Potentiostat for use in electro-chemical experiments

Summary In many electro-chemical experiments it is desirable that one of the current-carrying electrodes of an electrolytic bath has a constant potential. This may be realized by manual control of the bath current. Owing to the sudden and unpredictable potential fluctuations of the electrode concerned, this is very difficult and even impossible in some special cases, e.g. when passivating an Fe-electrode in aqueous H₂SO₄. In addition there may be a considerable time-lag between the potential and the current fluctuations. The different types of potentiostats described so far seem to have too large a time-lag or/and too low a sensitivity for this purpose. The authors describe a potentiostat with proportional and integrating properties resulting in a stabilization of the potential within 1/2 mV in favourable sections of the potential current characteristic. Even with an electrode with a very steep potential current characteristic (e.g. the above mentioned Fe-electrode) the ripple never exceeds a few mV. The bath current, the maximum value of which is 30 mA, is the differential current of two electronic tubes. This type of circuit offers various advantages.     

On the pressure dependence of the viscosity of aqueous sugar solutions

New experimental data for the pressure dependence of the viscosity of aqueous solutions of different sugars are presented. Measurements were carried out with a gravity driven high pressure viscometer with a maximum pressure of 700 MPa. The influence of both concentration and temperature on the pressure dependence of the viscosity is considered. Next to the data, a viscosity model based on a suspension model is introduced and it is shown that it is able to predict the viscosity for sugar solutions in a broad parameter range, including pressure. It is demonstrated that the relative viscosity for varying sugar mass fraction at constant pressure coincides with the pressure-dependent relative viscosity for a fixed sugar mass fraction, suggesting that there are no structural changes occurring under pressure. A brief interpretation of the viscosity model is given.     

Approximate analytical solutions for solute transport in two-layer porous media

Mathematical models for transport in layered media are important for investigating how restricting layers affect rates of solute migration in soil profiles; they may also improve the analysis of solute displacement experiments. This study reports an (approximate) analytical solution for solute transport during steady-state flow in a two-layer medium requiring continuity of solute fluxes and resident concentrations at the interface. The solutions were derived with Laplace transformations making use of the binomial theorem. Results based on this solution were found to be in relatively good agreement with those obtained using numerical inversion of the Laplace transform. An expression for the flux-averaged concentration in the second layer was also obtained. Zero- and first-order approximations for the solute distribution in the second layer were derived for a thin first layer representing a water film or crust on top of the medium. These thin-layer approximations did not perform as well as the binomial solution, except for the first-order approximation when the Peclet number,P, of the first layer, was low (P<5). Results of this study indicate that the ordering of two layers will affect the predicted breakthrough curves at the outlet of the medium. The two-layer solution was used to illustrate the effects of dispersion in the inlet or outlet reservoirs using previously published data on apparatus-induced dispersion.The U.S. Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

Schnelle Berechnung der mittleren Temperaturdifferenz in luftgekühlten Kreuzstromwärmeaustauschern

Zusammenfassung Es wird eine bekannte, explizite Näherungsgleichung mit zehn neuen Sätzen von jeweils sechzehn empirischen Konstanten vorgestellt zur schnellen Berechnung der mittleren Temperaturdifferenz von nunmehr insgesamt achtzehn Kreuzstromführungen. Die mittlere Temperaturdifferenz wird aus der dimensionslosen Abkühlung des Prozeßstromes und der Zahl der Übergangseinheiten auf der Luftseite berechnet.

---

Fast calculation of mean temperature difference in air-cooled cross-flow heat exchangers

A known approximate explicit equation together with ten new sets of sixteen empirical coefficients is presented for the fast calculation of the mean temperature difference of by now all together eighteen crossflow arrangements. The mean temperature difference is calculated from the effectiveness of the process stream and the number of transfer units on the air side.

---

Bezeichnungen b_{i, K} Konstanten in Gl. (6) - c_p spezifische Wärmekapazität der Luft bei Konstanten Druck - Wärmekapazitätsstrom der Luft - F Austauschfläche, Fläche - f mittlerer absoluter Fehler von r - i, k Exponenten und Summenzähler in Gl. (6) - K Wärmedurchgangskoeffizient - m, n Maximalwerte von i bzw. k - p, q dimensionslose Temperaturänderungen beider Fluide, definiert durch Gln. (2) und (3) - r dimensionslose mittlere Temperaturdifferenz, definiert durch Gl. (1) - s Zahl der Übergangseinheiten (NTU) auf der Luftseite, definiert durch Gl. (4) - T Temperatur des Prozeßstroms - w Anströmgeschwindigkeit der Luft - z Zahl der Punkte - endliche Differenz - Dichte der Luft Indices I eine Rohrreihe - 1, 2 Eintritt, Austritt - a Anströmquerschnitt - Ge Gegenstrom - Gl Gleichstrom - M Mittelwert - max Maximalwert (nicht Mittelwert) Herrn Prof. em. Dr.-Ing. Dr.-Ing. E. h. H. Hausen zur Vollendung seines 85. Lebensjahres     

Laminar source flow between parallel porous disks with equal suction and injection

An analysis is presented for laminar source flow between parallel stationary porous disks with suction at one of the disks and equal injection at the other. The solution is in the form of an infinite series expansion about the solution at infinite radius, and is valid for all suction and injection rates. Expressions for the velocity, pressure, and shear stress are presented and the effect of the cross flow is discussed.Nomenclature a distance between disks - A, B, ..., J functions of R _w only - F static pressure - p dimensionless static pressure, p(a ²/ ²) - Q volumetric flow rate of the source - r radial coordinate - r dimensionless radial coordinate, r/a - R radial coordinate of a point in the flow region - R dimensionless radial coordinate of a point in the flow region, R - Re source Reynolds number, Q/2a - R _w wall Reynolds number, Va/ - reduced Reynolds number, Re/r ² - critical Reynolds number - velocity component in radial direction - u dimensionless velocity component in radial direction, a/ - average radial velocity, Q/2a - u dimensionless average radial velocity, Re/r - ratio of radial velocity to average radial velocity, u/u - velocity component in axial direction - v dimensionless velocity component in axial direction, v - V magnitude of suction or injection velocity - z axial coordinate - z dimensionless axial coordinate, z a - viscosity - density - kinematic viscosity, / - shear stress at lower disk - shear stress at upper disk - ₀ dimensionless shear stress at lower disk, - ₁ dimensionless shear stress at upper disk, - dimensionless stream function     

Study on Deformation of Polycrystalline Aluminum Alloy Using Moiré Interferometry

Polycrystalline aluminum alloy is manufactured by annealing, compared to the width of the specimen, the size of the grains can not be omitted, which makes the specimen anisotropic. Under uniaxial tension, the deformation field is inhomogeneous. In this study, moiré interferometry is successfully applied to measure the deformation of the polycrystalline specimen. The experimental results presented the stress versus strain responses of the marked grains in different orientations and different shapes. By using fringe centering method, the strain distributions under certain load in the centers and on the boundaries of the grains are analyzed.     

Detonability of simple and representative components of pyrolysis products of kerosene: pulsed detonation engine application

The use of liquid fuels such as kerosene is of interest for the Pulse Detonation Engine (PDE). In this context, a representative gaseous mixture of the lighter products resulting from the decomposition of a kerosene of type JP-10 was studied. The detonability limits of simple components (hydrogen, ethylene, propylene) and mixtures of these components were tested in a 50 mm diameter and 2.5 m long detonation tube. This dimension is compatible with the applications of the aircraft industry and more particularly the PDE. The influences of the nitrogen dilution, geometry of the DDT device (Schelkin spiral), ignition energy and initial pressure were investigated. This paper was based on work that was presented at the 19th International Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27 - August 1, 2003. Communicated by J.E. Shepherd