Current Flow Conditions Research Articles

Study of the magnetic flux density distribution of nickel coated aluminum foams

Open cell aluminum foams are metal cellular structures with a large volume fraction of pores. Due to their high stiffness to weight ratio, they are commonly used in applications for energy absorption and mechanical damping. The stiffness of the aluminum foam was increased by a nanocrystalline nickel coating via an electrodeposition process. The deposition process and thus the coating thickness strongly depend on mass transport limitations. To visualize the coating thickness distribution of the foam, we measured the magnetic flux density distribution by scanning the surface of cuts of coated foams with a commercial Hall probe. By measuring the magnetic flux density distribution, deposition parameters as the current density and flow conditions could be optimized with regard to a more homogeneous coating thickness distribution. Furthermore, a model of the mass transport limitation at a complex three dimensional foam electrode could be evaluated from the magnetic flux density distribution of the nickel coated foam cuts.

Current Forecast for Tunnel-Element Immersion in the Bosphorus Strait, Turkey

The Bosphorus Tube Tunnel beneath the Bosphorus Strait is under construction in Istanbul, Turkey, using the tunnel-element immersed method. It is well known that the Bosphorus Strait has a two-layer current system with a rapidly changing flow. To maintain quality and safety of tunnel-element installation, it is important to accurately forecast the current-flow condition for periods of 48 h to identify windows of opportunity for the element-immersion process. In this study, a forecast system is developed based on the information collected and analyzed over an immediate past five-day period (120 h) to provide a two-day (48 h) period current prediction required for the element-immersion process. The system consists of a water-level forecast model and a current forecast model. The water level is forecasted from air pressure and wind information obtained from on-line monitoring data and meteorological service forecasts. The current is then forecasted from the water-level data taking the two-layer density interface effect and the immediate preceding 120-h current data (including its concurrent value) into consideration. This selected data-analysis period is determined based on our operational experience at the site. Both models are statistically constructed and calibrated using a year-long observation data set (October 2004–September 2005). It is found that the proposed forecast system gives sufficiently accurate current information for safe ongoing marine construction operation.

Rayleigh-Benard instability in a flat electrolyte solution layer between two horizontal ion-selective membranes

The convective stability of a flat layer of a binary electrolyte solution (an electrolyte containing cations of one type and anions of one type) between two horizontal ion-selective membranes under electric current flow conditions was studied theoretically. Membranes only permeable to anions were considered. Apart from changes in solution density resulting from solution concentration changes under the action of current, the effect of electroosmotic slip (violation of the adherence condition) at the interface between the membrane and electrolyte solution was taken into account. This effect was included in boundary conditions on hydrodynamic velocity. The critical Rayleigh and critical wave numbers were determined. It was shown that, in the electrochemical system under consideration, the critical Rayleigh number was significantly different from that for the problem of heat transfer in a horizontal liquid layer.

A numerical study on entropy generation induced by turbulent forced convection in curved rectangular ducts with various aspect ratios

The present paper analyzes the entropy generation induced by turbulent forced convection in a curved rectangular duct with external heating by numerical methods. The problem is assumed as steady, three-dimensional and turbulent. The flow features, including the secondary flow motions, the distribution of local entropy generation as well as the overall entropy generation in the whole flow fields, are analyzed. For a baseline case with Re = 20,000, external heat flux q⁎ = 0.112 and aspect ratio γ = 1, the results show the entropy generation induced by the frictional irreversibility concentrates within the regions adjacent to the duct walls, whereas the entropy generation resulted from the heat transfer irreversibility only significantly occurs near the outer wall of the duct where the external heat flux imposed. Except the baseline case, two additional cases with aspect ratio equal to 0.25 and 4 are calculated. Through the comparison of the three aspect-ratio cases, it is seen that the resultant entropy generations in the flow fields for the three cases are all dominated by the frictional irreversibilities. Among the three aspect-ratio cases, the resultant entropy generation is minimal in the γ = 1 case. Accordingly, the case with γ = 1 is concluded to be the optimal aspect ratio under the current flow condition based on the minimal entropy generation principle.

A simple method for testing the electromigration resistance of solders

Recently, the size of solder bump interconnects have been significantly reduced with the advent of high-density packaging, and thus the evaluation of electromigration in solder bumps has become necessary. The present paper proposes a simple method to test the electromigration resistance of Pb-free solders. One of the key points of the present method is the fabrication of a simple solder sample that can produce sufficient current density to cause electromigration. Moreover, the actual local temperature of a small area subjected to electromigration in the sample was measured by a direct method. A right-angled diamond-shaped hole was introduced in a thin film of solder using a focused-ion-beam system. A direct current was supplied to the film far from the hole, perpendicular to the diagonal of the hole. In this way, the current density was concentrated near to the corner of the hole, and the value of this was obtained through a theoretical analysis. It was noted that the steady temperature in the film along a line extending from the diagonal, remained constant, although the current density decreased gradually far from the corner. Therefore, the temperature at a position near the corner, where electromigration takes place, can easily be found by measuring the temperature far from the corner. The temperature was measured directly under current flow conditions by utilizing the chemical reagents with known melting points. Finally, by measuring the ratio of hillock volume to the time for the current supply as a measure of the atomic flux divergence due to electromigration, the corresponding resistances of some Pb-free solders to electromigration were evaluated.

Electrical resistivity and microstructural properties of concrete materials in conditions of current flow

The concrete matrix has a significant contribution to the global performance of reinforced concrete. In conditions of current flow (as in the case of impressed current cathodic protection (ICCP)) the concrete bulk material undergoes changes in chemical composition, electrical properties and microstructure, thus influences the overall response of the reinforced concrete system. Aiming at optimization of the ICCP technique, a comparison study is carried out in this contribution between pulse and conventional ICCP in terms of bulk matrix (concrete) response to applied electrical current regimes. Ordinary Portland cement concrete blocks were cast, differing in experimental set-up and chloride concentrations. All specimens were subject to resistance monitoring and ion concentrations determination. The favourable effects of pulse cathodic protection current flow on the concrete matrix were additionally explained by microstructural analysis of the specimens. It is found out that physicochemical changes due to ion transport as well as electrical properties of the concrete matrix are attributed to the structural alterations of the pore space induced by the cathodic protection current.

Numerical investigation on developing laminar forced convection and entropy generation in a wavy channel

The present paper investigates the developing laminar forced convection and entropy generation in a wavy channel with numerical methods. The effects of aspect ratio (W/H) and Reynolds number (Re) on entropy generation are the major concerns. The studied cases cover W/H=1, 2 and 4, and Re range from 100 to 400. The flow features, including secondary flow motion and temperature distribution as well as the detailed distributions of local entropy generation due to frictional and heat transfer irreversibilities are reported. Through the evaluations of entropy generation in the whole flow field, the case of W/H=1 is found to have the minimal entropy generation among all of the analyzed cases. Besides, the higher Re is found to be beneficial for obtaining the lower values of the total resultant entropy generation in the flow field. Accordingly, the case with W/H=1 and higher Re is suggested to be used under the current flow conditions, so that the irreversibility resulted from the developing laminar forced convection in the wavy channel could be least and the best exergy utilization could be achieved.

Design of Shape Memory Alloy Actuated Deformable Airfoil for Subsonic Flight

Aerodynamic surfaces for subsonic flight vehicles are usually designed primarily with the cruise condition in mind. With this objective in mind, the design of these aerodynamic surfaces, which usually exist as airfoils on the vehicles, are in general suboptimal for actual situation because they must be used for takeoff, landing, and maneuver in addition to cruise condition [1]. Therefore, it would be always desirable to design an airfoil with the ability to adapt to its current flow condition and alter its shape to remain the efficiency at any speed. The present paper reports a design of an airfoil with NACA 0012 profile which aims to deform the airfoil shape under the actuation the shape memory alloy (SMA) actuators embedded in it. The SMA actuators design to alter the aerodynamic lift and drag in a subsonic flow. The feasibility of the design is examined and discussed in the light of the change in lift to drag ratio and the power budget of the actuation.

Carbon deposition during propane dehydrogenation in a fuel cell

Carbon deposition in a high temperature proton-conducting fuel cell for selective propane dehydrogenation to propylene with co-generation of electrical power was investigated. Comparison of carbon deposition was made for catalytic propane dehydrogenation under an open circuit condition and electro-catalytic conversion of propane to propylene on the anode catalyst of synthesized chromium(III) oxide during fuel cell operation conditions with current flow. Carbon deposition under the fuel cell operating conditions was much less than that under the open circuit conditions. Chromium(III) oxide catalyst modified by potassium showed a better resistance to carbon formation under the open circuit conditions but was similar to the unmodified catalyst under current flow conditions.

Photoreflectance spectroscopy with nanometer spatial resolution under lateral current flow conditions in selectively doped heterostructures

AbstractThe simultaneous measurements of the photoreflectance spectra and I –V characteristics in GaAs/AlGaAs HEMT‐like structures allowed to determine the built‐in electric fields and a spatial dependence of the current density in direction transverse to heterostructure layers with nanometer spatial resolution. The variations of a spatial distribution of the current density with the lateral current were also investigated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interfacial tension between aluminum and cryolite melts during electrolysis of the systems Na3AlF6–AlF3 (NaF)–Al2O3

The interfacial tension between aluminum and cryolite melts containing different salt additions has been measured by the capillary depression method. The technique is based on the measurement of the capillary depression occurring when the capillary, which is moved vertically down through the molten salt layer, passes through the salt/metal interface. The depression is measured by simultaneous video recording of the immersion height of the alumina capillary. The interfacial tension was found to be strongly dependent on the n(NaF)/n(AlF3) ratio (cryolite ratio, CR). At the cryolite ratio 2.28 (80 wt.% Na3AlF6 + 10 wt.% AlF3 + 10 wt.% Al2O3 // Al, t = 1000 °C) the interfacial tension was 546 mN m−1, while it was 450 mN m−1 at the cryolite ratio 4.43 (80 wt.% Na3AlF6 + 10 wt.% NaF + 10 wt.% Al2O3 // Al, t = 1000 °C). Experiments under current flow conditions were also performed. During the electrolysis the interfacial tension at n(NaF)/n(AlF3) ratio 2.28 decreased from 546 mN m−1 at zero current to 518 mN m−1 at 0.112 A cm−2. The same trend was observed in the system with a cryolite ratio 4.43. The interfacial tension decreased from 450 mN m−1 at zero current to 400 mN m−1 at 0.112 A cm−2. The consequent increase in interfacial tension of these systems caused by interruption of electrolysis was observed. Electrolysis of the system 25 wt.% NaF + 75 wt.% NaCl (eutectic mixture)/Al indicated no influence of applied current on the interfacial tension at 850 °C.

Oxide Scale Formation and Stability of Fe–Cr Alloy Interconnects under Dual Atmospheres and Current Flow Conditions for SOFCs

Oxide scales formed on alloys were characterized by electrochemical resistance measurements and microstructure observation. alloy samples were exposed to air on one side and fuel gas (dilute with ) on the other side under current flow (dual atmospheres and current flow). The alloys were connected with Pt-mesh cathode and Ni-mesh anode at . The interface resistance changes at the alloy interfaces were monitored as a function of time, which increased with time in a parabolic relationship. The observed interface resistance after operation was less than at . The oxide scale phases formed were spinel, , and layer from surface to inner alloy, which is almost the same with the oxide scales formed in a single atmosphere. No significant effects of dual atmosphere and current flow was observed in the formed oxide scale phases of both anode and cathode side on alloys. Around the glass-sealing/alloy contact interfaces, a thick oxide scale was observed on the alloy. The reaction of glass components with alloy can affect the stability of oxide scales and alloys in the long-term operation.

Sub boundary-layer vortex generators for the control of shock induced separation

AbstractThe results of an investigation into the effects that sub-boundary layer vortex generators (SBVGs) have on reducing normal shock-induced turbulent boundary-layer separation are presented. The freestream Mach number and Reynolds number were M = 1·45 and 15·9 × 106/m, respectively. Total pressure profiles, static pressure distributions, surface total pressure distributions, oil flow visualisation and Schlieren photographs were used in the results analysis. The effects of SBVG height, lateral spacing and location upstream of the shock were investigated. A novel curved shape SBVG was also evaluated and comparisons against the conventional flat vane type were made. The results show that in all but two cases, separation was completely eliminated. As expected, the largest SBVGs with height,h= 55%δ, provided the greatest pressure recovery and maximum mixing. However, the shock pressure rise was highest for this case. The experiments showed that the mid height SBVG array with the largest spacing provided similar results to the SBVG array with the largest height. Reducing the distance to shock to 10δ upstream also showed some improvement over the SBVG position of 18δ upstream. It was suggested that total elimination of the separated region may not be required to achieve a balance of improved static pressure recovery whilst minimising the pressure rise through the shock. The effect of curving the SBVGs provided an improved near wall mixing with an improved static and surface total pressure recovery downstream of the separation line. The optimum SBVG for the current flow conditions was found to be the curved vanes ofh= 40%δ, with the largest spacing, located at 18δ upstream of the shock. Overall, it was apparent from the results that in comparison to larger vortex generators with a height comparable to δ, for SBVGs the parameters involved become more important in order to obtain the highest degree of mixing from a given SBVG configuration.

Transverse charge-carrier transfer in selectively doped GaAs/AlGaAs semiconductor heterostructures with longitudinal current flow

Photoreflectance spectra of selectively doped GaAs/AlGaAs heterostructures are studied under the conditions of direct current flow along the structure layers. Using a model developed for the spectra, variations of the internal transverse electric fields are calculated for longitudinal current flow. It is proved experimentally that even a weak heating of electrons in such structures leads to a spatial redistribution of electrons in the direction transverse to the heterostructure layers.

Examination of optimal separator shape of polymer electrolyte fuel cell with numerical analysis including the effect of gas flow through gas diffusion layer

This work concentrates on the effects of channel depth and separator shape on cell output performance, current density distribution and gas flow condition in various conditions with PEFC numerical analysis model including gas flow through GDL. When GDL effective porosity was small, the effect of gas flow through GDL which was changed by channel depth on cell output performance became large. However, current density distribution was ununiform. As GDL permeability became larger, cell output density increased, but current density and gas flow rate distribution were ununiform. From the results of changing the gas flow rate, it was found that the ratio of the minimum gas flow rate to the inlet flow rate depended on channel depth. Furthermore, the optimal separator, which increased output density and made the current density distribution and gas flow rate distribution uniform, was examined. It was also found that cell performance had possible to be developed by improving the turning point of the serpentine separator.

Liquid velocity in upward and downward air–water flows

Local characteristics of the liquid phase in upward and downward air–water two-phase flows were experimentally investigated in a 50.8-mm inner-diameter round pipe. An integral laser Doppler anemometry (LDA) system was used to measure the axial liquid velocity and its fluctuations. No effect of the flow direction on the liquid velocity radial profile was observed in single-phase liquid benchmark experiments. Local multi-sensor conductivity probes were used to measure the radial profiles of the bubble velocity and the void fraction. The measurement results in the upward and downward two-phase flows are compared and discussed. The results in the downward flow demonstrated that the presence of the bubbles tended to flatten the liquid velocity radial profile, and the maximum liquid velocity could occur off the pipe centerline, in particular at relatively low flow rates. However, the maximum liquid velocity always occurred at the pipe center in the upward flow. Also, noticeable turbulence enhancement due to the bubbles in the two-phase flows was observed in the current experimental flow conditions. Furthermore, the distribution parameter and the void-weighted area-averaged drift velocity were obtained based on the definitions.

LDA measurement in air–water downward flow

Local characteristics of the liquid phase in air–water downward flow were investigated in a 50.8 mm inner-diameter round pipe. A laser Doppler anemometry (LDA) system was used to measure axial liquid velocity and its fluctuations. To reduce the measurement uncertainty, the experiments were performed in flow conditions with low void fraction. Titanium dioxide particles with a mean diameter of 2 μm were used as seeding particles to enhance the data rate. Benchmark experiment in the single-phase liquid flow was first carried out to ensure good performance of the LDA system in the current setup. A total of 13 flow conditions were examined in air–water two-phase experiment. By applying a special setup of the LDA system, it was found that no further signal discrimination process was required to obtain the liquid velocity in the present low void fraction conditions. The comparisons between the liquid flow rates measured by the magnetic flow meter and those obtained from the local measurements showed good agreements, with differences less than 6.0%. The measurement results demonstrated that the presence of the bubbles tended to flatten the liquid velocity radial profile, and the maximum liquid velocity might occur off the pipe centerline, in particular at relatively low flow rates. Furthermore, the axial liquid velocity fluctuations were quite uniform in the radial direction. No significant turbulent reduction in the two-phase downward flow was observed in the current experimental flow conditions.

Characterisation of the oxygen transfer in BIMEVOX membranes under applied current conditions

Oxygen isotopic exchange has been studied for a number of materials in the BIMEVOX family of compounds. The exchanges were undertaken at 620 °C with gold grid electrodes on the samples and with a constant current flowing through the samples during the exchange anneals. These conditions simulate those used when these materials are employed in oxygen separation devices where substantial oxygen fluxes can be sustained using such simple gold grid electrodes. The results showed that samples exchanged under current flow conditions exhibit substantial oxygen exchange at the cathode, in contrast to samples where no electrical bias is applied. This effect was sustained in regions remote from the sputtered gold electrode. Complementary studies of the samples using X-ray diffraction revealed subtle changes in the diffraction patterns following experiments with current flow. These changes are ascribed to a reduction of V 5+ to V 4+ at the cathode locally transforming the BIMEVOX material into a mixed conducting material, and hence enhancing the oxygen isotopic exchange process.

Dependence of GaN photoluminescence on the excitation intensity

The dependence of the edge photoluminescence (PL) intensity on the excitation intensity in (0001) HVPE-grown GaN samples has been studied. The specific behavior found is that, at a low excitation level, the dependence is markedly superlinear, namely, superquadratic, and at high excitation levels it is nearly linear. A model accounting for the observed superquadratic behavior is proposed which is based on the identity of the recombination processes in the surface space charge region (SCR) under optical excitation with those in the SCR of the Schottky barrier or a p-n junction under current flow conditions. The superquadratic dependence is obtained analytically under the assumption that the nonradiative recombination channel is associated with multiple-hopping tunneling along a dislocation, which is formed by a chain of carrier localization centers and crosses the SCR. The experimental dependence of the PL intensity on the excitation intensity is a power-law function. The distance between the neighboring localization centers, i.e., the period of the potential along the dislocation, is determined as ∼4.1 nm.

Phase lags in oscillatory sheet flow: experiments and bed load modelling

Sheet flow corresponds to the high velocity regime when small bed ripples are washed out and sand is transported in a thin layer close to the bed. Therefore, it is often assumed that sand transport in oscillatory sheet flow behaves quasi-steady: time-dependent transport rates are assumed to be instantaneously related to the near-bed orbital velocity. However, new experimental results show that even in sheet flow, phase lags between sediment concentration and near-bed velocity can become so large that they lead to a reduction of the net (wave-averaged) transport rate. A phase lag parameter is defined, which shows that phase lags become important for fine sand, high velocities and short wave periods. A semi-unsteady model is developed that includes the effects of phase lags on the net transport rate. New experiments were carried out in a large oscillating water tunnel with three different sands ( D 50=0.13, 0.21 and 0.32 mm) for a range of prototype, combined wave–current flow conditions. Measured net transport rates were compared with predictions of a quasi-steady model and the new semi-unsteady model. This comparison indicates that net transport rates are reduced if phase lags become important: the quasi-steady model overestimates the net transport rates and the semi-unsteady model gives better agreement with the data. Time-dependent measurements of velocities and concentrations show that the reduced net transport rates can indeed be explained by phase lag effects, because for tests with smaller net transport rates than predicted by the quasi-steady model, considerable phase lags between velocities and concentrations were observed, even inside the sheet flow layer. Verification of the semi-unsteady model against a larger data set confirms the occurrence of phase lag effects in oscillatory sheet flow. Also for the larger data set the semi-unsteady model yields better agreement with the data than the quasi-steady model.