ACTA TECHNICA CSAV |

The paper deals with determination of the transient electromagnetic field in a conducting cylinder placed in external uniform magnetic field. Computations are realized by means of solution of the Bessel equation in cylindrical co-ordinates using the Laplace integral transformation. Then, the Poynting theorem is used to determine the superficial density of the instantaneous power flux diffused into the cylindrical charge and instantaneous volume density of the power converted into heat. That allows determining the instantaneous power converted into heat, and, therefore, the energy of the heat emitted into the charge. These results can be used to describe the transient temperature field inside the heated charge.

The article describes a method of computation of current density, potential and polarization distribution around metallic structures buried in soil. The first step consists of developing partial differential equations together with appropriate nonlinear boundary conditions that are consequently solved in 2D by the finite element method (FEM). The formulation is first tested on a simple problem with well-known analytical solution and then applied to practical problem of the cathodic protection of a buried pipe in soil. The main goal is to suggest a computational method that allows to find such a distribution of the sacrificial electrodes that the protected structure will not corrode. The method is illustrated on a typical example.

Boundedness of linear and nonlinear Boltzmann gain collision operators is established in weighted Lebesgue spaces for a class of collision models including rigid spheres and interaction potentials with angular cut-off.

Induction heating of metal work-pieces in transverse flux magnetic field (TFIHS) belongs to modern industrial technologies. Such kind of rapid heating is especially effective in case of thin, flat strips with a thickness of the same range as the depth of electromagnetic field penetration d. The arrangement makes it possible to achieve required parameters of the technological process at low frequency of current within the inductor, for some purposes even simply at mains frequency 50 Hz. The paper presents a mathematical model of induction heating of thin brass strips. Three-dimensional model analysis of weakly-coupled electromagnetic and temperature fields is taken into consideration. Particular example of TFIHS supplied directly from the 50 Hz network is analysed. Computations are supplemented with experimental results obtained on a laboratory stand. The differences between calculation results and measurements are quite small, but, unfortunately, distribution of temperature is non-uniform. In order to obtain more uniform shape of the temperature curve, higher frequencies of the field current should be applied.

The paper introduces a way to study structure of thermally sprayed coatings in detail and join it with the knowledge about thermal history of atomized droplets. Two microstructures are compared, which originate from two different spraying techniques used for the same material.

The material selected is stainless steel AISI 316L and the techniques are plasma spraying using water-stabilized plasma gun and wire arc spraying. Both these techniques could be distinguished from all other thermal projections by high-density stream of fully molten particles projected onto a substrate. Physical nature of the means used for heating and melting of feedstock material as well as the form of the feedstock differ between two studied techniques. An analytical technique is applied which was developed for a 3D reconstruction of the architecture of thermally sprayed coatings, revealing their main characteristic structural features such as unmelted particles and porosity. The technique is called the ``Volume Reconstruction method'' (VR) and describes the shape of pores, their interactions, etc. It is based on optical microscopy and software treatment of microstructural images. This method used successfully earlier to study structure of plasma sprayed coating is here applied first time also on wire arc sprayed coating. Velocity and temperature of the particles in flight are measured with special equipment and several aspects of the parameters in flight and its significance for coating microstructure are discussed.

Part I: Theoretical review and experimental results, pp.423-458

The aim of the paper is to compare the changes in dynamics of a free plasma flow consequent upon inherence of a wall barrier, considering different flow rates and different energy inputs. The determination of the velocity of macroscopic structures in a free plasma flow is achieved using both the correlation analysis and the testing procedures based on differential geometry. The obtained results indicate that the self-organization of the vortex structures is prompted by quasi-periodic changes of the anisotropy of heat distribution, given by the product of the enthalpy and velocity of the plasma flow. The enthalpy is represented by the intensity of radiation in the central region of the jet, which allows for the reconstruction of the heat flow. Four flow rates (20, 40, 50, 80 slm) and two values of electric power inputs (150 A and 200 A) are considered for analysing two configurations of the flow-the flow against a barrier wall, and a free plasma flow.

A new circuit for processing signals from ISFET (Ion Sensitive Field Effect Transistor) is proposed to allow for measuring hydrogen ion concentration in electrolyte. In comparison to ordinary approaches, a sensitive and an additional ISFET are incorporated into a switched flip-flop circuit that has two stable states, namely 'one' and 'zero' state. The principle of the measurement consists in the fact that the concentration of hydrogen ions changes the threshold voltage of the sensitive ISFET and thus breaks the symmetry of the switched flip-flop. This asymmetry is compensated by the voltage introduced to a flip-flop in such a way that the probability of the 'one' is equal to the probability of the 'zero'. This voltage consists of two parts. The first one corresponds to the concentration of hydrogen ions and the second one to mismatches in the elements of the flip-flop. First, the voltage without influence of hydrogen ions is obtained. Then it is subtracted from the voltage obtained by the measurement carried out with presence of hydrogen ions of electrolyte to compensate for these mismatches. In addition, as the voltage is set by using a feedback circuit that processes 'ones' and 'zeros' from the flip-flop output, the result is obtained directly in the digital form.