Small Rectangular Channels Research Articles

11007 周方向非均一濡れ性を用いたプラッギングフリー流路の開発

If the flow channel in the PEFC separator is plugged with the liquid product water, pressure loss increases and the flow is destabilized to cause serious deterioration of the power generation performance. It may be probable that occurrence of the channel plugging can be avoided if the surface wettability of the channel wall is circumferentially non-uniform. In this work, systematic pressure drop measurements were carried out for the gas-liquid two-phase flow in small rectangular channels of various wettability distributions. It was found that the fluctuation of pressure loss that is frequently encountered in uniform wettability channels can dramatically be mitigated by the use of the flow channel of circumferentially non-uniform wettability.

Flow pattern map and time–frequency spectrum characteristics of nitrogen–water two-phase flow in small vertical upward noncircular channels

We experimentally investigate the vertical upward gas–liquid two-phase flow in a 2×0.81mm small rectangular channel and an equilateral triangle channel with a 2mm side length. The two channels have same hydraulic diameters (Dh=1.15mm). We first present an experimental flow pattern map with nitrogen and water superficial velocities ranging from 0.08m/s to 11.82m/s and 0.12m/s to 1.52m/s, respectively. We also compare the flow pattern transition criteria between the triangle and rectangular cross sections by using the same hydraulic radius. We employ the influence rule that small passage geometry limits the flow pattern transition criteria. Thereafter, we comparatively analyze the transition boundaries of experiment flow patterns with other results in literature and classical prediction models. Results show that the cross-sectional shapes and experimental conditions of the experimental pipeline significantly affect the changes in the flow regime. Given the differential pressure signal of the two-phase flow, we propose two effective quadric time–frequency representations, namely, the adaptive optimal kernel time–frequency representation (AOK TFR) and data reduction sub-frequency band wavelet (DA SFBW) to investigate the complex behavior of vertical upward gas–liquid flows. We extract the positive power spectral density of the singular value–frequency entropy, singular value–frequency entropy, damping ratio, and vibration mode to characterize the evolution of flow patterns. The results suggest that the AOK TFR method can reveal flow dynamic details, whereas the DA SFBW based method is a powerful tool for characterizing the dynamical characteristics of different vertical upward gas–liquid flow patterns.

周方向非均一濡れ流路における気液二相流の圧力損失特性

If the flow channel in the PEFC separator is plugged with the liquid product water, pressure loss increases and the flow is destabilized to cause serious deterioration of the power generation performance. It may be probable that occurrence of the channel plugging can be avoided if the surface wettability of the channel wall is circumferentially non-uniform. In this work, systematic pressure drop measurements were carried out for the gas-liquid two-phase flow in small rectangular channels of various wettability distributions. It was found that the fluctuation of pressure loss that is frequently encountered in uniform wettability channels can dramatically be mitigated by the use of the flow channel of circumferentially non-uniform wettability.

The sliced trispectrum fluctuation characteristics and flow pattern representation of the nitrogen-water two-phase flow of small channel

A new method of nonlinear analysis is proposed by combining the sliced trispectrum method with the fluctuation characteristics, and the influences of the length of the time sequence and fast Fourier transform (FFT) on the fluctuation characteristic value of sliced trispectrum are discussed. It is found that in the absence of noise, the length of sequence and FFT are proportional to the vector distance value; in the presence of noise, it is approximate inversely related. In order to test the anti-noise ability and characterize complexity ability of the fluctuation characteristics method, the proposed method is applied to fractal sequence (Brown), chaotic sequence (Lorenz) and periodic sequence (sine signal). The results show that compared with other power spectral method, the method of the fluctuation characteristics has good noise immunity of fractal sequence, and the anti-noise ability is relatively weak with periodic sequence; but the fluctuation characteristic theory of sequence internal complexity representation has a good effectiveness. On this basis, the differential pressure signals of nitrogen-water two-phase flow in small rectangular channel (w×h=2 mm×0.81 mm) are studied. By analyzing the differential pressure sliced trispectrum of flow patterns, the secondary coupling phenomenon of main oscillation mode of different flow patterns is established. The fluctuation characteristic values of the sliced trispectrum of different flow patterns are extracted to accurately identify the typical flow patterns of small channel nitrogen-water two-phase flow. At the same time, the fluctuation characteristic theory can be used to provide a useful exploration for the further investigation of flowing mechanism of multi-phase flows.

Effect of Small Rectangular Channel Height on Bubble Moving Velocity in Gas-liquid Two-phase Flow through Sudden Contraction

An experimental investigation is carried out on the effects of a sudden contraction on the characteristics of an air-water two-phase flow in a millimeter-scale rectangular channel. The results reveal that the flow pattern transition types caused by sudden contraction can be basically classified into five. The scatter of the leading edge velocity of a bubble observed downstream of the sudden contraction increases as the channel height becomes smaller than a certain critical value. This phenomenon is not beneficial for practical usage of gas-liquid two-phase flows in micro channels.

Solutal transport in rectangular nanochannels under pressure-driven flow conditions

In this article, we investigate the effect of channel sidewalls on the transport of neutral samples through rectangular conduits under pressure-driven flow and small zeta potential conditions. Our analyses show that while these structures can significantly reduce the streaming potential in small aspect ratio rectangular channels, they introduce a very minor variation in the sample velocity with the extent of Debye layer overlap in the system. Moreover, the increase in sample dispersion due to the channel side-regions has been shown to be nearly independent of the Debye layer thickness and very comparable to that reported under simple pressure-driven flow conditions. Interestingly however, a simple one-dimensional (1D) model that decouples band broadening arising due to diffusional limitations across the depth and width of the rectangular conduit has been shown to capture the predicted dependence of the Taylor–Aris dispersion coefficient on the channel aspect ratio under all operating conditions with less than 3% error.

Two-phase flow pressure change subject to sudden contraction in small rectangular channels

This study investigates the pressure change and flow pattern subject to the influence of sudden contractions. The air and water mixture flows from small rectangular channels (2 × 4, 2 × 6, 4 × 4 and 4 × 6 mm, respectively) into a 2 mm diameter tube. The total mass flux ( G) ranges from 100 to 700 kg/m 2 s with gas quality ( x) being varied from 0.001 to 0.8. In general the contraction pressure change increases with the rise of mass flux, and gas quality but an unique deflection of contraction pressure change pertaining to liquid vena contracta at a very low gas quality is encountered at a very low gas quality in the 4 ± 6 mm test section. For a low gas quality, elongated bubble prevails after the contraction, yet the size of elongated bubbles is reduced when the aspect ratio is increased. Comparisons amid the pressure change data of this study and available literature data with the predictions of existing model/correlations indicate that none of them can accurately predict the data. It is found that the influence of surface tension and outlet tube size, or equivalently the Bond number plays a major role for the departure of various models/correlations. Among the models/correlations being examined, the homogeneous model shows a little better than the others. Hence by taking account the influences of gas quality, Bond number, Weber number and area contraction ratio into the homogeneous model, a modified homogeneous correlation is proposed that considerably improves the predictive ability over existing correlations with a mean deviation of 30% to all the data.

Two-phase flow pattern and frictional performance across small rectangular channels

This study presents flow visualizations and two-phase frictional pressure drop data for three rectangular channels with channel height of 3, 6 and 9 mm, and a fixed width of 3 mm. It is found that the stratified flow pattern still exists for an aspect ratio of unity at a low mass flux of 100 kg/m 2 s but it completely vanishes when G > 200 kg/m 2s. For the same plug flow of intermittent flow pattern, the number of plug increases whereas its length decreases when the aspect ratio is increased. This is especially pronounced when the mass flux is further increased over 500 kg/m 2 s. The major departure of the observed flow pattern relative to the conventional Mandhane flow map is the transition boundary for slug/annular had been moved to a much lower superficial vapor velocity. The two-phase frictional pressure drop data are compared to homogeneous and Chisholm method, Wambsganss and Ide-Fukano correlations. It is found that none of the existing methods or correlations can satisfactorily predict the two-phase pressure gradient in rectangular channels. A modified C factor of Chisholm method considering the effect of aspect ratio was proposed from the empirical fit with the data sets of Wambsganss et al., Ide-Fukano, and this study. The corresponding mean deviations of the proposed correlation against the datasets are 24.99%, 10.83% and 10.73%, respectively. This correlation is applicable in wide rages of mass flux (50 < G < 700 kg/m 2 s), vapor quality (0.001 < x < 0.95), Martinelli parameter (0.05 < X < 20) and aspect ratio (0.1 < A < 1.0).

Two-phase flow characteristics across sudden contraction in small rectangular channels

This study examines the two-phase flow pattern change and pressure drop at the sudden contraction from small rectangular channels (3 × 9 mm and 3 × 6 mm) into a 3 mm diameter tube. A unique deflection of contraction pressure drop vs. vapor quality is observed at a very low quality regime with an unusual “liquid like vena contracta”. The measured pressure drops are compared with existing correlations/models, but none of them can accurately predict the available database. By proposing a correction factor accounting for the influence of surface tension (Bond number and contraction ratio) to the original homogeneous model, considerable improvement of the predictive ability of homogeneous model is arrived. The mean deviation of this correlation is 26.18% applicable to the present data (80 points) and available literatures (357 points).

Two-phase flow characteristics across sudden expansion in small rectangular channels

This study examines the two-phase flow pattern change and frictional pressure loss pertaining to the sudden flow area expansion from a small diameter tube into small rectangular channels. It is found that a unique flow pattern “liquid jet-like flow pattern” occurs at a very low quality region of G = 100 kg / m 2 s in a test section of 3 mm × 9 mm . This flow pattern is almost inconceivable for the 3 mm × 6 mm test section. The pressure difference usually increases with vapor quality but a setback is encountered provided that the liquid jet flow pattern prevails. A phenomenal observation suggests that a conceivable reduction of the pressure difference is related to the liquid jet-like flow pattern, yet a flat or level-off of the pressure difference is seen when the liquid jet-like flow is less pronounced. By contrast, an appreciable increase of pressure difference is seen when the liquid jet-like flow pattern is completely gone. The measured pressure differences are compared with existing correlations/models, whereas none of them gives satisfactory predictions against the present data. It is found that a slight modification to the empirical constant K of Wadle’s correlation may give rise to a fair improvement of the predictive ability.

Two-phase frictional pressure drop in small rectangular channels

This study presents single-phase and two-phase frictional pressure drop data in three rectangular channels with channel width of 3, 6 and 9 mm, and a fixed gap of 3 mm. Adiabatic flows of air–water mixtures are tested at mass flux of 100–700 kg/m 2 s and gas quality ranging from 0.001 to 0.8. The single-phase friction factors agree satisfactorily with the predictions of laminar and turbulent flows. The two-phase frictional pressure drop data are compared to predictions of homogeneous and Chisholm method, Wambsganss and Ide–Fukano correlations. It is found that none of the existing methods or correlations can predict the two-phase pressure gradient in rectangular channels satisfactorily. A modified C factor of Chisholm method considering the effect of aspect ratio is proposed from the empirical fit with the data sets of Wambsganss et al., Ide–Fukano, and this study. The corresponding mean deviations of the proposed correlation against the datasets are 24.99%, 10.83% and 10.73%, respectively. This correlation is applicable in wide rages of mass flux (50 < G < 700 kg/m 2 s), gas quality (0.001 < x < 0.95), Martinelli parameter (0.05 < X < 20) and aspect ratio (0.1 < A < 1.0).

サブクール流動沸騰の高発熱密度電子素子冷却への応用について(OS.16 明日に向けた熱工学の技術)

Subcooled flow boiling of water is performed in a horizontal small rectangular channel with 5mm height and 14mm width for the purpose of high heat flux cooling of power electronics. A small heating surface of 10mm length or 20mm length for 10mm width is located on the bottom surface of the channel. Microbubble Emission Boiling, MEB, occurs in transition boiling and higher heat fluxes than 500W/cm^2 are obtained. For the longer heating surface, the heat flux in MEB decreases for the down stream of the heating surface because of falling the subcooling of liquid. At lower liquid subcooling of 20K and lower liquid velocity of 0.3m/s, unstable MEB is observed to transit easily to film boiling. The experimental results suggest how to keep the subcooling of liquid at least 20K for the down stream section of electronic devices.

Syntheses and Crystal Structures of Three-Dimensional Cyanocadmate Clathrates with Imidazole

The crystal structures of two clathrates with imidazole (=imH), [Cd3(CN)6(imH)2]·C6H12, I, and [Cd3(CN)6(imH)2]·C6H5Cl, II, have been analyzed by single-crystal X-ray diffraction methods. Clathrate I crystallizes in the monoclinic system, space group P21/n with a=13.416(2), b=14.140(2), c=14.151(1)Å, β=105.08(9)°, V=2592.1(5)Åsup3;, Z=4, R 1=0.0360, wR 2=0.0850 for 5931 independent reflections; II: monoclinic P21/n, a=12.369(2), b=14.313(2), c=14.416(4)Å, β=106.04(2)°, V=2452.7(9)Åsup3;, Z=4, R 1=0.0398, wR 2=0.1105 for 5642 independent reflections. The three-dimensional host structures of clathrates I and II are isostructural with each other. The host framework is built through cyanide bridges among two kinds of Cd ions, tetrahedral and octahedral, in a ratio of 2 : 1. The tetrahedral Cd(1) centre is linked to three octahedral Cd(3) atoms and one tetrahedral Cd(2) atom; the tetrahedral Cd(2) centre is ligated by one unidentate imidazole ligand and linked to one tetrahedral Cd(1) atom and two octahedral Cd(3) atoms; the octahedral Cd(3) centre is ligated by one unidentate imidazole ligand and linked to three tetrahedral Cd(1) atoms and two tetrahedral Cd(2) atoms. The novel host structure provides two types of channels, a large, elongated, rectangular channel and a small rectangular channel. The larger channel is occupied by imidazole ligands coordinated to tetrahedral Cd(2) and octahedral Cd(3). The smaller channel accommodates the guest molecules cyclohexane and chlorobenzene.

Review of saturated flow boiling in small passages of compact heat-exchangers

Intensification of process heat exchangers has led to considerable interest in compact heat exchangers for evaporation. These heat exchangers are characterised by multichannel passages with hydraulic diameters less than about 10 mm. In this article, two different categories of channels composing compact heat exchangers are reviewed: small circular and rectangular channels (including studies of individual channels and multichannel arrangements of parallel channels) and multichannel arrangements of straight perforated and serrated fin passages. The regimes of the two-phase flow which occur in these passages differ from those in larger systems. The typical trends of local heat transfer coefficient in small passages of compact heat exchangers show that it is easy to distinguish between the two dominant mechanisms of boiling, nucleate and convective boiling. The insights relative to heat transfer mechanisms in small passages and the flow patterns associated to convective and nucleate boiling regions are presented. In particular, the effects of vapour quality, mass flux, heat flux, channel geometry/size, fin type, gravity and surface tension in each boiling region are highlighted. Literature correlations valuable in small channels expressing the local boiling heat transfer coefficient as a function of vapour quality, mass flux, heat flux, fluid properties, fluid pressure and channels dimensions are reported.

Heat transfer and pressure drop in narrow rectangular channels

Recently, with the advent of more powerful electronic chips and the miniaturization of electronic circuits and other compact systems, a great demand exists for developing efficient heat removal techniques to accommodate these high heat fluxes. With this objective in mind, both single-phase forced convection and subcooled and saturated nucleate boiling experiments have been performed in small rectangular channels using FC-84 as test fluid. The test section used in these experiments consisted of five parallel channels with each channel having the following dimensions: hydraulic diameter ( D h)=0.75 mm and length to diameter ratio ( L/ D h)=409.8. The experiments were performed with the channels oriented horizontally and uniform heat fluxes applied at the top and bottom surfaces. The parameters that were varied during the experiments included the mass flow rate, inlet liquid subcooling, and heat flux. In each of the experiments conducted, the temperature of both the liquid and the wall were measured at various locations along the flow direction. Based on the measured temperatures, pressure drops, and the overall energy balance across the test section, the heat transfer coefficients for both single-phase forced convection and flow boiling has been calculated. Additionally, in these experiments, the single- and two-phase pressure drop across the channels was also measured. A correlation has been developed for two-phase flow pressure drop under subcooled and saturated nucleate boiling conditions. Furthermore, two new correlations are proposed – one for subcooled flow boiling heat transfer and the other for saturated flow boiling heat transfer.

Heat transfer correlation for boiling flows in small rectangular horizontal channels with low aspect ratios

In the present experimental study, a correlation is proposed to represent the heat transfer coefficients of the boiling flows through horizontal rectangular channels with low aspect ratios. The gap between the upper and the lower plates of each channel ranges from 0.4 to 2 mm while the channel width being fixed to 20 mm. Refrigerant 113 was used as the test fluid. The mass flux ranges from 50 to 200 kg/m 2 s and the channel walls were uniformly heated up to 15 kW/m 2 . The quality range covers from 0.15 to 0.75 and the flow pattern appeared to be annular. The modified Lockhart–Martinelli correlation for the frictional pressure drop was confirmed to be within an accuracy of ±20%. The heat transfer coefficients increase with the mass flux and the local quality; however the effect of the heat flux appears to be minor. At the low mass flux condition, which is more likely to be with the smaller gap size, the heat transfer rate is primarily controlled by the liquid film thickness. A modified form of the enhancement factor F for the heat transfer coefficient in the range of Re LF⩽200 well correlates the experimental data within the deviation of ±20%. The Kandlikar's flow boiling correlation covers the higher mass flux range ( Re LF>200) with 10.7% mean deviation.

Initial stages of the anodic etching of aluminum foils studied by atomic force microscopy

The modification of the surface morphology of two types of aluminum foils during galvanostatic etching in hot HCl⧹H 2SO 4 solution was studied as a function of etch time by Atomic Force Microscopy (AFM) and complementary electrochemical measurements. Particular emphasis was placed on the nucleation and growth of etch pits and tunnels. Three stages of the etch process with characteristic differences in the surface morphology can be identified: for etch times up to 5–10 s predominantly large etch pits with surface areas 0.5 mm 2 are formed (stage I), followed by the nucleation of small rectangular pits and channels (0.01–0.5 mm 2 area) for etch times up to 20 s (stage II), and finally by the formation and lateral growth of surface areas with a rough, strongly etched surface morphology for etch times >20 s (stage III). The distribution of smaller etch pits and tunnels and the rough morphology in stage III differ strongly for the two different materials, which can be correlated with differences in the sub-micrometer scale morphology of the foils prior to etching. The etch process is accompanied by the deposition and redissolution of characteristic hemispherical hills.

Electrophoretic separation schemes in ultrathin channels with capillary sample introduction

A continuous sample introduction and separation scheme is presented as a rapid, low volume technique for bioanalytical separations. This new technique involves continuous sample introduction via a conventional small bore capillary coupled to a small dimension rectangular channel. Both free zone and size based separations have been carried out in the rectangular channel. Additionally, micellar electrokinetic channel chromatography is demonstrated. Laser induced fluorescence and electrochemical array detection schemes have been employed with this technique. Some of the areas this technology has been used to investigate include monitoring dynamic events from microenvironments, monitoring analytes over long periods of time, kinetic analysis, and performing parallel DNA separations.

Multiple Separations in Microfabricated Channels: From Biological Microenvironments to DNA

A continuous sample introduction and separation scheme is presented as an alternative to the current slab gel and microfabricated chip technologies for biological separations. This new technique involves continuous sample introduction via a conventional small bore capillary coupled to a small dimension rectangular channel. Both free zone and size based separations have been carried out in the rectangular channel. Laser induced fluorescence and electrochemical detection schemes have been employed with this technique. Some of the areas this technology has been used to investigate include monitoring dynamic events from microenvironments, monitoring analytes over long periods of time, and performing DNA separations.

Convective heat transfer and flow friction for water flow in microchannel structures

The single-phase forced convective heat transfer and flow characteristics of water in microchannel structures/plates with small rectangular channels having hydraulic diameters of 0.133 – 0.367 mm and distinct geometric configurations were investigated experimentally. The results indicated that the geometric configuration had a significant effect on the single-phase convective heat transfer and flow characteristics. The laminar heat transfer was found to be dependent upon the aspect ratio and the ratio of the hydraulic diameter to the center-to-center distance of the microchannels. The turbulent heat transfer was found to be a further function of a new dimensionless variable, Z, such that Z = 0.5 will be the optimum configuration for turbulent heat transfer regardless of the groove aspect ratio. The friction factor or flow resistance reached a minimum value as Z approaches 0.5. The turbulent flow resistance was usually smaller than that predicted by classical relationships, and the Reynolds number for flow transition to fully developed turbulent flow became much smaller than the ordinary channel flow. Empirical correlations were suggested for calculating both the heat transfer and pressure drop.