Four-wire Probe Research Articles

Electrical characterization of CIGS thin-film solar cells by two- and four-wire probe technique

A precise characterization of thin-film solar cells is of huge importance for obtaining high open-circuit voltage and low recombination rates from the interfaces or within the bulk of the main materials. Among many electrical characterization techniques, the two- and four-wire probe using the Cascade instrument is of interest since the resistance of the wires and the electrical contacts can be excluded by the additional two wires in four-wire probe configuration. In this paper, both two- and four-point probes configuration are employed to characterize the CIGS chalcogenide thin-film solar cells. The two-wire probe has been used to measure the current–voltage characteristics of the cell which results in a huge internal resistance. Therefore, the four-wire connection is also used to eliminate the load resistance to enhance the characterization’s accuracy. The load resistance in the two-wire probe diminishes the photogenerated current density at smaller voltage ranges. In contrast, the proposed four-wire probe collects more current at higher voltages due to enhanced carrier collection efficiency from contact electrodes. The current conduction mechanism is also identified at every voltage region represented by the value of the ideality factor of that voltage region. It is observed that a longer time given to the charge collection results in increased current density at a higher voltage. According to the results and device characteristics, a novel double-diode model is suggested to extract the saturation current density, shunt and series resistances and ideality factor of the cells. These cells are shown to be efficient in terms of low recombination at the interfaces and with lower series resistance as the quality of the materials is in its most possible conductive form. The measured internal resistance and saturation current density and ideality factor of the two measurement configurations are measured and compared.

Design of a new TDR probe to measure water content and electrical conductivity in highly saline soils

The inappropriate irrigation is accelerating the soil salinization in western irrigation districts of China. The amelioration of salinized land must be based on large amount of water content and salinity data. Plastic coated TDR has been designed to measure water content accurately in highly saline soil, but the soil bulk electrical conductivity cannot be measured due to the coated materials. In order to measure the volumetric water content and bulk electrical conductivity in highly saline soils at the same time, a parallel three-wire TDR probe with central rod coated which was used to measure water content and a triangle three-wire TDR probe which was used to measure electrical conductivity were integrated in one probe with four rods and one slide switch. The influence of angle in triangle three-wire TDR probe and the non-working rod on water content or electrical conductivity measurement were fully discussed through HFSS simulation and NaCl solution test. In the soil column experiment, four levels of salinity, 0.2, 0.4, 0.6, and 1.0% were set, the soil water content decreased from 30% in mass through the evaporation and measured by TDR and electronic balance. Then the probe was calibrated by model of Topp and Evett with these data. The results show that probe has the largest EPA (polarization degree index) in angle from 97° to 138°; the non-working rod will enhance the EPA during this angle range and the four-wire probe with angle of 120° is optimal; the PVC is a better insulated material which can improve the effective salinity scope; the Evett model could improve the water content measurement greatly especially in soil with higher salinity. This new four-wire insulated probe can be applied as a beneficial use to monitor the moisture and electrical conductivity in highly saline soils.

An experimental study of the wake of a model wind turbine using phase-averaging

The wake of a model wind turbine with a rotor diameter of 0.9 m has been investigated at the design condition (tip speed ratio = 6) in a closed return wind tunnel with a cross-section of 1.8 by 2.7 m. The three bladed rotor was operated in a low turbulence intensity uniform flow. Velocity data were obtained in the wake using a four-wire hot-wire probe at 18 radial traverses from x/D=0.22 to 5. All three components of the velocity vector were resolved instantaneously for a wide range of flow angles and velocity magnitudes. In addition to time averaged data, phase-averages with respect to rotor position have been used to extract periodic phase-averaged motions from the flow. In this way the downstream development of the tip vortices was extracted. The phase-averaging method allows the local phase-averaged and turbulent parts of the stresses associated with the vortical motion to be studied, as well as their production terms. The results show that the periodic phase-averaged structures initially contains most of the turbulent kinetic energy and dominate the edge of the wake. The helical vortex structures interact and pair-up, merge and finally breaks up within x/D=3. From this point the phase-averaged motion is lost and the radial transport of momentum across the wake is seen to increase significantly. By x/D=5 the large scale diffusion has removed the sharp edge of the wake that was characteristic in the initial region, and developed the approximately Gaussian velocity defect which is characteristic of bluff body wakes.

A Study on Measuring Contact Resistance of Ball Bonds on Thin Metallization

Contact resistance measurement configurations for the evaluation of ball bond reliability on thin metallizations are studied. Samples with symmetrical and asymmetrical four-wire probe configurations are analyzed and compared with a calibrated finite-element model. It is found that the measured contact resistance strongly depends on the geometric placement of the probe wires, and the value closest to the true value is obtained from a probe placement that is opposite and away from high-current-density regions on the metallization. Probes should also be placed as close as possible to the contact interface. Sheet resistance components contributed by intermediate thin metallizations are of the same order of magnitude as the unaged reported $R_{C}$ . Therefore, they are considered. Symmetrical current distributions yield a more homogeneous potential drop across the bond interface being probed, but also capture a larger component of metallization sheet resistance. Experimental samples were aged, and the contact resistance increase was measured. The impacts of various probe placements on the sensed $R_{C }$ were studied.

Effect of Sr and Ca dopants on oxidation and electrical properties of lanthanum chromite-coated AISI 430 stainless steel for solid oxide fuel cell interconnect application

Abstract Lanthanum chromite coating occupies a noticeable position as a ceramic coating on metallic interconnects in solid oxide fuel cells because of its excellent electrical conductivity, high oxidation resistance and desirable chemical stability in both oxidizing and reducing atmospheres. In the present work, a sol–gel process based on the dip-coating technique was used to prepare dense and uniform coatings on metallic alloy for interconnect application (AISI 430 type). The effect of strontium and calcium doping into lanthanum chromite structure on electrical conductivity and oxidation behavior of the coated sample has been investigated. The oxidation behavior was evaluated by cyclic oxidation test at 800 °C in lab air. In addition, the area specific resistance (ASR) of the coated and uncoated samples after long-term oxidation was measured through a two-point, four-wire probe method. It was found that, for the coated sample, when compared to the bare metallic one, the addition of Ca into lanthanum chromite coating drastically lowered the oxidation rate and electrical resistance by approximately 2 and 6 times, respectively.

Development of an optimal data reduction scheme fora four-wire hot-wire probe

Abstract One of the most widely used measurement instruments in the turbulence community is the hot-wire probe. Amongst them, the four-wire probe is lately gaining popularity because of the enhanced accuracy and the extended directional working range due to the presence of the fourth ‘redundant’ wire. However, the need for exhaustive calibration makes the hot-wire probe a less preferred instrument to be used in atmospheric research. In the present study, development and testing of an effective data reduction scheme for a four-wire probe is reported. The robustness of the data reduction scheme enables one to obtain the same order of accuracy in measurement with reduced calibration points, and in turn reduced calibration effort. The data reduction scheme works based on the calculation of the directional sensitivity function of each wire, and then, minimization of an artificially constructed error function. Since the directional sensitivity function is smooth and continuous in space, the same order of accuracy can be obtained with less effort. In the present work, a four-wire probe was considered, and calibrated in a constant velocity jet facility. Following calibration, the data reduction scheme was applied to test the probe numerically for a random noise applied on four probes simultaneously, and the angular error was computed. The assessment of the involved accuracy of the data reduction scheme with reduced points of calibration was established.

Measurements of Surface-Roughness Effects on the Development of a Vortex Produced by an Inclined Jet in Cross-Flow

This study examines the effects of surface roughness on the streamwise development of a vortex created by an isolated circular jet injected at 45 deg pitch and 90 deg skew into a crossflow. The study is motivated by the typical surface conditions encountered on in-service turbine blades of gas-turbine engines. Detailed measurements of the velocity field have been performed with a miniature four-wire probe at the jet exit plane, in the oncoming cross-stream boundary layer, and in a series of planes that capture the streamwise development of the vortex in the crossflow boundary layer up to about 15 jet-discharge diameters downstream of the jet. The paper presents the effects of surface roughness on the structure of the dominant streamwise vortex created by the interaction of the inclined jet with the mainstream, and documents the changes in the location, streamwise rate of change of circulation, and streamwise rate of diffusion of this vortex. Through these results, the change in the effectiveness of the vortex in energizing the boundary layer in the presence of surface roughness can be quantified.

Survey of multi-wire probe data processing techniques and efficient processing of four-wire probe velocity measurements in turbulent flows

A general presentation of the two main approaches for the extraction of velocity information from multi-wire hot-wire probe measurements is first given, along with a short introduction to the different data processing techniques currently available. This global survey is intended to help researchers interested in using multi-wire probes (for velocity measurements) choose more easily a data processing technique adapted to their experiments and laboratory facilities.

Three-component turbulence measurements using a four-wire probe

A probe, consisting of two orthogonal X-wire probes, allows all three velocity fluctuation components to be measured simultaneously. Attractive features of the probe include its ease of construction, the use of a standard yaw–pitch calibration and the potential to correct for spatial resolution. The reliability of this probe was assessed using measurements in the near-field of a plane jet and in the self-preserving region of a cylinder wake. Statistics of all three components compare favourably with those obtained, though not simultaneously, using standard X-wires.

The relationship between coherent structures and heat transfer processes in the initial region of a round jet

This paper describes an experimental study of the relationship between coherent vortical structures and the intensity of heat transport in the initial region of a round, free jet. Simultaneous measurements of velocity and temperature were taken with a four-wire combined probe in a jet that was acoustically stimulated with a frequency corresponding to the jet-column mode. The obtained results suggest that the mutual phase relations between oscillatory and random components of velocity and temperature lead to substantial intensification of the radial heat transport. Due to the same reason the longitudinal heat flux does not reveal a significant change in the presence of coherent structures and, as a result, a much wider spread of the temperature field in comparison with velocity may be observed as a characteristic feature of this flow. It was also observed that heat transfer processes are realized in substantial part by random turbulence generated due to the action of coherent motion.

Simultaneous measurements of temperature and its dissipation using pairs of parallel cold wires

The link between fluctuations of a passive scalar and its dissipation is an important problem for various aspects of turbulent flow modelling both with and without chemical reaction. This paper first reports experimental methods for simultaneous measurements of the three derivatives involved in temperature dissipation using cold wire probes in a boundary layer over a weakly heated flat plate. Particular attention is paid to the determination of the optimal length and spacing between sensors, including a four-wire probe of fixed dimensions, for such gradient measurements. Corrections to be applied to account for the effect of the fixed geometry are analyzed. Results about joint statistics, including conditional temperature pdfs, between the scalar and its dissipation are then reported and compared to previous results which mainly concerned conditional dissipation rates.

Lateral vorticity measurements in a turbulent wake

The accurate measurement of vorticity has proven difficult because of the difficulty of estimating spatial derivatives of velocity fluctuations reliably. A method is proposed for correcting the lateral vorticity spectrum measured using a four-wire probe. The attenuation of the measured spectrum increases as the wavenumber increases but does not vanish when the wavenumber is zero. Although the correction procedure assumes local isotropy, the major contributor to the high-wavenumber part of the vorticity spectrum is the streamwise derivative of the lateral velocity fluctuation, and the correction of this latter quantity does not depend on local isotropy. Satisfactory support for local isotropy is provided by the high-wavenumber parts of the velocity, velocity derivative and vorticity spectra measured on the centreline of a turbulent wake. Second- and fourth-order moments of vorticity show departures from local isotropy but the degree of departure seems unaffected by the turbulence Reynolds number Rλ. The vorticity probability density function is approximately exponential and has tails which stretch out to larger amplitudes as Rλ increases. The vorticity flatness factor, which is appreciably larger than the flatness factor of the streamwise velocity derivative, also increases with Rλ. When Rλ is sufficiently large for velocity structure functions to indicate a r2/3 inertial range, two-point longitudinal correlations of lateral vorticity fluctuations give encouraging support for the theoretical r−4/3 behaviour.

Velocity measurements in 3D turbulent flows by means of a rotating X-wire probe

One of the problems encountered in 3D turbulent flow measurement when using a four-wire probe is that the equation system is overdetermined, bringing many sources of errors. In the present paper we discuss a method of resolution of such a problem by means of a rotating X-wire probe, and we define a well determined 6*6 matrix system in order to obtain the solution with a good accuracy. Comparisons with the normal single-wire and the rotating X-wire probe measurements show the validity of our method in the detached flow behind a 45 degrees swept backward-facing step. The advantage of the rotating X-wire probe is its good space resolution, giving a better accuracy in the 3D velocity and in the double-correlation determination.

Computer-aided calibration and measurements with a quadruple hotwire probe

A method for calibration and measurement with a four-wire probe is described. For each of the wires a three dimensional calibration field is determined, thus no assumption like King's law or the cosine law need to be made. The velocity vector can then be detected in a fairly large angular range (± 40°) with a numerical search algorithm. First measurements in a free jet and a confined, strongly swirling flow are presented.

Vortical motion contributions to stress transport in turbulent boundary layers

Four-wire hot-wire probe measurements are used to examine the relationships between the motions bearing spanwise vorticity, ωz, and the mechanisms responsible for turbulent stress transport. The measurements were carried out in thick zero pressure gradient turbulent boundary layers over the Reynolds number range, 1010<Rθ<4850. Because of the scale of the flow, the spatial resolution of the probe was, in general, very good. The capabilities of the probe allowed all of the terms in the balance equation for 〈u2〉, except the pressure strain correlation, to be directly measured. Regarding wall region vorticity dynamics, velocity–vorticity correlation measurements are used to investigate ωz motion contributions to the gradients of the turbulent stresses. In particular, the measured positive correlation between v and ωz for y+<15 is shown to be consistent with the large ∂〈−uv〉/∂y gradient in this region, and physically, with the outward motion of sublayer streaks. Relationships between the motions bearing ωz and the sweep and ejection bursting events are examined, and the results are discussed in connection with the turbulent diffusion term in the budget for 〈u2〉. The present results indicate that in the near-wall region (but outside the sublayer) approximately two-thirds of the contributions to the turbulent diffusion term come from the correlation between the uv shear product and ωz fluctuations. The overall results are discussed relative to coherent motion interactions and the known properties of the inner region vorticity field.

Quadruple hot-wire probes in a simulated wall flow

A method for calibration and measurement with a fourwire probe is described. The method does not require any assumption about the response of the wires and it is not necessary to know the exact probe geometry. Measurements in a two-dimensional turbulent boundary layer showed large errors in the \(\overline {\upsilon \prime w\prime } \) shear stress, although the probe had a wire arrangement practically insensitive to mean velocity gradients normal to the wall. The problem seems to be caused by the strong instantaneous spanwise velocity gradients, as suggested by the computed response of a simple probe model inserted into a numerical flow obtained by direct simulation.

On accurately measuring statistics associated with small-scale structure in turbulent boundary layers using hot-wire probes

Spanwise vorticity measurements have been performed in zero-pressure-gradient boundary layers over the range 1010 < Rθ < 4850 (Rθ ≡ U∞ θ/ν, where U∞ is the free-stream velocity and θ is the momentum deficit thickness) using a four-wire probe. In addition, experiments quantifying the spatial and temporal resolution required to obtain an accurate statistical representation of the small-scale structure of wall-bounded turbulence were performed. Furthermore, a thorough investigation of statistical convergence for a variety of fluctuating quantities was performed. Comparisons with earlier high-resolution studies indicate that the maximum value of u′/uτ increases with increasing Reynolds number over the given Rθ range (u′ ≡ r.m.s. u, and uτ is the friction velocity). It is suggested that detecting this dependence provides a good measure of probe resolution. In general it was found that statistics of velocity gradients were distinctly more sensitive to finite probe size than velocity statistics. Wire spacing experiments suggest that Wyngaard's (1969) criterion is to a good approximation valid even under anisotropic conditions. Furthermore, it was found that instantaneously spatial averaging of ∂u/∂t caused significant attenuation in the resulting r.m.s., and that this averaging procedure is sensitive to the level of mean shear. A simple method of estimating how noise in the u-velocity signals enters into the ∂u/∂y signals is presented. The convergence study shows that statistical convergence criteria developed from free-shear flows severely underestimates the averaging times required in boundary layers. A table of general convergence criteria is provided.

Basic considerations concerning the construction and usage of multiple hot-wire probes for highly turbulent three-dimensional flows

The authors discuss the use of multiple hot-wire probes in turbulent three-dimensional flows with the aid of a graphical analysis. It is shown that hot-wire sensors can be used to measure all three components of the velocity vector as long as its direction lies within the acceptability range, which can be extended to a hemisphere if a probe with at least four wires is used. The detection of flow reversal is, however, not possible with any number of simple wire sensors. A computer-aided calibration and data reduction method for a four-wire probe is described. Test measurements in an axisymmetric free jet prove the applicability of the method.

Computer-aided calibration and measurements with a quadruple hotwire probe

A method for calibration and measurement with a four-wire probe is described. For each of the wires a three dimensional calibration field is determined, thus no assumption like King's law or the cosine law need to be made. The velocity vector can then be detected in a fairly large angular range (± 40°) with a numerical search algorithm. First measurements in a free jet and a confined, strongly swirling flow are presented.

A hot-wire technique for simultaneous measurement of instantaneous velocities in 3D flows

A four-wire probe and a novel signal interpretation procedure were developed for the simultaneous measurement of three instantaneous velocity components in a three-dimensional, turbulent, flow field. The new probe consists of four sensors slanted at 50 degrees with respect to the centreline axis and arranged in a double V configuration; each V configuration is perpendicular to the other. The implementation of this technique and the inclusion of the fourth wire remove ambiguity and resolve most of the problems affecting the performance of the three-wire probe and, in particular, prevent the occurrence of conditions of non-uniqueness. The new technique was tested in turbulent pipe flow and in a swirling jet issuing into a co-flowing stream. Some of the results obtained in the swirling flow are compared with previous measurements, which were obtained using a five-tube pressure probe. The agreement between both sets of data is shown to be very good.