, the authors conduct flow-velocity measurements by UDM (Ultrasonic Doppler Method). Here, we consider the simplest geometry as a suction sump, that is, a straight channel with rectangle-cross section and a simple suction pipe near the end of the channel. Ultrasonic transducers are fixed outside the side, bottom and back walls with right/near-right angles and, we get three-dimensional time-mean velocity distributions and equi-vorticity contours. At first, measurement accuracy is checked by comparing velocity profiles by UDM with hydrogen bubble method. As a result, the authors show typical flow fields in the sump, and show the relation between flow pattern and air entrainment. Especially, we compare two cases where the air entrainment is often observed.

Flow Measurements in a Suction Sump by UVP

In the present study, to reveal the air entrainment mechanism into a suction pipe in a suction sump, the authors conduct flow-velocity measurements by UDM (Ultrasonic Doppler Method). Here, we consider the simplest geometry as a suction sump, that is, a straight channel with rectangle-cross section and a simple suction pipe near the end of the channel. Ultrasonic transducers are fixed outside the side, bottom and back walls with right/near-right angles and, we get three-dimensional time-mean velocity distributions and equi-vorticity contours. At first, measurement accuracy is checked by comparing velocity profiles by UDM with hydrogen bubble method. As a result, the authors show typical flow fields in the sump, and show the relation between flow pattern and air entrainment. Especially, we compare two cases where the air entrainment is often observed.

901 回転円柱まわりの流れ場に関する研究 : 円柱回転速度が一様でない場合(2)(OS9-1 超音波を利用した流動場の計測(流れ場計測),OS9超音波を利用した流動場の計測,オーガナイズドセッション)

The present study is designed to experimentally investigate the flow field past a circular cylinder rotating uniformly and oscillating rotationally in a uniform stream, and identify the wake vortex structures by varying the rotational speed ratio and oscillating frequency. Considering the Reynolds number dependency on flow structures, in this study, experiments were conducted with Re_d = 800, which is larger than a critical value. In order to observe and identify the flow patterns, the flow fields were visualized by the hydrogen-bubble method, and then the flow velocity pattern visualization was applied by using an ultrasonic velocity profile (UVP) monitor. Variation in the wake vortex structure according to the change of the oscillating frequency and the rotational speed ratio was examined.

Vortex Behavior of Pulsating Jets from a Rectangular Nozzle

This paper describes an experimental study on the vortex formation of a pulsating jet issuing from a rectangular nozzle of an aspect ratio of 5 which has a short parallel section upstream of the jet exit. Visualization with hydrogen bubble method reveals three-dimensional vortex nature. It was confirmed that vortices which originate from short sides of the nozzle flow downstream faster than vortices from long sides and shift into the jet axis. On the other hand the vortices from the long sides move away from the jet axis and downstream bend at their middle with the edges of the vortices going ahead. The three-dimensional vortex deformation that the major and the minor axis of the vortex ring are switched was confirmed further downstream.

VORTEX BEHAVIOR OF PULSATING JETS FROM A RECTANGULAR NOZZLE(Flow Control 2)

This paper describes an experimental study on the vortex formation of a pulsating jet issuing from a rectangular nozzle of an aspect ratio of 5 which has a short parallel section upstream of the jet exit. Visualization with hydrogen bubble method reveals three-dimensional vortex nature. It was confirmed that vortices which originate from short sides of the nozzle flow downstream faster than vortices from long sides and shift into the center axis of the jet. On the other hand the vortices from the long sides move away from the jet axis and downstream bend at their middle with the edges of the vortices going ahead. The three-dimensional vortex deformation that the major and the minor axis are switched was confirmed further downstream.

A111 脈動噴流に関する研究

The development of a large vortex structure in fluid flow is responsible for some of the most fascinating aspects of fluid dynamics, such as mixing, transport, and instability. This paper describes an experimental study on the vortex nature of a pulsating jet issuing from a rectangular nozzle of an aspect ratio of 5 which has short parallel section upstream of the jet exit. The visualized images by hydrogen bubble method revealed the influence of the bounded plates upon the vortex nature.

Visualization of three-dimensional flow structures in the wake of an inclined circular cylinder

In the previous measurements of the aerodynamic sound generated from an inclined circular cylinder, it is reported that the sound pressure level (SPL) changes with the aspect ratio and the inclined angle. Therefore, we have investigated the changes in the vortex structure of the wake considered as one of the causes of the SPL variation. Using the low-noise wind tunnel, the velocity fluctuation in the wake is measured to obtain the correlation length. Moreover, the flow visualization is performed with a hydrogen bubble method and a numerical analysis method in order to clarify how the wake structure changes by variations of aspect ratio and inclined angle. As a result of this investigation, it is shown that the spanwise structure of Karman's vortex is highly influenced by the interference of Karman's vortex with the bottom endplate, and that the influence on the spanwise structure in the wake becomes greater as the aspect ratio decreases and the inclined angle increases.

The sweep down flow and “contra-hairpin vortex” in a turbulent boundary layer

The sweep down flow and related flow structures in a turbulent boundary layer were visualized with hydrogen bubble method. The streamwise and transverse vortices induced by sweep down flow near the wall of a vessel were observed. A new kind of vortical structure, whose features are contrary to those of the hairpin vortex, was observed in the present experiments. It is termed “contrahairpin vortex” here. Its head points upstream, while the head of a hairpin vortex points downstream; lift-up flow is induced between the legs of a hairpin vortex, while between the legs of this new vortical structure, sweep down flow is induced.

流体力学における導入教育(OS.10 技術教育・工学教育)

In our laboratory, manufacture and flight tests of PET bottle rocket in the first semester of 2nd grade have been carrying out as introductive education of fluid engineering. The objective of this paper is to investigate a stall phenomenon arising in the PET bottle rocket at the optimum launching angle, which is calculated computationally. The separated vortex patterns over the PET bottle rocket at the attack incidence range 45 to 70 deg have been visualized by hydrogen bubble method and tungsten halogen lamp in the improved water channel experiment.