This study aims to develop practical tools for the mechanical design of porous media subjected to a broad gap in a hygrothermal environment. The one-dimensional transient hygrothermoelastic field in a porous strip is investigated considering the nonlinear coupling between heat and binary moisture. The derivation of the system of governing equations is summarized first, by considering the nonlinear relation between temperature, dissolved moisture content, and vapor concentration and diffusivities of both dissolved moisture and vapor. The nonlinearity between the temperature, dissolved moisture content, and vapor concentration is investigated qualitatively and quantitatively. Next, the system of governing equations is applied to the infinite strip subjected to broad gaps of temperature and dissolved moisture content and solved by the finite difference method. The distributions of temperature, dissolved moisture content, vapor concentration, and dissolution rate were illustrated numerically, and the moisture distribution was found to be highly nonlinear. Finally, based on hygrothermoelasticity, the distribution of the resulting in-plane stress in a strip free from mechanical constraints is analyzed theoretically. The occurrence of residual stress, which is quite unlike what it is under the linear theory, is confirmed. Finally, the effect of gaps in a hygrothermal environment on the residual stress is investigated qualitatively and quantitatively.
Marangoni convection is induced in liquids by surface tension gradient along a free surface. Such flows also develop in nano- and micro-scale systems and play important roles. To have a better understanding for the phenomena occurring in processing of such micro-scale systems, generally two kinds of numerical simulation approaches have been considered: continuum and discrete (molecular dynamics). While the continuum-based techniques cannot capture the intermolecular effects, the molecular dynamics approach requires huge computational cost. To address the adverse futures of these techniques, a new numerical method has been developed by combining the computational fluid dynamics (CFD) from the continuum side and Langevin dynamics from the discrete approach. The present simulation results have shown that this new numerical technique can successfully study and predict the phenomena occurring in macro-scale process applications.
Knowledge of transient critical heat flux (CHF) for subcooled boiling of water flow is important for the design of cooling devices such as fusion reactor divertor. Heating occurs rapidly in the divertor and the heat flux is estimated to reach 10 MW/m2 because of plasma disruption. To cope with such a heat flux under the transient condition, the use of a small inner diameter tube (< 3 mm diameter) is recommended, since the use of a smaller inner diameter tube results in a higher CHF. The transient CHF of subcooled boiling of water was measured for a 1.0 mm inner diameter tube, which was heated exponentially, with inlet subcoolings ranged from 82 to 145 K and a flow velocity of 10 m/s. The heat generation rate of the tube was exponentially increased according to the function Q0exp(t/τ). E-folding time, τ, ranged from 182 ms to 30.5 s. It was found that the transient CHF increased as the e-folding time decreased and that CHF of the small-diameter tube was higher than that of conventional channels. This indicated that CHF is affected by the e-folding time under transient conditions.
A humidity swing air cleaning method has been studied experimentally for improving a SPM (suspended particulate matter) removal effect. This method uses humidity swing of the air with evaporation of water and condensation of moisture, and removes the SPM by making a nuclear condensation with the SPM as nuclei and gravitational settling of the grown droplet. According to our previous study, the SPM removal effect of 90 % level was obtained at humidity swing of 0.2 kg/kgDA at flow rate of 5 L/min. The purpose of this study is to improve the SPM removal performance, especially in a high flow rate conditions. We have tested two ways; one is a modification of the dehumidifier geometry for promoting the nuclear condensation effect and the gravitational sedimentation effect. The other is an introduction of an inertial impaction as the grown droplets removal method using a coarse mesh filter as an obstacle in the fast flow condition. The geometry changes of the dehumidifier made the improvement in removal ratio up to 99.9% level at the flow rate of 5 L/min. Moreover, the introduction of the mesh filter resulted in the same removal ratio of 99.9% level at a fast flow condition of 10 L/min.
The cryopreservation of fish eggs is an important subject in the field of fishery and preservation of biological species. Thus far, there has been no success in the preservation of fish eggs because of the large size of the eggs and the thick external shell. This paper discusses the effectiveness of using the liquid meniscus formed around the egg for protecting its morphology. Freezing and thawing experiments of medaka eggs were performed under different freezing conditions, and the hatching rate of the egg was examined. Before freezing, the eggs were dehydrated at room temperature in order to reduce the effect of volume expansion caused by freezing. It was confirmed that 100% of the eggs dehydrated by 15% or less were successfully hatched. In the freezing process, a medaka egg was placed on a hydrophobic cooling plate and a thin liquid meniscus was formed around the hydrophilic egg surface. An aqueous solution of trehalose was used as the liquid meniscus as well as a cryoprotectant to prevent damage caused by freezing. Cryopreservation of the egg was not successfully performed for all processes, including intracellular freezing; however, 80% of the eggs were alive even after freezing of the external meniscus. Therefore, it is confirmed that the liquid meniscus is effective for the cryopreservation of the external shell. The liquid meniscus can reduce the physical stress due to extracellular ice growth. Moreover, since the liquid meniscus system has a low heat capacity, the thermal process is easy to control compared to the conventional method. We concluded that the present method can be used for the cryopreservation of fish eggs.
In the present study, an adjoint-based shape-optimization method is formulated for heat transfer enhancement in liquid-solid phase change problems, in which heat conduction is dominant. In the present shape-optimization scheme, extended heat transfer surfaces with constant wall temperature are allowed to deform based on variational information of a cost functional, which is obtained from the physical temperature and adjoint enthalpy fields. In the computation of the developed scheme with an enthalpy-based formulation, a meshless local Petrov-Galerkin (MLPG) method is implemented for dealing with the complex boundary shape. For high-resolution analyses, a bubble-mesh method for boundary-fitted node arrangement in a well-controlled manner is employed, combined with a high-efficiency searching algorithm for choosing the neighboring bubbles interacted with each other. In the shape evolution process for different initial fin shapes, it is demonstrated that, within a certain range of the initial state, the present shape-optimization scheme leads to tree-like fin shapes that achieve the temperature field with global similarity.
Near-field radiation intensity above a tungsten emitter surface was measured using a silica-glass fibrous optical microscope with chromium coating and emitter temperature of 950 K. The aperture size of the glass fibrous probe was 550 nm. The signal was detected using a photo-multiplier with an active wavelength range of 950 to 1700 nm and was amplified using a lock-in amplification system corresponding to the sinusoidal movement of the emitter surface with an average amplitude of 2.6 μm from the outside to the inside of near-field effect regions. Consequently, it was clearly seen that the detected intensity increased monotonically as the gap between the probe tip and the emitter surface decreased. The tendency for the intensity to increase was in agreement with that of the near-field radiation flux between two semi-infinite planes obtained through calculations using a framework of fluctuation electrodynamics.
Thermally induced two-phase oscillation in a capillary tube is known to effectively transport large amount of heat. However, the oscillating flow is very complicated, because acceleration, deceleration, evaporation and condensation take place simultaneously. In this study, heat transfer and flow characteristics of the thermally induced oscillating flow in a straight micro tube were experimentally investigated. The experimental setup was composed of heating, cooling, and moving sections. Circular and square tubes were compared to investigate the effects of the channel geometry. In addition, the circular tubes with roughed inner surfaces were tested to investigate the effect of roughness. Furthermore, different types of the moving section were compared. PF-5060 (C6F14) was used for a working fluid. The oscillating motion was detected by the pressure change in the test section and the displacement of the moving section. Effective thermal conductivities representing thermal performance were compared against the heat flux defined by the channel cross-sectional areas. The circular tube showed better performance than the square tube. The circular tubes with roughed inner surface showed better performance than the circular tube with no surface treatment. From the visualization, it is considered that the amount of wetted area affects the thermal performance.
Single-layer electrodes (SLEs) were used to replace conventional multi-layer electrodes for proton exchange membrane fuel cells (PEMFCs), and their performance in the analysis of mass transfer in an electrode was evaluated. The cell comprises a polymer electrolyte membrane, catalyst layers, and separators with a microchannel. The removal of conventional microporous and gas diffusion layers has the potential to enhance the through-plane mass transfer efficiency. The biggest challenge is the degradation of in-plane diffusivity of gas and water under ribs. Finding optimum microchannel and SLE structure by evaluating the cell performance under a rib and channel, respectively, is necessary. In this study, two types of cells were developed to obtain a fundamental understanding of gas and water transport in SLE-PEMFCs using microfabrication techniques. One cell had its cathode active area under a microchannel, and the other had it under a rib. A silicon wafer was used as a separator. A microchannel was fabricated on a silicon substrate. A current-collecting layer was coated on the substrate by Au sputtering. The reaction area was determined by the anode electrode, which was buried in the microchannel. The width of both the microchannel and rib was 100 μm. Performance evaluation was conducted using the cells, and the performance was compared under the cathode channel and rib. Results indicated that the overpotential under the rib was much larger than that under the channel. To achieve high performance as an overall cell, a large channel area is preferable. Therefore, performance evaluation was conducted under channels with widths of 50-400 μm. The channel performance over the range of widths studied (50-100 μm) was consistent, although the performance was low for widths above 100 μm. This could be because of the electrical conductivity and delamination of the electrode.
Heterogeneous nucleation is a prominent phenomenon in the environment and industry. Nanorods have been studied intensively due to a wide range of applications they possess. Here, molecular dynamics is used to analyze how different aspect ratios of a nanorod will affect spontaneous nucleation of droplets within a highly supersaturated vapor system. The results have been compared with previous studies conducted by Suh and Yasuoka (Journal of Physical Chemistry B, 115 10631; Journal of Physical Chemistry B 116 14637), which were about nanospheres and nanocubes, respectively. The previous results showed that the change in seed shape from a sphere to cube did not affect the spontaneous nucleation rate of droplets. This result is natural because the droplet nucleation rates are the number of nuclei that form and the addition of a seed will only act as a single nucleation site regardless of the shape. The addition of a nanorod, however, clearly increases the droplet nucleation rate and this trend is consistent for all aspect ratios exceeding unity. The reason for this increase was verified for greater system sizes and canceling any rotation the nanorod may exhibit. Both cases show that the increase in the nucleation rate persists. Further studies on the origin of the phenomenon will be conducted in future work.
Molecular dynamics simulations were carried out to elucidate the effect of the wettability of the carbon support used for Nafion ionomer thin films on proton transport in the ionomer, which is related to the power density of polymer electrolyte fuel cells. The Lennard-Jones wall model was used as the support model for the ionomer to generate two different hydrophobic walls: the high hydrophobic wall (H-wall) and the low hydrophobic wall (L-wall). The proton transport model, including the Grotthuss mechanism, was used to express real proton transport phenomena (In early work, we confirmed that it well reproduces the experimentally measured proton self-diffusion coefficient in a Nafion membrane.) The obtained proton self-diffusion coefficient (DH+) indicated that the DH+ for the H-wall case is larger than for the L-wall case. This is related to the morphology of the films. For the H-wall case, the sulfonic groups that form part of the hydrophilic Nafion side chains were confirmed to be oriented in the direction to the wall in the upper side of the film and opposite from the wall in the lower side of the film, which can lead to the alignment of Nafion molecules and also create lamellar water structures in the film. It was also confirmed that such water structures have better cluster connectivity and larger cluster size, meaning that they serve as better proton transport pathways.
This study proposes a prototype of a microscale engine swimming underwater powered by Marangoni convection. The engine is a layered disk with a hole and holds a bubble ring in a gap between the layers. When the liquid-gas interface at the hole edge has the surface tension gradient, the engine gains the thrust force as the reaction of Marangoni convection. In a temperature Marangoni convection experiment, one side of the engine was heated by a laser, to make temperature gradient on the liquid-gas interface. It was confirmed that Marangoni convection was generated while the expanding bubble plugged the hole, to prevent the flow penetrating through the hole. In a concentration Marangoni convection experiment, pure water and acetic acid were injected toward the hole at each side, to make concentration gradient. The engine successfully generated jet-like flow through the hole to drive itself by 1.2mm. PIV analysis visualized the flow field around the engine and the velocity profile of the jet. The jet direction was not stable because of the non-uniformity of the concentration on the gas-liquid interface and the magnitude of the jet velocity gradually diminished with the diffusion of acetic acid. The thrust force of the engine was estimated as 270nN by calculating the momentum conservation equation of the flow around the engine.
A 10 kHz simultaneous measurement of OH-CH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) is applied to a methane-air turbulent jet premixed flame. The measurement of the flame tip for high Reynolds number conditions shows that isolated fine-scale unburnt mixtures, so-called unburnt mixture islands or reactant pockets, are frequently generated. POD analysis shows that the separation of unburnt mixture from the upstream main reactants is the characteristic flame structure. In our previous study (Johchi et al. 2015), the consumption rates of isolated unburnt mixtures are estimated from changes of area of unburnt region detected in OH and CH PLIF images under the assumptions that the isolated unburnt mixtures are spherical and pillar shapes. The most expected consumption rate conditioned by curvature of flame front is about 0.71 m/s, which is much higher than the laminar burning velocity of the corresponding reactants. The consumption rate increases with the decrease of the radius of the isolated reactants. The reason that the consumption rates of the fine-scale isolated unburnt mixtures are much higher than the laminar burning velocity is discussed based on heat conduction in the isolated unburnt mixture by assuming that the heat release from mass difference between going out and coming in the preheat zone increases the mean temperature of remained reactants to consider enhancement of the effect of heat conduction. From the analysis, characteristic scale of fine-scale unburnt mixture in which heat conduction effect is significant is discussed.
Ammonia has promising features as a carbon-free fuel without greenhouse gas emission. However, due to its low combustion intensity, the combustion characteristics of ammonia have been rarely investigated. The design of ammonia based industrial applications requires the development of effective turbulent ammonia combustion models. Thus, a study of the flame stretch effect in ammonia/air premixed combustion is necessary. The objective of this research was to study the ammonia flame extinction stretch rate both experimentally and numerically and to investigate the effects of pressure on its extinction characteristics. Experiments were conducted using a counterflow flame burner. Numerical simulations were run on CHEMKIN-PRO, using the PREMIX opposedflow model, for various detailed chemistry mechanisms. The effects of pressure on the extinction stretch rate of ammonia/air premixed flame were compared with that of methane/air flame, and the effects of pressure on the detailed reaction paths were clarified. It was found that the extinction stretch rate of ammonia/air flame is low compared with that of methane/air flame, as could be expected from its low laminar burning velocity. However, the increase of extinction stretch rate with pressure was found to be greater in the case of ammonia/air flame. From detailed chemistry analysis, it was found that the different dependence on pressure of the reaction path of the two fuels could explain this difference. Indeed, the heat release process and flame strength are affected by a greater dependence on pressure of the reactions contributing the most to heat released in the case of methane/air flame. For methane/air flame, CH3 is consumed in the main heat releasing reactions, and they experience competition by the third body recombination, 2CH3+M⇋C2H6+M at high pressure. In the case of ammonia/air flame, the heat release process is mainly related to NH3+OH ⇋ NH2+H2O, NH2+OH ⇋ NH+H2O and NH2+NO⇋N2+H2O, which remain preponderant when pressure increases. Thus, the decrease of characteristic reaction time with pressure was found to be greater in the case of ammonia/air flame, explaining a larger increase in extinction stretch rate when pressure increases.
To develop accurate models for the numerical simulation of coal combustion field, detailed experimental data using laser techniques, which can figure out the basic phenomena in a coal flame, are necessary. In particular, soot is one of the important intermediate substances in a coal flame. This paper is the first paper in the world reporting soot particle size distributions in a coal flame. The spatial distribution of the primary soot particle diameter were measured by the combination of the time-resolved laser induced incandescence (TiRe-LII) method and the thermophoretic sampling (TS) method. The primary soot particle diameter distribution was expressed by the log normal function based on the particle diameter measurement using SEM images obtained from the TS samples. The relative function between the signal decay ratio obtained by TiRe-LII and the primary soot particle diameter was defined based on the log normal function. Using the relative function, the spatial distributions of the primary soot particle diameter with the soot volume fraction were obtained. The results show that the small isolated soot regions instantaneously exist in the entire combustion field. This characteristics is different from spray combustion field. From the ensemble-averaged TiRe-LII images, it was found that the soot volume fraction and the primary soot particle diameter increases with increasing the height above the burner in any radial distance. It was also found that the volumetric ratio of small particles decreases with increasing radial distance at the region close to the burner exit. However, the variation of the soot particle diameter distribution along the radial direction becomes small in the downstream region. This tendency is caused by the turbulent mixing effect. It is expected that the accurate soot formation model will be developed in the near future by using the data reported in this paper.