Lubomír Klimeš

MELTING FRONT PROPAGATION IN A PARAFFIN-BASED PHASE CHANGE MATERIAL – LAB-SCALE EXPERIMENT AND SIMULATIONS

The paper reports experimental and numerical investigation of the melting front propagation in a paraffin-based phase change material (PCM). The investigated case was a block of PCM with a heat flux introduced at one of its sides. The PCM block was contained in a transparent container and thus the propagation of the melting front could be monitored with a camera. The melting temperature of the PCM was 28 °C and the container was located in an environmental chamber where the ambient temperature was maintained at 27 °C during the experiment. The natural convection in the melted PCM played an important role and it had to be considered in the heat transfer models. The numerical models taking into account natural convection in liquid PCM require long computation times, and therefore they are impractical if the fast computation of the melting front position is needed. The effective heat conductivity approach can be used to overcome this issue. Two numerical models were compared: an in-house heat transfer model using effective conductivity approach developed in MATLAB and a more advanced model created in the off-the-shelf simulation tool COMSOL, which accounts for the natural convection in liquid PCM.

An accuracy analysis of the front tracking method and interface capturing methods for the solution of heat transfer problems with phase changes

Materials undergoing a phase change have a number of applications in practice and engineering. Computer simulation tools are often used for investigation of such heat transfer problems with phase changes since they are fast and relatively not expensive. However, a crucial issue is the accuracy of these simulation tools. Numerical methods from the interface capturing category are frequently applied. Such approaches, however, allow for only approximate tracking of the interface between the phases. The paper presents an accuracy analysis and comparison of two widely used interface capturing methods—the enthalpy and the effective heat capacity methods—with the front tracking algorithm. A paraffin-based phase change material is assumed in the study. Computational results show that the front tracking algorithm provides a significantly higher accuracy level than the considered interface capturing methods.

A Validated TRNSYS Model of Thermally Activated Layer With Phase Change Material

Simulations of building performance or HVAC systems performance usually cover a time period of several weeks, months or even a year. Therefore, the computational demand of simulation models of buildings or HVAC systems can be quite constraining for their practical application. A substantial simplification of the simulated problem is usually necessary to reduce the computational demand. The paper reports the development of a quasi 1D model of a thermally activated layer with phase change material. The model was developed in MATLAB and subsequently implemented as a TRNSYS type. The model was validated with data obtained from experiments with thermally activated panels. The experimental panels contained a 15 mm thick layer of gypsum plaster comprising 30 wt.% of microencapsulated phase change material. Plastic tubes for liquid heat carrier (water in the presented study) were embedded at the bottom of the plaster layer. Thermal imaging was used to acquire the average surface temperatures of the panels in the experimental investigations. The experimental and numerical results were in a good agreement.Copyright

Various Approaches to Numerical Discretization of Thermal Model With Phase Change

Phase change materials are nowadays used in a wide range of technical applications such as energy storage, building temperature comfort, food-freezing, etc. The accurate modelling of this process is important for the performance and usability of many technologies. Analytical solutions can be used only for simple heat transfer problems which make the numerical methods the only possible way how to deal with complex multidimensional cases. This paper deals with the comparison of commonly used numerical schemes for heat diffusion with phase change and its parallel computation possibilities.Copyright

Steel Structure Prediction for Continuous Steel Casting by Means of a Parallel GPU-Based Heat Transfer and Solidification Model

Continuous casting of steel is currently a predominant production method of steel, which is used for more than 95% of the total world steel production. An effort of steelmakers is to cast high-quality steel with a desired structure and with a minimum number of defects, which reduce the productivity. The paper presents our developed GPU-based heat transfer and solidification model for continuous casting, which is coupled with a submodel used for the prediction of the steel micro-structure. The model is implemented in CUDA/C++, which allows for rapid computing on NVIDIA GPUs. The time-dependent temperature distribution calculated by the thermal model is iteratively passed to the submodel for the steel micro-structure prediction. The structural submodel determines the spatially-dependent rates of temperature change in the strand, for which the interdendritic solidification model IDS predicts the micro-structure of steel. The paper presents preliminary simulation results for the steel grade used for pressure vessel plates, which is sensitive to rapid cooling rates.Copyright

Parameter Identification of Rheological Models Using Optimization Algorithms

The paper presents results of the application of optimization software GAMS and CONOPT solver for nonlinear optimization problems to identify multi-parameter elastomer rheological models. The solution is based on the experimentally set frequency-dependent dynamic stiffness of the elastomeric parts. Measured waveforms of dynamic stiffness are approximated using multi-parameter rheological models. The CONOPT solver has shown a significantly faster convergence and better accuracy of the calculation compared with the classical non-linear Gauss-Newton least square optimization technique.

Mathematical Model of Multi-Layer Wall with Phase Change Material and its Use in Optimal Design

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.