Steady-state Resistance Research Articles

Effect of Interfacial Water Transport Resistance on Coupled Proton and Water Transport Across Nafion

Dynamic and steady-state water flux, current density, and resistance across a Nafion 115 membrane-electrode-assembly (MEA) were measured as functions of temperature, water activity, and applied potential. After step changes in applied potential, the current, MEA resistance, and water flux evolved to new values over 3000-5000 s, indicating a slow redistribution of water in the membrane. Steady-state current density initially increased linearly with increasing potential and then saturated at higher applied potentials. Water flux increases in the direction of current flow resulting from electro-osmotic drag. The coupled transport of water and protons was modeled with an explicit accounting for electro-osmotic drag, water diffusion, and interfacial water transport resistance across the vapor/membrane interface. The model shows that water is dragged inside the membrane by the proton current, but the net water flux into and out of the membrane is controlled by interfacial water transport at the membrane/vapor interface. The coupling of electro-osmotic drag and interfacial water transport redistributes the water in the membrane. Because water entering the membrane is limited by interfacial transport, an increase in current depletes water from the anode side of the membrane, increasing the membrane resistance there, which in turn limits the current. This feedback loop between current density and membrane resistance determines the stable steady-state operation at a fixed applied potential that results in current saturation. We show that interfacial water transport resistance substantially reduces the impact of electro-osmotic drag on polymer electrolyte membrane fuel cell operation.

Impedance characteristics of grounding electrodes on earth surface

This paper deals with the comparison of impedance characteristics of grounding electrodes on earth surface and those that are buried underground. The impedance characteristics are investigated analytically, experimentally and numerically. It has been found that there exists no significant difference of a transient resistance between a grounding electrode on the earth surface and a conventional buried electrode, when the electrode length is greater than some 10m. The result agrees with that evaluated by well-known Sunde's formula for a steady-state resistance of a grounding electrode. The observation suggests that a grounding electrode does not necessarily need to be buried at a large depth, but can be laid on the earth surface covered by soils in a place where the touch and step voltages are not significant.

Integrated methodology for evaluation of energy performance of the building enclosures: part 3 — uncertainty in thermal measurements

Thermal performance of building enclosure is typically based on laboratory tests performed on dry materials without consideration of air and moisture movement through the assembly. To address the field performance of the assembly, however, one must combine measurements and hygrothermal modeling. Hygrothermal models are necessary to include effects of climate and in particular the effects of moisture movement on thermal performance. Full-scale testing is necessary to relate findings of the models to actual construction details. The first paper in this series (Bomberg and Thorsell, 2008) introduced a test program that starting with benchmarking of the R-value of the tested wall in standard conditions, examined the effects of air flow on its thermal performance. This test program does not use the average R-values of the wall as it is in the ASTM testing, but measures local thermal resistance in selected places and calculates the average R-values. The second paper in this series (Thorsell and Bomberg, 2008) applied this integrated testing and modeling approach to selected wood framed residential walls, identifying the magnitude of air flow effects on steady-state thermal resistance, as well as demonstrating that the thermal performance of the top of the wall differs from that at the bottom. It also showed a systematic and time dependent shift in thermal performance caused by moisture movement. In this, third paper in the series, we present measurements performed on metal frame walls that introduce additional sources of uncertainty in the experimental results. We end with a discussion on the need of improvements to testing procedures for evaluation of energy performance of building enclosures.

Structure Evolution and Deformation Resistance in Production and Application of Ultrafine-Grained Materials – the Concept of Steady-State Grains

Severe plastic predeformation of crystalline materials leads not only to formation of a steady-state dislocation structure including low-angle boundaries, but also brings the high-angle boundary structure into a steady state. When the steady-state flow stress is high enough, the material becomes ultrafine-grained or even nanocrystalline. The change from coarse-grained to ultrafine-grained is accompanied by a distinct change in the steady-state deformation resistance that is measured after predeformation. This is explained by two opposing effects of high-angle boundaries, namely enhanced dislocation storage and accelerated dislocation recovery. The first one causes net hardening at high temperature-normalized strain rate Z (Zener–Hollomon), the second one net softening at low Z. This means that the rate-sensitivity of the flow stress is enhanced, which causes the paradoxon of enhanced strength at enhanced ductility. Tests with abrupt large changes of deformation conditions bring the strain associated with dynamic recovery into the focus. The results of such tests indicate that the boundaries, low-angle as well as high-angle ones, migrate under concentrated stress during deformation and thereby contribute to straining and recovery. The corresponding system of differential equations needed to model structure evolution and deformation kinetics on a semi-empirical basis is briefly discussed.

Direct Connections Between the Spinal Epidural Space and the Venous Circulation in Humans

Our previous studies in pigs indicate that direct connections exist between the spinal epidural space and the venous circulation. We wondered if similar connections occur in humans and have extended our investigations to human cadavers awaiting autopsy. We studied 10 recently dead human bodies. We inserted 2 Tuohy needles into the epidural space of the lower thoracic spine at adjacent interspaces. We infused saline with a constant-flow pump into 1 needle and measured the resulting pressure through the other. Epidural pressure increased to a steady plateau during fluid infusion, and this value was recorded at several flow rates. The pressure decay after flow stopped was also recorded. Then we infused radiopaque contrast, removed the needles, and obtained a computed tomographic scan of the spine from the foramen magnum to the coccyx. Pressure in the epidural space increased to a plateau during saline infusion. Higher flow rates produced higher plateau pressures. Plots of plateau pressure versus infusion rate were linear in all bodies. The slope of the flow-pressure plot gave a steady-state resistance (543±638 mm Hg·s/mL). The time constant of the pressure decay curve allowed calculation of initial capacitance (0.090±0.062 mL/mm Hg). Contrast could be identified in veins around the spinal column in all bodies. Contrast was found most commonly in the deep veins of the neck (7 bodies) and in veins originating in the area of the brachial plexus (7 bodies). Contrast was found less commonly and in smaller amounts in veins draining into the azygous system (5 bodies) and the lumbar veins (5 bodies). No contrast was found in veins in the sacral area. A direct connection between the spinal epidural space and the venous circulation has been demonstrated in human cadavers. The connection is most commonly found in the cervical and upper thoracic spine.

Unsteady flow field under surge and rotating stall in a three-stage axial flow compressor

The unsteady flow structure between rotor blade-to-blade passages in a three-stage axial flow compressor is experimentally investigated by detailed measurements of unsteady performance characteristics, casing wall pressure fluctuations and their wavelet analyses. The main feature of the test compressor is a capacity tank facility connected in series to the compressor outlet in order to supply compression and/or expansion waves from downstream of the compressor. Research attention is focused on the post-stall characteristics of the surge and rotating stall which occur simultaneously. The influence of the compressor operating point on the unsteady performance curve shows that the surge cycle changes irregularly depending on the steady-state resistance characteristics, and the results of the wavelet analyses of the wall pressure fluctuations suggest that the surge cycle may selectively be determined by the rotating stall cell structure within the rotor cascade.

接地電極過渡応答理論式の導出と測定結果の検討

This paper has derived analytical formulas of a voltage and a current on a grounding electrode by adopting a simple model circuit composed of R, L and C elements. Based on the formulas, its transient characteristic is discussed, and approximate formulas of the surge impedance and the propagation velocity are proposed. Also, this paper summarizes measured results of transient responses on grounding electrodes with horizontal and vertical arrangements. The characteristics of the measured results are estimated based on the analytical formulas derived in the paper. It becomes clear that most of the measured results show either an inductive or a resistive characteristic as far as the investigated cases in the paper are concerned. In the capacitive case, the transient resistance is not much different from the steady-state resistance. Also, it has been shown that measured steady-state resistances agree well with those evaluated by Sunde's formula for a horizontal electrode.

A new dynamic sweating hotplate system for steady-state and dynamic thermal comfort measurements

This paper discusses design details and measurement principles of a dynamic sweating hotplate system used for both steady-state thermal and water vapor resistance and dynamic water vapor transmission rate measurements. Dynamic and steady-state transfer measurements were carried out by separate systems in preceding studies. Water vapor resistance and dynamic water vapor transmission rate values were determined according to the new principles put forward in this study. Relationships between properties obtained by the new system and standard permeability (water vapor and air) parameters were determined for different types of fabrics used for sports clothing. The materials and physical/constructional properties of the investigated fabrics varied in a wide range as the aim was to determine if the developed system could differentiate their comfort performances. According to the results, water vapor resistance is significantly correlated with fabric weight and water vapor resistance is the best in differentiating the water vapor permeability characteristics of the fabrics among other transfer properties.

Experimental Investigation of the Thermal Impedance of Interstitial Material at a Junction

Using a quasi-steady-state T-type probe, experimental evidence of the different behavior of the thermal impedance of a junction with different interstitial material (interposer) was presented. In the T-type probe, a short hot wire serves both as a heater and a thermometer, which is subjected to an alternating current, and a thermally infinite long test wire is attached to the midpoint of the hot wire with an interposer. The thermal impedance of the interposer was introduced, which was taken to be the product of the steady-state thermal resistance and a complex ratio function. A complete expression for the thermal impedance of the interposer was derived, and the effects of the radiation heat loss as well as the deviation of the contact junction position were theoretically estimated. A microscale Pt wire with two interposers was measured, including solidified platinum black and Apiezon N vacuum grease. Experimental results showed that the platinum black contact with high thermal effusivity served as a negative thermal impedance, while a positive thermal impedance was observed for the Apiezon N contact. The obtained thermal impedance of the Apiezon N contact could be equivalent to its thermal resistance, which was verified by measuring the thermal conductivity of a Cu wire using the steady-state T-type probe.

High Frequency Grounding Impedances of Vertically-Driven Ground Rods

Grounding impedance depends on the frequency of current flowing into a grounding system. Lightning in particular has a broad frequency spectrum from some tens of ㎐ to a few ㎒. So the grounding impedance related to transient currents such as lightning should be measured. In this paper, the grounding impedances of vertically-driven ground rods of 10, 30 and 48[m] long are measured and analyzed as functions of the frequency of injected current and the feeding point. As a result, the longer the ground rod is, the lower the steady-state ground resistance is. However the grounding impedance of a vertically-driven ground rod at a high frequency is significantly increased. It is not always true that low grounding impedance follows from a low steady-state ground resistance. It is important to evaluate the high frequency performance of grounding systems for protection against lightning.

Unsteady finite-depth effects during resistance tests on a ship model in a towing tank

This paper covers an extension of the study of Doctors et al. (J Ship Res 52(4):263–273, 2008) on oscillations in wave resistance during the constant-velocity phase of a towing-tank resistance test on a ship model to the case of relatively shallow water. We demonstrate here that the unsteady effects are very prominent and that it is essentially impossible to achieve a steady-state resistance curve in a towing tank of typical proportions for a water-depth-to-model-length ratio of 0.25. This statement is particularly true in the speed region near a depth Froude number of unity. However, on the positive side, we show here that an application of unsteady linearized wave-resistance theory provides an excellent prediction of the measured total resistance, when one accounts for the form factor in the usual manner. Finally, a simple application of the results to the planning and analysis of towing-tank tests is presented.

Dynamic pressure–flow relationship of the cerebral circulation during acute increase in arterial pressure

The physiological mechanism(s) for the regulation of the dynamic pressure-flow relationship of the cerebral circulation are not well understood. We studied the effects of acute cerebral vasoconstriction on the transfer function between spontaneous changes in blood pressure (BP) and cerebral blood flow velocity (CBFV) in 13 healthy subjects (30 +/- 7 years). CBFV was measured in the middle cerebral artery using transcranial Doppler. BP was increased stepwise with phenylephrine infusion at 0.5, 1.0 and 2.0 microg kg(-1) min(-1). Phenylephrine increased BP by 11, 23 and 37% from baseline, while CBFV increased (11%) only with the highest increase in BP. Cerebrovascular resistance index (BP/CBFV) increased progressively by 6, 17 and 23%, demonstrating effective steady-state autoregulation. Transfer function gain at the low frequencies (LF, 0.07-0.20 Hz) was reduced by 15, 14 and 14%, while the phase was reduced by 10, 17 and 31%. A similar trend of changes was observed at the high frequencies (HF, 0.20-0.35 Hz), but gain and phase remained unchanged at the very low frequencies (VLF, 0.02-0.07 Hz). Windkessel model simulation suggests that increases in steady-state cerebrovascular resistance and/or decreases in vascular compliance during cerebral vasoconstriction contribute to the changes in gain and phase. These findings suggest that changes in steady-state cerebrovascular resistance and/or vascular compliance modulate the dynamic pressure-flow relationship at the low and high frequencies, while dynamic autoregulation is likely to be dominant at the very low frequencies. Thus, oscillations in CBFV are modulated not only by dynamic autoregulation, but also by changes in steady-state cerebrovascular resistance and/or vascular compliance.

First in situ determination of the ground thermal conductivity for boreholeheat exchanger applications in Saudi Arabia

The paper deals with the in situ experimental determination of the thermal properties of the underground soil for use in the design of borehole heat exchangers (BHE). The approach is based on recording the unsteady thermal response of a BHE that has been installed for the first time in Saudi Arabia. In this approach, the temperature of the circulating fluid has been recorded at the inlet and outlet sections of the BHE with time following the start of its operation. Severe fluctuations in these temperatures occur at small times (up to 8 h) due to the transient effects inside the borehole and must be excluded. A method has been developed for estimating the time period characterized by these severe fluctuations. The recorded thermal responses together with the line source theory are used to determine the thermal conductivity, thermal diffusivity and the steady-state equivalent thermal resistance of the underground soil.

Validation of a Simple Steady-State Forecast of Minimum Nocturnal Temperatures

Abstract A two-layer steady-state resistance model is compared with routine meteorological data collected from the Western Australian wheat belt during 2000–06. Major difficulties in implementing such a model are the correct parameterization for the incoming longwave radiation and estimation of daily soil moisture, neither of which are routinely measured. These difficulties are addressed by testing parameterizations for incoming longwave radiation calibrated to local conditions and incorporating a soil–water balance model based on routine weather data. The modified model has RMSE and biases ranging from 2.4° to 3.1°C and −0.2° to 0.8°C, respectively, across the wheat belt when comparing all minimum nocturnal temperatures. The model is shown to predict frost events approximately 55% of the time and illustrates that frost damage to foliage may occur when screen temperatures are < 2°C.

Shrinkage Effects of the Conduction Zone in the Electrical Properties of Metal Oxide Nanocrystals: The Basis for Room Temperature Conductometric Gas Sensor

The influence of charge localized at the surface of minute metal oxide nanocrystals was studied in WO3 and In2O3 nanostructures, which were obtained replicating mesoporous silica templates. Here, it is shown that the very high resistive states observed at room temperature and dark conditions were originated by the total shrinkage of the conductive zone in the inner part of these nanocrystals. On the contrary, at room temperature and under UV illumination, both photogenerated electron‐hole pairs and empty surface states generated by photons diminished the negative charge accumulated at the surface, enlarging the conductive zone and, as a consequence, leading to a reduction of the electrical resistance. Under these conditions, empty surface states produced by UV light reacted with oxidizing gaseous molecules. The charge exchange associated to these reactions also affected the size of the inner conductive zone, and leaded to a new steady‐state resistance. These chemical, physical and geometrical effects can be used for gas detection, and constitutes the basis for developing novel room temperature conductometric gas sensors responsive to oxidizing species.

A Study on Effective Length of Long Grounding Conductor in Windfarm

This paper describes a study of grounding system of wind turbine generators in a windfarm. A horizontal grounding conductor is often used to obtain a low steady-state grounding resistance. The grounding resistance of the grounding conductor shows transient characteristics. Effective length of the grounding conductor is an important parameter to estimate peak value of lightning overvoltages on the grounding system. This paper proposes approximate formulas to investigate an influence of such parameters as soil resistivity and boundary condition of the grounding system on the effective length.

Thermal Characterization of a Three-Dimensional (3D) Chip Stack

In order to determine appropriate cooling solutions for 3D chip stacks in various cases, it is important to have a better understanding of the total thermal resistance of a 3D chip stack. For this purpose, precise thermal resistance measurements and modeling of each component of a 3D chip stack are important. The thermal resistance of interconnection is considered to be one of the thermal resistance bottlenecks of a 3D chip stack. In this study, a steady-state thermal resistance measurement method is employed for the thermal resistance measurement of interconnection. The thermal resistance of the 200 μm pitch C4's (Pb97Sn3) jointed samples are measured and the thermal conductivity of C4's is derived to be 18–24 W/mK. With regard to the thermal resistance of a silicon substrate, the thermal resistance of a silicon substrate with various interconnection pitches and diameters has already been modeled by considering the concentrated heat flow to interconnection, as presented in ICEP 2008.[15] Based on the modeled data, the thermal resistance reduction by underfill with various interconnection pitches and diameters is also studied.

Acceleration of chemo-sensory information processing using transient features

A snapshot from the steady-state response of chemical sensors conveys, on average, more mature and relevant information regarding the analyte than a snapshot from the transient can provide. Nevertheless, time constraints in many applications make it infeasible to wait for and extract steady-state features. Substituting them by transient ones is the only viable solution to accelerate odor processing. Based on measurements recorded from metal-oxide sensors, we point to a correlation between a transient feature and the steady-state resistance that are observed in response to fixed analyte concentration. We utilize this correlation to expedite standard quantification and classification substantially while ensuring the performance that the steady-state feature can provide.

Strength scaling of adhesive joints in polymer–matrix composites

The fracture of adhesive joints between two glass-fibre laminates was studied by testing double cantilever beam test specimens loaded by uneven bending moments. A large-scale fracture process zone, consisting of a crack tip and a fibre bridging zone, developed. The mixed mode fracture resistance increased with increasing crack length, eventually reaching a steady-state level ( R-curve behaviour). The steady-state fracture resistance level increased with increasing amount of tangential crack opening displacement. Cohesive laws, obtained from fracture resistance data, were used for prediction the load carrying capacity of 2-m long “medium size” adhesive joint specimens subjected to four point flexure. Medium size specimens were manufactured and tested. A good agreement was found between the predicted and measured strength values of the medium-size specimens. Thus, the scaling from small specimens to medium-size specimens was successfully achieved.

A Fixed-Angle Dynamic Heat Spreading Model for (An)Isotropic Rear-Cooled Substrates

During a period of almost 40 years already, various fixed-angle heat spreading models have been developed in the literature. These models are commonly used by thermal engineers as approximations for the thermal steady-state resistance of a heat source on a rear-cooled substrate. In this paper, an extension of these models to dynamic (time-dependent) phenomena is proposed. The heat dissipated by a square source (side a) is assumed to spread out into the substrate (thickness b) under an angle ϕ. An analytical solution for the complex thermal impedance Zth(jω) in phasor notation is derived. The obtained expression, in which ϕ is used as a fitting parameter, is compared with accurate analytical results. A very good agreement is observed (average relative error less than 6%) for a wide range of the normalized thickness λ=b/a. A compact expression for the optimal heat spreading angle as a function of λ is given. Also the temperature response to a heat power step is investigated, and a simple formula for the thermal rise time is provided. Finally, the model can be easily extended to anisotropic media, which often appear in electronic packaging applications. Overall the proposed model allows a thermal designer to make quick yet accurate estimations about the dynamic behavior of the device.