Papers by Author: Josef Stetina

Abstract: Phase change materials have a wide range of application including thermal energy storage in building structures, solar air collectors, heat storage units and exchangers. Such applications often utilize a commercially produced phase change material enclosed in a thin panel (container) made of aluminum. A parallel 1D heat transfer model of a container with phase change material was developed by means of the control volume and effective heat capacity methods. The parallel implementation in the CUDA computing architecture allows the model for running on graphics processing units which makes the model very fast in comparison to traditional models computed on a single CPU. The paper presents the model implementation and results of computational model benchmarking carried out with the use of high-level and low-level GPUs NVIDIA.

Abstract: The quality of the working rollers from ductile-cast-iron used for rolling rails is determined by the chemical and structural composition of the material of the rollers and the production technology. The requirements of the quality cannot be ensured without perfect knowledge of the course of solidification, cooling and heat treatment of the cast rollers as well as the kinetics of the temperature field of the casting and mould. An original application of ANSYS simulated the forming of the temperature field of the entire system comprising the casting, the mold and ambient. In the experimental investigation of temperature field, an original methodology for the measurement of the distribution of temperatures and heat flows in the roller-mould system had been developed and verified in the operation. The kinetics of the solidification has a measurable and non-negligible influence on the chemical and structural heterogeneity of the investigated type of ductile-cast-iron. Tying on to the results of the model of the temperature field of the cast rollers, an original methodology was developed for the measurement of chemical microheterogeneity. The chemical and structural heterogeneity of the cast roller is therefore a significant function of the method of melting, modification and inoculation and the successive procedures of risering, casting and crystallization after cooling.

Abstract: The paper deals with the mathematical model of the multi-layer wall containing the phase change material (PCM). The model utilizes the effective heat capacity method for modeling the latent heat of phase change and the control volume method is used for the discretization of the model. The utilization of the model is then demonstrated on the problem of the optimal design of the multi-layer wall with the PCM. The TMY2 data for the city of Brno were used in simulations as operational conditions. The main attention is aimed at the determination of the optimal thickness of the PCM layer for the multi-layer wall design with various thicknesses of the masonry.

Abstract: This paper deals with the causes of a transversal crack in a steel slab with an l300×l45 mm cross-section by means of results from two models. Samples were taken from and around the crack in order to analyze the concentration (chemical heterogeneity) of the selected elements. Simultaneously, the concentration of elements at the surface of the crack was measured after the crack was opened. The chemical heterogeneity of elements was analyzed by means of the JEOL JXA 8600/KEVEX analytical equipment. The measurement results were processed using mathematical statistics procedures. The results proved that there was an internal crack initiating immediately below the curve of the solid-state temperature and consecutively propagating.

Abstract: In the secondary-cooling zone, where the slab is beginning to straighten out the breakout of the steel can occur in points of increased local chemical and temperature heterogeneity of the steel, from increased tension as a result of the bending of the slab and also from a high local concentration of non-metal, slag inclusions. Especially dangerous are the changes in the chemical composition of the steel during the actual concasting. In the case of two melts one immediately after the other, this could lead to immediate interruption in the concasting and a breakout. The material, physical, chemical and technological parameters, which both melts differed in were determined. If the dimensionless analysis is applied for assessing and reducing the number of these parameters, then it is possible to express the level of risk of breakout as a function of five dimensionless criteria.

Abstract: An original three-dimensional (3D) model of solidification is used to describe the process of solidification and cooling of massive 500x1000x500 mm cast-iron castings in sand moulds. The calculated model of the kinetics of the temperature field of the casting is verified during casting with temperature measurements in selected points. The following dependences are later determined according to the experimental and calculated data: the average size of the graphite spheroids rg, graphite cells Rb and the average distances among the particles of graphite Lg – always as a function of the local solidification time θ [xi, yi, zi]. Furthermore, it has been found that the given basic characteristics of the structure of the cast-iron (rg, Rb and Lg) are a linear function of the logarithm of the local solidification time θ. The original spatial model of solidification can therefore be used in its first approximation for the assessment of the pouring structure of massive cast-iron castings.

Abstract: This paper deals with an analysis of the possible causes of a transversal crack in a steel slab with a l300×l45 mm cross-section using results from two models. Samples were taken from and around the crack in order to analyze the concentration (i.e. chemical heterogeneity) of the relevant elements. Simultaneously, the concentration of elements at the surface of the crack was measured after the crack was opened. The chemical heterogeneity of elements was analyzed with the aid of the JEOL JXA 8600/KEVEX analytical equipment. The measurement results were processed using mathematical statistics procedures. The results proved that there was an internal crack initiating immediately below the curve of the solid-state temperature and consecutively propagating.

Abstract: Solidification and cooling of a continuously cast steel slab and the heating of the mould is a very complicated problem of transient heat and mass transfer. This original three-dimensional (3D) numerical model is capable of simulating the temperature field of a caster. The numerical computation has to take place simultaneously with the data acquisition—not only to confront it with the actual numerical model, but also to make it more accurate throughout the process. The utilization of the numerical model of solidification and cooling plays an indispensable role in practice. An important step in this analysis is to determine the necessary quantities in the course of concasting. The software enables data acquisition in real time, which is necessary for optimization. This is ensured by the correct process procedure: real process → input data → numerical analysis → optimization → correction of process. This procedure is necessary for optimization (i.e. maximization of the quality of the process)—especially when reacting to specific needs and conditions in the operation.